The RSS is usually not a separate instrument; its functions are usually "piggybacked" on the existing telecommunications subsystem.
Radio science is commonly used to determine the gravity field of a moon or planet by observing Doppler shift.
This requires a highly stable oscillator on the spacecraft, or more commonly a "2-way coherent" transponder that phase locks the transmitted signal frequency to a rational multiple of a received uplink signal that usually also carries spacecraft commands.
As the spacecraft moves behind the planet, its radio signals cuts through successively deeper layers of the planetary atmosphere.
Measurements of signal strength and polarization vs time can yield data on the composition and temperature of the atmosphere at different altitudes.