Last weekend I did some tests with a UBLOX Neo6 GPS running next to the V2 camera to see if it’s better than the V1 camera.
The GPS had a clear view of half of the sky with a building to one side so not necessarily ideal conditions. The GPS was powered from the Pi with the cable wrapped through a small ferrite bead.
Here is a picture of the setup:
Here is the graph showing the Satelite number, PDOP, Position accuracy and velocity accuracy reported by the GPS:
There is a clear jump when turning on the camera where PAcc goes from ~2 to ~3.2. With the V1 camera this jumped to 30-40.
The jamming indicator showed 15-20% regardless if the camera was off or on. With V1 the indicator jumped to 100% when turning the camera on.
The number of satellites is the same, as is the PDOP and VAcc.
The spectrum measured with the SDR was crystal clear even when the antenna was almost touching the camera ribbon cable – which btw is unshielded.
So my conclusion is that the V2 interferes with the GPS way less than the V1 and with a bit of (optional) ribbon cable shielding it should be ok for a multirotor.
I took the quad for a spin yesterday and figured out 4 things:
- It flew, quite good. Very stable, video quality was good and the signal was very strong even 4-500 meters away
- I crashed a few times. See video here: https://www.youtube.com/watch?v=AjwiUQltHaE&feature=youtu.be
- Flight time was around 20-25 min with a 3000mAh Multistar LiHV battery. Average amps for hover was around 6-7 which is great. AUW around 630g
- Eventually it flew away in a near-by forest and I couldn’t find it. The reason was a sudden loss of signal which triggered the fail safe routine. The quad started climbing to 50m, went home and then dropped to 5m. Problem was that ‘home’ was a few hundred meters into the near-by forest due GPS interference from the camera….
So it’s gone, have to build another one.
The lesson here is that I have some software issues to iron out and that I really need another signal path for the control. I plan to use a rfm22b for this.
The material loss is not that huge – a raspberry pi, 4 rctimer 1806 motors, a brand new battery and a Navio board. A total of 250 euros plus about 2 weeks of work to print all the pieces, etc.
So the next one will need to fix these issues:
- Rock solid RC link for control
- Solid software without glitches. My GS crashed once in mid-flight (see video) due to the h264 decoder
- More portable GS. A laptop with 2 wifi cards, a ps4 controller and many wires is not a fun way to go to the field and fly
- The raspicam interferes with the GPSsomething fierce. Even after copper shielding the H Acc went from 2m to 50m after starting the camera…
A few days ago I got a RTL-SDR dongle to play with and I thought to give it a shot and measure the interference levels coming form the raspberry pi camera.
This is what I got around the GPS L1 frequency.
Notice the 2 peaks at 1,574.996 Mhz of -68db and 1,576.468 Mhz, -70db. They are very close to the GPS L1 frequency of 1575.42 Mhz and I’m sure the are the cause of the interference.
The big issue is that they radiate a long distance around the quad, dropping by about 10db 0.5m away from the quad. The pattern is not uniform, there are gaps where the interference disappears and I assume it’s due to reflections.
Note that changing the resolution and bitrate of the camera doesn’t change anything…
Next step – since I can now measure it – is to see how I can eliminate it.
A few options come to mind:
- tin foil (although last time I tried this there was no change)
- replace the flex cable with a shielded hdmi cable
- replace the camera with another one? Not sure if this will change anything