This display shows the radio signal as received from 2001 Mars Odyssey during the spacecraft's critical engine firing just before entering Mars' orbit on October 24, 2001 (Universal Time). Due to the spacecraft's rapidly changing speed and the resulting shift in the frequency of its radio signal, drop-outs in the received signal occurred,as expected.
This plot shows by how much Mars Odyssey's velocity along the line of sight between us and the spacecraft has changed, compared with what the velocity would be if there was no Mars Orbit Insertion (MOI) burn. Since this shows the change in velocity relative to some baseline, this is called "residual" data, because it's the velocity that is left after the baseline is subtracted out--that is, the entire velocity change along the line of sight.
Most people are familiar with the phenomenon of a car horn or train whistle changing its frequency as it moves towards or away from them. Electromagnetic radiation (e.g. light waves or radio signals) also experience this effect. The size of the frequency shift, or "Doppler shift," depends on how fast the light source is moving relative to the observer. Astronomers often refer to the "redshift" and "blueshift" of visible light, where the light from an object coming towards us is shifted to the blue end of the spectrum (higher frequencies), and light from an object moving away is shifted towards the red (lower frequencies).
Mars Odyssey commmunicates with controllers on the ground by radio signal. Ground controllers know the frequency of the signal that is emitted from the spacecraft. However, since the spacecraft is moving away from (or towards) us, this frequency is being Doppler shifted to a different frequency. So, engineers (or, more accurately, computers) compare the received frequency with the emitted frequency to get the Doppler shift. It's then straightforward to find the velocity that would cause the resulting Doppler shift.
At the time of the Orbit Insertion Burn, the spacecraft velocity is changing radically, due to the pull of Mars' gravity, as well as the thrusting of the engines. As a result, the difference between the velocities for a flyby (no burn) and an Orbit Insertion burn does not change in a linear fashion (follow a straight line). Therefore, some variation will be seen in the residual (which, again, is the difference between the actual spacecraft velocity, and the velocity the spacecraft would have if no burn had occured).
The gaps shows when the Deep Space Network ground stations lost lock with the spacecraft. There were two events when this was expected to occur. The first was at the beginning of the burn. The ignition of the main engine caused the spacecraft to shake enough such that the signal became more chaotic, and harder to lock on to. The second event was when the spacecraft passed behind Mars (as viewed from Earth), and the planet blocked the signal from the spacecraft.