WAAS/EGNOS and Accuracy - Detailed information

Wide Area Augmented System (WAAS) and European Geostationary Navigation Overlay Service (EGNOS) both do the same thing, provide a more accurate signal for your GPS. This is done by having a network of geostationary satellites around the equator. These broadcast a signal to your GPS which corrects the normal GPS satellite signal you receive. This correction provides a greater degree of position accuracy. To achieve this ground stations called RIMS (Ranging and Integrity Monitoring Stations) receive the normal GPS satellite signal and make necessary corrections for position and atmospheric disturbance. This corrected signal is then sent to the geostationary EGNOS satellites from NLES (Navigation Land Earth Stations) and a MCC (Mission Control Centre) keeps the whole thing ticking over. For the UK region the RIMS are Cork, Gatwick, Glasgow and the NLES  at Goonhilly Downs in Cornwall. The MCC is at Swanwick (Hants). When you switch your GPS on it can take 20 minutes sometimes for the GPS to download the almanac from the EGNOS satellite - when done you'll get the D's in the display (Garmin). I've had the accuracy normally down to 6' or 7', the best being 5'. But please note this is not a precisely accurate value. Read the section on accuracy below for details.

The picture below shows the WAAS/EGNOS satellites we can normally pick up in the UK and Europe and their 'footprint'. The GPS display is showing the satellites being received. The numbers shown on the map by each satellite is the number that is displayed on Garmin GPS receivers satellite display. Due to their location over the equator they only appear some 25° to 30° or so above the horizon so it may be difficult to get a clear view of them. When the GPS has a satellite lock on the WAAS/EGNOS satellite a letter D appears in each normal satellite bar (see picture). This tells you the GPS is receiving a corrected signal. In the picture of the GPS satellite screen it shows the GPS is picking up EGNOS satellite 37 (Artemis) and the normal satellites being received (3, 6, 8, 16, 18, 19, 22 & 24) are being corrected - D in the display. The accuracy is 9 feet.

GPS Accuracy

The accuracy figure displayed on your GPS is not an exact value. If it says '10 feet' it does not mean you are within 10 feet of the location you asked the GPS to go to. To know how accurate the GPS is you need to be a mathematician and understand all about HDoP - Horizontal Dilution of Precision! To try and put it simply. Your GPS is accurate for approximately 95% of the time. This means for 5% of the time it could be very inaccurate. When you switch it on and it starts to download the data from the satellites it starts to work out where it is and depending on how many satellites are visible and how good the atmospheric conditions are it will give you an accuracy figure of lets say 30 feet. If you leave it for a few minutes longer this may (or may not) reduce to say 20 feet. Now with the 95% figure you could be within 20 feet of your target but there is a chance you are not. You might be up to 30 feet away. Of course the person who marked the waypoint would have had an accuracy figure too and their error, coupled with yours means you could be 60 feet away! Does all this matter? Not really, if your GPS shows an accuracy of say 18 feet then you can be reasonably confident it is 'about' right but do not take it as a precise figure, it could be 15 feet or as much as 30 feet.

If you enable WAAS then you should get an improvement of accuracy as now your GPS should be accurate for 98% of the time. Typically your GPS may give you 10 feet or even less. But be careful, it can sometimes take around 20 minutes to download the data almanac from the EGNOS satellites.

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