When the DME ground station is out of range it would be inefficient for your DME radio to transmit interrogation pulses. To do so would waste electric power. To solve this problem your DME will enter auto-standby mode until such time that it "hears" response pulses from the ground station. To ensure that there are always response pulses to be heard the ground station transmits "squitters" even if there are no airplanes in the area. This system is explained in the simulation below.