A UAS may implement multiple radios for control, video, telemetry, etc. Range testing can be done on the ground but outdoor flight testing is necessary to establish how the various radio links will behave in flight. The motors and electronic speed controls on a UAS may generate electronic noise that can interfere with its ability to receive a control signal.
The largest multicopter vendors including DJI, Parrot, and 3DR all use the crowded 2.4 Ghz spectrum for multicopter control. In some cases they are using Wi-Fi and in others it is one of the RC protocols. Some systems use the 5.8 Ghz spectrum for either dual-band Wi-Fi or analog/digital video transmission. In any case it will be important to perform flight testing to see how interfering devices such as Wi-Fi access points, phones, etc. will effect range and bandwidth. Systems that use the 5.8 Ghz band are likely to be more problematic when traversing behind a structure in a first person view (FPV) scenario. This is because higher frequencies generally have poor penetration and reflection characteristics. They therefore suffer greater attenuation in situations where there is no direct line of sight.
Performing in-flight range testing inside a net is essential to ensure the UAS will not run away if/when one of the RF links fails. Repetitive testing can help establish maximum safe operation ranges with different platforms and radios.