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Galileo Europa 18 Doppler Plot



The Doppler Plot above will be updated live every minute during the encounter day On November 22, 1998 from 11:00 UTC to 14:00 UTC.

This plot presents the velocity change that the Europa encounter gravity assist imparts to the Galileo spacecraft. This velocity change is measured as a "Doppler shift."

The Doppler shift of a radio 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 Doppler shift is a frequency shift measured in Hertz (Hz) and the relationship between Hertz and velocity change for the Galileo 2-way S-band radio signal is: 1 Hz = 0.065 m/sec (for additional information on Doppler data, see the question and response below).

The gravity of Europa changes the velocity (magnitude and direction) of the spacecraft, and this change in velocity shows up as a Doppler shift of the radio signal sent from the spacecraft. The magnitude of the Doppler shift indicates the magnitude of the velocity change that the gravity assist provides.

The plot shows how the gravity assist changes the velocity (both direction and magnitude) of the Galileo spacecraft. If the Doppler shift increases (goes up) this indicates that Galileo is being accelerated away from the Earth. If the Doppler shift decreases (goes down) then Galileo is being accelerated towards the Earth. Again, note that this measurement does not show the total spacecraft velocity change, only the Earthline component.

For this encounter, the total velocity (vector magnitude) change due to the gravity assist is 279 m/s.

Doppler plot is courtesy of the Galileo Navigation Team

Doppler Display Mini-FAQ

What is Doppler data, and what does it have to do with velocity?

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).

Galileo commmunicates with controllers on the ground by radio signal. Ground controllers know the frequency of the signal that is transmitted from the spacecraft. However, since the spacecraft is always moving away from or towards us, the transimitted signal is being Doppler shifted to a different frequency. Engineers then compute the Doppler shift by comparing the frequency received on the ground to the known transmitted frequency. It is then straightforward to find the velocity change that would cause the resulting Doppler shift. (Note that this gives us only the line-of-sight velocity.) Again, the frequency shift is measured in Hertz (Hz), and the conversion for Galileo (2-way at S-band) is: 1 Hz = 0.065 m/sec.

What does the blue line represent?

This line is the predicted behavior of the Doppler data. The actual data will follow (and be plotted over) this line in black.

What does the "Last Updated" represent?

This plot is being updated in "real time" as the events occur. This plot will update about once every minute during the encounter activities. This label shows you how "current" the plot is. Recall that it takes about 50 minutes for a radio signal from Galileo to reach the ground at this time, so the events you see here actually took place almost an hour earlier at the spacecraft.

Europa Quick-Look Orbit Facts


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