The SBAS system is expanding with India's GAGAN system ( GPS Aided Geo Augmented Navigation ) developed by the Indian Space Research Organization and the Airport Authority of India, acronyms not repeated. GSAT-8 at 55å¡E and GSAT-10 at 83å¡E are already transmitting and GSAT-15 at 93.5å¡E is in orbit. GAGAN joins WAAS (US), EGNOS (Europe) and MSAS (Japan). Russia and China are still developing their systems.
The US WAAS was a cheap way to get SBAS stared as only a transmitter payload was added to civilian communication satellites in geostationary orbits. The correction signal was developed at earth stations then sent to the satellites for rebroadcast. That leads to a delay which is OK because the geostationary satellites were not moving relative to the earth. SBAS instead uses geosynchronous satellites which are more or less in Polar orbits over Australia and Japan. That requires that the signal be generated on the satellite to decrease the delay. The Chinese plan a mixture of geostationary and geosynchronous satellites.
The US is now studying how space based generation of signals can become cost effective in improving performance. The plan includes broadcasting the L1 and L5 signals from the same satellite which will greatly improve the current ranging precision using L1 only when mixed in with a typical GNSS constellation or constellations.
Paul in PA