Spread-spectrum signals used in global satellite navigation

Abstract

Both the Navstar GPS and GLONASS satellite navigation systems developed respectively by the United States and the Soviet Union are now planned to become operational by the 1995. By means of spreadspectrum signals transmitted in L-band, both systems are capable of providing the civil community with high-precision position-fixes and/or timiig references on a continuous, world-wide basis. Global satellite navigation systems Both the Navstar GPS and GLONASS satellite navigation systems developed respectively by the United States and the Soviet Union are now planned to become operational by 1995. They are intended to replace earlier satellite navigation systems (Transit and Cicada) also operated by the USA and USSR which provide limited daily coverage and are unable to provide the user with velocity information. These two systems employ Similar orbits with a small number of low altitude (1100 lun) polar-orbiting satellites transmitting information at dual frequencies around 150 and 400 MHz. The user waits for a single satellite (possibly as long as two hours) and then makes a series of doppler measurements during the short period (e16 minutes) when the satellite remains above the horizon. The satellites’ position and velocity are transmitted in the navigation message and these, together with the doppler measurements are sufficient to allow the user to compute his position. Transmissions on two frequencies are used to allow an ionospheric group delay correction to be applied. The two systems have two major drawbacks, the system is not available 24 hours a day and the user velocity must be known. In an effort to overcome the difficulties associated with the earlier systems both the US and the USSR plan to introduce precise, glob4 continuous position-fii capabilities by using navigation satellites transmitting dual-frequency spread-spectrum signals in Lband (1.2 and 1.6 GHz). In contrast to the earlier VHF systems, the primary navigation mode is based on range measurement rather than integrated doppler. At present, both systems are in the pre-operational stage with a small number of satellites in two orbital planes. The two systems, both of which possess a military and a civil role, are the USSR’s GLONASS and the US Navstar GPS [l], designed to provide accurate position, velocity and time information.