Aradio science subsystem (RSS) is asubsystem placed on board aspacecraft forradio science purposes.
The RSS usesradio signals to probe amedium such as a planetaryatmosphere. The spacecraft transmits a highly stable signal toground stations, receives such a signal from ground stations, or both. Since the transmitted signalparameters are accurately known to the receiver, any changes to these parameters are attributable to thepropagation medium 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 existingtelecommunications subsystem. More advanced systems use multiple antennas withorthogonalpolarizations.
Radio science is commonly used to determine thegravity field of amoon orplanet by observingDoppler shift. This requires a highly stableoscillator on the spacecraft, or more commonly a "2-way coherent"transponder thatphase locks the transmitted signal frequency to a rational multiple of a receiveduplink signal 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 strength and polarization vs time can yield data on the composition and temperature of the atmosphere at different altitudes.
It is also common to use multipleradio frequencies coherently derived from a common source to measure the dispersion of the propagation medium. This is especially useful in determining the freeelectron content of a planetary ionosphere.