Why can I not use the aircrafts GPS Rx position when testing the ADS-B?

Why can’t I use the aircraft’s GPS or my phone for position testing?

You cannot use the aircraft's GPS receiver as a source for the test location because this would create a circular reference. If the GPS receiver being tested is faulty, it would still provide a location, making it impossible to detect issues and ensuring the system never fails improperly. Using a cell phone for location input is also not acceptable because its accuracy is not sufficient for a valid test. While some devices may have higher accuracy, the regulations specifically require the use of a surveyed or known location to ensure compliance and reliability during testing.