|Assisted GNSS systems rely on the visibility of GPS satellites (left). |
Together, GPS and GLONASS satellites could improve location effectiveness.
Location-based services (LBS) is one of the fastest growing segments in mobile device applications, so it is easy to understand the urgency to provide accurate-everywhere location (5 to 10-m accuracy) in any environment. Considering some of the data about how and where mobile devices are used, it is apparent why indoor positioning is becoming a much higher-value item (and a much bigger challenge) than outdoor tracking, from both a regulatory and commercial perspective.
Assisted-GPS (A-GPS) uses the GPS satellite constellation, which is controlled by the U.S. military and consists of 30+ satellites in medium earth orbit. The wireless network provides “assistance” data to the handset, which includes information to speed up the process of locking on to the satellites. The final position can be calculated at the user equipment (UE), termed UE-based positioning, or at the network, termed UE-assisted positioning. To speed up the process of obtaining a GPS fix, the network provides satellite constellation information, including:
- Current GPS constellation for the UE’s location
- Current GPS time
- Information on satellite orbits
- Frequency shifts in GPS frequencies because of Doppler effects
The availability of access to the GPS system chiefly governs A-GPS performance. The reference signals are very weak, easily attenuated, or even outright blocked by environmental obstructions. Any remaining satellite signals suffer heavy multipath, and any visible satellites may display poor geometry, which causes a higher positioning error. These issues are most prominent in urban and indoor situations.
Assisted GNSS (A-GNSS) uses satellite constellations other than GPS to improve overall satellite availability. Increasing the number of visible satellites in the sky would cause fewer of them to be blocked out and provide better geometry, increasing performance in urban situations. Currently, the Russian GLONASS system is available for use. With A-GPS, the network can choose to provide assistance data for the additional satellite systems for enhanced performance. With clear visibility, A-GNSS provides very high accuracy, as high as 5 meters under 16 seconds in cold start.
Combined GPS and GLONASS signals, along with similarly configured repeater system could be used to provide indoor GNSS tracking services, providing indoor-outdoor accurate and ubiquitous location coverage for consumer, and commercial tracking needs.