Much of the communication between Earth and the spacecraft during a flight was carried out using the S-band communication system, either through the highly directional high-gain antenna, soon to be discarded along with the service module, or through one of the four omnidirectional antennae placed around the command module's periphery. However, re-entry was a highly dynamic event, with the spacecraft travelling at high speed, relatively close to the ground and below the horizon of the major S-band stations. Also, as it entered, it would roll regularly from side to side to steer a course towards the recovery forces.
As the directional nature of S-band communications made it impractical for use during entry, the CM reverted to the shorter range VHF radio that had been used by earlier Earth-orbiting missions and was used for communication with the lunar module during operations around the Moon. This enabled them to talk to mission control via ARIA communications aircraft deployed on the ground track, and later directly to the recovery aircraft carrier and its associated helicopters. In preparation for this, the VHF communication system was powered up to enable it to be tested when the spacecraft came within range.
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