Hey Chris,
Here are some figures from the sonar.
Its working, but not as accurately as I hoped. The higher it gets the error in the reading grows by about the same percentage. The error may be due to the quad not being level during my tests. That would probably explain why the closer it gets to the ground the more accurate the reading becomes, maybe, read on.
I've hung it from my pergola and got the following measurements:
Measured height Sonar Height Difference Error %
180cm 228cm +48cm +26%
124cm 158cm +34cm +27%
75cm 96cm +21cm +28%
45cm 57cm +12cm +26% (not much to worry about at this height)
Importantly the readings seem to only move about by a centimetre or two during the test in the Mission Planner CLI. So they are pretty consistent and were probably due to the slight breeze outside moving the quad around which is hanging from a piece of string.
The sonar has a separate board with an Atmel chip on it that must be taking a reading from the HC-SR04 ultrasonic module and tweaking the reading into something the APM can read, so I'm thinking they have an error in the firmware on the sonar Atmel chip given that the error percentage seems reasonably consistent.
Or because the quad is not 100% level, the reading is taking a longer path from the trigger to the echo transducer on the ultrasonic module. I reckon there is a high probability that this is what the problem is.
I think as long as the APM software can keep up with the readings then it should be a good sonar for assisted landing. I think I read somewhere that the sonar reading is only used under 2 metres anyway, I'll check some of the forums and see if I can find where I read that.
I've gotta say, I'm very happy with the Reptile-Aphid frame. I've pulled it partially apart numerous times to put more hardware on it and as can be seen from the picture below, it has a purpose made fibre glass mini board for the sonar that is connected to the main board by standoffs. I'm going to install the Optical Flow sensor that arrived with the sonar now, then I'll give it a flight.
Cheers, Mark