Radio Science

Radio Science
John Anderson, Celestial Mechanics Principal Investigator
Jet Propulsion Laboratory
H. Taylor Howard, Propogation Principal Investigator
Stanford University

MISSION OBJECTIVES

Determine masses and internal structures of bodies from spacecraft tracking.
Determine satellite radii and atmospheric structure from radio propagation.

SUMMARY

There are two scientific experiments that use Galileo's radio telecommunications system. "Radio science" has been used for several decades within the space science community to denote experiments conducted in this manner. The two categories of radio science that will be done at Jupiter are celestial mechanics and radio propagation.

Celestial Mechanics

The celestial mechanics experiments use the radio system to sense small changes in the trajectory of the spacecraft. The spacecraft's radio transmitter sends a signal at a well-known stable frequency. Any change in speed that the spacecraft experiences will cause the frequency of the radio signal received on Earth to change. The amount of change is dependent on the change in speed of the spacecraft, relative to Earth. When the spacecraft passes close to Jupiter or one of the Galilean satellites, that body pulls on the spacecraft, causing its speed to change. The amount of change in speed depends not only upon the mass of the body and the distance of the spacecraft from that body but also on how that mass is internally distributed. Thus, by measuring the change in frequency of the Earth-received radio signal, the mass and internal structure of Jupiter or one of the Galilean satellites can be estimated.

The results should allow us to make a better selection of models for the interior of the satellites. This is possible because Galileo will approach the satellites much closer than did any earlier spacecraft, so that gravitational effects will be stronger and easier to observe. Arrival Day data have already confirmed that Io has a giant iron core.

Radio Propagation

The spacecraft radio signal will be used to investigate Jupiter's neutral atmosphere and ionosphere, Io's ionosphere, and to search for ionospheres on the other Galilean satellites (Europa, Ganymede, and Callisto). This is done during radio occultation experiments, when the Galileo orbiter passes behind the planet or satellite as viewed from Earth.

The radio signal propagating from the spacecraft to Earth experiences both refraction and scattering in the atmosphere of the occulting body. (The atmosphere will bend and slow the radio signal by the process of refraction; additionally, the atmosphere will diffuse the electromagnetic waves of the signal by the process of scattering.) This causes changes in the frequency and amplitude of the signal received at a DSN tracking station on Earth. Analysis of these changes will yield information about the atmospheres and ionospheres of the Jovian system.

Anticipated results include profiles of electon number density versus radius in the ionosphere - and profiles of refractive index, pressure, and temperature versus radius in the neutral atmosphere. Of particular importance will be the multiplicity of measurements of Jupiter's ionosphere at a variety of latitudes and magnetic longitudes.

The 18-month tour of the Jovian system includes 8 occultations of the Earth by Jupiter and more than 10 occultations of the Earth by the four Galilean satellites.

DESIGN DETAILS

The Galileo Radio Science Team consists of two groups organized around separate categories of scientific investigation: The Radio Propagation Group and the Celestial Mechanics and Gravitational Group.

The Radio Propagation Group will seek to determine atmospheric temperature and pressure profiles and electron densities as the spacecraft passes behind Jupiter and its satellites. During the Jupiter occultations, the team will sample the downlink in a frequency bandwidth of 2500 Hz at a rate of 5000 samples per second. The sampling bandwidth and sampling rate for the satellite occultations is still TBD. For these occultations the spacecraft downlink must be in residual carrier (i.e., non-suppressed) mode and referenced to the on-board Ultra Stable Oscillator (USO). The Radio Science Digital Signal Processor (DSP-R) is also required for these experiments.

The Radio Propagation Group will also study the solar wind during the January 1997 solar conjunction by observing the Doppler and amplitude scintillation effects on the radio signal as the spacecraft passes behind the Sun. For this activity, a residual carrier downlink, referenced to the USO, is strongly preferred, but not essential. The DSP-R is required for this experiment only when using the residual carrier.

Experiments performed by the Radio Propagation Group will make use of the Multimission Ground Data System (MGDS) to obtain data in near-real time via monitor displays and in non-real time from the Galileo Project Data Base.

The Celestial Mechanics and Gravitational Group will seek to determine the gravity fields of Europa, Ganymede, and Callisto by measuring changes in the spacecraft trajectory, as reflected in the downlink Doppler, as the spacecraft passes close to these bodies. Neither a residual carrier downlink nor the DSP-R are required for these experiments.

The Celestial Mechanics and Gravitational Group will also use the Very Long Baseline Interferometry DSP to obtain Delta Differenced One-Way Ranging (Delta-DOR) measurements of the spacecraft 3-4 times during the cruise phase of each orbit to refine estimates of the ephemerides of Jupiter. A quasar will be used as a standard point of reference for these measurements.

Data collected by the Celestial Mechanics and Gravitational Group will be placed on tape at the DSN station and shipped to JPL for analysis.

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