(note: Excerpts from a now outdated NASA/CloudSat page, updated for the present, introduces CloudSat)
CloudSat was selected as a NASA Earth System Science Pathfinder satellite mission in 1999 to provide observations necessary to advance our understanding of cloud abundance, distribution, structure, and radiative properties.
CloudSat [has] the first satellite-based millimeter-wavelength cloud radar—a radar that is more than 1000 times more sensitive than existing weather radars.
Unlike ground-based weather radars that use centimeter wavelengths to detect raindrop-sized particles, the CloudSat radar [allows] us to detect the much smaller particles of liquid water and ice that constitute the large cloud masses that make our weather.
CloudSat is co-manifested with the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite [and was launched from] a Delta II rocket. CloudSat and CALIPSO [joined] three satellites already in orbit (Aqua, PARASOL, and Aura) to form a constellation of satellites known as the A-Train.
The satellites fly in a nearly circular orbit with an equatorial altitude of approximately 705 km. The orbit is sun-synchronous, maintaining a roughly fixed angle between the orbital plane and the mean solar meridian.
CloudSat maintains a close formation with Aqua and a particularly close formation with CALIPSO, providing near-simultaneous and collocated observations with the instruments on these two platforms. Combined data products use data streams from CloudSat + Aqua and CloudSat + CALIPSO.