Preventing Busy Signals
The Deep Space Network (DSN) communicates with nearly all spacecraft flying throughout our solar system. Many spacecraft are cruising in space, observing Saturn, the sun, asteroids and comets. In addition, the Mars Exploration Rovers are still busy on the surface of Mars and NASA's Mars Reconnaissance Orbiter has joined the other martian orbiters. The DSN antennas are extremely busy trying to track all of these space missions at once. The Mars Science Laboratory 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 Mars Reconnaissance Orbiter track the signals from the spacecraft to minimize risk of loss of communication. During the landing operations phase on the martian surface, the Mars Science Laboratory is 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 rover's downlink sessions (when the rover sends 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). MSPA allows only one spacecraft at a time to have the uplink, and it is expected that the rover will command early in each sol (martian day) for roughly an hour to provide the instructions for that sol's activities.