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.
Interplanetary communications problem
