Since its inception in the early 1970s, satellite-based passive microwave sounding of the atmosphere has evolved considerably to where it now provides the single greatest positive impact to numerical weather forecast anomaly scores relative to any other single source of data [English et al. 2013]. In addition, passive microwave sensors of different architectures also produce key environmental data records (EDRs) such as Ocean Surface Vector Winds (OSVW), Tropical Cyclone Intensity (TCI), and atmospheric temperature, water vapor, and precipitation EDRs. This evolution has occurred due to the fundamental advantage of microwave observations relative to visible and infrared observations in being able to observe the atmospheric column through clouds [Staelin, 1981], and despite the somewhat slower advance of the receiver and antenna technology necessary to implement precision passive microwave sounding and imaging systems. Historically, satellite passive microwave instruments have come in two primary types: single ended total power radiometers and polarimeters. The key difference between these is that polarimeters measure the complex coherence (full stokes vector including 3rd and 4th components) while total power radiometers only measure radiometric power in the 1st and 2nd stokes vector components. In addition, microwave radiometers have employed two primary modes: sounding and imaging. Sounding radiometers exploit quantum mechanical resonances of atmospheric gasses to perform vertical profiling of the atmosphere where imaging radiometers perform total column measurements to produce imagery products such as OSVW or TCI. To date, all instruments developed for and flown on satellite missions have employed one architecture or the other but with the rise of proliferated Low Earth Orbit (pLEO) small satellite constellations and increasingly complex space mission architectures it is desirable to develop modular instrument architectures that can operate in either imaging or sounding modes as fungible payloads for a Space Based Environmental Monitoring (SBEM) pLEO constellation.
