부산대학교 도서관

High resolution (~100m) remote sensing of Soil Moisture (SM) and Vegetation Water Content (VWC) is critical for investigations of land surface hydrological and ecological processes along with applications to water resource management, agriculture yield, flood prediction and weather forecasts. Due to their importance, L-band (~1 GHz) microwave passive microwave missions, including the ESA Soil Moisture and Ocean Salinity (SMOS) and NASA Soil Moisture Active Passive (SMAP) missions with a swath width of 1000 km have been providing these two observables from space with a global coverage of every 2–3 days, however at a coarse resolution of about 30 km. In contrast the L-band Synthetic Aperture Radar (SAR) missions, such as the Japanese PALSAR-series and future NASA-ISRO SAR (NISAR), can provide high resolution, but with a narrower swath of about 250 km, limiting the temporal revisit of 6 to 12 days. To achieve a temporal revisit of 1–3 days with the nominal active SAR technologies, like PALSAR or NISAR, will require a constellation of satellites, which will require a high implementation cost. The Signals-of-Opportunity (SoOp) reflectometry, which is the basis of the NASA CYGNSS mission, can provide low-cost passive radar observations of earth surfaces, however with a limited spatial resolution of mostly about 20 km and no swath coverage. To address the scientific needs with an affordable cost, we have investigated the design of L-band GNSS-based SoOp SAR for high resolution and frequent coverage from space. The design employs an electronic scanning antenna to enable the tracking of reflections of multiple GNSS satellites, which can possibly allow an effective total swath of 1000 km or greater, and thus a temporal revisit of better than 3 days and a high spatial resolution. We have completed a signal to noise ratio (SNR) analysis. The SNR analysis is based on the SoOpSAR design equations (S. Yueh R. Shah X. Xu, B. Stiles, and X. Bosch-Lluis, “A Satellite Synthetic Aperture Radar Concept Using P-Band Signals of Opportunity,” IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Vol. 28, Feb 2021), which require the values of normalized bistatic radar cross sections (NBRCS) of land surfaces near the specular direction. According to the CYGNSS NBRCS data and theoretical estimation based the Kirchhoff scattering model for scattering off the azimuth from the specular point, the range of NBRCS is mostly in the range of 10 to 40 dB. With the NBRCS of 10 dB, 32 dBW for the Equivalent Isotropically Radiated Power (EIRP) of GNSS satellites, and a receiver antenna gain of 30 dB, a positive SNR can be achieved with a swath of 200 km for a low earth orbit altitude of 600 km. The antenna beamwidth for an antenna aperture of 2 m for the antenna gain of 30 dB is about 6 degrees, which can enable a dwell time of greater than 10 seconds, and therefore a resolution of about 2 m on the ground by Doppler filtering. The range resolution for a bandwidth of 10 MHz will vary across track. We will present the design of the GNSS-SoOpSAR along with conceptual extension to use multiple satellites to achieve single-pass interferometric SAR processing for detection of surface elevation and 3-D mapping by tomography.