GPS

Global Positioning System (GPS) The Global Positioning System (GPS) was developed by the US government for the Department of Defense. It is essentially a US military system, but it offers navigation services to civilians; however, at present, there is no law which mandate' the service to be made available for commercial application (Chien, 1991). Position fix is obtained through passive receivers by the triangulation method, wherein estimated ranges from four satellites are used to derive the position and altitude of a point. Ranges from three satellites can provide the latitude and longitude of a point on the Earth; the addition of a fourth satellite can provide a user's attitude and correct receiver clock error. It is possible to derive the velocity of the user and precise time information originating from onboard atomic clocks, which have a drift rate of I s, per 70,000 years. There are two rubidium and two caesiurn clocks aboard each satellite.

Two types of pseudo random codes are transmitted on two frequency bands, L, (1,575.42 MHz) and L 2 (1,227.6 MHz) the encrypted Precise Code (P Code) meant for US military operation is available on both frequencies, and the unencrypted coarse acquisition code (C/A code), available without restriction, is transmitted only in the Li band, where it is combined with Mode in phase quadrature. Codes have low cross correlation, allowing transmissions from each satellite on the same frequency. Spread spectrum modulation also provides some resistance against multipath and immunity to interference. The C/A code, operating at 1.023 Mbps, is a 1023 bit pseudo random code repeating each millisecond, and the P code, operating at 10.023 Mbps, has a cycle of 267 days but it is reset every seven days. Each code is combined with a navigation message comprising the status of the satellite, time synchronization information for transferring from coarse to fine code, clock correction, satellite ephemeris, propagation delay correc. tions and approximate ephemeris and status of the constellation used for signal acquisition. Next generation (Block IIF) satellites are expected to transmit on up to five frequencies, providing unencrypted transmissions for civilian use on three frequencies. We will see later that transmissions on multiple frequencies are used for improving the accuracy of navigation fixes. The P Code can provide accuracy of less than 8 m, whereas the C/A code, after selective availability (S/A), has a nominal accuracy of around 35 m. To avoid the system being used against itself, the military can degrade the accuracy by using S/A, which increases the error probability by satellite clock dithering and corruption of navigation message data. The S/A function is switched on or off as required by events. According to a recent report, the US government has decided to switch off the S/A function, giving improved accuracy for civilian use. Nevertheless, several techniques exist for improving the accuracy of C/A code fixes.

The GPS constellation comprises 21 satellites and three in orbit spares. Satellites are in a circular orbit at an altitude of about 20,200 km (orbital period of about 12 hours), controlled by a master control station in Colorado Springs, with five monitoring stations and three ground stations dispersed around the world. The master control station estimates the orbital parameters of each satellite from ranging data collected by the monitoring stations, formats them and transmits them to each satellite. Table 10.2 (Daly, 1993) summarizes the main orbital para. meters of GPS,, comparing it with the GLONASS system.

To estimate range precisely, each user clock must be synchronized to the satellite, which makes the receiver complex and conflicts with the need for a simple receiver. In the GPS system, the problem is resolved by estimating the range from a fourth satellite, which then allows resolution of user clock uncertainty. Range is measured by measuring the time shift between identical codes generated at the satellite and the receiver. The code generated at the receiver is time shifted until a maximum correlation is achieved between the transmitted and receiver codes; the time shift provides an approximate range or 'pseudo' range comprising a number of errors, listed below. Note that user must have knowledge of code to be able to use the system; this feature permits the military to use the higher accuracy P Code for its purpose. The received signal is around 22 dB below the receiver noise, and therefore it is necessary for the code to be acquired and signals despread prior to clock recovery.