sorry for the late response, I didn’t see your post just until now. The main requirement for an instrument to be accepted are:
1. It must be able to produce a signed (with G record) IGC file
2. It must use a standard plug for track download, like Mini-USB or something like that
3. The device should support the USB filesystem schema, so that the files can be downloaded directly, without having to install any drivers on the score-keeper’s computer.
4. It would be nice if the device allowed getting the device’s serial number the same way. A read-only file somewhere in the file system would be great.
5. The manufacturer supplies the software to validate the G-Record.
6. The device must store GPS altitude values in the IGC file. If possible, it should also store barometric altitude data.
I think that’s pretty much it