## 2 Description of the GNSS-R Experiment  
The SMAP mission was successfully launched on 31 January 2015 into a Sun-Synchronous Orbit (SSO) with a local time of ascending node (LTAN) of 6:00 h, and an orbit height of \(\sim\)685 km. The main mission instruments are an active L-band radar and a passive radiometer which share a \(\sim\)14 rev/min rotating 6-m aperture reflector antenna scanning a wide 1000 km wide swath (Figures 1 and 2). The dual-polarization reflector (H and V) points to Earth's surface with an approximately fixed elevation angle of \(\theta_{\rm e}\sim\)55\({}^{\circ}\). The elevation angle of the collected signals was approximately constant \(\sim\)55\({}^{\circ}\) during the complete experiment because the attitude of the SMAP satellite is the same in nominal operations. Data acquisition used a 50 km cut on (footprint area \(\sim\)7800 km\({}^{2}\)) the closest approach to the specular point, and calibrated using the antenna radiation pattern. This possibly is cutting off the waveform trailing edge over some regions. Figure 3 shows enough sensitivity from Greenland to tropical forests. The High-Power Amplifier (HPA) failed on 7 July 2015, preventing the emission of radar pulses and thus the use of the instrument as an active device. As an experiment of opportunity, the radar receiver was tuned to the GPS L2 frequency (1227.6 MHz) in order to collect Earth-reflected GPS signals. Raw data were downloaded to the ground and then a geometric model was used to select SMAP's Level L1 A binary data (orbit, attitude, antenna pointing, etc.) extraction from their archive to only those time periods with potential specular reflections (Figure 2). These data, combined with GPS orbital information, were used to predict when the SMAP's antenna pointed to a nominal specular reflection point. This timing information was then used by a L1 A Block Floating Point Quantization (BFPQ) for In-phase (I) and Quadrature-phase (Q) conversion, where H-Pol and V-Pol data were extracted. This converter read in SMAP Level 1A data, and converted the compressed BFPQ numbers to I and Q 8-bit signed integers. These I/Q data had continuity gaps when the radar transmitter was supposed to be transmitting. The converter computed the required number of zero-valued samples to be inserted to provide a temporally continuous data set. These continuous I/Q samples were passed to the GPS L2 acquisition software, which searched in Doppler and delay for the L2 Civilian Medium-length code (L2 CM), which repeats every 20 milliseconds. Once this code was found, the software aligned with the L2C Long code (L2 CL), which repeats every 1.5 s. The precise Doppler and range information was passed on to the DDM processor, creating Delay Doppler Maps (DDMs) spanning \(\pm\)10 kHz in Doppler and \(\pm\)3000 km in range, and centered at the strongest detected signal, for both the H and V polarization data. After inspection, the data set was seen to be free of potential effects of Faraday rotation. Two signal peaks were selected because the direct GPS signal was almost always present in the data: timing between the two signals unambiguously determined which was the reflected signal used here.