On the weekend I had some time to go hunting for WAAS / EGNOS satellites. Equipped with a GPS receiver (the Holux GPSlim240) I went to the highest (well – in Berlin it won’t get any better than the Teufelsberg with 115 meters) mountain around and looked if I could receive any of the EGNOS satellites. Unfortunately none of them (not AOR-E, nor IOR-W) showed up despite the good position (all visible obstables had a ground elevation below 10 degrees).

(above you see the position of the EGNOS satellites I should have seen in Berlin at that time)

Well, this makes EGNOS unusable – at least for city streets – where you have a shadowing effect through surrounding property that normally does not give you the nice conditions I had on the Teufelsberg. Seems I have to live with an accuracy of 15 meters for now – if there are enough satellites around to determine a position at all!

Let’s take a closer look at the shadowing effect. After getting home I have taken the Trimble Planning Software and prepared 2 examples (all screen shots are generated with this tool):

In the first example I have chosen a 30 degree shadowing all around like that:

How about the number of GPS satellites you can receive with this shadowing during a whole day?

The visibility does not drop below 3 satellites needed for a 2D-fix, but for a 3D-position fix you will have problems around 3 and 4 p.m. and 2 and 3 p.m. (as a side note: conventional consumer GPS receivers have a suboptimal accuracy when it comes to 3D-fixes – jumping up and down tenth of meters is sometimes only a matter of seconds).

So much for the visibility. How about the accuracy of the signal? Here the “Dilution of Precision” or DoP comes into play, which gives you a “soft” scale on which to rate the current accuracy. The basis for this position quality indicator is each satellite’s location relative to the other satellites in the current constellation and their position in relation to the GPS receiver.

Here are the DoP values with their rating and description:

DOP Value Rating Description
1 Ideal This is the highest possible confidence level to be used for applications demanding the highest possible precision at all times
2-3 Excellent At this confidence level, positional measurements are considered accurate enough to meet all but the most sensitive applications
4-6 Good Represents a level that marks the minimum appropriate for making business decisions. Positional measurements could be used to make reliable in-route navigation suggestions to the user
7-8 Moderate Positional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended
9-20 Fair Represents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location
21-50 Poor At this level, measurements are inaccurate by as much as half a football field and should be discarde

(taken from Wikipedia)

Below you have the DoP precision graph for Berlin for that day (don’t ask me which was the exact position – it must have been something around N52.5 and E13.3):

The moderate precision level in average does did not provide much confidence to me.

On to the second example: I have chosen a valley shadowing of about 60 degrees which is still (thanx to a colleague for the hint) very optimistic when roaming the streets of Berlin:

The effect was foreseeable: The satellites vanish…

…which has a negative (or positive – depending on how you like to see it) impact on the DoP:

But there is still hope! The local mass transit carrier (BVG) and the shipping carriers also had that problem – and they solved it using MW-DGPS, SAPOS and RDS to access more accurate DGPS data, right? So how about John Doe? Can we get into the exclusive DGPS circle without spending large amounts of money on industrial strength equipment?

More on this in the next installment on GPS accuracy.
Have a nice weekend – and don’t get lost 😉

Links:

  1. Holux Technology Inc., “GPSlim 240”, August 19, 2007, http://www.holux.com/JCore/en/products/products_content.jsp?pno=253
  2. Luisenstädtischer Bildungsverein e.V., “Berge und Erhebungen”, August 19. 2007, http://www.luise-berlin.de/Novitaeten/landschaften/TEXTE/BERGE.HTM
  3. Dr. Anja Köhne und Dr. Michael Wößner, “GPS-Infos”, August 19. 2007, http://www.kowoma.de/gps/waas_egnos.htm
  4. Dr. Anja Köhne und Dr. Michael Wößner, “Sources of Errors in GPS”, August 19. 2007, http://www.kowoma.de/en/gps/errors.htm
  5. Trimble Navigation Limited, “Trimble Planning Software”, August 19. 2007, http://www.trimble.com/planningsoftware_ts.asp?Nav=Collection-8425
  6. Wikipedia, “Dilution of precision (GPS)”, August 19. 2007, http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)
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