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SBAS made easy

Updated: Apr 16

To understand how Space Based Augmentation Systems (SBAS) such as WAAS/EGNOS/MSAS/GAGAN work, the conventional method of real-time differential correction (LBAS) will first be presented along with the factors affecting local DGPS accuracy. (Note that this section does not deal with carrier-phase differential). Local Based Augmentation System (LBAS) Space-Based Augmentation System (SBAS) Error Handling – Differences between LBAS and SBAS Local Based Augmentation System (LBAS) Conventional DGPS involves setting up a reference GPS receiver with the antenna set at a point of known coordinates. This receiver makes distance measurements, in real time, to each of the GPS satellites. The measured ranges include the errors present in the system. The base station receiver calculates what the true range is, without errors, knowing its coordinates and those of each satellite. The difference between the known and measured range for each satellite is the range error. This error is the amount that needs to be removed from each satellite distance measurement in order to correct for errors present in the system.

Local Based Augmentation System (LBAS)

The base station transmits the range error corrections to remote receivers in real time. The remote receiver corrects its satellite range measurements using these differential corrections, yielding a much more accurate position. This is the predominant DGPS strategy used for a majority of real-time applications. Positioning using corrections generated by DGPS radio beacons, for example, will provide a horizontal accuracy of less than 1m to 5 meters with a 95% confidence depending on the quality of the GPS receiver used. Under the same principle, more sophisticated, short-range DGPS systems (10 to 15 km) can achieve centimeter-level accuracy using the carrier phase. In this case, we commonly refer to such a system as RTK instead of DGPS.

Space-Based Augmentation Systems (SBAS) The US Federal Aviation Administration has developed a Wide Area Augmentation System (WAAS) for the purpose of providing accurate positioning to the aviation industry. In addition to providing high quality and accurate service for this industry, this service is available free of charge to all other civilian users and markets in Central and North America. This service falls into the greater category of the Space-Based Augmentation System (SBAS).

Upon the successful completion of a 21-day test on August 24, 2000, the Federal Aviation Administration of the United States of America announced that its Wide Area Augmentation System (WAAS) would be running 24 hours per day, seven days per week from then on. Testing has shown since that this signal is accurate and reliable. Since the date it was commissioned (July 10, 2003), WAAS has undergone a few changes in its satellite constellation and coverage (PRN 122 and 134 phased out and replaced by PRN 135 and 138 at new locations; and with the addition of ground monitoring stations in Canada and Mexico in September 2007).

Other government agencies have followed the pace and developed compatible SBAS systems for their respective geographic regions. In Europe, the European Space Agency, the European Commission, and EUROCONTROL have jointly developed the European Geostationary Overlay System (EGNOS). EGNOS is now fully deployed and in its pre-operational phase. The system will undergo certification for safety-of-life applications before becoming fully operational. Furthermore, on June 28, 2007, the European Space Agency and the Agency for Security of Air Navigation in Africa and Madagascar have signed a cooperation agreement with the objective of using satellite navigation to improve air traffic safety over the African continent.

In Japan, the MTSAT Satellite-based Augmentation System (MSAS) has been deployed by the Japan Civil Aviation Bureau (JCAB). Successful launches of MTSAT-1R and MTSAT-2 were followed by system integration for MSAS ground system and MTSATs by transmitting test signals from MTSATs. Purposes of test signal transmission were to optimize system performance and then to verify that augmentation information meets safety and performance requirements. Since those tests had been accomplished successfully, MSAS for aviation use was commissioned on September 27, 2007.

In India, the Indian Space Research Organisation (ISRO) and Airports Authority of India have successfully completed the final system acceptance test of the GPS Aided GEO Augmented Navigation system (GAGAN) as announced on November 20, 2007, by Raytheon Company. With the completion of the final system acceptance test, the stage is set for India to embark on the next phase of the program, which will expand the existing ground network, add redundancy, and produce the certification analysis and documentation for safety-of-flight commissioning. Inmarsat 4f1 was used during the system acceptance test. Awaiting the launch of its own communication satellite, the GSAT-4 (scheduled for June 2009), the Indian ISRO has stopped the broadcast of GAGAN test signals. China has a similar program for an SBAS and the service is named the Chinese Satellite Navigation Augmentation System (SNAS). The SXBlue GPS series is capable of receiving correction data from all compatible SBAS.

How it Works SBAS incorporates a modular architecture, similar to GPS, comprised of a Ground Segment, Space Segment, and User Segment:

The Ground Segment includes reference stations, processing centers, a communication network, and Navigation Land Earth Stations (NELS) The Space Segment includes geostationary satellites (For example, EGNOS uses Inmarsat transponders) The user segment consists of the user equipment, such as an SXBlue II GPS receiver and antenna SBAS uses a state-based approach in its software architecture. This means that a separate correction is made available for each error source rather than the sum effect of errors on the user equipment’s range measurements. This more effectively manages the issue of spatial decorrelation than some other techniques, resulting in a more consistent system performance regardless of geographic location with respect to reference stations. Specifically, SBAS calculates separate errors for the following:

The ionospheric error GPS satellite timing errors GPS satellite orbit errors Figures below show the ground segments of the WAAS, EGNOS, and MSAS systems, respectively. In 2007, a total of 13 monitoring stations were added to the existing WAAS network, increasing the ionospheric coverage for this SBAS constellation. The location is shown in red: 4 in Alaska, 4 in Canada and 5 in Mexico.

WAAS Ground Segment WAAS Ground Segment (end of 2007) EGNOS Ground Segment EGNOS Ground Segment (end of 2009) MSAS Ground Segment MSAS Ground Segment (end of 2007)

Provided that a GPS satellite is available to the SBAS reference station network for tracking purposes, orbit, and timing error corrections will be available for that satellite. Ionospheric corrections for that satellite are only available if the signal passes through the ionospheric map provided by SBAS (ex. the WAAS ionospheric map covers the entire Central and North American region). As an example, if a satellite is South of your current location at a low elevation angle, the pierce point of the ionosphere will be considerably South of your location since the ionosphere is at an altitude of approximately 60 km. There must be sufficient ionospheric map coverage beyond your location to have ionospheric correctors for all satellites.

To enhance the information provided by SBAS, the SXBlue GPS has a unique ability to extrapolate the ionospheric information beyond the broadcast grid. This feature increases the usable geographic coverage area of an SBAS system.

Signal Information An SBAS transmits correction data on the same frequency as GPS from a geostationary satellite (the space segment), allowing the use of the same receiver equipment used for GPS. Another advantage of having SBAS transmit on the same frequency is that only one antenna is required.

Reception Since SB