As previously mentioned, we've experienced two GPS failures in the past few weeks. I've been trying to google the issue today, with little luck. There is a lot of chatter about different sources of GPS signal disturbances, and it seems to be picking up the pace, but there's nothing specific relating to our region. Here's a detailed description of the problem:
We have three GPS systems on board. They are all completely separate, from the power supply through to the antenna. The signal loss occurs suddenly and simultaneously on all three devices. The signal strenght drops to zero for all sattelites, and the units lose their fix. The first incident (off the Brittany coast) lasted for about ten minutes, the second (in the northern part of the bay of Biscaya) approximately twenty minutes. During the second incident, we were out of sight of land, and there were no other craft on the radar, nor were there any aircraft visible. When the signal comes back, it appears just as suddenly as it disappears.
Due to the separate nature of the affected units, I'm certain that the source of the problem is some sort of signal disruption. After the first incident we assumed that we were the victims of a jamming signal, but given our remote location during the second signal loss, that would have to be a long range signal indeed. I'm also reasonably certain that the disturbance doesn't originate from anywhere on board, as no equipment was switched on or off during or previous to the anomalies. The TV antenna amplifiers on board have all had their power supplies disconnected, as the system is decommissioned.
So far I'm drawing a total blank on this, and I'm getting a little worried. The Man expects a definitive answer to the problem. Any help or feedback wanted!
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Theory 1: Some anomaly in the power supply - like an improper voltage or an imperfect AC sine curve might be having an impact on a common component in the GPS receiver systems. Do they work on DC 110V or are they wired through one of the green converters? One way to test this theory is to have a battery operated backup device available. Another would be to use good measuring device, and REALLY scout out the reliability of the green converter(s).
ReplyDeleteTheory 2: There is some source of radio signal disturbance on board. The disturbance can be quite weak: The average GPS signal is about –160dBw (1 x 10–16 watts), which is roughly equivalent to viewing a 25-watt light bulb from a distance of 10,000 miles. However, since all three units conk out and come back in at the same time, it's hard to imagine the disturbance as something that's often (or persistently) there, and periodically gets just a LITTLE stronger. And since they conk out so infrequently, you're probably looking at a circuit that comes on automatically only at very long intervals. Or ... could it be that the GPS systems fail more regularly, and that you usually don't notice?
The most probable source of radio disturbance would be a radio transmitter. If the problem happens when you approach the shore, it could have something to do with mobile phones (or sat phones or radio equipment?) starting to engage automatically with the shore network. How close together are the GPS units? How well are their antenna systems shielded? Is it possible to move one system (or its antenna) temporarily to a different location?
ReplyDeletehttp://www.pprune.org/private-flying/290409-gps-loss-signal-resolved.html
Communication radios can "kill" GPS units also when they're not sending, because they contain internal oscillators. The phenomenon depends on what frequency the set is tuned to.
ReplyDeletehttp://www.scn.org/~bk269/gps.html
Thanks for the feedback, Dad. I'll look into the comms radio angle, though all the systems on board are certified / professional grade, so I have my doubts.
ReplyDeleteThe power supplies are entirely separate. One GPS is powered from the service batteries, one from the radio batteries and the last from a separate AC-DC trafo. The battery banks are fed by an SMPS. If it wasn't for that, an input voltage disturbance would be the obvious thing to look for.
That rules out power disturbance. I think we can also rule out a sudden loss of GPS transmissions from Space. If the signal is there, and the GPS units are working, then you're probably looking for something that disturbs or jams out the signal. Since it's happened twice, the source is probably somewhere on board. Remember Grandpa's story about his car's injection system that was sensitive enough that radio noise from poorly shielded traffic lights could be enough to kill the system? After he realized what it was, he installed 4 cut-off switches that enabled him to "kill" the injectors long enough that the cylinders dried out, and he could restart the car normally.
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