Constellations of CubeSats to exploit signals-of-opportunity for Earth system science

Signals-of-Opportunity (SoOp) reflectometry is a concept for reutilizing existing microwave satellite transmissions, in bands allocated for space-to-Earth communications and navigation, for the purpose of Earth remote sensing. SoOp uses a bistatic radar geometry employing forward scatter, which views Earth differently from traditional monostatic (backscatter) radars and radiometers. Additionally, SoOp enables use of microwave frequencies outside the limited number of narrow and protected bands allocated for remote sensing. A powerful example of the science returns achievable using SoOp is currently being demonstrated on orbit by the NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission, which measures ocean surface wind speed using reflected Global Positioning System (GPS) signals. Over the past decade we have seen a growing number of communication and navigation signals coming into existence spanning the range of frequencies from Very High Frequency (VHF) to K Bands, which could be used to measure different variables using wavelengths optimally suited to specific interactions with the Earth system, including: Soil Moisture; Sea Surface Height; Ocean Vector Winds; Snow Water Equivalent; Root Zone Soil Moisture; and Vegetation Water Content. The use of SoOp for earth remote sensing could enable these measurements from small passive receivers in Low Earth Orbit (LEO) on CubeSat constellations to improve the temporal frequency of the observations. This paper describes different architectures of CubeSats constellations that could be used to measure different science measurements as well as identifying some of the challenges of the SoOp technique.