aparently some radio frequencies can cause a problem with some GPS.
True...from AC20-138:
Evaluate the following VHF frequencies (25 kHz channels):
121.150 MHz 131.250 MHz
121.175 MHz 131.275 MHz
121.200 MHz 131.300 MHz
For VHF radios with 8.33 kHz channel spacing, evaluate the following VHF frequencies: 121.185 MHz 130.285 MHz
121.190 MHz 131.290 MHz
For installations on rotorcraft, ensure that the rotorblades do not interfere with the GPS/WAAS received signals. This problem has been experienced in some rotorcraft and varies with the rotation rate.