The US has the global positioning satellite system, Russia has GLONASS, Europe hopes to have Galileo operational by 2020 and China has a similar timescale for its COMPASS system. Even Japan and India are getting in on the act with QZSS, the Quasi Zenith Satellite System of Japan, and the Indian Regional Navigational Satellite System or IRNSS.
The GPS system alone is estimated to have cost $1.5 billion in 2012 alone. A cheaper idea is to transmit the highly accurate time signals from the ground and simply route them through geostationary satellites that are already in orbit. Receivers can then triangulate their position in the same way. That’s much easier and cheaper since it uses existing communications infrastructure
China tested this idea in 2005 using communications satellites to provide positioning signals for several Chinese cities.
There is a problem, however. Geostationary satellites sit exactly in the equatorial plane so their signals can only give an east-west position. To get a north-south position requires satellites above and below the equatorial plane.
The Chinese Area Positioning System (CAPS), a navigation system based on GEO communication satellites, was developed in 2002 by astronomers at Chinese Academy of Sciences. Extensive positioning experiments of CAPS have been performed since 2005. On the basis of CAPS, this paper studies the principle of navigation constellation composed of Slightly Inclined Geostationary Orbit (SIGSO) communication satellites. SIGSO satellites are derived from end-of-life Geostationary Orbit (GEO) satellites under inclined orbit operation. Considering the abundant frequency resources of SIGSO satellites, multi-frequency observations could be conducted to enhance the precision of pseudorange measurements and ameliorate the positioning performence. The constellation composed of two GEO satellites and four SIGSO satellites with inclination of 5 degrees can provide the most territory of China with 24-hour maximum PDOP less than 42. With synthetic utilization of the truncated precise (TP) code and physical augmentation factor in four frequencies, navigation system with this constellation is expected to obtain comparable positioning performance with that of coarse acquisition code of GPS. When the new approach of code-carrier phase combinations is adopted, the system has potential to possess commensurate accuracy of precise code in GPS. Additionally, the copious frequency resources can also be used to develop new anti-interference techniques and integrate navigation and communication
Due to a variety of perturbations, end-of-life GEO communication satellites will drift to SIGSO satellites under inclined orbit operation. SIGSO satellites are favorable to be deployed in navigation constellation to achieve 3D positioning. When SIGSO satellites have relatively large inclination angles (e.g., 5◦ ) and four-frequency PAFP is enforced, the system composed of SIGSO communication satellites is capable to obtain approximately equal positioning performance of C/A code in GPS. If the new approach of code-carrier phase combinations with three frequencies is further employed, the positioning accuracy will be enhanced remarkably and would be as good as that of GPS P code.