Inside GNSS: Engineering Solutions from the Global Navigation Satellite System Community
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About GPS

SPAC_GPS_NAVSTAR_IIA.jpgThe fully operational GPS constellation has 24 satellites. In 2007, 30 are actually in orbit.

GPS 21st Century Milestones (2001-2009)

The Global Positioning System (GPS) is the first and only fully functional Global Navigation Satellite System. Developed and operated by the U.S. Air Force for the Department of Defense (DoD), GPS is designated by executive and congressional action as a dual-use system available without user fees for civil and military use.

After many technical problems and delays, the GPS IIR-20(M) satellite carrying a demonstration payload of the new L5 signal 
launched on March 24 2009. The first GPS signal in the L5
frequency band successfully transmitted on April 10.

Intended for safety-of-life applications including civil aviation, the
GPS L5 signal is located in an aeronautical radionavigation service
(ARNS) band centered at 1176.45 MHz. Compared to legacy GPS signals,
including the L1 C/A-code, L5 has an improved signal structure, higher
transmitted power (about 3dB), wider bandwidth, and longer spreading

The first production-grade GPS satellite was launched February 22, 1978, although an earlier spacecraft — the second Navigation Technology Satellite (NTS-2) placed into middle earth orbit (MEO) the preceding year — is sometimes referred to as the original GPS space vehicle (SV). Up to 31 operational GPS satellites have been in orbit at one time, but the official specification for a fully operational capability (FOC) constellation calls for only 24 SVs.

GPS spacecraft transmit navigation messages on several L-band frequencies, with military signals able to be encrypted and civil signals broadcast in the open. All signals are spread spectrum and use code division multiple access (CDMA) technology in which different SVs are distinguished by unique pseudorandom noise (PRN) codes. The so-called coarse acquisition (C/A) code — broadcast at the Link-1 (or L1) band centered at1575.42 MHz and employing bi-phase skip keying (BPSK) modulation — is the original civil signal, but modernized civil signals are being introduced at the L2 (1227.60) and L5 (1176.45 MHz) bands. The military precision (or P-code) and its encrypted form (Y-code) is broadcast at L1 and L2 frequencies; a modernized M-code signal for DoD and allied users is currently being implemented.

Design, development, and acquisition of the GPS space, ground, and military user-equipment segments is handled by the GPS Wing (formerly NAVSTAR GPS Joint Program Office), which is part of the Space and Missile Systems Center at Los Angeles Air Force Base, California. A thorough documentation of the signals, which can be used as the basis for designing civil GPS receivers and related products, resides in the GPS Interface Specifications (formerly called Interface Control Documents or ICDs) maintained by the GPS Wing. The current GPS documentation, including IS-GPS-200 and the draft IS-GPS-800 for the new L1 civil signal (L1C), can be found and downloaded here.

Operational control of the satellites and ground facilities is maintained by the 2nd Space Operations Squadron, 50th Space Wing, at the GPS Master Control Station at Schriever AFB, Colorado. Policy and management oversight resides with the National Space-Based Positioning, Navigation, and Timing Executive Committee cochaired by deputy secretaries of defense and transportation.

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