Efficiency Improvement of the Current SBAS System by Removal of RRC and Fast Correction

The Satellite Based Augmentation System (SBAS) is an enhancement to augment Global Positioning System (GPS) by broadcasting differential correction information and integrity information from SBAS geostationary satellite (GEO). These information are packed on messages on the GPS-like L1 ranging signal that were designed on the basis of L1 SBAS Minimum Operational Performance Standards (MOPS) DO-229. DO-229 was developed by Working Group 2 of RTCA Special Committee 159 (SC-159) in 1998 when the Selective Availability (SA) was included to the GPS signal. SA was the largest fluctuation error in pseudorange, which was included in satellite clock error. To remove the error effectively, the SBAS GEO satellite was designed to broadcast the Pseudo Range Correction (PRC) in fast correction message, Message Type (MT) 2~5, every 6 seconds. On user side, the SBAS receiver has derived the Rate Range Correction (RRC) from PRC as a linear rate, and then has compensated for the time delay of a new PRC arrival. After the removal of SA on May 2, 2000, the fluctuation in pseudorange error had been much smaller, and it becomes gradually more stable because of the performance improvement on the satellite atomic clocks. Even though the pseudorange error characteristic has been totally changed after the SA removal, the SBAS GEO and user receivers still hold on the initial design discussed when the SA was activated. In this paper, we suggest practical methods to improve the efficiency of the SBAS system which are reducing the receiver computing load and saving the SBAS message bandwidth. First, in user side we suggest not to use RRC to compensate for the latency of the fast correction. Secondly, in infrastructure side we suggest not to use the fast correction, MT 2 to 5. Instead, we convert PRC of the fast correction into the unit of second and then add it to the satellite clock error correction term in MT 25 while pulling out the fast correction from the current message format. It was confirmed that we can save about 50% of the bandwidth without significant accuracy degradation.