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 SBAS broadcasts in the L-band, the signal requires a line of sight in the same manner as GPS to maintain signal acquisition. Let’s select WAAS for an example. Currently, two communication satellites are transmitting WAAS data for public use. Due to their location, these satellites may appear lower on the horizon, depending on your geographic position on land. In regions where the satellites appear lower on the horizon, they may be more prone to being masked by terrain, foliage, buildings, or objects, resulting in signal loss. The further that you are away from the equator and the satellite’s longitude will cause the satellite to appear lower on the horizon. Fortunately, the COAST Technology helps alleviate this problem by maintaining system performance when SBAS signal loss occurs for extended periods.
Worldwide SBAS Coverage The figure below depicts the current world SBAS coverage. This figure is only an approximation of signal coverage by each of the SBAS constellations. Although there is geographic coverage at higher latitudes, practical usage of SBAS will be limited to environments where a relatively consistent line of sight to the satellites from the SXBlue GPS system is available.
Worldwide SBAS Coverage
Worldwide SBAS Coverage The map below shows the ionospheric coverage for each of the SBAS constellations. For WAAS, EGNOS, and MSAS, the grids shown are actual grids with no extrapolation. The grid shown for GAGAN was recorded by an SXBlue II GPS during the Final Acceptance Test phase and is not an official grid from the Indian authorities.
Worldwide SBAS Ionospheric Coverage Worldwide SBAS Ionospheric Coverage The following table shows a list of all the PRN numbers allocated to each of the SBAS Constellations.
Worldwide SBAS Constellation (July 2012)
SBAS Constellation PRN Number NMEA Number Satellite Name Longitude WAAS 133 46 Immarsat 4F3 98.0° W WAAS 135 48 Galaxy XV 133.0° W WAAS 138 51 Anik F1R (Telesat) 107.3° W EGNOS 120 33 Inmarsat 3f2 (AOR-E) 15.5° W EGNOS 124 37 ESA (Artemis)* 21.5° E EGNOS 126 39 Immarsat 3F5 (IND-W) 25.0° E MSAS 129 42 MTSAT 1R 140.0° E MSAS 137 50 MTSAT 2 145.0° E GAGAN 127 40 GSAT-8 55.0° E GAGAN 128 41 GSAT-10 83.0° E SDCM 125 38 Luch-5B 16.0° W** SDCM 140 53 Luch-5V 95.0° E** SDCM 141 54 Luch-5A 167.0° E** EGNOS PRN 124 normally used to perform industry tests, is currently set to test mode. * SDCM is not operational. Luch-5B might be repositioned to a more eastern location. Luch-5A is currently at 95.0°E and will be moved to 167.0°E. The luch-5V launch is planned for Q1 2014. SBAS Ionospheric Map Extrapolation To improve upon the ionospheric map provided by SBAS, the SXBlue GPS receivers extrapolate a larger ionospheric map from the broadcast coverage map, extending its effective coverage. This allows the SXBlue GPS to be used successfully in regions where competing products may not. For example, the extrapolation provides enhanced coverage through the northern part of South America for WAAS, through the northern part of Africa for EGNOS, etc.
Please note that the process of estimating ionospheric corrections beyond the SBAS broadcast map would not be as good as having an extended SBAS map in the first place. This difference may lead to minor accuracy degradation. As an example, the figure below depicts the broadcast WAAS and EGNOS ionospheric map extent and the extrapolated version. As can be seen, the extrapolated grid extends further in all directions than the broadcast one, enhancing usable coverage.
Broadcast and Extrapolated Ionospheric Correction Map Broadcast and Extrapolated Ionospheric Correction Map Error Handling – Differences between LBAS and SBAS Besides Dilution of Precision (DOP) and Multipath (that are errors that cannot be removed by differential correction), many factors affect the positioning accuracy that a user may expect from a DGPS system. The most significant of these influences include:
The proximity of the remote user to the reference station (atmospheric and orbit errors) Age of the received differential corrections Atmospheric conditions at the base station and remote user locations Quality of the GPS receiver being used at both the reference and remote stations The proximity of the Reference Station In an LBAS, the distance between a remote user and the reference station can sometimes be considerable, such as when using 300 kHz DGPS radio beacons. Consequently, some of the errors associated with GPS at the base station differ somewhat from those at the remote user’s location. This spatial decorrelation of errors can result in a relative position offset from the absolute coordinates of the remote receiver. This offset may be as much as one meter for every 100 km (62 miles) between the base station and remote receiver.