Mixed Signals
Any sufficiently advanced information is indistinguishable from noise
Friday 19 June 2009, 12:29 PM
Reports of GPS' death are greatly exaggerated
The latest GPS satellite is in trouble, which has triggered another round of the "Satnav is going to die!" rumours we had a couple of weeks ago. But it turns out that the satellite - GPS IIR-20(M) - isn't in very much trouble at all, and neither is the whole system.
GPS IIR-20(M) was launched in March. It's a fairly standard GPS satellite with one significant addition, the L5 signal. This is a so-called 'Safety of Life' signal, which adds higher power, better performance and more resilience to GPS - making the quality of the service sufficient to act as the primary guidance system for aeronautical and other areas where human life is at stake. The new satellite is supposed to demonstrate this, and pave the way for L5 to finish testing by October this year.
After launch, everything seemed fine. The L5 transmitter was switched on, the signal observed and tested, and all was well. However, subsequent testing showed that the other GPS functions of the satellite had been degraded somewhat with what Lockheed -- the satellite suppliers -- are calling "an elevation dependent bias in ranging measurements". In other words, how far away the satellite appears to be depends on how high you are when you look - something not entirely ideal in navigation.
It turns out that this is "related to the interface for the L5 demonstration". The L5 signal continues to be fine, and by changing certain aspects of the satellite's configuration the vanilla GPS signals can be brought into compliance. There will be lots more testing - until October, in fact - to make sure that these changes don't cause any further problems, but if that's all OK not only will the satellite be able to join the GPS constellation but the rest of the new, L5 equipped birds can go up as planned.
(Well, if a small problem with the booster rockets is fixed. But that's space for you).
The Air Force, which runs GPS, says that there are thirty operation satellites up there already and everything is tickety-boo. I'm inclined to believe them.
GPS IIR-20(M) was launched in March. It's a fairly standard GPS satellite with one significant addition, the L5 signal. This is a so-called 'Safety of Life' signal, which adds higher power, better performance and more resilience to GPS - making the quality of the service sufficient to act as the primary guidance system for aeronautical and other areas where human life is at stake. The new satellite is supposed to demonstrate this, and pave the way for L5 to finish testing by October this year.
After launch, everything seemed fine. The L5 transmitter was switched on, the signal observed and tested, and all was well. However, subsequent testing showed that the other GPS functions of the satellite had been degraded somewhat with what Lockheed -- the satellite suppliers -- are calling "an elevation dependent bias in ranging measurements". In other words, how far away the satellite appears to be depends on how high you are when you look - something not entirely ideal in navigation.
It turns out that this is "related to the interface for the L5 demonstration". The L5 signal continues to be fine, and by changing certain aspects of the satellite's configuration the vanilla GPS signals can be brought into compliance. There will be lots more testing - until October, in fact - to make sure that these changes don't cause any further problems, but if that's all OK not only will the satellite be able to join the GPS constellation but the rest of the new, L5 equipped birds can go up as planned.
(Well, if a small problem with the booster rockets is fixed. But that's space for you).
The Air Force, which runs GPS, says that there are thirty operation satellites up there already and everything is tickety-boo. I'm inclined to believe them.
PreviousComments on this post
GPS is one of those services that everybody is "listening to". The slightest hiccup and the Tweets start flying between the twits. A lot of Senators and Congressmen have boats, so yes the GPS system is going to continue to work.
