In broad terms, a SoOp (Signals of Opportunity) receiver opportunistically re-utilize various anthropogenic microwave signals for remote sensing, which are not originally intended for remote sensing. In other words, this approach re-purposes (recycles) satellite communications and navigation transmissions to enable microwave remote sensing at frequencies, fundamentally different than those used in multispectral imaging for unmanned aircraft system (UAS) or small satellite -based mapping and analytics. The key principle of the SoOp approach is similar to that of multispectral imaging where the sensor receives and further extracts information from a “source” whose signal reflects off the earth surface. The key difference is the type and number of “sources”, penetration, and their spectrum diversity. While multispectral imaging uses visible and near infrared wavebands where the sun is the “source” for illumination, the SoOp approach uses large portions of the microwave / radio wave spectrum and takes advantage of an abundance of anthropogenic signal “sources” illuminating multiple regions of the globe all the time. This is equivalent to having thousands of “suns,” under all weather conditions, day and night, to monitor environment. Furthermore, these satellites are positioned into unique orbital planes transmitting at different frequency bands, making their signals much more diverse in frequency and direction. Hence, each one of these satellites is a potential candidate for doing different environmental sustainability, especially agriculture applications in the microwave regime. We envision that the microwave remote sensing from small UAS as well as small satellite platforms can enable non-intrusive high resolution geophysical retrievals at multiple depths of soil and vegetation as it has the potential to be a powerful augmentation to traditional thermal and multispectral imaging capabilities due to its complementary features.
Figure above shows an illustration of the physics behind spectral use of shortwave/longwave optical remote sensing and RF/microwave remote sensing via Signals of Opportunity (SoOp). The key difference between optical and SoOp reflectometry is the type and number of “sources”, penetration, and their spectrum diversity. For instance, while reflected shortwave solar radiation and infrared longwave emission provide information about the surface characteristics, low-frequency (e.g., I-, P-, and L-bands) microwave reflectivity carries information at multiple depths within vegetation and soil. The main motivation to use microwave signals is due to its high sensitivity to water within plants and soil. This high sensitivity can be seen in everyday life. For example, kitchen microwave ovens use microwave radiation to warm food by making water molecules vibrate.
RF sensors offer many benefits over the more traditionally used optical sensors such as cloud-penetration, direct interaction with water content, and vegetation resilience. Diversity of RF spectrum use, differing signal power, system geometry, and parametrization tactics make different system designs and modeling techniques more tractable for a given application. Broadly speaking, the three primary branches of microwave remote sensing are (1) active, (2) passive, and (3) SoOp methodologies as illustrated above. Radars and microwave radiometers are the active and passive instruments, respectively, which have been extensively utilized from space platforms and are considered as traditional microwave remote sensing approaches. Active microwave remote sensing is the measurement of microwave signals where the user has significant control over the transmitter, receiver, and signal processing. Passive microwave remote sensing is the measurement of naturally emitted microwave radiation, and while the user contains a great amount of control over the receiver, no transmitted signals are involved. As a third way of microwave remote sensing, SoOp reflectometry uses of man-made signal transmissions in a bistatic radar configuration and has been gaining considerable attention within scientific communities in the last decade . Unlike traditional radar sensing, SoOp only requires the development of advanced receiver systems to detect microwave signal interaction with vegetation and land surfaces. While the SoOp reflectometry technique can be applied to any reliable signal transmission, the use of spaceborne sources such as communication and navigation satellites has been the focus of growing research in recent years due to its potential for global remote sensing at a low size, weight, power, and cost (SWaP-C). SoOp measurements generally make use of forward-scattered signals from anthropogenic sources, and this can provide an advantage over traditional active sensors since scattered signals will typically be dominantly scattered in the forward direction. An example SoOp implementation for a small UAS using GNSS signals is illustrated below.
Source: Mehmet Kurum, Ali C. Gurbuz, Spencer Barnes, Dylan R. Boyd, Matthew Duck, Md. Mehedi Farhad, Austin Flynt, Nathan Goyette, Preston Peranich, Mia Scheider, and Volkan Senyurek “A UAS-based RF testbed for water utilization in agroecosystems”, Proc. SPIE 11747, Autonomous Air and Ground Sensing Systems for Agricultural Optimization and Phenotyping VI, 117470J (12 April 2021); https://doi.org/10.1117/12.2591895