The flight testing of communications systems is avionics flight testing in its most basic form, but it does illustrate many of the common factors seen in the more complex systems. The testing of communication systems occurs more often than one would expect. Radios are often upgraded, which requires a flight test program, but these upgrades may be many years apart. What occurs on a more frequent basis is the relocation of antennas or the installation of stores or other obstructions that may affect communications. Some flight testing may be required to ensure that no degradation of communications due to these modifications has occurred. Radios, because they transmit as well as receive radio frequency (RF) energy, are also key players in electromagnetic interference/electromagnetic compatibility (EMI/EMC) testing.