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Galileo Europa 6 Doppler Plot

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The Doppler Plot above is updated live every minute during the encounter day on February 20, 1997

This plot indicates the velocity change that the Europa gravity assist imparts to the Galileo spacecraft along the Earthline direction. 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. Note that this measurement is not the total spacecraft velocity change, only the Earthline component.

The plot shows that as the spacecraft approaches Europa it is accelerated along the Earthline direction (i.e., away from the Earth), and the Earthline velocity increases. It reaches its peak (Earthline) velocity change of approximately 91 m/s (1396 Hz) about six minutes before closest approach. Then as the Galileo continues past Europa, its gravity accelerates the spacecraft in the anti-Earthline direction, and the Earthline velocity decreases. The net (Earthline) velocity change of -341 m/s (-5221 Hz) is reached about three hours after the encounter.

Once the flyby is complete, Europa will have increased the speed of the spacecraft (with respect to Jupiter) by 218 m/sec. Remember that the gravity assist changes the spacecraft velocity magnitude and direction. The total velocity (vector magnitude) change that Europa imparts to Galileo is 519 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?

Just after the encounter, the spacecraft will pass behind Europa, as viewed from the Earth. This is called an "occultation" (the occultation entry and exit times are marked on the plot). For about 13 minutes, the spacecraft will not be able to communicate with the Earth, since Europa will blocking the radio signal. You will therefore notice a gap in the data during the occultation.

Why is there a gap in the data?

At the time of the encounter, the spacecraft will pass behind Europa, as viewed from the Earth. This is called an "occultation" (the occultation entry and exit times are marked on the plot). For about 12 minutes, the spacecraft will not be able to communicate with the Earth, since Europa will blocking the radio signal. You will therefore notice a gap in the data during the occultation.

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.

What is TWNC ON and TWNC OFF?

TWNC stands for "two-way non-coherent". This refers to the method that the spacecraft uses to generate the signal that is transmitted to earth. In "one- way" mode, the spacecraft generates its own radio frequency from the onboard oscillator and transmits this signal to the ground. In "two-way" mode, the spacecraft is receiving a signal sent from the ground, turning it around, and "coherently" transmitting a signal back to earth (with the TWNC OFF). When the spacecraft is in two-way mode and the TWNC is turned ON, this means that the spacecraft is no longer using the uplink to coherently generate the downlink signal frequency. Therefore (for doppler navigation) the signal is effectively 1-way. (Note that the TWNC state also has implications on signal strength and telemetry capability, but that is not discussed here.)

For this encounter, the spacecraft sequence will turn the TWNC ON when the spacecraft is in occultation so that when it comes out, the DSN will be able to acquire in one-way mode (which is easier than two-way acquisition). Once the signal is acquired, the TWNC will be turned back OFF and two-way doppler will resume.

Europa 6 Quick-Look Orbit Facts

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