# The Use of SMAP-Reflectometry in Science Applications: Calibration and Capabilities
## 2 Capabilities
### SMAP-R Characteristics  
SMAP was not designed to operate as a reflectometer measuring GPS signals. Consequently, some of the characteristics of SMAP-R, such as its poor spatial sampling, are not ideal for some science applications. However, some characteristics, such as the dual-polarization and the high gain antenna, provide an opportunity to study the potential benefits of these unique features. In addition, SMAP provides global measurements, which are important for cryosphere investigations. SMAP-R characteristics include:  
1. Low temporal coverage and low sampling limits utility in applications that require high temporal repetition. However, applications that do not require frequently updated information are suitable for SMAP-R. For example, SMAP-R data are suitable for vegetation water content estimation, since these observables are less variable on sub-seasonal timescales (i.e., 15 days-1 month). Figure 2 provides an example of the coverage obtained for 1 day, 15 days, 1 month, and 2 months of data. Figure 2 illustrates that sampling over 1 day is scarce, but after 1 month, the accumulated dataset is suitable for spatial analysis at regional scales. In order to leverage this unique, yet sparse dataset, data must be aggregated over time. After an accumulation of measurements over long periods with low variation, we can define two extreme states as references from which we can investigate transitional periods in time between these two states. These data synthesis techniques have been implemented in [32] for freeze and thaw seasonal transitions in Alaska and in [33] for vegetation water content estimation on the US Corn Belt agricultural area. The technique allows for an understanding of how the observed surfaces transition from one state to another, for example, from freeze and thaw states [32], or from bare soil to peak vegetation growth [33].  
SMAP-R provides measurements at vertical (V-) and horizontal (H-) polarizations. Polarimetric information is utilized with the calculation of a polarimetric ratio, which enables the estimation of vegetation water content and crop characteristics. Other applications include freeze/thaw state characterization and sea ice type classification; both freeze/thaw states and different ice types have polarization dependent signatures. Additionally, studies of surface roughness in arid/semi-arid regions (very low vegetation) are possible because roughness also has clear polarimetric components. Polarimetry is also a key element in oil spill detection, but SMAP-R may have limited utility in this application as the poor spatial sampling may not be able to capture these events.
3. SMAP-R has a higher gain antenna than other GNSS-R missions (36 dB compared to the CYGNSS down-looking gain \(\sim\)14.5 dB), allowing for better signal-to-noise ratio (SNR) at shorter integration times. Shorter integration times translate into better along-track spatial resolution for each specular reflection measured. The along-track spatial resolution is the result of the scattering area being elongated by the distance SMAP moved during the integration time. Better SNR leads therefore to improved spatial resolutions.  
In the next two subsections we specifically compare SMAP-R to the other two GNSS-R missions, CYGNSS and TDS-1. The differences shown are derived from each mission's design requirements and/or in the case of SMAP-R, the repurposing of a mission to use its radar receiver for something it was not originally intended to do.