The pictures from Mars rovers Spirit and Opportunity have focused interest in the planet. You may be wondering what frequencies are being used to transmit telemetry and scientific data back to Earth and how they were coordinated. Based on information I found on the Web, the rovers are using frequencies around 430 MHz to communicate with the orbiting spacecraft at data rates up from 128 kbps to 2 Mbps. The link from Mars to Earth uses X-band frequencies in the 8.4 to 8.45 GHz band, while the return link from Earth to Mars uses frequencies around 7.2 GHz. See Frequencies for Mars Local High-Rate-Links for an excellent description of the design of future communications systems between Mars and the orbiter and the orbiter and Earth and a limited description of existing links. Also see Mars Relay Telecommunications Design Notes for has a detailed explanation of Mars-Orbiter and Mars-Earth communications system. Although the document was created in 1995, it appears many of the systems are still in use.
If you watched the landing of Opportunity on Mars last week you heard many references to "tones" that the lander used to send status information back to Earth. These tones are generated by a radio called the Small Deep Space Transponder using a special form of 256-tone MFSK modulation. Note that frequency shift keying rather than phase modulation is used to allow better frequency tracking during periods of high dynamics present during the entry, descent and landing (EDL) process. The radio operates at 8.4 GHz communicates with Earth using the back shell low-gain antenna, switching to the rover low-gain antenna when the lander separates from the back shell. Even with the 70 meter Deep Space Network (DSN) antenna, the signal to noise ratio can drop as low as 22 dB-Hz and the demodulator has to deal with a Doppler rate up to 1200 Hz/second.
As you can imagine, this required an extremely robust modulation system. I found an excellent paper that describes the design and performance of the system -- Direct-to-Earth Communications and Signal Processing for Mars Exploration Rover Entry, Descent, and Landing. The paper describes the operation of the system this way: "There will be 256 different signal frequencies, modulated one at a time onto a subcarrier, using the spacecraft capability to switch the subcarrier frequency. During hypersonic entry, the signal frequency can be switched every 10 s, resulting in the communication of 8 bits of information each 10 s. When the lander is suspended from the bridle, and the UHF link is prime, the duration of the modulation frequencies may be extended to 20 s to better facilitate detection during this period of highly varying SNR. This would result in fewer messages of higher reliability than would the use of the 10-s duration." The paper includes a detailed mathematical analysis of the system. NASA's Spotlight series has a simpler explanation of the tones. See Spotlight: Tones Break Silence During Mars Exploration Rover Landings.
With all the spacecraft converging on Mars this year, you may be wondering how frequencies were coordinated to keep them from interfering with each other. That coordination is the responsibility of the Space Frequency Coordination Group. While some of the documents on the Web site require a password for access, Recommendation 22-1R1 - Frequency Assignment Guidelines for Communications in the Mars Region as well as the STCG Handbook 2002 and other Resolutions and Recommendations do not. Recommendation 22-1R1 includes a summary of the frequency bands available for communications on and with Mars and outlines how frequencies are to be selected and coordinated.
Communication with Mars poses unique challenges. For more information on how engineers are dealing with them, take time to browse through the links I've provided here.
