WAAS and EGNOS
Very much simplified, WAAS (Wide Area Augmentation System) is a satellite based differential GPS system (DGPS). The difference is, that no additional long-wave receiver is necessary to receive the correction data and there is no need for an endless number of DGPS beacons that transmit these correction data.
Differences between WAAS, EGNOS and MSAS
In principle, all three systems are the same and even more astonishing, the three systems are compatible to each other. This can be called astonishing since WAAS is maintained by north America, EGNOS (European Geostationary Navigation Overlay Service) is maintained by the European community and MSAS (Multi-Functional Satellite Augmentation System) is developed by Japan and other Asiatic countries.
While WAAS is operational (IOC = Initial Operational Capability) since January 2003 (although not yet approved by the FAA), the EGNOS system made some huge steps forward during 2002 but is still (May 2005) in test operation (ESTB = EGNOS Satellite Test Bed). The development of the MSAS system had a major drawback in 1999 after the first of two satellites planned for the system was lost during launch because of a malfunction of the rocket. The start for the replacement satellite was initially planned for August 2004 but was delayed until further notice to investigate the reasons for the malfunction of the rocket in 1999. Up-to-date information on the project are sparse (see here)
While alle systems can be called SBAS (Satellite Based Augmentation Systems), this name is seldomly used.
How the SBAS work
It is no surprise that WAAS, EGNOS and MSAS have not been developed to the increase the accuracy of GPS for hikers and geocachers. The main reason is to increase the safety for aviation. The GPS system is neither accurate nor reliable enough to be accepted as a sole means of navigation. One of the reasons is that there is no reliable and quick (within seconds) information to the user if problems with the system occur. As a consequence, for landing approaches, GPS can’t be used. Airplanes still have to use ILS-systems (Instrument Landing Systems) if visibility is poor. But the installation and maintenance of ILS-systems on every airport is expensive. With the SBAS systems, CAT I approaches (limited visibility) will be possible without additional ILS systems. For CAT III approaches (zero visibility) eventhe SBAS will not suffice and ILS are still required.
Infrastructure and Principle of the System
The SBAS shall provide additional accuracy and reliability for the GPS system. To achieve this, a number of GPS receiving stations are necessary. In the US , 25 station are used, Europe uses 10 stations during the test operation and will have 34 when EGNOS is fully operational. The position of these RIMS (Ranging and Integrity Monitor Stations) must be known very precise. This means that the position of the receiving antenna needs to be know exactly to a few centimetres. The RIMS station receive the standard GPS signal (and also the signal from the russion GLONASS system and the GALILEO system in future). That way it is possible to calculate the difference between the known position of the station and the position as calculated by the GPS receiver. And since the RIMS use receivers that use both GPS frequencies (L1 and L2), the signal delay through the ionosphere can be calculated for every single satellite.
Additionally, if the signals from more than four satellites are received, more information than needed for a position determination is available and this information may be used to check for possible problems with the satellites or deviations in their orbits or time.
The data from all RIMS are sent to a Central Processing Centre. For the EGNOS test bed (ESTB) this centre is in Toulouse ( France ) and a backup system is located in Hønefoss ( Norway ). Once EGNOS is fully operational there will be control centers, called MCC (Mission Control Centre) in Germany (Langen near Frankfurt ), Spain (Torrejon near Madrid ), Italy (Ciampino near Rome ) and Great Britain (Swanwick near London ). At these stations, the data will be collected and the following data will be calculated:
- Long term errors of the satellite orbits
- Short term and Long term errors of the satellite clocks
- IONO correction grids
- Integrity information
By use of the integrity information, it is possible to inform the users within 6 seconds on problems that occur with the GPS system.
The most important feature of the SBAS for common GPS users is the IONO correction grid. Since SA (selective availability) is deactivated, the largest single source of error in GPS position determination is the signal delay in the ionosphere. Being able to correct these errors significantly increases the accuracy of every GPS receiver that is able to process WAAS/EGNOS data.
From the measured data of the RIMS, a ‘map’ of the Total Electron Content (TEC) in the ionosphere for the area covered by the RIMS station is calculated. With decreased accuracy the area where the TEC map is calculated can even be expanded further.
This TEC map is now transmitted to a geostationary satellite that itself acts like a GPS satellite, that means can be used for position determination but also provides the receiver with the information it needs for the correction of the ionospheric effects.
For the EGNOS test system (ESTB) from Aussaguel (near Toulouse in France ) data is transmitted to INMARSAT AOR-E and from Fucino ( Italy ) to INMARSAT IOR. Later when EGNOS is fully operational, data will be sent from Aussaguel and Goonhilly ( Great Britain ) to AOR-E and from Fucine and Goonhilly to IOR-F5. From the stations Torrejon ( Spain ) and Scanzano ( Italy ) data will be transmitted to the satellite Artemis.
The satellite Artemis had an interesting history when in January 2003 it finally arrived at its designated position. After problems with the fourth stage of the Ariane-5 rocket after the launch in July 2001, Artemis had almost to be abandoned when but the engineers managed to “walk” the satellite to its planned position by making extensive use of its newly developed ion propulsion system.
The geostationary satellites do provide a signals very similar to that of the GPS-satellites and on the same frequency. Therefor these satellites may be used for position calculation and additionally, the correction data sent out can be used to improve accuracy for position calculation with all GPS satellites.
Using the TEC map transmitted by the geostationary satellites, the GPS receiver can now calculate the ‘pierce point’ and signal delay of the signal of each satellite used for position calculation and then correct the data for higher accuracy in position determination.
The ionosphere is not static but depends on the sun’s activity. For example it is known that single frequency receivers are more accurate shortly after midnight than they are during the day.
The other functions that the SBAS provide like integrity check of the GPS system and transmission of warnings in case of problems with the system will probably be never evaluated by standard handheld GPS receivers since the calculations are complex and the information is of not much interest to the common GPS user.
Differences to DGPS
For land-based people, the main difference between D GPS and the SBAS systems ist he calculation of the TEC map for ionospheric corrections. This brings some of the benefits of an expensive dual frequency receiver to a cheap single frequency receiver.
With D GPS , every single reference station compares its own precisely known position with the position calculated from the GPS signals. The station then transmits this information on a certain long wave band as correction data. A D GPS receiver receives the correction information and applies this correction to the signals received from the GPS satellites. With increasing distance of the receiver to the D GPS reference station, the atmospheric influences on the signals get more and more different and the correction get less and less accurate. If the distance between the reference station is large, the signals from the satellites travel through different parts of the atmosphere, being influenced in different ways. Even worse, due to the large distance the receiver may receive data from completely different satellite where no correctional information are provided in the correctional data. This effect where the reference station does not provide the right data for correction due to large distance to the receiver is called ‘spatial decorrelation’. Because of these phenomena, the typical range for DGPS stations is 70 – 200 km with good accuracy.
For the SBAS systems (WAAS, EGNOS, MSAS) this is different. Here, the monitor stations do not provide single isolated corrections but from all stations together a correction map is calculated for a wide area. Every single receiver then corrects its own position itself by use of this data. That way, the accuracy that can be achieved is even better than with D GPS .
If the receiver is outside of the area where valid TEC map data are provided, the receiver should use the build in standard ionospheric correction and then there should be no difference between WAAS/EGNOS switched on or off in the receiver setup menu in these cases. But since most manufacturers recommend switching WAAS/EGNOS off if not within the area where the signals are provided for, it may be assumed that the receivers do not make full use of all information provided and do not check whether the correction should be used or not. That way, the position may be even worse when WAAS/EGNOS is switched on when no proper correction signal is provided.
Coverage of the geostationary satellites
The area covered by WAAS, EGNOS and MSAS depends on where RIMs station are located and if signals from geostationary satellites are being received. For the transmission of the SBAS signals currently some INMARSAT satellites are used. These satellites are positioned in geostationary orbits (about 36000 km) and are primarily being used as relay satellites for telephone calls from and to ships. The following map shows the ‘footprint’ of these satellites, that means the area where their signals may be received. Before EGNOS is completely operational, there will be some changes, especially concerning the coverage over Europe.
The following table lists the WAAS/EGNOS satellites and their identification numbers:
|Satellite||Satellite location||GPS PRN No.||Garmin Sat ID|
|INMARSAT 3 F2 (AOR-E)
(Atlantic Ocean Region East)
|INMARSAT 3 F4 (AOR-W)
(Atlantic Ocean Region West)
|East coast of Brasil||122||35|
|INMARSAT 3 F1 (IOR)
(Indian Ocean Region)
|INMARSAT 3 F3 (POR)
(Pacific Ocean Region)
|INMARSAT IOR-W (III-F5)
(Indian Ocean Region West)
(Multifunction Transportation Satellite)
If you have the WAAS/EGNOS function activated on your GPS and you receive signales of other satellites than no. 33 or no. 44 while being in Europe you should be careful. Under certain circumstances, signal of satellites being designated for the America (especially no. 35) may be received in Europe . But these satellites only transmit correctional data for northern America so you do not have any advantages.
As already said, the satellite constellation over Europe will change before EGNOS is fully operational. It is planned that the ESA (European Space Agency) satellite ARTEMIS will be used for EGNOS while AOR-E won’t be used in future. The satellite IOR will be moved towards the pacific.
There is one major disadvantage of the correction systems that are based on geostationary satellites. For a GPS near ground and in central Europe or northern america , all geostationary satellites are located in the south and quite low over the horizon. For example, if you are located in munich , AOR-E is about 35° above the horizon, IOR on at 16°. This easily leads to blocking of the signals by buildings or trees. In forested or hilly areas, WAAS or EGNOS probably will never work perfectly. This disadvantage results in the system being developed for aviation where it does not matter if the satellites are a little low over the horizon.
The INMARSAT satellite III-F5 that will also be used for EGNOS, will be located about 35° over the horizon viewed from munich. How good this will be for signal reception will be seen in future.
WAAS, EGNOS and Garmin GPS
Beginning with April 1st, 2003 , the EGNOS or ESTB signal is transmitted in WAAS compatible data format (SBAS mode 0/2). Since that time, the receivers from Garmin are able to use the data. It should be repeated that Garmin receivers are only capable of doing WAAS/EGNOS corrections if they are set to normal mode. It is not enough to switch to WAAS-yes, you also have to got to normal mode. Being in enery-saving mode they do not evaluate the WAAS/EGNOS data. Unfortunately, normal mode does use significantly more power and thus greatly reduces the life time of your battery. The picture on the right shows the satellite screen of Garmins extrex vista with EGNOS correction applied. The letter “D” in the signal bar indicates that the data from this particulate satellite is being corrected by the EGNOS signal. With an accuracy of 2 meters (RMS) the positioning is quite good.
Current Status of ESTB/EGNOS
as of June, 5th 2005:
ESTB tests on satellite IOR (PRN 131; ID 44) have been stopped on Mai, 27 th. The satellite will now be moved towards the pacific and there will be a transition from ESTB to EGNOS. At the moment ESTB signals are being transmitted through AOR-E (PRN 120; ID 33).
The final EGNOS constellation will consist of the satellites ARTEMIS (PRN 124; ID 37) , Inmarsat AOR-E (PRN 120; ID 33) and Inmarsat IOR-W (PRN 126; ID 39). Until the official start of EGNOS these satellites do send signals from time to time that will not be evaluated by end-user receivers, although they sometimes show up as grey bars in the skyview with a GPS signal.
The current transmission status of the ESTB-Satellites can he seen here.
The following diagram shows the planned activity for ESTB/EGNOS.
For more information click here.
SBAS-page of the FAA (Federal Aviation Administration)
ESA satellite navigation
EGNOS-page of ESA
ESA-list of GPS-Receivers capable to receive SBAS-signals (WAAS and EGNOS) as pdf-File
(Special thanks go to Michael Baguhl for additional hints and corrections)