GPS

灰色血小板症候群 gray platelet syndrome

WordNet

1. the 7th letter of the Roman alphabet (同)g

PrepTutorEJDIC

1. general practitioner一般開業医

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/06/08 01:10:24」(JST)

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Navstar=GPS衛星（図は初期のBlock-I衛星）

舶用GPS受信機

グローバル・ポジショニング・システム（Global Positioning System, GPS, 全地球測位網）は、アメリカ合衆国によって運用される衛星測位システム（地球上の現在位置を測定するためのシステムのこと）を指す。

ロラン-C (Loran-C: Long Range Navigation C) システムなどの後継にあたる。

概要

アメリカ合衆国が軍事用に打ち上げた約30個のGPS衛星のうち、上空にある数個の衛星からの信号をGPS受信機で受け取り、受信者が自身の現在位置を知るシステムである。

元来は軍事用のシステムであったが、現在では非軍事的な用途（民生的用途）でもさかんに用いられている。

GPSは地上局を利用するロラン(LORAN)-Cと異なり、受信機の上部を遮られない限り、地形の影響を受けて受信不能に陥る事が少ない。

GPS衛星からの信号には、衛星に搭載された原子時計からの時刻のデータ、衛星の天体暦（軌道）の情報などが含まれている。GPS受信機にも正確な時刻を知ることができる時計が搭載されているならば、GPS衛星からの電波を受信し、発信-受信の時刻差に電波の伝播速度（光の速度と同じ30万km/秒）を掛けることによって、その衛星からの距離がわかる。3個のGPS衛星からの距離がわかれば、空間上の一点は決定できる。

実際のGPS受信機に搭載されている時計はクオーツなどを利用しているため、あまり正確ではない。時刻の誤差がたとえ100万分の1秒であったとしても（100万分の1秒の時刻精度を維持することは非常に難しい）、距離の誤差は300mにも及んでしまう。そこで、4つのGPS衛星からの電波を受信し、GPS受信機内部の時計の校正を行いつつ測位を行う。

GPS衛星は約20,000kmの高度を一周約12時間で動く準同期衛星である（静止衛星ではない）。軌道上に打ち上げられた30個ほどの衛星コンステレーションで地球上の全域をカバーできる。また中地球軌道なので信号の送信電力としても有利であり、ある地域からみても刻々と配置が変化するため、全地球上で誤差を平均化できる（地域によってはカバーする衛星の個数が常に少ない場合もある）。

原理

二つの球面が交わるとき、一般に共通部分は円になる。

球面と円周が交わるとき、一般に二つの交点ができる。

3次元測位

GPS測位の原理は、局所慣性系で光速cが一定であることによる。

=2.99792458 × 10⁸ m/s

GPS衛星と受信機がともに正確とみなせる時計をもっていれば、送信時刻と受信時刻の差の時間tに光の速度cを掛けると距離rがわかる。

衛星の位置を座標 ()、受信機の位置を () とすると、

GPS衛星の位置X, Y, Zは受信データに重畳された航法メッセージ信号を復調して得る。

受信機の位置である三つの変数、x, y, zを得るには最低三本の連立方程式を要する。このため三つ以上のGPS衛星を受信する。つまり、

• 1個の衛星からの電波が受信でき距離が決定できたとすると、そこから距離が一定である点は球面上のどこかになる。一定の長さの糸におもりをつけて振り回すと、おもりは糸の長さを半径とする球面上のどこかにいることになる。
• 2個の衛星からの距離が決定できた場合、二つの球面が定まる。受信機は、二つの球面の交わりである円周上のどこかにいることになる。
• 3個の衛星からの距離が決定できれば、二つの球面による円にさらに一つの球面が加わる。円と球面は2点で交わり、受信機はそのうちの一点に位置する（もう一つの点は衛星が位置する3点を通る平面に対し対称な位置、すなわち地球から離れた位置にある）。三脚は3本の脚（長さは異なってもよい）があるため固定できるのと同様である。

GPS衛星には原子時計が搭載されているため、時刻は正確とみなせる（現実には遅れ等の誤差が生じるので補正する）。一方、GPS受信機は原子時計ほど正確な時計をもたず、通常のクォーツ時計程度の精度である。そこで生じる時間の誤差をδとすると、式は

四つの未知数、を求めるには最低4本の連立式を要する。つまり、4つの衛星から受信することで、原理的に受信側の時計の誤差を除くことができる。

ただし、実際には誤差が定数とは限らず、衛星によって多少異なるかもしれないし、誤差の要因は他にもある。そこで、衛星との距離 r ではなく、二つの衛星との距離の差 を考える。

δ₁ とδ₂は等しくは無くても、差は小さい。打ち消しあうことでの誤差は r の誤差よりはるかに小さくなることを利用する。
2衛星からの距離の差が一定な点は、2衛星を焦点とする回転双曲面をなす。上記同様、3衛星について双曲面の交わりから、受信機の位置を知ることができる（双曲線航法）。 それでも残る誤差の差分に応じて幅を持たせておく。さらに、より多くの衛星から受信することで誤差を減らすことができる。一般には、衛星iに対し、

i本の連立を、を用いて解くことになる。

SS変調による測距

GPS衛星からの情報を変調する方式であるSS (spread spectrum) 変調方式（CDMA: code division multiple access方式などともよばれる）は、人工的に作った（真の乱数と区別が付かない）コードである擬似雑音系列に送信データを掛けて送信信号を生成する。

このため、FMやAM変調などに比べて広いバンド幅で低電力で送信でき、秘話性（擬似雑音系列がわからなければデータを復調できない）や秘匿性（白色雑音と区別がつかないため送信していること自体がわからない）、同一バンドを異なる擬似雑音系列で多重利用できることなどの特徴がある。

擬似雑音系列の開始位置の時刻を定めておけば、復調時に精度よく送出時刻を知ることができることも特徴のひとつで、GPSではこの特徴を活かして測位とデータ（天体暦（軌道）の情報などが含まれる）の送信を同時に行っている。

GPS衛星からのL1電波 (1.57542GHz) には公表されているC/Aコードを擬似雑音系列に用いた信号と、公表されていない擬似雑音系列であるP(Y)コードの2種類の信号が載せられている。P(Y)コードは軍事目的を想定しており、系列の生成多項式の次数が大きい（擬似雑音系列が一巡するのに長時間かかる）ため、精度は非常に高く（16cm程度）、ミサイルや誘導爆弾の誘導に用いられている。

民間利用が許されている暗号化されていないC/Aコードのデータを用いると、95%以上の確率で正確な緯度経度から10m以内の座標が得られる程度の精度となる。これは短時間での精度であり、長期間受信し続けることにより精密な測量も可能である。

測位法

GPSの測位方法は、コード（搬送波の変調）に基づく方法（コード測位方式）と、搬送波の位相に基づく方法（搬送波測位方式）に分けられる。一般にはコード測位が用いられているが、高精度の測量には搬送波測位が用いられる。

単独測位 

コード測位。誤差10m程度（民生用）。

DGPS 

Differential GPS（相対測位方式）。コード測位。測位対象となる移動局のほかに、位置のわかっている基地局でもGPS電波を受信し、誤差を消去する方法。基地局で生成された補正情報を送信し、移動局で受信すれば、実時間でDGPSの補正処理を行うことができる。日本国内では、海上保安庁の中波ビーコンにより補正情報を送信しており、これを使用するのが一般的である。誤差数m。

RTK測位 

Real Time Kinematic GPS。干渉測位方式。DGPSと同様に、電子基準点から受信する電波の位相差を計測し、測位計算。測位時間1分以下、誤差数cmが可能。測量地点では、基準受信機を参照基準点（既知）に設置し、（複数の）移動受信機で測位。

高速スタティック測位 

干渉測位方式。測量地点で、複数のアンテナを固定設置し、測位時間30分以下、誤差1cm以下が可能。

VRS-RTK測位 

Virtual Reference Station RTK-GPS。RTK測位の弱点（初期待ち時間、複数の受信機が必要、移動範囲が限定、電子基準点から電波が届かない等）を改良。仮想基準点方式。複数の電子基準点と通信回線を結ぶVRSセンターがあり、測量地点では、1つの移動受信機から得られるデータを、携帯電話等によりVRSセンターと送受信し（データを持ち帰り後日計算も可能）、RTK測位を実施。

精度

GPS受信機の測位精度には、原理的な誤差による要因・人為的要因などさまざまな要因がある。ここではその他の不具合も含め列挙する。

下記のうち誤差要因については、GPS受信機である程度推定し表示することができる。GPS受信機がある円内にいる確率が50%であるところの円を、CEP (Circular Error Probability) とよぶ。地図を表示するタイプのGPS受信機では、この円も同時に表示し利用者への参考としているものもある。

電波伝播経路の特性

GPS衛星からGPS受信機まで電波が達する経路では、電離圏や対流圏での電波特性の変化により、若干の電波伝播速度の遅延が生じる場合がある。これによって、計算で定めたはずの空間上の一点の信頼性が損なわれる。一般的に受信機からみてGPS衛星が低仰角の場合、この誤差は増加する傾向がある。これは大気中を電波が伝播するときの遅延による影響が、高仰角（薄い大気を通過する）よりも低仰角（厚い大気を通過する）で大きいからである。またそもそも低仰角衛星からの信号は減衰が大きい。

このための補正手段として、正確な時計をもち座標のわかっている固定局を設置し、GPS受信データから計算した位置と固定局の位置の差から、精度を上げるなどの仕組み（ディファレンシャルGPS、Differential GPS、DGPS）も確立されている。DGPSの補正信号は、かつてFM放送の利用されていない帯域で送信するシステム（JFN系列の放送局で実施）があり、カーナビなどでの利用には有用であった（1997年5月〜2008年3月）。また、WAASやMSAS（MTSATを利用した日本の運用）では、静止軌道の衛星からDGPSの補正信号を各受信機に送信している（WAAS/MSAS静止衛星自体もGPS衛星同様、測位にも使われる）。

このほか、ビル街や谷山ではマルチパス（ひとつの衛星からひとつの受信機までの電波経路が反射などによって多数存在すること・テレビのゴースト現象同様）により、信号の時間差が生じたりS/N比が低下し、精度が落ちる。

受信可能な衛星の個数・配置による影響

GPS衛星の軌道アニメーション。数字は北緯45度（北海道付近）から同時受信可能な衛星数

通常日本（本州）では、理想的に空がひらけている場合、受信可能な衛星は6〜10個程度である。位置の計算に最低必要な4個より多い衛星がみえている場合は、複数の衛星からの情報で測位精度を向上させることができる。それぞれの衛星からの信号強度（S/N比）を観測したりDGPS情報から衛星ごとの信頼度を与え、また4つ組みの取り方をなるべく計算誤差が大きく出ないように取ったり、さらに複数の測位結果の信頼度が低いものを棄却・平均化するなどの方法がとられる。

受信可能な衛星の個数・配置により、電波伝播の誤差が大きく利いてくる場合がある。原理での三脚での喩えを用いると、三脚の脚が固定の長さではなく、ある程度伸び縮みしたとしよう。すると三脚の頭が動く範囲（推定誤差範囲）は、三脚の脚の開き具合によって異なる。計算に用いる衛星のみかけの位置が接近していると、計算に用いる推定誤差が大きくなる（脚を閉じた三脚ではぐらつきが大きい）。また計算に用いる衛星が一直線に並んでいたりする場合は、ある方向への信頼度が大きく低下する（三脚の脚が並んでいると垂直方向にぐらつきが大きい）。

補助手段による精度の向上

GPSは原理的には最低4つの人工衛星がみえていることが必要であるが、空がひらけていない場合などは、補助手段で精度を向上させることも可能である。

まず、GPS受信機内部の時計が正確な時刻に校正された後の一定時間は、時刻情報は内部の時計を用いて3つの衛星で3次元の位置を知ることができる（#原理参照）。ただしクォーツ程度の進み遅れがあると、これも数分で信頼できない時刻になってしまう。

また、地球の形が分かっており、地表（あるいは一定の高度）を移動していると考えられる場合、さらに1つの衛星からの距離を省略しても位置は求められる。地球の形（平均海面）は球体ではなく赤道付近が膨らんだ回転楕円体（扁球）であることは知られているが、これをよく近似した3次元曲面（WGS84など）を多くのGPS受信機がデータとして持っている。

さらに受信機のドップラーシフトを観測すると、C/A信号の1ビット送信時間未満の距離の観測もできる。衛星と受信機の距離が接近または乖離している場合、ドップラー効果により受信周波数の上昇または低下（これは信号の位相変化として観測される）がおきる。これを用いれば、受信機が等速直線運動しかしていないか、それ以外の方向に動いたかも推定できる（1つないし2つの衛星からの信号でもある程度の位置は推定できる）。なお長時間の位相観測によりC/A信号の精度限界以上に精度を上げる方法は、測地用のGPS受信機などでも用いられている。

またカーナビなど大きなGPS受信機では、GPSで初期位置を決定した後は、ジャイロ・加速度センサなどから得られる情報で自律位置推定できるGPS受信機もある。この場合完全に空が塞がれている状態（トンネル内に入ったとき）などもある程度の位置は分かる。航空機の慣性航法装置と同様であるが、精度は3桁程度低いため、数分程度の自律位置推定で測位は大きく外れる。

登山用のGPS受信機では、気圧高度計で高度方向の位置推定の補助手段としたり（GPS信号の信頼度が高いときには逆に気圧を校正したり）、磁気コンパスを併用するものもある。なおもともとGPSでは、高度方向は精度が低い場合が多い。空間の (x, y, z) 方向の誤差は均等であるが、前述のようにGPS受信機の多くは地表に沿って動く（地表と鉛直方向には動かない）ため、計算アルゴリズムを工夫して、地表に沿った方向の位置推定の精度を上げる代わりに高度方向の位置推定を犠牲にしているためである。

携帯電話・モバイル情報機器搭載のGPSでは、携帯電話の基地局の位置情報（精度数百m〜数km程度）を補助情報として用いることができる。このため初期捕捉を速くしたり、高速移動時に衛星を見失わないための補助手段とすることができる。

GIS情報を補助手段として用いる場合もある。カーナビでは地図を搭載しているため、道路情報と照らし合わせることで誤差を修正しているものもある（車は道路以外を走れない・水面を走れない、などという制約を利用している）。

測地系

詳細は「測地系」を参照

GPS受信機に経緯度を直接入力してナビゲーションする場合、測地系をあわせることにも注意が必要。（例えば、WGS84若しくは日本測地系を選択）

原子時計の遅れ

受信機側での信号処理には、さまざまな要因によるものが含まれるが、高速で運動するGPS衛星の運動による発振信号の時間の遅れと、地球の重力場による時間の遅れである。後者は、衛星軌道の擾乱や信号到達距離の湾曲、発振信号の時間の遅れなどを引き起こす。 地上の時計は、GPS衛星の時計よりわずかに遅れるので、GPS衛星の時計は、これを補正するため遅く進むように設計されている。この時間の遅れは相対論効果を考慮した計算結果と高い精度で一致しており、身近な相対性理論効果の実証の一つとして挙げられる。

GPS時刻

GPS時刻（GPSの基準時刻系）は1980年1月1日のUTC（TAI−19秒）を開始時刻（基準）とし、その後は、UTCのような閏秒調整は施されない。したがって、現在、GPS時刻はUTCから16秒進んでいる（2012年7月1日の修正後）。

このGPS時刻とUTCとの差はGPS信号の中に含まれているため、受信機ではこの差を補正してUTC時刻を出力することができる。このUTCとのオフセット信号は255（8ビット）の値まで持てるため現状の閏秒挿入のペースであれば2300年頃まで問題ないと考えられる。

人為的に加えられた誤差とその解除

1990年から2000年までは、米国の軍事上の理由（敵軍に利用されることを防止する）で、C/Aコードにおいて民間GPS向けのデータに故意に誤差データを加える操作（Selective Availability、略称 SA）が行われ、精度が100m程度に落とされていた。

SAが加えられていたときから既にGPSは民生用として有用であることが知られていたため、2000年5月2日4時5分 (UTC) から米国はGPS技術を広く役立てて欲しいという主旨でこれを解除した。競合技術であるガリレオ（EUが主体となって推進している）が提案された理由のひとつに、GPSのSAによる誤差により民生用で精度が上がらないということがあるが、これに対して優位を保ち続けリーダシップを取るという米政府の意図も含まれている。また民間GPS機器の軍事転用により調達コストを抑える目的もあると見られている。SA解除以降は、民間GPSでもC/Aコードの技術的な限界までの精度が得られるようになっている。

2000年以降は米国の政策上の必要に応じて、有事があった際など特定地域において精度低下の措置がとられる可能性があるとされていた。しかし、米国のジョージ・W・ブッシュ大統領と国防総省は2007年9月18日に、次世代GPS (GPS III) にはSA機能を搭載しない（正確には、「SAを持つ衛星を調達しない」）との大統領決定を発表した。したがって、この決定が将来覆されない限り、SAの操作は永久に実施されないこととなった。

多くの天文観測設備では天体追尾にGPSに同期させることで補正するクォーツ時計やルビジウム時計を用いている。このため、米国が秘密裏にSAを加えようとしても、少なくともSAが加えられたこと自体はエラーとして検出される。

1999年8月21日問題

1999年8月21日問題（GPSの時計が桁溢れする日）などが1024週ごとにある。これは、GPS衛星に搭載されている時計の週の積算データが10ビットで管理されているため、GPS時計の周期開始日である1980年1月6日から1024週後の1999年8月21日（JSTでは1999年8月22日午前9:00）にリセットされて内部で0週に戻ってしまう仕様となっていたのを無視してカーナビゲーションシステムを製造したために、発生した問題である。当時、対応が迫られていた2000年問題と同根の問題であることもあり、こう呼ぶ。

日本国内において修正ミスが原因の不都合が、一部のカーナビゲーションシステムで生じた。

その次にGPSの週の積算が0になるのは基点から2048週間後の2019年4月7日午前9:00 (JST) である。

様々な用途

民間利用の一例、タクシーにて。（2004年7月16日京都）

民生用GPS受信機は当初航空機、船舶、測量機器、登山用（個人携帯等）に利用されてきたが近年は自動車（カーナビゲーション・システム、以下カーナビ）や携帯電話などにも搭載され利用されている。

• カーナビゲーション
• 携帯端末（ハンドヘルドGPS、PDA、携帯電話、スマートフォン、PSP用GPSレシーバー）
• ノートパソコン
• レーダー探知機
• デジタルカメラ

個人携帯用

GPSを備えたタブレットの一例「Nexus7」

携帯電話にGPSを組み合わせた製品も出現している。この種の製品では、地図情報・GIS情報をサーバ側にもつことにより詳細な地図を提供したり付加サービス（例えば最寄の料理店を検索し電話を掛けて予約する）の可能性が拡がっている。また情報を送信できないGPSと送受信機である携帯電話を組み合わせ、セキュリティ（児童誘拐や徘徊老人対策等）への応用も拡がっている。

さらに位置情報を併用したメッセージ交換システムも登場している。携帯端末利用者が、ある地点に特有の情報（例えば地理的な注意喚起、お店の感想、写真など）を残し、後から来たひとが検索・表示できるものがある。また検索を時刻で制約することで地点・時刻に特有な情報（例えば天候や交通機関の遅れ状態など）を参照することもできる。GPSで測位しつづけたトラッキング情報は、前記のように登山などの記録のため自己のみが参照できる・セキュリティのため限られたひとが参照できるのが普通であったが、最近ではあるコミュニティ（または完全にオープン）の中で自分の位置情報を配信し続けることができるサービスがある。これを利用すると仲間の行動が把握できたりするが、プライバシー漏洩防止の観点からは十分に注意しなければならない。これらのメッセージ交換システムは、あくまでもGPSで測位した地点と補助情報を（地理的に関係ない位置にある）サーバに蓄積し参照するものであるが、うまく作られている場合はあたかも地点にメッセージが結びついているかのように利用できる。

現在の携帯電話ではA-GPS (Assisted Global Positioning System) を利用して空が開けていない場所でも携帯電話の基地局から衛星の軌道データを受け取ることで測位までの時間の短縮を行っている。

携帯電話との組み合わせならではの技術として、空が開けていない場所でも携帯電話の基地局の位置情報を補助情報として利用する方式があげられる。 これは、基地局の既知の位置と、GPSから計算される位置を比較することで得られる誤差情報を基地局から携帯電話に送ることで、携帯電話の位置誤差を小さく出来るA-GPSと呼ばれる技術である。ちなみに、GPSチップ・携帯電話搭載のプロセッサの能力が低かった時代は、位置計算が高速でできない携帯電話のために、測距情報をホストに送り、緯度・経度・高度情報を携帯端末に送り返してもらうというシステムも存在した（現在はワンチップ型のGPSチップの中で位置計算まで完了できるのでこの種のサービスはない）。

登山用・単体モジュールを問わず、NMEA信号を出力することができるGPS受信機（GPSロガー等）は、プロ用一眼レフカメラなどと連動し、撮影記録を自動的に残すためにも使われている。対応機をGPS受信機と組み合わせれば、JPEG画像ファイルなどのExifフィールドに自動的に撮影地の緯度・経度・時刻などが記録される仕組みである。単体のコンパクトデジタルカメラ、GPSとカメラ双方を内蔵したスマートフォンでもこれが可能なものが出現している。

その他、ラップトップ型のPCやPDA、携帯ゲーム機をカーナビとして使えるようにするGPSユニットとソフトも発売されている。なお単体のGPSユニットは（電源内蔵でない場合、本体から電源を供給されなければ動作しないが）、測位等はすべてユニット内で完結しており、NMEAなどの標準フォーマットで緯度・経度その他の情報を送り出すものが多い。PCやPDA本体ではこれを受信し、地図ソフトなどと組み合わせてカーナビ同様に使ったり、トラックの記録をすることができる。PCやPDAの接続も、かつてはRS-232C（シリアル）接続やPCMCIA（PCカード）・CFカード規格が多かったが、現在はBluetoothで測位情報を本体に転送するものもある。

ゲーム・スポーツ

登山用

登山用はもともと数値的に経度と緯度を表示するだけのものであった。これは経度と緯度が細かく記してある正確な地図がなければ役に立たない。この種のものはトラッキング（移動経路）を記録することができるものが多く、登山のみならずグライダー等での競技にも用いられている。最近では登山用でも、白黒の低解像度の地図を内蔵するものから比較的詳細なカラー地図を表示できるものへと進化が進んでいる。いずれにしても電池切れや故障に備え地図と磁気コンパスを携行することは必要である。

フィールド用

アスリート用に走行距離・ラップなどを表示する、腕時計のような形態の非常に小型の製品も実用化されている。これはナビゲーション用途ではなく、従来は歩数計（振動の情報を利用）などを利用していた測位の精度向上のためである。

ジオキャッシング

ジオキャッシングはネット上で公開されたキャッシュ（宝）の緯度・経度とヒントを元にGPSを利用しキャッシュを探すゲームである。キャッシュはバケツなどの容器に入れて隠されており、見つけた人はキャッシュをもらい（その代わり自分で1個キャッシュを入れる）、ログブックに記録を残したりする。

GPSドローイング（GPS絵画）

GPSドローイング（GPS絵画）はGPSロガーで描いた緯度・経度の軌跡により、文字やイラスト等のイメージを制作する行為である。軌跡はGoogle Earthなど地図ソフトウェア上に表示ができる。

パラグライディング競技・パラシューティング競技

これらのスポーツでは、(1) 競技者がGPSを公式な記録として提出することができる、(2) GPSを用いてより良い記録を出すことを目指す、などの利用がなされている。滑空距離や落下位置の正確さを競うこれらの競技では、従来写真による記録や審判員による目視で記録がなされることが多かった。プレイヤーが所有するGPSの記録を提出し、それが改ざんされていないと認められれば、有力な記録の証拠となる。また単体機のGPSレシーバでは、立体的にリアルタイムの移動距離・降下率を計算し、事前に設定した目標へ近づく参考となる機能を備えたものもある。

エリア奪取

エリア奪取はauのGPS携帯電話の位置情報サービスを使って、陣取り合戦をするゲームである。地球上を緯度1分×経度1分の四角形で区切ったものを「エリア」と呼び、「エリア」を参加者が奪い合う。ただし、位置情報サービスはEZwebの通信を使っておこなうため、auの電波状態が良好な地域に限られる。逆に、auのEZwebで通信できるなら、険しい山岳や海上も「エリア」となる。

迷路・ビーストハントなど

ガーミンの単体機GPSレシーバには、GPSを利用したゲームが内蔵されているものがある。ある程度以上広いところでGPSが受信できるならば、実際にプレイヤーが動くことでゲームをプレイする（レシーバが表示した仮想の迷路をプレイヤーが動いて脱出する、など）のゲームを行うことができる。

船舶

船舶にとってGPSは重要な航法支援設備である。航空機同様、陸から離れたら目印をもたない海上において、遭難・衝突や座礁を免れるために、精度の良い航法支援システムを利用することは重要であった。そもそもGPSはロラン-Cに取って代わるためにつくられたシステムである。

漁業用船舶や個人用のレジャーボートに搭載されるGPSでは、魚群探知機と組み合わせ、漁場をマークするなどの機能が付加されているものもある。

カーナビゲーションシステム

カーナビゲーションはGPSの実装において技術的に有利な応用である。自動車からは安定した大容量の電源が供給でき、GPS用アンテナを良い位置に設置できる。また本体が大きくてもよいため、詳細なカラー地図を内蔵できる。

しかし目的地への案内を目的としたカーナビゲーションの特性から、地図上の自動車道の新設・廃止等に随時追従する必要性や、近年では地図のみならず、例えば電話番号やレストランのリストなどGIS情報まで活用するようになり、ROMなどで固定データを本体に内蔵するのは不利になってきた。そのためデータを任意で更新できるような仕組みが備わっているものが標準的である。これには大きく分けてCDやDVD等の記録媒体でデータを供給し必要に応じて記録媒体そのものを交換して更新するものと、機器に内蔵されたハードディスクやフラッシュメモリのデータを何らかの手段で書き換えることで更新を行うものとがある。

また、レーダー等による速度規制取り締まりやシートベルト着装取り締まり等を行っている場所の緯度・経度をデータとして持ち、その近辺で警告を発する機器も存在する（レーダー探知機の項を参照）。

航空分野

GPSやGLONASSなどの位置情報を航空機にも使用することが促進されている。

従来の航空機航法は、VOR・DMEなどの地上航法支援施設を用い、いわば電波の灯台への方位・距離を測定して現在位置を知る方法だった。これに対し、衛星が4個以上見えていればある程度の精度で絶対位置がわかるGPSは、航空機向けの測位方式であるとも言える。

しかしながらGPS信号をそのまま航空航法に使用するには、測位の安全性・信頼性・精度等に問題がある。具体的には、低高度、特に精度がもっとも必要とされる着陸寸前の地形による遮蔽・マルチパス、機体の姿勢変更に伴いロックした衛星（測位に用いている衛星）が変化すること、一般にGPSによる測位では航空機にとって重要な高度方向の精度が緯度・経度方向の精度より低いこと（ただしこれは計算方法にもよる）、ジェット機などは高速移動するためドップラーシフト・衛星コンステレーションの時間的変化が無視できないこと、などである。

ただし、大型機ではINS（慣性航法装置）や従来の測位方式などと併用すること、小型機ではVFR（有視界飛行方式）が主であることなどから、実際の運用では（制度上は認められていないものの）機長判断の参考として用いられている場合が多かった。

こういった流れを受けて、また近年では航空機運航の高密度化により定められた航空路以外の経路を飛ぶための一手段として、GPS情報を航法に利用することが国際民間航空機関 (ICAO) や国土交通省航空局 (JCAB) でも検討されてきた。その成果として日本では、一部の空港の離着陸手順においてRNAV (GPS) 航法の実施が2007年9月27日より開始された。航空機はウェイポイントとよばれる架空の点を結ぶ線を経路とするように飛行する。従来のVOR/DME航法では、VOR/DMEの位置、あるいは1つまたは2つのVOR/DMEから一定の方位角・距離にある架空の点をウェイポイントとしていた。これに対しRNAV航法では、地上施設に拠らない自由な点をウェイポイントとして定めることができるため、飛行経路の短縮による運航時間の短縮、燃費の節約などが見込まれる。

航空機での精度向上を一次目的とした、静止衛星型衛星航法補強システム (SBAS: Satellite Based Augmentation System) の運用が以下の各国で開始され、あるいは計画されている。

• 米国：WAAS (Wide Area Augmentation System)
• ロシア：SDCM (System for Differential Corrections and Monitoring)
• 欧州：EGNOS (European Geostationary Navigation Overlay Service)
• 日本：MSAS (MTSAT-based Satellite Augmentation System)
• インド：GAGAN
• 中国：CNSS (Compass Navigation Satellite System)

SBASでは、GPS衛星の補正情報（特に高度情報の補正）や信頼性情報を送信し、またSBAS衛星自体も測位のためのひとつの衛星として働く。さらにSBAS衛星は静止軌道にあるため、中〜低緯度地方では天頂に近い高仰角でみえているのも有利な点である（北緯35度では仰角55度）。航空以外の分野でも、例えばビル街でのカーナビの精度向上にも役立つと考えられている。SBASを補助情報として用いることができるGPS受信機はすでにSBAS対応（WAAS対応）受信機として広く普及し始めている。

日本のMSASについては、航空機でのRNAV運用に伴い、2007年9月27日から試験信号フラグ (MT0) が運用モード (MT2) となり、正式に供用開始となった。ただし初期のWAAS対応機など一部のSBAS対応受信機では、MSASの衛星番号を設定・処理できないため測位に利用できないものがある。

科学技術分野

科学技術分野では、もちろん国土の形状を明らかにしたり、cm単位で地球の動きを知り地震予知に役立てるなどの、いわばGPS本来の用途のほかに、トラッキングや時刻の高精度同期などにも利用されている。

大型の渡り鳥にGPS発信機を装着して、その渡りの過程を追跡することに利用されている。山階鳥類研究所は絶滅が危惧されているアホウドリの繁殖活動を行っており、その一環として伊豆諸島の鳥島で生まれたアホウドリを聟島へ移住させて繁殖地の拡大を図っているが、そのうちの7羽にGPS発信機を装着してその後の足取りを追跡した。その中の1羽がカムチャッカ半島、アリューシャン列島、アラスカ湾、カナダ西海岸を経由してアメリカ・カリフォルニア州のサンフランシスコ沖に辿り着いていることが人工衛星による追跡で判明し、現地での写真撮影によりその個体が確認された。アホウドリの2万km以上におよぶ渡りの経路の詳細がGPSの技術により明らかになった。

GPS衛星搭載の原子時計からの時計情報も科学分野を中心に広く活用されている。GPSの時計情報はGPS衛星に搭載されている原子時計の精度とほぼ一致し、クォーツ時計の精度よりもはるかに高い。そのため、野外で正確な時刻を知る必要がある場合や、複数点で時計情報を高精度で一致させる（同期する）ために用いられる。GPS本来の目的である位置決定とは異なる利用法であるが、とくに地球科学や土木工学分野に大きな効果を与えている。

たとえば地震を監視しその震源を高精度に決定するためには、広範囲に多数設置された地震計すべての時計を秒未満の精度で一致させ、かつ数ヶ月から数年間にわたりその状態を維持する必要がある。そのために従来はJJY信号を同時に記録し時刻を記録していたり、各地震計に原子時計を接続する必要があり、コスト負担が大きかった。しかしGPS受信機を接続することにより、GPS衛星からもたらされる高精度の時計情報を受信できるようになったため、すべての地震計を容易に時刻同期させることが可能となった。

地震計に限らず精度の高い時刻情報が必要な場合、計測機器に小さなGPS受信機を取り付けることが多い。また、こうした用途のために各計測機器にGPS受信機が付属している場合がある。

コンピュータの時刻をネットワークで高精度に同期させるプロトコルであるNTPサーバでは、大元となる超高精度のサーバ (stratum 0) は従来、構築が容易ではなかったが、GPS受信機との接続により比較的容易にstratum 0サーバを構築できるようになった。

このほか、位置が既知の基地局で高精度にGPS測位を行い、その誤差情報からGPS電波伝播経路の大気の状態を知るGPS気象学なども実用化を目指して研究されている。

防犯

窃盗や誘拐等の特定の物や人物に対する犯罪を防止するために、GPSが活用されている。例えば、建設機械は高価な機械も少なくなく、開発途上国などで需要が高いため日本等において窃盗されることが多いが、メーカーがGPSで現在位置を報告する装置を一台ごとに組み込んだところ、窃盗された建機の位置が特定し犯人が検挙された事例が報道され、建機の窃盗が減っている。また、誘拐等の児童に対する犯罪が社会的関心が高まる中で、保護者が児童の位置を管理しそれらの犯罪を防止するためにGPS付の携帯端末が販売されており、一部の携帯端末（mamorino等）は警備会社と提携して、問題行動があれば保護者に代わりに即応できる体制のサービスも提供されている。近年は、GPS携帯端末を徘徊行動をする認知症を患った高齢者や、一人暮らし若しくは持病のある高齢の親に持たせて、何かあった場合に位置を確認して親類が保護したり、警備員が駆けつけるサービスを利用して保護している家庭もある。それ以外に重要なモノを管理するために活用したり、児童や高齢者同様成年男性や女性の居場所を探すために利用されている。しかし、これらの機能がストーカーの付きまとい行動に悪用されている事件も発生している。

軍事用途

勿論、本来の目的である軍事用途においてもGPSは活用されている。湾岸戦争やイラク戦争では、アメリカ軍の地上部隊はGPSのおかげで何の目印もない広大な砂漠での進軍を可能にした。誘導爆弾もGPSを利用したタイプが登場し、安価でレーザーなどによる誘導操作が不要である反面、命中精度に劣る事や標的座標エラーによる誤爆の危険があるなどの問題点がある。

高周波信号の生成

従来水晶振動子を用いて生成していたPLLの基準信号をGPSの受信信号に置き換えることにより、PLLの要素機能をほぼそのまま流用しながら、GPS信号とほぼ同等の精度および安定性を持つ高周波信号の発振回路を作ることが出来る。このような回路はGPSDOと呼ばれ、基準信号であるGPS信号と同じく時刻および周波数基準に使用されたり、QRSS等の超狭帯域無線通信(おおむね数Hz以下)の信号生成に用いられる。

GPSにまつわる誤解

以下の言説は、すべて誤解である。

• GPS衛星はGPS受信機（カーナビ等）に直接その座標値 (x, y, z) を送っている。
• GPS衛星がカーナビのルートを作成している。
• GPS衛星はGPS受信機の位置を逆探知できる。
• GPS衛星とGPS受信機が相互に通信をしている。
• GPS情報によって方位も決定している。

正しくは、GPS衛星は宇宙空間に向けて基本的に時報と自衛星の天体暦（軌道）情報を発しているだけであり、一方、GPS受信機はそれらを受信することで、そのGPS受信機の現在位置を計算する。また、ジャイロ機構や方位コンパス等を使用していない場合、静止している受信機器の向いている方位を知ることはできない。

GPS受信機は、外部に電波を発する装置を有していないため、位置情報をGPS衛星に通知するのは原理上そもそも不可能である。つまり、GPS受信機は受信するだけ、GPS衛星は送信するだけなのである。

実際、大半の航空会社では離着陸時を除いてGPS受信機の機内での使用を認めている。

この誤解の元となった技術に、運輸業などの車両位置監視システムや、児童・徘徊老人のセキュリティシステムなどがあるが、これらではGPS受信機の位置情報を外部に通知するために、携帯電話等による通信を行っている。2008年2月5日に岡山市で現金自動預払機 (ATM) が盗まれた事件では、事件発生後約45分でGPSによって盗難ATMを発見するという成果を挙げている。このケースでも機器組み込み型の携帯電話モジュールでセキュリティ会社への位置通報をしていたとみられる。

同様の技術・関連技術

詳細は「衛星測位システム」を参照

前述の様にGPSは元々アメリカの軍事用システムであるため、民間や他国の利用には一定の制限が設けられる事が多い。そのため、より自身の利益に適った独自のシステムを保有しようとする動きがみられる。下記以外にも、中国やナイジェリア、トルコなどにも他の衛星ナビゲーションシステムの開発の動きがある。

日本

日本には、3基の人工衛星からなりGPSの位置情報を補正して高精度の測位を可能とする準天頂衛星システム (Quasi-Zenith Satellite System, QZSS) と呼ばれる計画がある。すでに事業化を検討する民間の主体として新衛星ビジネス株式会社が2002年に設立されており、高速で移動する車輛の内部で精度25cmとされる測位精度を用いた各種事業が検討されていた。最初の人工衛星は2008年に打ち上げられる予定だが予算の都合で通信・放送との複合機能衛星となっており、それらのサービスのシナジー効果が期待されていたが、採算性の面から2006年3月に放送・通信の事業化が断念され、純粋な測位衛星として利用されることになった（新衛星ビジネス株式会社は2007年8月2日に解散し財団法人衛星測位利用促進センターが測位分野のみ継続）。

ちなみに日本では2005年第44回衆議院議員総選挙の自由民主党マニフェストである「政権公約2005」の52項目に「国家基盤としての衛星測位の確立と骨格的空間情報の整備」との記載があり、日本独自の高精度な位置測定衛星を打ち上げる可能性が出ている。

日本ではその後、内閣官房に測位・地理情報システム等推進会議が設置され、2006年3月には「準天頂衛星システム計画の推進に係る基本方針」を発表した。それによると、国家が衛星測位の重要性を認識し、民間の資金負担がないとしても、国家が衛星測位システムを整備することを宣言している。

2010年9月11日に、準天頂衛星の実用試験機「みちびき」が打ち上げられてシステムの有効性が検証され、今後2019年までに衛星3基が追加で打ち上げられ、4基体制でシステムが運用されることが決定された。

ロシア

旧ソ連は米国との対抗上、GPSと同様のGLONASSを構築しようとしたが必要な衛星を全て打上げる前にソ連が崩壊してしまい、予算の縮小から衛星打ち上げが頓挫した。ロシアになってから計画が再開され、2005年には再開後初の衛星を打ち上げ、2010年までに24基の衛星を打ち上げる予定とされる。2011年には全世界で測位可能となり、現在は測位精度を高めるためにGLONASSとGPSを併用する受信機が登場している（GLONASS#受信機も参照のこと）。

欧州連合

GPSを使用する上で米国に頼ることを嫌ったEUは独自のGalileo（ガリレオ）を計画、中国も計画に参加している。2005年にはロシアのソユーズロケットを用いて最初のジオベ衛星を打ち上げたが、共同事業体の体制がととのわず、民間企業も採算の見込みが立たないと手を引いたため、本格運用開始の目処が立たない状況となっている。

インド

「インド地域航法衛星システム」を参照

脚注

参考文献

関連項目

• 補助GPS
• GPS衛星
• 準天頂衛星（準天頂軌道）
• 電子基準点
• 地図
• 地理情報システム (GIS)
• 屋内GPS
• ジオキャッシング
• 緊急通報位置通知
• GPS気象学
• 九州南西海域工作船事件（いわゆる「不審船」が東シナ海に出現し、海上保安庁の巡視船と銃撃戦になった事件。自沈し後に引き揚げられた不審船からは、日本製のGPSプロッターが見つかっている）
• アビオニクス
  • 広域航法
• 航空保安施設
• NMEA 0183
• レジナルド・ゴリッジ - 視覚障害者向けのGPSを開発。
• GLONASS - ロシア軍が運用する位置測位システム
• ロジャー・イーストン

外部リンク

• GPSデータクリアリングハウス
• GPS固定点データ提供サービス
• 電子基準点データ提供サービス
• WAAS（英語）
• MSAS（英語）
• EGNOS（英語）
• 「交点プロジェクト」サイト

wiki en

[Wiki en表示]

"GPS" redirects here. For other uses, see GPS (disambiguation).

Geodesy
Fundamentals
Geodesy Geodynamics Geomatics Cartography
Concepts
Datum Geographical distance Geoid Figure of the Earth Geodetic system Geodesic Geographic coordinate system Horizontal position representation Latitude / Longitude Map projection Reference ellipsoid Satellite geodesy Spatial reference system
Technologies
Global Navigation Satellite System (GNSS) Global Positioning System (GPS) GLONASS (Russian GPS) Galileo Indian Regional Navigational Satellite System (IRNSS) Beidou-2
Standards
ED50 (European Datum 1950) SAD69 (South American Datum 1969) GRS 80 (Geodetic Reference System 1980) NAD83 (North American Datum 1983) WGS84 (World Geodetic System 1984) NAVD88 (North American Vertical Datum 1988) ETRS89 (European Terrestrial Reference System 1989) Spatial Reference System Identifier (SRID) Universal Transverse Mercator (UTM)
History
NAVD29 (Sea Level Datum 1929)

Artist's conception of GPS Block II-F satellite in Earth orbit.

Civilian GPS receivers ("GPS navigation device") in a marine application.

Automotive navigation system in a taxicab.

U.S. Air Force Senior Airman runs through a checklist during Global Positioning System satellite operations.

The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil and commercial users around the world. It is maintained by the United States government and is freely accessible to anyone with a GPS receiver.

The GPS project was developed in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. GPS was created and realized by the U.S. Department of Defense (DoD) and was originally run with 24 satellites. It became fully operational in 1995. Bradford Parkinson, Roger L. Easton, and Ivan A. Getting are credited with inventing it.

Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next Generation Operational Control System (OCX). Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the modernization effort, GPS III.

In addition to GPS, other systems are in use or under development. The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s. There are also the planned European Union Galileo positioning system, India's Indian Regional Navigational Satellite System and Chinese Compass navigation system.

History

The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s and used by the British Royal Navy during World War II.

Predecessors

In 1956, the German-American physicist Friedwardt Winterberg proposed a test of general relativity (for time slowing in a strong gravitational field) using accurate atomic clocks placed in orbit inside artificial satellites. (Later, calculations using general relativity determined that the clocks on GPS satellites would be seen by Earth's observers to run 38 microseconds faster per day, and this was corrected for in the design of GPS.)

The Soviet Union launched the first man-made satellite, Sputnik, in 1957. Two American physicists, William Guier and George Weiffenbach, at Johns Hopkins's Applied Physics Laboratory (APL), decided to monitor Sputnik's radio transmissions. Within hours they realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their UNIVAC to do the heavy calculations required. The next spring, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem—pinpointing the user's location given that of the satellite. (The Navy was developing the submarine-launched Polaris missile, which required them to know the submarine's location.) This led them and APL to develop the Transit system. In 1959, ARPA (renamed DARPA in 1972) also played a role in Transit.

Official logo for NAVSTAR GPS

Emblem of the 50th Space Wing

The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite that proved the ability to place accurate clocks in space, a technology required by GPS. In the 1970s, the ground-based Omega Navigation System, based on phase comparison of signal transmission from pairs of stations, became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.

While there were wide needs for accurate navigation in military and civilian sectors, almost none of those was seen as justification for the billions of dollars it would cost in research, development, deployment, and operation for a constellation of navigation satellites. During the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded. It is also the reason for the ultra secrecy at that time. The nuclear triad consisted of the United States Navy's submarine-launched ballistic missiles (SLBMs) along with United States Air Force (USAF) strategic bombers and intercontinental ballistic missiles (ICBMs). Considered vital to the nuclear-deterrence posture, accurate determination of the SLBM launch position was a force multiplier.

Precise navigation would enable United States submarines to get an accurate fix of their positions before they launched their SLBMs. The USAF, with two thirds of the nuclear triad, also had requirements for a more accurate and reliable navigation system. The Navy and Air Force were developing their own technologies in parallel to solve what was essentially the same problem. To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (such as Russian SS-24 and SS-25) and so the need to fix the launch position had similarity to the SLBM situation.

In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D LORAN. A follow-on study, Project 57, was worked in 1963 and it was "in this study that the GPS concept was born". That same year, the concept was pursued as Project 621B, which had "many of the attributes that you now see in GPS" and promised increased accuracy for Air Force bombers as well as ICBMs. Updates from the Navy Transit system were too slow for the high speeds of Air Force operation. The Naval Research Laboratory continued advancements with their Timation (Time Navigation) satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clock into orbit.

Another important predecessor to GPS came from a different branch of the United States military. In 1964, the United States Army orbited its first Sequential Collation of Range (SECOR) satellite used for geodetic surveying. The SECOR system included three ground-based transmitters from known locations that would send signals to the satellite transponder in orbit. A fourth ground-based station, at an undetermined position, could then use those signals to fix its location precisely. The last SECOR satellite was launched in 1969. Decades later, during the early years of GPS, civilian surveying became one of the first fields to make use of the new technology, because surveyors could reap benefits of signals from the less-than-complete GPS constellation years before it was declared operational. GPS can be thought of as an evolution of the SECOR system where the ground-based transmitters have been migrated into orbit.

Development

With these parallel developments in the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in a multi-service program.

During Labor Day weekend in 1973, a meeting of about twelve military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that "the real synthesis that became GPS was created." Later that year, the DNSS program was named Navstar, or Navigation System Using Timing and Ranging. With the individual satellites being associated with the name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify the constellation of Navstar satellites, Navstar-GPS, which was later shortened simply to GPS. Ten "Block I" prototype satellites were launched between 1978 and 1985 (with one prototype being destroyed in a launch failure).

After Korean Air Lines Flight 007, a Boeing 747 carrying 269 people, was shot down in 1983 after straying into the USSR's prohibited airspace, in the vicinity of Sakhalin and Moneron Islands, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good. The first satellite was launched in 1989, and the 24th satellite was launched in 1994. The GPS program cost at this point, not including the cost of the user equipment, but including the costs of the satellite launches, has been estimated to be about USD$5 billion (then-year dollars). Roger L. Easton is widely credited as the primary inventor of GPS.

Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded (Selective Availability). This changed with President Bill Clinton ordering Selective Availability to be turned off at midnight May 1, 2000, improving the precision of civilian GPS from 100 to 20 meters (328 to 66 ft). The executive order signed in 1996 to turn off Selective Availability in 2000 was proposed by the U.S. Secretary of Defense, William Perry, because of the widespread growth of differential GPS services to improve civilian accuracy and eliminate the U.S. military advantage. Moreover, the U.S. military was actively developing technologies to deny GPS service to potential adversaries on a regional basis.

Over the last decade, the U.S. has implemented several improvements to the GPS service, including new signals for civil use and increased accuracy and integrity for all users, all while maintaining compatibility with existing GPS equipment.

GPS modernization has now become an ongoing initiative to upgrade the Global Positioning System with new capabilities to meet growing military, civil, and commercial needs. The program is being implemented through a series of satellite acquisitions, including GPS Block III and the Next Generation Operational Control System (OCX). The U.S. Government continues to improve the GPS space and ground segments to increase performance and accuracy.

GPS is owned and operated by the United States Government as a national resource. Department of Defense (DoD) is the steward of GPS. Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from 1996 to 2004. After that the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems. The executive committee is chaired jointly by the deputy secretaries of defense and transportation. Its membership includes equivalent-level officials from the departments of state, commerce, and homeland security, the joint chiefs of staff, and NASA. Components of the executive office of the president participate as observers to the executive committee, and the FCC chairman participates as a liaison.

The DoD is required by law to "maintain a Standard Positioning Service (as defined in the federal radio navigation plan and the standard positioning service signal specification) that will be available on a continuous, worldwide basis," and "develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses."

Timeline and modernization

Main article: List of GPS satellite launches

Summary of satellites

Block	Launch Period	Satellite launches				Currently in orbit and healthy
		Suc- cess	Fail- ure	In prep- aration	Plan- ned
I	1978–1985	10	1	0	0	0
II	1989–1990	9	0	0	0	0
IIA	1990–1997	19	0	0	0	9
IIR	1997–2004	12	1	0	0	12
IIR-M	2005–2009	8	0	0	0	7
IIF	From 2010	5	0	7	0	5
IIIA	From 2014	0	0	0	12	0
IIIB	—	0	0	0	8	0
IIIC	—	0	0	0	16	0
Total		62	2	7	36	32
(Last update: April 8, 2014) PRN 01 from Block IIR-M is unhealthy PRN 25 from Block IIA is unhealthy PRN 32 from Block IIA is unhealthy PRN 27 from Block IIA is unhealthy For a more complete list, see list of GPS satellite launches

• In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB), conducted developmental flight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.
• In 1978, the first experimental Block-I GPS satellite was launched.
• In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace because of navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that GPS would be made available for civilian uses once it was completed, although it had been previously published [in Navigation magazine] that the CA code (Coarse Acquisition code) would be available to civilian users.
• By 1985, ten more experimental Block-I satellites had been launched to validate the concept. Command & Control of these satellites had moved from Onizuka AFS, CA and turned over to the 2nd Satellite Control Squadron (2SCS) located at Falcon Air Force Station in Colorado Springs, Colorado.
• On February 14, 1989, the first modern Block-II satellite was launched.
• The Gulf War from 1990 to 1991 was the first conflict in which GPS was widely used.
• In 1991, a project to create a miniature GPS receiver successfully ended, replacing the previous 50 pound military receivers with a 2.75 pound handheld receiver.
• In 1992, the 2nd Space Wing, which originally managed the system, was inactivated and replaced by the 50th Space Wing.
• By December 1993, GPS achieved initial operational capability (IOC), indicating a full constellation (24 satellites) was available and providing the Standard Positioning Service (SPS).
• Full Operational Capability (FOC) was declared by Air Force Space Command (AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning Service (PPS).
• In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
• In 1998, United States Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety and in 2000 the United States Congress authorized the effort, referring to it as GPS III.
• On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.
• In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europe's planned Galileo system.
• In 2004, United States President George W. Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.
• November 2004, Qualcomm announced successful tests of assisted GPS for mobile phones.
• In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.
• On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the new Architecture Evolution Plan.
• On May 19, 2009, the United States Government Accountability Office issued a report warning that some GPS satellites could fail as soon as 2010.
• On May 21, 2009, the Air Force Space Command allayed fears of GPS failure saying "There's only a small risk we will not continue to exceed our performance standard."
• On January 11, 2010, an update of ground control systems caused a software incompatibility with 8000 to 10000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, Calif.
• On February 25, 2010, the U.S. Air Force awarded the contract to develop the GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.

Awards

On February 10, 1993, the National Aeronautic Association selected the GPS Team as winners of the 1992 Robert J. Collier Trophy, the nation's most prestigious aviation award. This team combines researchers from the Naval Research Laboratory, the USAF, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation honors them "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."

Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003:

• Ivan Getting, emeritus president of The Aerospace Corporation and an engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).
• Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinson served twenty-one years in the Air Force, from 1957 to 1978, and retired with the rank of colonel.
• GPS developer Roger L. Easton received the National Medal of Technology on February 13, 2006.

In 1998, GPS technology was inducted into the Space Foundation Space Technology Hall of Fame.

Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Force Space and Missile Pioneers Hall of Fame at Lackland A.F.B., San Antonio, Texas, March 2, 2010 for his role in space technology development and the engineering design concept of GPS conducted as part of Project 621B.

On October 4, 2011, the International Astronautical Federation (IAF) awarded the Global Positioning System (GPS) its 60th Anniversary Award, nominated by IAF member, the American Institute for Aeronautics and Astronautics (AIAA). The IAF Honors and Awards Committee recognized the uniqueness of the GPS program and the exemplary role it has played in building international collaboration for the benefit of humanity.

Basic concept of GPS

A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include:

• the time the message was transmitted and,
• satellite position at time of message transmission.

The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using the speed of light. Each of these distances and satellites' locations defines a sphere. The receiver is on the surface of each of these spheres when the distances and the satellites' locations are correct. These distances and satellites' locations are used to compute the location of the receiver using the navigation equations. This location is then displayed, perhaps with a moving map display or latitude and longitude; elevation or altitude information may be included, based on height above the geoid (e.g. EGM96).

Basic GPS measurements yield only a position, and neither speed nor direction. However, most GPS units can automatically derive velocity and direction of movement from two or more position measurements. The disadvantage of this principle is that changes in speed or direction can only be computed with a delay, and that derived direction becomes inaccurate when the distance travelled between two position measurements drops below or near the random error of position measurement. GPS units can use measurements of the doppler shift of the signals received to compute velocity accurately. More advanced navigation systems use additional sensors like a compass or an inertial navigation system to complement GPS.

In typical GPS operation, four or more satellites must be visible to obtain an accurate result. Four sphere surfaces typically do not intersect. Because of this, it can be said with confidence that when the navigation equations are solved to find an intersection, this solution gives the position of the receiver along with the difference between the time kept by the receiver's on-board clock and the true time-of-day, thereby eliminating the need for a very large, expensive, and power hungry clock. The very accurately computed time is used only for display or not at all in many GPS applications, which use only the location. A number of applications for GPS do make use of this cheap and highly accurate timing. These include time transfer, traffic signal timing, and synchronization of cell phone base stations.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible.

Structure

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US). The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.

The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit. The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations. The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service (see GPS navigation devices).

Space segment

See also: GPS satellite and List of GPS satellite launches

Unlaunched GPS block II-A satellite on display at the San Diego Air & Space Museum.

A visual example of a 24 satellite GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time.

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits, but this was modified to six orbital planes with four satellites each. The six orbit planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbital period is one-half a sidereal day, i.e., 11 hours and 58 minutes so that the satellites pass over the same locations or almost the same locations every day. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which sum to 360 degrees.

Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi), each SV makes two complete orbits each sidereal day, repeating the same ground track each day. This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

As of December 2012, there are 32 satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail. About nine satellites are visible from any point on the ground at any one time (see animation at right), ensuring considerable redundancy over the minimum four satellites needed for a position.

Control segment

Ground monitor station used from 1984 to 2007, on display at the Air Force Space & Missile Museum.

The control segment is composed of:

1. a master control station (MCS),
2. an alternate master control station,
3. four dedicated ground antennas, and
4. six dedicated monitor stations.

The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC. The tracking information is sent to the Air Force Space Command MCS at Schriever Air Force Base 25 km (16 mi) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the U.S. Air Force. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.

Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.

The Operation Control Segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports global GPS users and keeps the GPS system operational and performing within specification.

OCS successfully replaced the legacy 1970s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported the U.S. armed forces. OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System (OCX), is fully developed and functional.

The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS's mission capabilities, and enabling Air Force Space Command to greatly enhance GPS operational services to U.S. combat forces, civil partners and myriad domestic and international users.

The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50% sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX expected to cost millions less than the cost to upgrade OCS while providing four times the capability.

The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program.

• OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
• Built on a flexible architecture that can rapidly adapt to the changing needs of today's and future GPS users allowing immediate access to GPS data and constellations status through secure, accurate and reliable information.
• Empowers the warfighter with more secure, actionable and predictive information to enhance situational awareness.
• Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
• Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
• Supports higher volume near real-time command and control capabilities and abilities.

On September 14, 2011, the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development.

The GPS OCX program has achieved major milestones and is on track to support the GPS IIIA launch in May 2014.

User segment

Further information: GPS navigation device

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as these.

The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels.

A typical OEM GPS receiver module measuring 15×17 mm.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.

A typical GPS receiver with integrated antenna.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA), references to this protocol have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth.

Applications

While originally a military project, GPS is considered a dual-use technology, meaning it has significant military and civilian applications.

GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance. GPS's accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids by allowing well synchronized hand-off switching.

Civilian

See also: GNSS applications and GPS navigation device

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.

Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer.

• Astronomy: both positional and clock synchronization data is used in Astrometry and Celestial mechanics calculations. It is also used in amateur astronomy using small telescopes to professionals observatories, for example, while finding extrasolar planets.
• Automated vehicle: applying location and routes for cars and trucks to function without a human driver.
• Cartography: both civilian and military cartographers use GPS extensively.
• Cellular telephony: clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The U.S. Federal Communications Commission (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.
• Clock synchronization: the accuracy of GPS time signals (±10 ns) is second only to the atomic clocks upon which they are based.
• Disaster relief/emergency services: depend upon GPS for location and timing capabilities.
• Meteorology-Upper Airs: measure and calculate the atmospheric pressure, wind speed and direction up to 27 km from the earth's surface
• Fleet Tracking: the use of GPS technology to identify, locate and maintain contact reports with one or more fleet vehicles in real-time.
• Geofencing: vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate a vehicle, person, or pet. These devices are attached to the vehicle, person, or the pet collar. The application provides continuous tracking and mobile or Internet updates should the target leave a designated area.
• Geotagging: applying location coordinates to digital objects such as photographs (in Exif data) and other documents for purposes such as creating map overlays with devices like Nikon GP-1
• GPS Aircraft Tracking
• GPS for Mining: the use of RTK GPS has significantly improved several mining operations such as drilling, shoveling, vehicle tracking, and surveying. RTK GPS provides centimeter-level positioning accuracy.
• GPS tours: location determines what content to display; for instance, information about an approaching point of interest.
• Navigation: navigators value digitally precise velocity and orientation measurements.
• Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to compute phasors.
• Recreation: for example, geocaching, geodashing, GPS drawing and waymarking.
• Robotics: self-navigating, autonomous robots using a GPS sensors, which calculate latitude, longitude, time, speed, and heading.
• Surveying: surveyors use absolute locations to make maps and determine property boundaries.
• Tectonics: GPS enables direct fault motion measurement in earthquakes.
• Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems

Restrictions on civilian use

The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 kilometers (11 mi) altitude and 515 meters per second (1,001 kn) or designed, modified for use with unmanned air vehicles like e.g. ballistic or cruise missile systems are classified as munitions (weapons) for which State Department export licenses are required.

This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A (Coarse/Acquisition) code.

Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule refers to operation at both the target altitude and speed, but some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches that regularly reach 30 kilometers (19 mi).

These limits only apply to units exported from (or which have components exported from) the USA – there is a growing trade in various components, including GPS units, supplied by other countries, which are expressly sold as ITAR-free.

Military

Attaching a GPS guidance kit to a dumb bomb, March 2003.

As of 2009, military applications of GPS include:

• Navigation: GPS allows soldiers to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the Commanders Digital Assistant and lower ranks use the Soldier Digital Assistant.
• Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile. These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-to-ground roles, use GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be viewed with specialized software).
• Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles, precision-guided munitions and Artillery projectiles. Embedded GPS receivers able to withstand accelerations of 12,000 g or about 118 km/s² have been developed for use in 155 millimeters (6.1 in) howitzers.
• Search and Rescue: Downed pilots can be located faster if their position is known.
• Reconnaissance: Patrol movement can be managed more closely.
• GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System. General William Shelton has stated that this feature may be dropped from future satellites in order to save money.

Communication

Main article: GPS signals

The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military.

Message format

GPS message format

Subframes	Description
1	Satellite clock, GPS time relationship
2–3	Ephemeris (precise satellite orbit)
4–5	Almanac component (satellite network synopsis, error correction)

Each GPS satellite continuously broadcasts a navigation message on L1 C/A and L2 P/Y frequencies at a rate of 50 bits per second (see bitrate). Each complete message takes 750 seconds (12 1/2 minutes) to complete. The message structure has a basic format of a 1500-bit-long frame made up of five subframes, each subframe being 300 bits (6 seconds) long. Subframes 4 and 5 are subcommutated 25 times each, so that a complete data message requires the transmission of 25 full frames. Each subframe consists of ten words, each 30 bits long. Thus, with 300 bits in a subframe times 5 subframes in a frame times 25 frames in a message, each message is 37,500 bits long. At a transmission rate of 50-bit/s, this gives 750 seconds to transmit an entire almanac message (GPS). Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite.

The first subframe of each frame encodes the week number and the time within the week, as well as the data about the health of the satellite. The second and the third subframes contain the ephemeris – the precise orbit for the satellite. The fourth and fifth subframes contain the almanac, which contains coarse orbit and status information for up to 32 satellites in the constellation as well as data related to error correction. Thus, in order to obtain an accurate satellite location from this transmitted message the receiver must demodulate the message from each satellite it includes in its solution for 18 to 30 seconds. In order to collect all the transmitted almanacs the receiver must demodulate the message for 732 to 750 seconds or 12 1/2 minutes.

All satellites broadcast at the same frequencies. Signals are encoded using code division multiple access (CDMA) allowing messages from individual satellites to be distinguished from each other based on unique encodings for each satellite (that the receiver must be aware of). Two distinct types of CDMA encodings are used: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P(Y)) code, which is encrypted so that only the U.S. military can access it.

The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The almanac is updated typically every 24 hours. Additionally, data for a few weeks following is uploaded in case of transmission updates that delay data upload.

Subframe #	Page #	Name	Word #	Bits	Scale	Signed
1	all	Week Number	3	1–10	1:1	No
1	all	CA or P On L2	3	11,12	1:1	No
1	all	URA Index	3	13–16	1:1	No
1	all	SV_Health	3	17–22	1:1	No
1	all	IODC(MSB)	3	23,24	1:1	No
1	all	L2Pdata flag	4	1	1:1	No
1	all	ResW4	4	2–24	N/A	N/A
1	all	ResW5	5	1–24	N/A	N/A
1	all	ResW6	6	1–24	N/A	N/A
1	all	ResW7	7	1–16	N/A	N/A
1	all	TGD	7	17–24	2^-31	Yes
1	all	IODC (LSB)	8	1–8	1:1	No
1	all	TOC	8	9–24	2^4	No
1	all	AF2	9	1–8	2^-55	Yes
1	all	AF1	9	9–24	2^-43	Yes
1	all	AF0	10	1–22	2^-31	Yes

Subframe #	Page #	Name	Word #	Bits	Scale	Signed
2	all	IODE	3	1–8	1:1	No
2	all	CRS	3	9–24	2^-5	Yes
2	all	Delta N	4	1–16	2^-43	Yes
2	all	M0 (MSB)	4	17–24	2^-31	Yes
2	all	M0 (LSB)	5	1–24
2	all	CUC	6	1–16	2^-29	Yes
2	all	e (MSB)	6	17–24	2^-33	No
2	all	e (LSB)	7	1–24
2	all	CUS	8	1–16	2^-29	Yes
2	all	root A (MSB)	8	17–24	2^-19	No
2	all	root A (LSB)	9	1–24
2	all	TOE	10	1–16	2^4	No
2	all	FitInt	10	17	1:1	No
2	all	AODO	10	18–22	900	No

Subframe #	Page #	Name	Word #	Bits	Scale	Signed
3	all	CIC	3	1–16	2^-29	Yes
3	all	Omega 0 (MSB)	3	17–24	2^-31	Yes
3	all	Omega 0 (LSB)	4	1–24
3	all	CIS	5	1–16	2^-29	Yes
3	all	i0 (MSB)	5	17–24	2^-31	Yes
3	all	i0 (LSB)	6	1–24
3	all	CRC	7	1–16	2^-5	Yes
3	all	Omega (MSB)	7	17–24	2^-31	Yes
3	all	Omega (LSB)	8	1–24
3	all	Omega Dot	9	1–24	2^-43	Yes
3	all	IODE	10	1–8	1:1	No
3	all	IDOT	10	9–22	2^-43	Yes

Satellite frequencies

GPS frequency overview

Band	Frequency	Description
L1	1575.42 MHz	Coarse-acquisition (C/A) and encrypted precision (P(Y)) codes, plus the L1 civilian (L1C) and military (M) codes on future Block III satellites.
L2	1227.60 MHz	P(Y) code, plus the L2C and military codes on the Block IIR-M and newer satellites.
L3	1381.05 MHz	Used for nuclear detonation (NUDET) detection.
L4	1379.913 MHz	Being studied for additional ionospheric correction.
L5	1176.45 MHz	Proposed for use as a civilian safety-of-life (SoL) signal.

All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The actual internal reference of the satellites is 10.22999999543 MHz to compensate for relativistic effects that make observers on Earth perceive a different time reference with respect to the transmitters in orbit. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code. The P code can be encrypted as a so-called P(Y) code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

The L3 signal at a frequency of 1.38105 GHz is used to transmit data from the satellites to ground stations. This data is used by the United States Nuclear Detonation (NUDET) Detection System (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth's atmosphere and near space. One usage is the enforcement of nuclear test ban treaties.

The L4 band at 1.379913 GHz is being studied for additional ionospheric correction.

The L5 frequency band at 1.17645 GHz was added in the process of GPS modernization. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that provides this signal was launched in 2010. The L5 consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. "L5, the third civil GPS signal, will eventually support safety-of-life applications for aviation and provide improved availability and accuracy."

A conditional waiver has recently been granted to LightSquared to operate a terrestrial broadband service near the L1 band. Although LightSquared had applied for a license to operate in the 1525 to 1559 band as early as 2003 and it was put out for public comment, the FCC asked LightSquared to form a study group with the GPS community to test GPS receivers and identify issue that might arise due to the larger signal power from the LightSquared terrestrial network. The GPS community had not objected to the LightSquared (formerly MSV and SkyTerra) applications until November 2010, when LightSquared applied for a modification to its Ancillary Terrestrial Component (ATC) authorization. This filing (SAT-MOD-20101118-00239) amounted to a request to run several orders of magnitude more power in the same frequency band for terrestrial base stations, essentially repurposing what was supposed to be a "quiet neighborhood" for signals from space as the equivalent of a cellular network. Testing in the first half of 2011 has demonstrated that the impact of the lower 10 MHz of spectrum is minimal to GPS devices (less than 1% of the total GPS devices are affected). The upper 10 MHz intended for use by LightSquared may have some impact on GPS devices. There is some concern that this will seriously degrade the GPS signal for many consumer uses. Aviation Week magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system.

Demodulation and decoding

Demodulating and Decoding GPS Satellite Signals using the Coarse/Acquisition Gold code.

Because all of the satellite signals are modulated onto the same L1 carrier frequency, the signals must be separated after demodulation. This is done by assigning each satellite a unique binary sequence known as a Gold code. The signals are decoded after demodulation using addition of the Gold codes corresponding to the satellites monitored by the receiver.

If the almanac information has previously been acquired, the receiver picks the satellites to listen for by their PRNs, unique numbers in the range 1 through 32. If the almanac information is not in memory, the receiver enters a search mode until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern. There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data.

Processing of the navigation message enables the determination of the time of transmission and the satellite position at this time. For more information see Demodulation and Decoding, Advanced.

Navigation equations

The receiver uses messages received from satellites to determine the satellite positions and time sent. The x, y, and z components of satellite position and the time sent are designated as [x_i, y_i, z_i, t_i] where the subscript i denotes the satellite and has the value 1, 2, ..., n, where When the time of message reception indicated by the on-board clock is , the true reception time is where is receiver's clock bias (i.e., clock delay). The message's transit time is . Assuming the message traveled at the speed of light, , the distance traveled is . Knowing the distance from receiver to satellite and the satellite's position implies that the receiver is on the surface of a sphere centered at the satellite's position with radius equal to this distance. Thus the receiver is at or near the intersection of the surfaces of the four or more spheres. In the ideal case of no errors, the receiver is at the intersection of the surfaces of the spheres.

The clock error or bias, b, is the amount that the receiver's clock is off. The receiver has four unknowns, the three components of GPS receiver position and the clock bias [x, y, z, b]. The equations of the sphere surfaces are given by:

or in terms of pseudoranges, , as

.

These equations can be solved by algebraic or numerical methods.

Least squares method

When more than four satellites are available, the calculation can use the four best or more than four, considering number of channels, processing capability, and geometric dilution of precision (GDOP). Using more than four is an over-determined system of equations with no unique solution, which can be solved by a least-squares or weighted least squares method. Errors can be estimated through the residuals. With each combination of four or more satellites, a GDOP vector can be calculated, based on the relative sky directions of the satellites used. The location is expressed in a specific coordinate system or as latitude and longitude, using the WGS 84 geodetic datum or a country-specific system.

Bancroft's method

Bancroft's method involves an algebraic as opposed to numerical method and can be used for the case of four or more satellites. Bancroft's method provides one or two solutions for the four unknowns. However, when there are two solutions, only one of these two solutions will be a near earth sensible solution. When there are four satellites, we use the inverse of the B matrix in section 2 of. If there are more than four satellites then we use the Generalized inverse (i.e. the pseudoinverse) of the B matrix since in this case the B matrix is no longer square.

Error sources and analysis

Main article: Error analysis for the Global Positioning System

GPS error analysis examines the sources of errors in GPS results and the expected size of those errors. GPS makes corrections for receiver clock errors and other effects but there are still residual errors which are not corrected. Sources of error include signal arrival time measurements, numerical calculations, atmospheric effects, ephemeris and clock data, multipath signals, and natural and artificial interference. The magnitude of the residual errors resulting from these sources is dependent on geometric dilution of precision.

Artificial errors may result from jamming devices and threaten ships and aircraft.

Accuracy enhancement and surveying

Main article: GPS enhancement

Augmentation

Integrating external information into the calculation process can materially improve accuracy. Such augmentation systems are generally named or described based on how the information arrives. Some systems transmit additional error information (such as clock drift, ephemera, or ionospheric delay), others characterize prior errors, while a third group provides additional navigational or vehicle information.

Examples of augmentation systems include the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Differential GPS (DGPS), Inertial Navigation Systems (INS) and Assisted GPS. The standard accuracy of about 15 metres (49 feet) can be augmented to 3–5 metres (9.8–16.4 ft) with DGPS, and to about 3 metres (9.8 feet) with WAAS.

Precise monitoring

Accuracy can be improved through precise monitoring and measurement of existing GPS signals in additional or alternate ways.

The largest remaining error is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but some errors remain. This is one reason GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.

Military receivers can decode the P(Y) code transmitted on both L1 and L2. Without decryption keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. However, this technique is slow, so it is currently available only on specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). This corrects the error that arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. CPGPS uses the L1 carrier wave, which has a period of , which is about one-thousandth of the C/A Gold code bit period of , to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to 2–3 metres (6.6–9.8 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1.2 in) of ambiguity. By eliminating this error source, CPGPS coupled with DGPS normally realizes between 20–30 centimetres (7.9–11.8 in) of absolute accuracy.

Relative Kinematic Positioning (RKP) is a third alternative for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to a precision of less than 10 centimeters (3.9 in). This is done by resolving the number of cycles that the signal is transmitted and received by the receiver by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).

Timekeeping

Leap seconds

While most clocks derive their time from Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time (GPST; see the page of United States Naval Observatory). The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI − GPS = 19 seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC. As of July 2012, GPS time is 16 seconds ahead of UTC because of the leap second added to UTC June 30, 2012. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits).

Accuracy

GPS time is theoretically accurate to about 14 nanoseconds. However, most receivers lose accuracy in the interpretation of the signals and are only accurate to 100 nanoseconds.

Format

As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-bit field that only repeats every 8,192 weeks (157 years), thus lasting until the year 2137 (157 years after GPS week zero).

Carrier phase tracking (surveying)

Another method that is used in surveying applications is carrier phase tracking. The period of the carrier frequency multiplied by the speed of light gives the wavelength, which is about 0.19 meters for the L1 carrier. Accuracy within 1% of wavelength in detecting the leading edge reduces this component of pseudorange error to as little as 2 millimeters. This compares to 3 meters for the C/A code and 0.3 meters for the P code.

However, 2 millimeter accuracy requires measuring the total phase—the number of waves multiplied by the wavelength plus the fractional wavelength, which requires specially equipped receivers. This method has many surveying applications.

Triple differencing followed by numerical root finding, and a mathematical technique called least squares can estimate the position of one receiver given the position of another. First, compute the difference between satellites, then between receivers, and finally between epochs. Other orders of taking differences are equally valid. Detailed discussion of the errors is omitted.

The satellite carrier total phase can be measured with ambiguity as to the number of cycles. Let denote the phase of the carrier of satellite j measured by receiver i at time . This notation shows the meaning of the subscripts i, j, and k. The receiver (r), satellite (s), and time (t) come in alphabetical order as arguments of and to balance readability and conciseness, let be a concise abbreviation. Also we define three functions, :, which return differences between receivers, satellites, and time points, respectively. Each function has variables with three subscripts as its arguments. These three functions are defined below. If is a function of the three integer arguments, i, j, and k then it is a valid argument for the functions, :, with the values defined as

,

, and

 .

Also if are valid arguments for the three functions and a and b are constants then is a valid argument with values defined as

,

, and

 .

Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with that from satellite 2 at the same epoch. This difference is designated as

Double differencing computes the difference of receiver 1's satellite difference from that of receiver 2. This approximately eliminates satellite clock errors. This double difference is:

$\begin{align} \Delta^r(\Delta^s(\phi_{1,1,1}))\,&=\,\Delta^r(\phi_{1,2,1} - \phi_{1,1,1}) &=\,\Delta^r(\phi_{1,2,1}) - \Delta^r(\phi_{1,1,1}) &=\,(\phi_{2,2,1} - \phi_{1,2,1}) - (\phi_{2,1,1} - \phi_{1,1,1}) \end{align}$

Triple differencing subtracts the receiver difference from time 1 from that of time 2. This eliminates the ambiguity associated with the integral number of wavelengths in carrier phase provided this ambiguity does not change with time. Thus the triple difference result eliminates practically all clock bias errors and the integer ambiguity. Atmospheric delay and satellite ephemeris errors have been significantly reduced. This triple difference is:

Triple difference results can be used to estimate unknown variables. For example if the position of receiver 1 is known but the position of receiver 2 unknown, it may be possible to estimate the position of receiver 2 using numerical root finding and least squares. Triple difference results for three independent time pairs quite possibly will be sufficient to solve for receiver 2's three position components. This may require the use of a numerical procedure. An approximation of receiver 2's position is required to use such a numerical method. This initial value can probably be provided from the navigation message and the intersection of sphere surfaces. Such a reasonable estimate can be key to successful multidimensional root finding. Iterating from three time pairs and a fairly good initial value produces one observed triple difference result for receiver 2's position. Processing additional time pairs can improve accuracy, overdetermining the answer with multiple solutions. Least squares can estimate an overdetermined system. Least squares determines the position of receiver 2 which best fits the observed triple difference results for receiver 2 positions under the criterion of minimizing the sum of the squares.

Regulatory spectrum issues concerning GPS receivers

In the United States, GPS receivers are regulated under the Federal Communications Commission's (FCC) Part 15 rules. As indicated in the manuals of GPS-enabled devices sold in the United States, as a Part 15 device, it "must accept any interference received, including interference that may cause undesired operation." With respect to GPS devices in particular, the FCC states that GPS receiver manufacturers, "must use receivers that reasonably discriminate against reception of signals outside their allocated spectrum.". For the last 30 years, GPS receivers have operated next to the Mobile Satellite Service band, and have discriminated against reception of mobile satellite services, such as Inmarsat, without any issue.

The spectrum allocated for GPS L1 use by the FCC is 1559 to 1610 MHz, while the spectrum allocated for satellite-to-ground use owned by Lightsquared is the Mobile Satellite Service band. Since 1996, the FCC has authorized licensed use of the spectrum neighboring the GPS band of 1525 to 1559 MHz to the Virginia company LightSquared. On March 1, 2001, the FCC received an application from LightSquared's predecessor, Motient Services to use their allocated frequencies for an integrated satellite-terrestrial service. In 2002, the U.S. GPS Industry Council came to an out-of-band-emissions (OOBE) agreement with LightSquared to prevent transmissions from LightSquared's ground-based stations from emitting transmissions into the neighboring GPS band of 1559 to 1610 MHz. In 2004, the FCC adopted the OOBE agreement in its authorization for LightSquared to deploy a ground-based network ancillary to their satellite system - known as the Ancillary Tower Components (ATCs) - "We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service."  This authorization was reviewed and approved by the U.S. Interdepartment Radio Advisory Committee, which includes the U.S. Department of Agriculture, U.S. Air Force, U.S. Army, U.S. Coast Guard, Federal Aviation Administration, National Aeronautics and Space Administration, Interior, and U.S. Department of Transportation.

In January 2011, the FCC conditionally authorized LightSquared's wholesale customers, such as Best Buy, Sharp, and C Spire, to be able to only purchase an integrated satellite-ground-based service from LightSquared and re-sell that integrated service on devices that are equipped to only use the ground-based signal using LightSquared's allocated frequencies of 1525 to 1559 MHz. In December 2010, GPS receiver manufacturers expressed concerns to the FCC that LightSquared's signal would interfere with GPS receiver devices although the FCC's policy considerations leading up to the January 2011 order did not pertain to any proposed changes to the maximum number of ground-based LightSquared stations or the maximum power at which these stations could operate. The January 2011 order makes final authorization contingent upon studies of GPS interference issues carried out by a LightSquared led working group along with GPS industry and Federal agency participation.

GPS receiver manufacturers design GPS receivers to use spectrum beyond the GPS-allocated band. In some cases, GPS receivers are designed to use up to 400 MHz of spectrum in either direction of the L1 frequency of 1575.42 MHz, because mobile satellite services in those regions are broadcasting from space to ground, and at power levels commensurate with mobile satellite services. However, as regulated under the FCC's Part 15 rules, GPS receivers are not warranted protection from signals outside GPS-allocated spectrum. This is why GPS operates next to the Mobile Satellite Service band, and also why the Mobile Satellite Service band operates next to GPS. The symbiotic relationship of spectrum allocation ensures that users of both bands are able to operate cooperatively and freely.

The FCC adopted rules in February 2003 that allowed Mobile Satellite Service (MSS) licensees such as LightSquared to construct a small number of ancillary ground-based towers in their licensed spectrum to "promote more efficient use of terrestrial wireless spectrum." In those 2003 rules, the FCC stated "As a preliminary matter, terrestrial [Commercial Mobile Radio Service (“CMRS”)] and MSS ATC are expected to have different prices, coverage, product acceptance and distribution; therefore, the two services appear, at best, to be imperfect substitutes for one another that would be operating in predominately different market segments... MSS ATC is unlikely to compete directly with terrestrial CMRS for the same customer base...". In 2004, the FCC clarified that the ground-based towers would be ancillary, noting that "We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service." In July 2010, the FCC stated that it expected LightSquared to use its authority to offer an integrated satellite-terrestrial service to "provide mobile broadband services similar to those provided by terrestrial mobile providers and enhance competition in the mobile broadband sector." However, GPS receiver manufacturers have argued that LightSquared's licensed spectrum of 1525 to 1559 MHz was never envisioned as being used for high-speed wireless broadband based on the 2003 and 2004 FCC ATC rulings making clear that the Ancillary Tower Component (ATC) would be, in fact, ancillary to the primary satellite component. To build public support of efforts to continue the 2004 FCC authorization of LightSquared's ancillary terrestrial component vs. a simple ground-based LTE service in the Mobile Satellite Service band, GPS receiver manufacturer Trimble Navigation Ltd. formed the "Coalition To Save Our GPS."

The FCC and LightSquared have each made public commitments to solve the GPS interference issue before the network is allowed to operate. However, according to Chris Dancy of the Aircraft Owners and Pilots Association, airline pilots with the type of systems that would be affected "may go off course and not even realize it." The problems could also affect the Federal Aviation Administration upgrade to the air traffic control system, United States Defense Department guidance, and local emergency services including 911.

On February 14, 2012, the U.S. Federal Communications Commission (FCC) moved to bar LightSquared's planned national broadband network after being informed by the National Telecommunications and Information Administration (NTIA), the federal agency that coordinates spectrum uses for the military and other federal government entities, that "there is no practical way to mitigate potential interference at this time". LightSquared is challenging the FCC's action.

Other systems

Comparison of GPS, GLONASS, Galileo and Compass (medium earth orbit) satellite navigation system orbits with the International Space Station, Hubble Space Telescope and Iridium constellation orbits, Geostationary Earth Orbit, and the nominal size of the Earth. The Moon's orbit is around 9 times larger (in radius and length) than geostationary orbit.

Other satellite navigation systems in use or various states of development include:

• GLONASS – Russia's global navigation system. Fully operational worldwide.
• Galileo – a global system being developed by the European Union and other partner countries, planned to be operational by 2014 (and fully deployed by 2019)
• Beidou – People's Republic of China's regional system, currently limited to Asia and the West Pacific
• COMPASS – People's Republic of China's global system, planned to be operational by 2020
• IRNSS – India's regional navigation system, planned to be operational by 2014, covering India and Northern Indian Ocean
• QZSS – Japanese regional system covering Asia and Oceania

See also

• GPS/INS
• GPS navigation software
• High Sensitivity GPS
• Indoor positioning system
• Local positioning system
• Military invention
• Mobile phone tracking
• Navigation paradox
• S-GPS

Notes

References

• "NAVSTAR GPS User Equipment Introduction" (PDF). United States Coast Guard. September 1996. 

Further reading

• Parkinson; Spilker (1996). The global positioning system. American Institute of Aeronautics and Astronautics. ISBN 978-1-56347-106-3. 
• Jaizki Mendizabal; Roc Berenguer; Juan Melendez (2009). GPS and Galileo. McGraw Hill. ISBN 978-0-07-159869-9. 
• Nathaniel Bowditch (2002). The American Practical Navigator – Chapter 11 Satellite Navigation. United States government. 
• Global Positioning System Open Courseware from MIT, 2012

External links

• Schriever Air Force Base – GPS Operations Center Responsible for operation of the Global Positioning System
• Global Positioning System at DMOZ
• FAA GPS FAQ
• GPS.gov—General public education website created by the U.S. Government
• USCG Navigation Center—Status of the GPS constellation, government policy, and links to other references; includes satellite almanac data.
• The GPS Program Office (GPS Wing)—Responsible for designing and acquiring the system on behalf of the United States Government.
• U.S. Army Corps of Engineers manual: NAVSTAR HTML and PDF (22.6 MB, 328 pages)
• National Geodetic Survey Orbits for the Global Positioning System satellites in the Global Navigation Satellite System
• GPS PPS Performance Standard—The official Precise Positioning Service specification
• GPS and GLONASS Simulation (Java applet) Simulation and graphical depiction of space vehicle motion including computation of dilution of precision (DOP)
• Navipedia information on GPS

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