Step Eleven: Data transmission to the Mars Global Surveyor orbiter begins
The descent UHF (ultra-high frequency) antenna, mounted on the lander, begins transmitting once the lander descends from the backshell, which stays attached to the parachute. From the lander's position at the bottom of a tether (aka bridle) connected to the backshell, this antenna can transmit to the Mars Global Surveyor orbiter that will be passing overhead at this time. The link allows Spirit to supplement the tone-coded information with additional status reports that can be forwarded to Earth by Mars Global Surveyor almost immediately. The window for relaying information to Mars Global Surveyor closes by about 8:42 p.m., when the orbiter sets below the landing site's horizon, about seven minutes after Spirit hits the ground. Although this communication link has been extensively planned and simulated, there is no guarantee that Mars Global Surveyor will successfully relay information from Spirit. When the lander descends from the backshell, a low-gain X-band antenna mounted on the rover itself takes over from the backshell antenna the job of transmitting tones to Earth. It transmits tones until retro rockets fire, six seconds before impact. Then, during the spacecraft's impact, bouncing, and rolling, it transmits a carrier signal only.

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