Galileo Ganymede 1 Doppler Plot
This plot shows the velocity change that the Ganymede gravity assist imparted to the spacecraft along the Earthline direction (see below). This is shown as a doppler shift. The doppler shift of a signal sent by Galileo is proportional to the line-of-sight (direction from the earth to the spacecraft) or earthline velocity of the spacecraft. The gravity of Ganymede changes the velocity of the spacecraft, and so this change in velocity shows up as a doppler shift. The magnitude of the doppler shift indicates the magnitude of the velocity change that the gravity assist provides. Note that this measurement is not the total spacecraft velocity change, only the earthline component.
The plot shows that as the spacecraft approaches Ganymede it is accelerated along the earthline direction, and the velocity increases. It reaches its peak (earthline) velocity change of approximately 539 m/s about four minutes after closest approach. Then as the Galileo continues past Ganymede, its gravity accelerates the spacecraft in the anti-earthline direction, and the velocity decreases. The net (earthline) velocity change of 338 m/s is reached several hours after the encounter. The total velocity change that Ganymede imparts to Galileo is 728 m/s. The doppler shift reflects the earthline component or about 46% of the total change.
It should be pointed out that wherever gaps are evident in the data, this indicates where the DSN had difficulty maintaining "lock" with the signal coming from the spacecraft. Since the signal strength is very low, and the dynamics of the encounter are quite large, it is not unusual for the signal to be dropped from time to time. The gaps do not indicate any problem with the spacecraft.
Doppler plot is courtesy of the Galileo Navigation Team
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