GNSS Satellite (GIOVE-A)

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Thursday, 14 May 2009

A detailed look at GPS Satellite SVN-49/PRN-01

As promised in my post "Problem with new GPS Satellite SVN-49/PRN-01"here some details regarding what we observe when we analyse the data of this satellite.

Below are two plots. The first one showing the pseudo range residuals of a "normal" GPS satellites. We randomly picked satellite SVN-38/PRN08 for this. The residuals are based on all the observations taken by the stations observing this satellite. The data used was from day 100 in 2009, or in normal date April 10, 2009. The residuals are plotted as a function of the elevation of the satellite above the local horizon of the observing station. That means that at 90 degrees elevation the satellite stands right above the station whereas at 10 degrees elevation the satellite is very low on the horizon. In this figure one can clearly see the increase of the noise of the observations at low elevations which is a well known phenomenon mainly caused by so called "multipath" effects.

The second figure shows the same picture but for our the new GPS satellite SVN-49/PRN01. The behavior of the residuals shows a clear signature which is obviously elevation dependent. Interestingly enought the carrier phase observation residuals do not show such a signature. This indicates that the problem is in the pseudo range observations, e.g., in the so called group delay.

However, the residuals show that the problem is only at the few meter level. But somehow the GPS operators see much larger problems. How and where the see those is still unclear to me. However, to my understanding it is possible to measure the differences between the observations on board of the satellite. Possilbe the >100 meter effects are observed in such measurments. However, for normal users as us there is only a few meter problem which we could live with, more or less. The really big problem comes from the fact that the GPS system operators are trying to solve this problem by changing the broadcast ephemerides. This becomes clear if we compare an orbit estimated by ourselves with the orbit information broadcasted by the GPS satellites. Note that our estimated orbits have a precission of a few centimeters whereas the broadcast ephemeris is normally at the few meter level. The figure below shows such a orbit comparison of the GPS satellites with PRN 1, 2, 3, 4, and 5. The differences for the "normal" GPS satellites is at the few meter level. For PRN 1 the differences amount up to 60 kilometers!!! This of course makes the satellite completely useless for any processing.

All in all the problems with this satellite do not seem to be very severe except for the broadcast ephemerides which are truely horrid!

So whatever you do with GPS do not use the data of the new SVN-49/PRN01 unless you know what you are doing!

(ps. I appologize for the poor quality of the plots. Have to find a better tool to do these plots...)

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Blogger -Ted said...

Nice analysis, I should point out that SVN 49 is PRN 1, not PRN 20 however

15 May 2009 22:12

Blogger Springinhetveld said...

Hi Ted,
Thanks for the note. I have corrected PRN20 to PRN01. The designation IIR-20, which is also used to this satellite, caused me to put down the wrong PRN. I corrected it now.

17 May 2009 11:27


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