wiki/ news/ 2005-03-14 - GPS, ATV & 2m oh my: Tim and Andrew freeze their asses off testing 2 GPS boards

Who: Tim and Andrew

Why: We love being outside on really, really cold late Winter nights. Oh, and to figure out how different systems affect the GPS receivers.

We tried two GPS receivers, the spare Jupiter GPS board and a SigNav MG5001. The MG5001 has a SAW filter (in the IF) so we hypothesized it might to better against the jamming interference of the 2m and ATV systems.

What we did

First, we used the Jupiter GPS receiver on a development board. We used Connexant's DOS-based "LABMON" software to look at it's output.

  1. the 2m system jams the Jupiter board completely.
  2. the ATV system with the exciter and power amp on jams the Jupiter board completely.
  3. the ATV system with power amp but no exciter, the Jupiter board works.
  4. The CPA works almost as well as the external 2 in2 patch antenna. "CN0" numbers were close, although the small path antenna's numbers we better.

Next, we used the SigNav MG5001 on its development board with Tera Term at 9600 bps 8N1:

Conclusions:

  1. Turning on the FC did seem to degrade the satellite signals slightly; this may be worse with the internal GPS.
  2. The wifi bits seem to have no affect on the GPS bits.
  3. The ATV exciter + power amp always jams any GPS receiver with any antenna.
  4. the 2m jams the Jupiter GPS (via the CAN bus wires!) but not the MG5001.

Next steps:

  1. Tim kills the 2m spurs using Paul's spectrum analyzer
  2. We try injecting a ~1.253 GHz pure tone into the power amp and see if that still jams the GPS units.
  3. If it does jam them, we're screwed.
  4. If it doesn't jam them, then we can filter the exciter's output to keep it in band.
  5. We don't think a filter on the input of the GPS preamp is going to help since the Jupiter board already has pre-filtering.
  6. We do this all again.