Intermittent communications

When Apollo was blazing its pioneer­ing trail to the Moon, the nascent space industry had yet to set up a comprehensive, worldwide communi­cations network using geostationary satellites and ground stations. It would take the efforts of another generation to arrange an infrastruc­ture that would allow crews to at least talk to mission control at any point in their orbit. Apollo crewmen could talk to mission control only for intervals of up to seven minutes at each ground station as they passed over a scattering of them along their orbital path. As with many aspects of Apollo, the exact configuration of these stations changed from mission to mission as operational experience was gained and priorities changed.

Early missions supplemented their coverage with extra ground sites. A scattering of specially equipped ships filled the gaps between the main sites.

Stations were sited on islands or on board ships strung across the Atlantic Ocean leading from Cape Canaveral to provide coverage for the ascent to orbit. A station on one of the Canary Islands off the coast of Africa permitted communications on the opposite side of the Atlantic, and another on Madagascar was used during the early missions for coverage heading out over the Indian Ocean. An outpost near Canberra in eastern Australia gave coverage on the opposite side of the world. An important station was set up on Hawaii, in the middle of the Pacific Ocean, which covered at least part of the spacecraft’s departure for the Moon. This was supplemented with ships and Apollo range instrumentation aircraft (ARIA) which filled in the gaps before a siring of stations across the continental United States gave constant coverage to the Atlantic. The ARIA were EC-135 jets – similar in structure to the Boeing 707 jetliner that were specifically equipped to support Apollo communications by relaying voice and recording telemetry.

During each short period of communication, data about the state of the crew and spacecraft were exchanged with updates from mission control. Another vital job for some of the ground stations at this time was to use large radar antennae to track the speed and position of the spacecraft as accurately as possible by reflection off its skin. This refined mission control’s knowledge of iis trajectory; information that was necessary to ensure an accurate burn towards the Moon. In particular, the station on the Canaries could provide an initial orbital determination and Carnarvon in Australia refined the determination antipodal to insertion.