Monday, July 23, 2007

Its all about the systems...

Its all about the systems integration.
I'm trying to diagnose the Helicopter systems to work on flight software. I have a GPS on the roof of my house and one mounted on the back of the helicopter, when working on the system I unplug the one on the helicopter and plug in the one on the roof We get ZERO gps reception inside. The one on the roof works really well.

So I take the helicopter out to fly it and after waiting a long time get no GPS lock at all. So I bring it back to the house and replace the GPS receiver with another one. (assuming the old GPS receiver got broken in the last heli cras h.) ,butI'm getting smarter I don't drive all the way to the flying field to test the new receiver. I go out in the back yard still no GPS.

So now I take a 6 foot cable and the "broken" gps and plug it into the helicopter it works from 6 feet away! So I then set the broken GPS on top of the gps on the helicopter and it still works! Then I remove the 6 foot cable and it does not work.

So then I plug in the 6 foot cable between the electronics and the GPS currently mouted on the helicopter. It works.....

So I now have 2 GPS receivers that have decided that they need 6 feet of cable to operate. 1 foot is not enough.

First working hypothesis: the 900Mhz telemetry radio is putting EMI on the wiring. The long cable is enough to filter it out, the short cable is not.

Experimental test don't transmit until you get GPS lock.
Still no GPS, add 6foot cable to verify the don't transmit till you get lock code works, the code works.

Second working hypothsis:
Some other component of the system (likely the CPU) is putting enough noise on the wire to kill the GPS, Next step put noise filter on all the cables and try again.

Well let you know......


John Carmack said...

We have found that a GPS antenna that is very close to a PC104 module will not work, but a foot of distance between the antenna and the CPU clears it up. Even with our grounded and shielded electronics box, our GPS reception was a little spotty until we added the little "antenna masts" for the GPS that you see on our recent boxes.

We also found that the higher-end GPS antennas from the septentrio won't work with either another GPS unit operating nearby, or our wireless video cameras.

John Carmack

Anonymous said...

if its definitely cable length and not antenna position or something, then ferrite is your friend.

Something like this:

Digikey P/N 28A0392-0A0, only one of many possibilities.


John Kasunich