Reach height value seems incorrect

I just got my reach modules up and running and have both modules out on the patio about 5 feet apart. The rover is in float mode and bounces back to fixed mode several times a minute. The latitude and longitude numbers look stable but the height has gone from 270 to 325 over the last few hours. Occasionally I will see it jump 50 points in one scan. What is going on with the height? neither module is moving.

Thanks,
Dave

It might be catching reflections from the ground. Have you installed the antennas on 100mm groundplanes with good sky view?

I did make a larger ground plane and things seem to be much better. I have a 17x10 cm ground plane but still get an occasional 3-4m change in height. It is only momentary and then returns to the correct value.