There are five broad classes of future Mars exploration systems which will require telecommunication relays similar to that carried by the Pathfinder lander for the Sojourner microrover:
These five types of Mars exploration systems in fact require two distinctly different relay paths. The nanoRovers will operate in the vicinity of their host lander, just as Sojourner does. These "lander-local" relay applications employ the infrastructure of their landers to accomplish the long-range transmission to and from Earth. Because nanoRovers are of a size such that an antenna at the 459.7 MHz frequency used by Sojourner would overwhelm the basic vehicle, a move to higher frequencies (shorter wavelengths) is necessary. The system baselined for the nanoRover operates at 1902 MHz. It takes advantage of technology and components developed for the Japanese Personal Handyphone System and the European Digital Cordless Telephone system, much in the same way that Sojourner utilized Motorola commercial hardware in volume production targeted for the terrestrial short-haul data transmission market.
Of the remaining types of Mars exploration systems, all will need "comm-to-orbit", that is, communication relay to and from Earth, not by way of a lander but by way of an overhead orbiting satellite or group of satellites. The majority will require a two-way communication link with Earth, to send data and receive commands. Additionally, most will require radiometric position determination, referenced to the instantaneous and known position of the orbiting relay platform to localize their surface position and thereby to give context to the data they are acquiring. The first comm-to-orbit system will be demonstrated by the Mars Microprobes, utilizing a French-designed Mars Balloon Relay System which is currently enroute to Mars aboard the Mars Global Surveyor spacecraft. This system is quite limited in its capabilities, for it does not support the forward command path or radiometric position determination, only data relay back to Earth. It operates at 401 MHz and 437 MHz, and it has set a de facto operating-frequency standard for all near and intermediate timeframe comm-to-orbit relay systems. Fortunately, the Martian exploration systems discussed so far that will need comm-to-orbit are all large enough to accommodate antennas for this frequency range.
One can appreciate upon reflection that the very nature of comm-to-orbit relay systems involves not only the surface exploration elements below but also an orbiter or orbiters overhead. There must be a synergy between the infrastructure capabilities of the orbiter (hardware and orbit as chosen) with the data volume, rates, and allowable latency established by the surface (or airborne) elements. Detailed studies are being conducted right now by the Telecommunications and Mission Operations Directorate at JPL in concert with their consulting engineering firm, Stanford Telecom, to assess these needs and to design an evolutionary system architecture that will grow to fulfill the needs of upcoming missions, both robotic and (eventually) manned. Along these lines, a recently discussed goal is to have a dedicated Mars Relay Satellite system in place to support the first manned mission to Mars which is capable of live High Definition Television transmissions of the surface operations: we at JPL are ready for the challenge.
Thank You and Farewell from the Mars Microrover Telecommunications Team