Radio science subsystem

Aradio science subsystem(RSS) is asubsystemplaced on board aspacecraftforradio sciencepurposes.

Function of the RSS

The RSS usesradio signalsto probe amediumsuch as a planetaryatmosphere.The spacecraft transmits a highly stable signal toground stations,receives such a signal from ground stations, or both. Since the transmitted signalparametersare accurately known to the receiver, any changes to these parameters are attributable to thepropagationmedium or to the relative motion of the spacecraft and ground station.

The RSS is usually not a separate instrument; its functions are usually "piggybacked"on the existingtelecommunicationssubsystem. More advanced systems use multiple antennas withorthogonalpolarizations.

Applications

Radioscience is commonly used to determine thegravity fieldof amoonorplanetby observingDoppler shift.This requires a highly stableoscillatoron the spacecraft, or more commonly a "2-way coherent"transponderthatphase locksthe transmitted signal frequency to a rational multiple of a receiveduplinksignal that usually also carries spacecraft commands.

Another common radio science observation is inradio occultation,performed as a spacecraft is occulted by a planetary body. As the spacecraft moves behind the planet, its radio signals cuts through successively deeper layers of the planetary atmosphere. Measurements ofsignal strengthand polarization vs time can yield data on the composition and temperature of the atmosphere at different altitudes.

It is also common to use multipleradio frequenciescoherently derived from a common source to measure the dispersion of the propagation medium. This is especially useful in determining the freeelectroncontent of a planetary ionosphere.

Spacecraft using RSS

• Cassini–Huygens
• Mariner 2,4,5,6,7,9, and 10
• Voyager1 and 2
• MESSENGER
• Venus Express

Functions

• Determine composition ofgas cloudssuch as atmospheres,solar coronas.
• Characterizegravitational fields
• Estimate masses of celestialsatellitesthat do not have satellites of their own.
• To estimate particle size ofparticle fields
• Estimate densities ofion fields.

Specifications

• Given adeep space network(DSN) of receivers and/or transmitters.
• AKa-bandtraveling wave tubeamplifier(K-TWTA) amplifies signals to a transmittingantennato be received by a distalradio telescope.
• Ka-band translator (KAT) receives signal from a high gain antenna and retransmits the signal back to DSN. In this way the phase and phase-shift resulting from signal modification
• Ka-band exciter (KEX) it suppliestelemetrydata.
• S-band transmitter is used for radio science experiments. The transmitter receives signal from the RFS, amplifies and multiplies the signal, sending a 2290 MHz signal to the antenna.
• Filter microwave emitter allow onlymicrowavesof a givenfrequencyto be emitted, there is a polarizing element. There are two-bypass filters and a wave-guide. The bypass filters allow different feed polarizations, receiving and transmitting.