# Mapping Rice Fields in Urban Shanghai, Southeast China, Using Sentinel-1A and Landsat 8 Datasets
## Methodology
### Sentinel-1A Data  
Launched in April 2014 by the European Space Agency (ESA) on a SOYUZ rocket from the Guiana Space Center in French Guiana, Sentinel-1A is the first of a series of earth observation satellites for the Copernicus Initiative. The Sentinel-1 mission is the European Radar Observatory for the joint initiative on environment and security between the European Commission and ESA. The Sentinel-1 synthetic aperture radar (SAR) constellation carries two C-band instruments (1A and 1B) operating at a center frequency of 5.4 GHz, sharing the same orbital plane. Sentinel-1A is a sun-synchronous, near-polar circular orbit at a height of 693 km and an inclination angle of 98.18\({}^{\circ}\). It offers a 12-day repeat cycle at the equator with 175 satellite orbits per cycle. With Sentinel-1A and Sentinel-1B operating, a 6-day repeat cycle at the equator will be achieved [41]. These polar-orbiting satellites acquire images day and night regardless of weather conditions. They continue and compliment C-band imaging missions of ESA's Earth Resources Satellites (ERS 1 and ERS 2), Environment Satellite (ENVISAT), and Canada's RADARSAT-1 and RADARSAT-2 [41]. Sentinel-1 presents significant advancement over previous C-band missions as stated above, in terms of spatial and temporal resolution, geographical coverage, reliability and data dissemination [41]. The mission seeks to benefit numerous services as in sea-ice monitoring and maritime surveillance, land monitoring of forest, water, soils and agriculture, monitoring of land surface motions, and mapping to support environment, crisis and natural disaster management [41]. Sentinel-1 operates in four exclusive acquisition modes; Stripmap (SM), Interferometric Wide swath (IW), Extra-Wide swath (EW), and Wave mode (WV). Each mode can produce products at SAR Level-0, Level-1 single look complex (SLC), Level-1 ground range detected (GRD) and Level-2 Ocean (OCN). Level-1 SLC products have been focused and geo-referenced using satellite altitude and orbit data, and are provided in zero-Doppler slant range geometry. Level-1 GRD products are detected, multi-looked and projected to ground range using an Earth ellipsoid model, and phase information is lost. The products have approximately square resolution and pixel spacing with reduced speckle effects at the cost of reduced geometric resolution. GRD products can be in full, high and medium resolution. Level-2 OCN products include Ocean Swell spectra (OSW), Ocean Wind Fields (OWI), and Surface Radial Velocities (RVL). OSW provides an estimate of wind speed and direction per swell spectrum. OWI gives a ground range gridded estimate of surface wind speed and direction at 10 m above the surface. RVL is a ground range gridded difference between the measured Level-2 Doppler grid and the level-1 calculated geometrical Doppler. Of these four acquisition modes of Sentinel-1A, the IW mode is regarded as the primary or default operational mode over land, supporting agriculture, forestry and other natural resource applications [41]. Hence, the IW mode is selected in this study to map rice fields in urban Shanghai. The IW mode combines a large swath (250 km) with a moderate geometric resolution (5 m by 20 m), and images in three sub-swaths using the Terrain Observation with Progressive Scans SAR (TOPSAR). In addition to steering the beam in range as in SCANSAR, the TOPSAR technique also steers the beam electronically from backward to forward in azimuth direction for each burst, thus, avoiding scalloping and resulting in higher quality images [41]. Interferometry is maintained by the sufficient overlap of the Doppler spectrum (azimuth domain) and the wave number spectrum (elevation domain). TOPSAR ensures homogenous image quality throughout the swath [41]. In this study, we have acquired five Sentinel-1A GRD images in IW mode during the 2015 rice growing season in Shanghai.