PRN01 … not quite working as expected

PRN01 is one of the latest GPS satellites launched on 24 March 2009. Over the summer we have been investigating what the deal is with this satellite which is still not set to healthy.

When a GPS satellite is launched it goes through a period of a few weeks of commissioning. This period involves in-orbit checkout, etc, including the activation of the navigation payloads and others. As soon as the navigation payloads are activated many of the stations of the IGS network will start recording observations for the satellite, so in the IGS we will produce precise clocks and orbits for the satellite. A few days after that normally the satellite is declared healthy for all navigation users and the satellite is available.

For PRN01 the situation has been a bit peculiar as you can read here GPS SVN49 and L5 Signal. The satellite was essential for GPS to lock the L5 frequency for GPSIII use in the future. A reserved frequency to be used in space applications has to be used from space within a certain number of years or it is released by the ITU for anyone else that needs it. Therefore as SV-49 was going up anyway to completent the GPS constellation they decided to add an L5 demonstration payload to the satellite so that the L5 frequency filing would not be in jeopardy. But after months and months the satellite never became “healthy” so it is not available for general navigation use and it is a doutbful “spare” satellite in the GPS constellation.

My wonderful colleagues at ESOC Tim Springer and Florian Dilssner have now investigated the situation an published the results in InsideGNSS, one of the best industry publications. Read their investigations online: SVN49 and Other GPS Anomalies, Saving GPS SVN49, GPS Signal Anomalies. There are other further investigations out there of this issue and possible solutions, but these three from my colleagues at ESOC are the first three I saw published.

I will not try to summarize all the issues which are very well explained above, basically they connected the L5 demonstration payload in a spare port to feed the signal to the transmitting antenna. In doing so they have introduced significant elevation dependent errors mainly in the L1 signal. While some elevation dependent issues are sometimes observed with different GPS satellites, none have ever had it as bad as this one!

The issue continues to be under investigation and the satellite will remain unhealthy until the Air Force is satisfied the problem is completely understood and a workaround implemented.

Good positioning!!
Ignacio Romero (Nacho of the IGS!)

One Response to “PRN01 … not quite working as expected”

  1. Pedro Says:

    Hi Nacho,

    So at least 6 more months without PRN01… Lets hope they are able to find a workaround, and will manage to set the satellite usable after this period!!!

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