# The Use of SMAP-Reflectometry in Science Applications: Calibration and Capabilities
## 3 Calibration of SMAP-R Signals
### SMAP Antenna High Gain Effect  
Another important aspect of SMAP-R data is the projection of the high gain antenna beam on the scattering surface. The high gain antenna acts as a spatial filter on the total area contributing to the received power and limits the extent of the measured DDMs to those delay and Doppler bins that fall within the SMAP antenna footprint, which account only for the \(-\)3 dB beamwidth with a size of 40 km diameter. Figure 9 shows three measurements obtained over different surfaces: Ocean (incoherent with a typical glistering zone of \(\sim\)250 km, for nominal winds of 10 m/s), land (incoherent with a glistering zone of \(\sim\)5-50 km, depends on the surface roughness), and sea ice (coherent with a typical glistering zone of \(\sim\)1 km). Figure 8: Temporal analysis of the variations in the combined transmitted power and gain observed for four GPS satellites over a period of 12 months. Mean variability observed in the vertical axis is due to antenna gain variations with respect to incidence angle. The standard deviation of the variability observed in the vertical axis is due to antenna gain variations with respect to azimuth angle. As can be seen in Figure 9a to Figure 9c, the spread of the reflected signal varies depending on the surface type and therefore the incidence angle impact observed will be more or less critical depending on the surface type. The scattering covers a different size for each one of the DDMs shown in Figure 9. The SMAP antenna moves through this surface partially observing the GPS scattered signal, resulting in the observed DDM not being a full-DDM but a portion of it. Note that the signal does not spread significantly in the delay dimension, because of the limiting SMAP antenna footprint. Next, we describe more in further detail what is the effect of this filtering for a coherently and an incoherently dominated scattering surface. Coherently dominated surfaces:  
For surfaces as the one shown in Figure 9a-c (sea ice surface in the Kara Sea, Russia), the dominant type of scattering is coherent. Figure 10 shows measurements of a single track for three different incidence angles, 37.5\({}^{\circ}\), 40\({}^{\circ}\) and 42.5\({}^{\circ}\) within the \(-\)3 dB beamwidth range, over some other sea ice surfaces near the Arctic, along with corresponding sketches on where the specular point is located (center of blue ellipses) versus the SMAP antenna's boresight (red dot). The effects of the SMAP high gain antenna on the measurements are small for most incidence angles and DDM bins. In other words, over the range of incidence angles considered here, the coherent scattering area typical of sea ice/lake surface (1 km \(\times\) 1.180 km) is within SMAP's footprint. In the incidence angle range extremes, Figure 10a at 37.5\({}^{\circ}\) and Figure 10c at 42.5\({}^{\circ}\), when the scattering area is near the edges of the SMAP antenna footprint--the specular point is inside the SMAP footprint, but the scattering area is only partially measured--we will observe a loss of power on the DDM. The loss of power observed in Figure 10 is \(\sim\)13 dB at both extremes and it is caused by three combined effects:  
Figure 9: Delay–Doppler Maps of the scattered power measured by SMAP-R for (**a**) ocean surface in the edges of the Caribbean Sea between Puerto Rico and Caracas, (**b**) arid land surface in the middle of Australia, near the Neale Junction Nature Reserve, and (**c**) sea ice surface in the Kara Sea, Russia. * The SMAP antenna beam pattern at 2.5\({}^{\circ}\) away from the boresight has an impact of \(-\)2.8 dB on the total power. Note that these measurements come from the same GPS satellite. * The GPS antenna beam pattern at the specific incidence angles causes a loss between 0.4 and 1 dB. * The partially measured scattering area that falls inside the SMAP antenna footprint, i.e., between 9.2 and 9.8 dB, approximately. This partially measured scattering area causes a shape change on the DDM, since the symmetry of delay and Doppler bin cells is lost.