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The Cassini Plasma Spectrometer Subsystem (CAPS) measures the flux of ions as a function of mass per charge and the flux of ions and electrons as a function of energy per charge and angle of arrival relative to the CAPS instrument. The CAPS Subsystem consists of three major instruments: the ion mass spectrometer, the ion beam spectrometer and the electron spectrometer.

The ion mass spectrometer (IMS) provides species-resolved measurements of the flux of positively charged atomic and molecular ions as a function of energy/charge vs aperture entry direction. The IMS uses a toroidal top-hat electrostatic analyzer combined with a linear electric field time-of-flight mass spectrometer (for mass/charge and other species-resolving data).

The second major CAPS instrument is the ion beam spectrometer (IBS). The IBS measures the flux of positively charged ions of all species as a function of energy/charge and aperture entry direction. The IBS based on a hemispherical electrostatic analyzer.

The third major CAPS sub-instrument is the electron spectrometer (ELS), which measures the flux of electrons as a function of energy/charge and aperture entry direction.

An important subassembly of CAPS is the actuator (ACT). The ACT rotates the CAPS instrument at a steady rate over a maximum range of 184 degrees with the acceleration/deceleration over a further 12 degrees (minimum) at either end of this range.

The CAPS Key Parameter data set consists of four different files. (1) MOMT: which contains calculated moments of the ion and electron distributions including ion density, velocity, pressure and and electron density and temperature. In addition the file includes error estimates. (2) SPEC: this file includes energy spectra data for ions (3) EPSC: electron energy spectra (4) PADS: finally, this file contains information about the pitch angle distribution of the ions summed over energy into four different energy bins.