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Monday, May 10, 2004
Armadillo Aerospace News: Position hold, big jet vanes
We did several more flight tests to track down the GPS loss-of-lock issue. We replaced the internal 802.11b antenna, which was very close to the GPS antenna, with an external one several feet removed, but that had no impact. We actually moved the GPS antenna off the vehicle to a point ten feet up the tether to the lift, and it still lost lock almost immediately after liftoff, so we conclude it has nothing to do with the antenna. We then took the GPS circuit board off of its mounting standoffs and wrapped it in foam and loosely wire tied it down. We made two perfect flights in a row with this arrangement, so it seems conclusive that some kind of vibration is hurting the RF processing on the board itself.
We then tried an auto-hover with the GPS, and it worked fine. The vehicle was bouncing up and down a lot in place, because the control authority was a lot higher than the responsiveness of the 10 hz GPS update, but it worked properly.
I had another theory on the GPS problems that sounded plausible: while the entire electronics board is mounted on a foam isolation ring that dampens any vibrations from the frame, it is mounted right above the engine, and the base is directly exposed to the ground reflections from the engine. Acoustic vibrations might be getting transmitted directly to the board. We remounted the GPS on the standoffs and glued a foam pad on the bottom of the electronics honeycomb and did more tests. This seemed to be better, as I was able to get a full flight off, but it still had a cutoff once right as it was leaving the ground, so we went back to the loose mounting of the GPS. I am fairly appalled that this $8000 GPS system intended for solid fueled missile applications is this sensitive to vibrations. It sounds like a cracked trace on the board, but I’m not sure if I want to send it back to Thales for another $500 service check that will take two weeks and probably result in them just flashing the bios again and saying it is fine. Read More
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