A global positioning system is composed of three segments - the user segment, the space segment and the control segment.
The user segment is the user and a GPS receiver. A GPS receiver is a specialized radio receiver. It is designed to listen to the radio signals being transmitted from the satellites and calculate a position based on that information. GPS receivers come in many different sizes, shapes and price ranges.The features and costs of GPS receivers is generally dependent on function that the receiver is intended for. Receivers intended for marine and aviation use often have the ability to interface with memory cards containing navigation charts. Receivers intended for mapping are capable of very good accuracy and have user interfaces that allow rapid data collection.
The space segment is composed of the GPS satellites that are transmitting time and position to you, the user. The whole set of satellites is called a "constellation".
NAVSTAR
The NAVSTAR constellation is composed of 24 satellites in six orbital planes. The satellites operate in circular 20,200 km (10,900 nm) orbits at an inclination angle of 55 degrees and with a 12-hour period. The spacing of satellites in orbit is arranged so that a minimum of five satellites will be in view to users worldwide, with a position dilution of precision (PDOP) of six or less. Each satellite transmits on two L band frequencies, L1 (1575.42 MHz) and L2 (1227.6 MHz). Each satellite transmits on exactly the same frequency; however, each satellites signal is doppler-shifted by the time it reaches the user. L1 carries a precise (P) code and a coarse/acquisition (C/A) code. L2 carries only the P code. A navigation data message is superimposed on these codes. The same navigation data message is carried on both frequencies. The P code is normally encrypted so that only the C/A code is available to civilian users; however, some information can be derived from the P code. When encrypted, the P code is known as Y code.There have been three distinct groups of NAVSTAR satellites so far, with one sub-group. The groups are designated as blocks. The block I satellites were intended for system testing. The block II satellites were the first fully functional satellites, including cesium atomic clocks for timing as well as the ability to implement selective availability. They also have radiation hardened electronics, allowing for longer lifetimes in space. In addition, the block II satellite can detect certain error conditions, automatically sending a code indicating that it is out of service. Block II satellites can operate for 3.5 days between corrections from the ground. The block IIa satellites are identical to the standard block II but continue to operate for 180 days between uploads from the ground.
The latest satellites, the block IIR versions, include autonomous navigation. These satellites can operate for 180 days between uploads like the block IIa. Unlike the block IIa, they can generate their own navigation information. Thus, the accuracy of the system can be maintained longer between uploads.
Each satellite has two identifying numbers. First is the NAVSTAR number which identifies the specific satellite hardware. Second is the space vehicle (sv) number. This number is assigned in order of launch. The third is the psuedo-random noise code number. This is a unique integer number which is used to code the signal from that satellite. Some receivers identify the satellites that they are listening to by SV, others by PRN.
GLONASS
The GLONASS constellation is composed of 24 satellites, eight in each of three- orbital planes. The satellites operate in circular 19,100 km orbits at an inclination angle of 64.8 degrees and with a 11-hour, 15 minute period. Each satellite transmits on two L frequency groups. The L1 group is centered on 1609 MHz while the L2 group is centered on 1251MHz. Each satellite transmits on a unique pair of frequencies. The GLONASS signals carry both a precise (P) code and a coarse/acquisition (C/A) code. The P code is encrypted for military use while the C/A code is available for civilian use.
The control segment is composed of all the ground-based facilities that are used to monitor and control the satellites. This segment is usually unseen by the user, but is a vital part of the system. The NAVSTAR control segment, called the operational control system (OCS) consists of a monitor stations, a master control station (MCS) and uplink antennas.The passive monitor stations are nothing more than GPS receivers that track all satellites in view and thus accumulate ranging data from the satellite signals. There are five passive monitor stations, located at Colorado Springs, Hawaii, Ascencion Island, Diego Garcia and Kwajalein. The monitor stations send the raw data back to the MCS for processing.
The MCS is located at Falcon Air Force Base, 12 miles east of Colorado Springs, Colorado and is managed by the U.S. Air Force's 2nd Space Operations Squadron (2nd SOPS). The MCS receives data from the monitor stations in real time 24 hours a day and uses that information to determine if the satellites are experiencing clock or ephemeris changes, and to detect equipment malfunctions. New navigation and ephemeris information is calculated from the monitored signals and uploaded to the satellites once or twice per day.
The information calculated by the MCS, along with routine maintenance commands are transmitted to the satellites by ground-based uplink antennas. The ground antennas are located at Ascencion Island, Diego Garcia and Kwajalein. The antenna facilities transmit to the satellites via an S-band radio link.
In addition to it's main function, the MCS maintains a 24 hour computer bulletin board system with the latest system news and status. The civilian contact for this is the United States Coast Guard's (USCG) Navigation Center (NAVCEN).
References
Chris Shank, Chris and John W. Lavrakas. "Inside GPS: The Master Control Station"
GPS World 5, no. 9 (September 1994) : 46-54Gouzhva, Yuri, Igor Kourdryavtsev, Vladimir Korniyenko and Irina Pushkina.
"Glonass Recievers: An Outline"
GPS World 5, no. 1 (January 1994) : 30 - 36FRP-GPS.TXT, downloaded from the GPSIC BBS
CONSTELL.TXT, downloaded from the GPSIC BBS
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