I received my 3DR radio kit last week and started to configure them for operation. My current configuration on both of them is:
Firmware SiK 1.6 on HM-TRP
Notice that S8-S11 should keep me in the legal range in Germany. I generally prefer to have the frequency hopping vs. single channel and reduced duty cycle since that way any small-banded RF problems will hopefully only be temporary. The only parameter I would consider playing with at this point is S10 (has to stay higher than 30, though).
For now I have turned off MAVLINK and OPPRESEND to have a better feeling of what is going on when looking at the serial output of the ground station. They will come back on before I go flying.
I did experience the same problems with sketchy links that have been mentioned here and noticed that sometimes I get a rock-solid link at 15 m across my house (plenty of steel in the walls) and sometimes it was just too lossy to have a mavlink connect with APM.
Upon closer inspection in that second state the green LEDs do something like 8 seconds on, 5/10s off. Looking at the serial output of the ground station, I can see the MAVLINK heartbeat come in at irregular intervals but most of the time not at all.
A third state I found is solid green LEDs but heartbeat coming in at 2-10s intervals or again not at all. When the heartbeat comes, it changes interval times in a seemingly sinusoidal manner.
In that scenario I use a serial terminal (screen under OSX, 3DR radio config under Win) to issue +++ and reset the radio via ATZ commands (you can also unplug/replug the radio in the FTDI cable), until I get a stable 1Hz heartbeat - once I have that, I can start APM and the connection seems to hold no matter what.
Another observation: When doing '+++', then just a single AT, then an ATZ, I occasionally get panics from the radio module.
To me the symptoms look like out of sync hopping patterns... According to the documentation the radios sync by hopping through the channels based on a pattern calculated from parameters S3 and S8-S10, but the unsynchronised hopping rate is different for the two radios. Here are a few questions:
- How do the identical radios determine, which one is supposed to hop fast and which one is supposed to hop slow?
- In the algorithm used to determine the hopping pattern, is there a possibility for ambiguous states that would seem synced but really is not?
- Can the radios even separate 50 narrow channels within the min and max frequency given above or will channels partially overlap leading to wrong synchronisation? Within the given legal boundaries is it maybe necessary to modify the parameters to get a more stable link?
Hope this helps to spot the problem.