A diagram of the many antennas (high-gain phased-array antennas, medium-gain fanbeam antennas, and low-gain antennas) that the MESSENGER spacecraft uses to communicate with Earth. Additional forward and aft low-gain antennas not shown
MESSENGER’s X-band coherent communications system includes two high-gain, electronically steered, phased-array antennas – the first ever used on a deep-space mission; two medium-gain fanbeam antennas; and four low-gain antennas. The circularly polarized phased arrays – developed by APL and located with the fanbeam antennas on the front and back of the spacecraft – are the main link for sending science data to Earth. For better reliability the high-temperature environment antennas are fixed; they “point” electronically across a 45° field without moving parts, and during normal operations at least one of the two antennas points at Earth.
High-gain antennas send radio signals through a narrower, more concentrated beam than low-gain antennas and are used primarily to send larger amounts of data over the same distance as a low-gain antenna. The fanbeam and low-gain antennas, also located on MESSENGER’s front and back sides, are used for lower-rate transmissions such as operating commands, status data, or emergency communications. MESSENGER’s downlink rate ranges from 9.9 bits per second to 104 kilobits per second; operators can send commands at 7.8 to 500 bits per second. Transmission rates vary according to spacecraft distance and ground-station antenna size.