Doppler Plot

During Mars orbit insertion, or "MOI," this data display showed changes in for changes in Odyssey's signal that indicate a change in speed, including a change 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 keep tabs on Odyssey's speed and location. One of the key methods for determining that the spacecraft is where it's supposed to be is by watching for changes in the radio signal frequency.

Odyssey's ground controllers know the frequency of the signal that is transmitted from the spacecraft. But because the spacecraft is always moving away from or toward us, the transmitted signal is "Doppler-shifted" to a different frequency in the same way the 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 actual change in velocity.

Mission engineers call this a "Doppler plot." It shows Odyssey's signal, measured in Hertz (Hz), and any motion-related shifts 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 eyes were fixed on it during Odyssey's Mars orbit insertion actitivites. The data display was the only way flight controllers could see the spacecraft's behavior during the critical engine firing that successfully delivered Odyssey into Mars orbit on October 24, 2001. When the engine fired to slow the spacecraft's speed relative to Mars, the readout on the plot dipped indicating the sudden change. Outages in the signal seen in the display were anticipated The Doppler plot readout halted when the spacecraft passed behind Mars, and resumed when Odyssey reconnected with ground controllers after the spacecraft reemerged from behind the planet.

Doppler Plot Images