The maxim that 'more is better' could well apply to satellite positioning, as signals from multiple systems like GPS, GLONASS and augmentation satellites can combine to improve availability and accuracy
This year, the GPS satellite constellation reached a new height: 31 active satellites in space and thus more GPS ranging signals available than ever before. But, for many applications, this still is not enough. As we all know from our own experiences, signals are often obstructed at many locations. More satellite signals can help increase GNSS availability. And more ranging signals improve positioning accuracy as well. Hence, the use of all available GNSS signals generally improves positioning performance.
Over the past year, the number of active GLONASS satellites increased from about 12 to 16. Most of the GLONASS satellites operating are the so-called M-type, which have a longer expected lifetime of about seven years. Nevertheless, this is still much shorter than the lifetimes of GPS satellites. Another six GLONASS satellites are expected to be launched in 2008. Presently, due to lack of satellites, GLONASS cannot be used reliably as a stand-alone system. However, its signals are very useful in combination with those of GPS. In recent years, the combination of GPS and GLONASS has become widely accepted among high-precision users, although the full potential of combined GPS and GLONASS ambiguity resolution has not often been exploited.
Then there are geostationary satellite-based augmentation system (SBAS) satellites, the main objective of which is to broadcast GPS augmentation information. Several independent but compatible systems exist: the American Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), the Indian GPS Aided GEO Augmented Navigation System (GAGAN), and the Japanese MTSAT Satellite-based Augmentation System (MSAS). Today, seven active SBAS satellites broadcast their signals. One more satellite, the EGNOS Artemis satellite, is set unhealthy since it is being used by industry to perform tests on the system. The SBAS satellites are located in geostationary orbits. Most SBAS satellites are 'stationary' (keeping pace with the Earth's rotation) to within fractions of a degree in latitude and longitude. Others perform deviations from an equatorial position or even follow slightly inclined orbits.
More and more receivers are capable of not only decoding the SBAS messages (to improve standalone code-based positioning accuracy and integrity) but also of making code and carrier-phase measurements on the signals. Whereas the service areas for GPS augmentation are limited to regions within the corresponding ground-station network, SBAS for ranging works wherever the signals can be received. Users of the SBAS ranging signals may consider WAAS, EGNOS, GAGAN, and MSAS as one GNSS, although this GNSS is not a standalone positioning system because of the small number of satellites and their distribution in the sky.
At mid-latitude locations, the geostationary satellite signals are received from fairly low elevation angles - often below 30 degrees - and thus are prone to signal attenuation and blockage by obstructions. On the other hand, if you are able to receive the signal at a particular location, the satellite is not going to disappear but provides you with ranging information 24 hours per day. The satellite geometry is more favorable closer to the Earth's equator, where SBAS satellites are typically observed at much higher elevation angles. Three or more SBAS satellites are visible at low to mid latitudes throughout most of the African, European, and Asian continents.
Although ranging to the SBAS satellites is one of the main objectives of these augmentation systems, the services have seldom been used for precise positioning.