sar104, msinger is correct, and his suggestion has solved this error in the past. Also, I'm sure you realize that there really isn't a working and reporting barometer system on board the Phantom. It uses a barometric sensor to gauge the take-off point, then uses the IMU to adjust the height (among other flight parameters) indicated in the DJI GO app (or other app being used). The calculated IMU data is probably compared to the pressure sensor data at some point, but the pressure data isn't very accurate and isn't used to limit the altitude. There really isn't a constant altitude being calculated and displayed by the barometric sensor. I do not know for a fact that the GPS also contributes, but it is often used when installed on commercial and military aircraft - including helicopters, as a comparison tool. That is used to verify the IMU data and the barometric system data comparison to see which system to use. Depending on the needs of the crew, other systems can be brought into the mix to fine tune the position in the sky and over which patch of ground. The Phantom is a great aircraft, but just isn't always as sophisticated as we might like to think it is.