Preventing "busy signals"

The Deep Space Network (DSN) communicates with nearly all spacecraft flying throughout our solar system. About 28 spacecraft are cruising in space, observing Jupiter, the sun, asteroids, and comets, and even more spacecraft are traveling to Saturn and the outer reaches of our solar system. The DSN antennas are extremely busy trying to track all of these space missions at once. The Mars Exploration Rover spacecraft must therefore share time on the DSN antennas. A sophisticated scheduling system with a team of hundreds of negotiators around the world ensures that each mission´s priorities are met.

During critical mission events, such as landing on Mars, multiple antennas on Earth and the MGS orbiter will track the signals from the spacecraft to minimize risk of loss of communication. During the landing operations phase on the Martian surface, the Mars Exploration Rovers are expecting to utilize the Multiple Spacecraft Per Aperture (MSPA) capability of the DSN, which allows a single DSN antenna to receive downlink from up to two spacecraft simultaneously.

The rovers´ downlink sessions (when the rovers send information back to Earth) will generally be limited to a couple of hours at a stretch, with perhaps two downlink sessions per Martian day (sol) per rover. MSPA allows only one spacecraft at a time to have the uplink, and it is expected that the rovers will command early in each sol (Martian day) for roughly an hour each to provide the instructions for that sol's activities.

In late 2003 and early 2004, the DSN resources will be extremely busy. The United States, Europe (Mars Express/Beagle 2 mission), and Japan (Nozomi mission) will each have missions arriving at Mars, two other spacecraft will be encountering comets (Deep Impact & Stardust), and a third comet mission (Messenger) will launch. Several other missions, including Cassini at Saturn, will have continuing telecom needs as well.