The SINCGARS series of radios were introduced in the 1980s as a solution to jamming of radio transmissions on the battlefield, as well as the risk of the enemy overhearing these messages. SINCGARS uses frequency hopping (rapidly changing frequencies according to a pre-arranged pattern) when sending messages. The three radios in the SINCGARS family have a range of from 8-35 kilometers. Unfortunately, these are FM (line of sight) radios that lose a lot of their range in hilly or urban terrain. Operators have also found that the range is halved when the frequency hopping is used. So when a user find the signal fading, they will switch to single frequency mode to keep the connection. This allows the enemy to jam the signal, and to listen in.