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2 July, 2014 9:32

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The Technical Stuff!

This page explains some of the more technical aspects of the Global Positioning System

Differential GPS (DGPS)

DGPS uses both the normal satellite signals as well as ground based radio signals to achieve a much greater degree of accuracy. Only commercial GPS devices such as used by surveyers have the capability of receiving DGPS signals. The popular handheld GPS devices geocachers use as made by Garmin and Magellan cannot directly pickup the DGPS signal but you can get an adaptor that plugs in to the GPS to give you DGPS. The sort of accuracy you can expect is down to about 1cm or so. A normal GPS receiver without DGPS would have an accuracy of anywhere between 2 and 5 meters (6 to 15 feet).

Do you need DGPS? Not really because the person who placed a cache and recorded the coordinates would have used a normal GPS device so the accuracy they get won't be that good so having your GPS at 1cm accuracy would just be a waste of time.


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.

Satellite Name Location Bearing Garmin #
Indian Ocean Region West IOR-W - Inmarsat East Africa 25.0°E 39
Artemis - European Space Agency Central Africa 21.3°E 37
Atlantic Ocean Region East - IOR-E - Inmarsat Eastern Atlantic Ocean 15.5°W 33

 

 

 


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 understand DoP - Dilution of Position! Your GPS picks up signals from several satellites as they orbit the earth. The signal takes a certain amount of time to get to your GPS. The satellites orbit 12,000 miles high so if it is directly above your GPS the signal takes about 6 miliseconds to get to you. If the satellite is further round on it's orbit it will be further away and the signal will take longer to get to you. The GPS uses this difference in the signal time to work out exactly where it is in relation to the satellites and thus where it is on the surface of the earth. If the GPS is picking up several satellites which are all very close together the time difference is not that much but if the satellites are more 'spread out' the signal differences are much greater. The greater the difference the greater the accuracy the GPS will have. This is one factor that determines the DoP. The lower the DoP the greater the position accuracy, the higher the DoP the weaker the position accuracy.



If you are in a valley with steep mountains either side or an 'urban canyon' with tall buildings around you the GPS is only going to be able to see satellites directly overhead which means the DoP is higher and your accuracy figure is not so good. This is why on some days your GPS may appear to indicate a different accuracy on the display.

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 a bit less or a bit more.

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 WAAS/EGNOS satellites.

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