Special signal tones the DSN received during entry, descent, and landing
During the entry, descent and landing phase of the Mars Exploration Rover mission, engineers listened anxiously for 128 distinct tones that indicated when steps in the process were activated; one sound indicated the parachute deployed, while another signaled that the airbags had inflated. These sounds were a series of basic, special individual radio tones.
The Mars Science Laboratory spacecraft transmitted in X-band during its entry, descent and landing process, which was the expected path for confirmation of the initial events in the process. Due to signal strength constraints, these transmissions were simple tones, comparable to semaphore codes, rather than full telemetry. The Deep Space Network listened for these direct-to-Earth transmissions. However, Earth went out of view of the spacecraft, “setting” below the Martian horizon, partway through the descent, so the X-band tones were not available for confirming the final steps in descent and landing. By then, the bent-pipe relay via Odyssey had begun.