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GPS Accuracy : Post-Processing and SBAS
Posté le 23 avril 2006 à 00:54:09 par gpspassion.

1. Introduction - GPS Accuracy and SBAS

Updated 20060705 - PRN126

This article started out as an attempt to come up with some real life data to answer the recurring questions about SBAS (Satellite Based Augmentation System - known in various parts of the world as WAAS, EGNOS, MSAS) with everyone being logically convinced they need it after reading the technical specs of GPS systems where SBAS accuracy is generally shown as being much better than non-SBAS accuracy, typically 10 meters down to 5 meters or even 2 meters...For some fair testing, I had to find a way to measure a reference point with the best possible accuracy and that's where the Delorme GPSPostPro suite comes in, so I added a page in this article describing how I used it.

Accuracy vs Integrity
Typically for a road navigation system, SBAS brings little to no advantage as vector maps don't have that type of accuracy and the position is "snapped to the road" for cosmetic purposes and also for the routing engine to be able to track the progress in real time.

It is also often overlooked that SBAS was originally designed to monitor the integrity of the satellite system in real time for critical use, such as flying planes or saling ships, the accuracy improvement is really a side effect, unless you go to LAAS (Local Area Augmentation System - details) for precision approach to replace ILS. Interestingly enough, what took away the usefulness of SBAS for consumer use is the removal of Selective Availibility (SA) in May 2000, as the residual GPS error of approximately 10 meters is mostly due to GPS signal deflection by the ionosphere that SBAS can only marginally correct. While I haven't been able to test it myself, I've heard from reliable sources that with SBAS, SA's accuracy of 100 meters could be improved down to...10 meters. We won't even get into the sources that degrade the accuracy of GPS, building, trees, etc...more about this here

Side by Side Testing
As usual, you can't beat side by side testing to try to make some sense of what's going on, so here's what I did :
  1. I Determined the exact position of a particular spot using Delorme's Delorme GPSPostPro setup with an accuracy of about 80 centimeters - tested on page 2

  2. Logged data simultaneously with a SBAS fix (being currently located in Europe that means EGNOS) and a non-SBAS fix with the same GPS Bluetooth GPS receiver - page 3 - page 4 - page 5

  3. Compared thre results using SA Watch, a wonderful GPS data plotter.

  4. SBAS tools : a fair bit of tweaking of the SBAS features can be done on SiRF based receivers both on PCs and PocketPCs (see this thread) but not on Nemerix chipsets as far as I'm aware, where SBAS is set at the factory
If you have questions or comments, you can use this thread of the forums

  2. Determining an Accurate Reference Point with Delorme PostPro and Garmin Accuracy Estimate >>

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