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The Jet Propulsion Laboratory has developed a 170 GHz airborne radar for cloud and humidity remote sensing. Called VIPR (Vapor Inside-cloud Profiling Radar), the system uses differential absorption at closely spaced frequencies near the 183 GHz water vapor resonance to obtain range-profiling measurements of absolute humidity inside clouds, and partial-water- column measurements in clear skies. VIPR transmits around 300 mW over 167-174.8 GHz, has a system noise figure of 8 dB, and uses a 60-cm diameter aperture. The radar has been deployed both on the ground pointing toward zenith, and from an aircraft with nadir pointing. Based on architectures originally developed for submillimeter-wave security imaging, VIPR uses ultra-high-isolation transmit/receive duplexing with a single primary antenna. This approach achieves thermal noise limited sensitivity even while using frequency-modulated continuous-wave ranging methods, and even when the radar is mounted in an aircraft with its beam emerging from an open-air viewport. Here we present a validation measurement of VIPR’s ability to sense humidity in clear skies using ground reflection magnitudes at different altitudes and frequencies. These results have also motivated a new investigation of using a higher-frequency 557 GHz differential absorption radar for water vapor sensing in the low pressure, cold, and dry conditions on Mars. We have developed a 552-558 GHz RF source with several mW of output power that could be used for making local humidity measurements on Mars out to several kilometer ranges.