Interplanetary communications problem


A Problem

Facing the international community involved in the study of other planets is the fundamental problem of bandwidth. At present, only a few percent of data capable of collection at Mars can be transmitted back to Earth because the data rates supported by the available communications links are much too low. Missions currently planned will further exceed communications limitations, such that within five years only 0.3% of available data can be sent home. The 'cost per bit' of data recovered from Mars is very high, so increasing downlink rates is critical to financially justify continued international investment in exploration.

With an array of separate exploration packages at Mars, an efficient communications network topology requires establishment of a high speed link between Mars and Earth, capable of carrying all the transmitted data. The local data collectors use smaller, shorter range and lower rate transmitters optimised for their specific application, to send their data to the Martian communication centre.

As part of the 'smaller, faster, cheaper' initiative, NASA's approach is to use a large number of smaller, lighter probes - both orbiting and on the surface - that can use communications relay satellites in Martian orbit to transmit their data to Earth, instead of requiring deep space communications systems of their own. Microwave data link systems on existing spacecraft consume in the order of 25 kg of payload - a significant fraction of the total mass of the new generation of small exploration probes, and quite untenable for small rovers.

The Mars Global Surveyor, orbiting Mars since 1997, is operating a 25-watt microwave transmitter with a high-gain antenna to provide a 128 kb/s downlink to Earth, of data collected by future lander packages and communicated to the MGS via UHF channels near 400 MHz. Mars Odyssey, orbiting Mars since 2001, carries a similar data relay capability, as do the currently planned orbiters.

The philosophy is already in place, but current technologies fall several orders of magnitude short of requirements.

The problem is of such magnitude that NASA's Deep Space Network is seriously considering a microwave link requiring deployment of 3,600 radio dishes, each of 12 metre aperture, to receive data from communications satellites in Martian orbit.

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Originally created by Andrew McGrath on July 10, 2002