GNSS Satellite (GIOVE-A)

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Monday, 11 May 2009

Problem with new GPS Satellite SVN-49/PRN-01

It looks as if the newest GPS satellite has a significant problem. This is the GPS satellite which also broadcast signals on the new L5 frequency to ensure that frequencies for the GPS system. The L5 signals were planned to be available from the new GPS Block II-F satellites but due to delays in building these satellites it became necessary as kind of an "emergency rescue" of the L5 frequency band to launch launch some other satellite to use the frequency. The "perfect" solution was found to add the L5 capability to one of the Block II-R satellites. So this was done for the SVN-49/PRN-01 satellite that was launched on March 29, 2009.

However, it seems that something has gone wrong with the signals of this satellite. In my work where we try to get the highest precission out of the GPS system, meaning orbits at the ~20mm level and receiver positions at the ~1mm level, we can see a clear pattern in the pseudo range residuals of this satellite. Now since we typically use the carrier phase observations and not the code this is not really a problem for our type of work. However, the few meter (!!!!) pseudo range residuals are disturbing and they show a clear dependency on elevation.

However, what is more disturbing is that on the GPS system side they seem to be getting even larger effects then the few meters we see in our work. So far this is something I have not yet been able to understand. It seems the GPS system operators see problems with this satellite at the 100 to 200 meter level!!! A true catastrophy! As work around for this problem they GPS system is applying a huge antenna offset when broadcasting the ephemerides and clock corrections of this satellite. These problems also explain why this satellite is still not declared healthy. In fact there is some speculation that because of the observed problems it may never be set healthy.

So in summary the facts are:
  1. There is a significant problem difference the code and the phase observations
  2. The phase observations seem to be fine as we can do integer ambiguity resolution without too much problems (but of course one can not use the code observations for aiding the ambiguity resolution)
  3. The observed differences point in the direction of a significant "group delay". This is a delay which effects the code observations but not the phase observations
  4. The GPS system is observing even larger problems (>100m)!? We do not see those problems however!?
In any case it may be that this GPS satellite will never be set healthy making the L5 experiment a rather costly one...

As soon as I find out more details I will let you know!


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