# High Spatio-Temporal Resolution CYGNSS Soil Moisture Estimates Using Artificial Neural Networks  
[PERSON]  
1 Department of Electrical and Computer Engineering, Mississippi State University, Starkville, MS 39759, USA; [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (A.C.G.) 1  
[PERSON]  
1 Department of Electrical and Computer Engineering, Mississippi State University, Starkville, MS 39759, USA; [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (A.C.G.) 1  
[PERSON]  
1 Department of Electrical and Computer Engineering, Mississippi State University, Starkville, MS 39759, USA; [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (A.C.G.) 1  
[PERSON]  
1 Department of Electrical and Computer Engineering, Mississippi State University, Starkville, MS 39759, USA; [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (A.C.G.) 1  
Footnote 1: email: {eef: {eef: {eef: {eef: {eef: {eef: {eef: {eef: {eef: {eef: {eef }}}}}}}}}} : {} {} {} {} {} {}  
due to the atmospheric losses, surface roughness, and vegetation cover [5]. ESA's Soil Moisture and Ocean Salinity (SMOS) [6] and NASA's Soil Moisture Active Passive (SMAP) [7] satellites are current missions that have on-board L-band radiometers and provide SM retrievals with a spatial resolution of around 40 km and revisit time of 2-3 days. On the other hand, radar backscattering generally offers finer spatial resolutions (tens of meters to few km) within longer revisit times except SMAP's radar instrument (L-band), which was capable of providing a spatial resolution of 3 km and a revisit time of 2-3 days with the help of its rotating antenna before a hardware failure in mid 2015. SMAP mission was designed to make use of a 6 m mesh reflector antenna for both radar and radiometer instruments to provide high spatio-temporal resolution SM products [7; 8]. In addition, the radar backscattering data of ESA's Sentinel-1 (C-band) [9] and DLR's TERRASAR-X (X-band) [10] were used for global SM estimates. Global Navigation Satellite System Reflectometry (GNSS-R) is an alternative microwave remote sensing approach, which is based on reception of the reflected GNSS signals from the Earth surface in a bistatic geometry [11]. This approach shows a great potential for remote sensing of SM because it operates at L-band. Moreover, it can offer high spatial resolutions with low revisit times by using constellations of small satellites due to being strictly \"receive-only\" [12]. GNSS-R applications have seen advancements for various Earth science areas over the past two decades that have resulted in the launch of new satellite missions [13; 14; 15; 16]. For instance, the first dedicated spaceborne GNSS-R receiver was a secondary payload on-board the UK Disaster Monitoring Constellation (DMC) [17]. It has demonstrated the potentiality of GNSS-R for the remote sensing of ocean, ice, and land geophysical parameters [18]. The UK Technology Demonstration Satellite (TDS-1) was launched in 2014 with an improved primary GNSS-R payload, Space GNSS Receiver-Remote Sensing Instrument (SGR-ReSI), which provided more data that were used to study GNSS-R sensitivity to SM [19; 20]. NASA's Cyclone GNSS (CYGNSS) was launched in December 2016 to improve weather predictions by estimating ocean winds between 38\({}^{\circ}\) north and 38\({}^{\circ}\) south latitudes [21]. CYGNSS has eight small satellites in orbit, each with four channels, allowing simultaneous measurements from up to 32 channels. It has a mean revisit time of seven hours over the ocean. The key orbital and instrumental specifications of the CYGNSS mission are listed in Table 1. The constellation records a considerable amount of land observation data as well. CYGNSS measurement sensitivity to the surface SM has been reported by multiple studies [12; 22; 23; 24]. These efforts demonstrated GNSS-R's potential to complement the traditional passive and active instruments for monitoring surface SM at global scales for improved spatio-temporal resolutions.