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 landed operations phase on the martian surface, the Mars Science Laboratory utilizes the Multiple Spacecraft Per Aperture (MSPA) capability of the DSN, which allows a single DSN antenna to receive downlink from up to four spacecraft simultaneously, as well as using the relay capabilities of the Mars Odyssey (ODY) and Mars Reconnaissance Orbiter (MRO) spacecraft.
The rover's downlink sessions (when the rover sends information back to Earth) are generally roughly 15 minutes each, with usually two downlink sessions per relay orbiter (ODY, MRO) per martian day (sol), with two sessions overnight and two sessions in the late martian afternoon. MSPA allows only one spacecraft at a time to have the uplink, and Curiosity commands early in each sol (martian day) for roughly 30 minutes to provide the instructions for that sol's activities.