During Mars orbit insertion, or "MOI," this data display showed changes in forchanges in Odyssey's signal that indicate a change in speed, including achange caused by the spacecraft's engine firing to enter Mars' orbit.
Odyssey's communications with controllers on the ground are via radio. The team uses various radio communications techniques to keeptabs on Odyssey's speed and location. One of the key methods fordetermining that the spacecraft is where it's supposed to be is bywatching for changes in the radio signal frequency.
Odyssey's ground controllers know the frequency of the signal that istransmitted from the spacecraft. But because the spacecraft is alwaysmoving away from or toward us, the transmitted signal is"Doppler-shifted" to a different frequency in the same waythe pitch of a car horn changes as it moves closer or farther away. Those changes in signal frequency are used to compute the spacecraft's actualchange in velocity.
Mission engineers call this a "Doppler plot." It shows Odyssey's signal, measured in Hertz (Hz), and any motion-relatedshifts that occur in its frequency.
› The horizontal axis (left to right) of the plot shows the time the signal is received from the spacecraft.
› The vertical axis (up and down direction) shows the Doppler shift in the "residual" frequency.
To give a clear picture of the spacecraft's velocity alone, other motions that can shift the frequency of the received radio signal are subtracted from the data displayed in the Doppler plot. These other motions include the rotation of Earth, which causes the Deep Space Network antennas to move as they receive the spacecraft's signals, and Earth's own motion around the Sun and the spacecraft's planned interplanetary trajectory. The information left, called "residual data" or simply "residuals," shows only the changes in the spacecraft's speed caused by the engine firing and Mars gravity, and displays the information as though Odyssey were traveling in a straight line from Earth.
The Doppler plot is watched throughout the entire mission, but all eyeswere fixed on it during Odyssey's Mars orbit insertion actitivites. The datadisplay was the only way flight controllers could see the spacecraft's behaviorduring the critical engine firing that successfully delivered Odyssey into Marsorbit on October 24, 2001. When the engine fired to slow the spacecraft'sspeed relative to Mars, the readout on the plot dipped indicating the suddenchange. Outages in the signal seen in the display were anticipated TheDoppler plot readout halted when the spacecraft passed behind Mars, andresumed when Odyssey reconnected with ground controllers after thespacecraft reemerged from behind the planet.