# Geolocation, Calibration and Surface Resolution of CYGNSS GNSS-R Land Observations
## 5 Discussion  
This manuscript addresses several of the underlying requirements unique in the processing of GNSS-R land observations: geolocating the observations in diverse topography, calibration the observations into both reflectivity and NBRCS estimates, and assessing the surface resolution for the coherent reflection case.  
We have shown that in a majority of CYGNSS land observations we were able to predict the surface observation point with high confidence. This can generally be attributed to many regions of the Earth having relatively low and moderate surface topographic variations, and those variations often being more or less consistent over kilometer scales. However, the exceptional cases in extreme environments will often be incompatible with a generalized algorithm and will require detailed per-DDM analysis across larger regions to identify the locations generating the received power at the instrument.  
As discussed above, the increased likelihood of complexities associated with the presence of coherent (and mixed) observations as well as diffuse scattering from land surfaces, can be mitigated with a dual approach for land observations that estimates both surface reflectivity and NBRCS,  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Distance** & **Mean** & **STD** \\ \hline
**1 Hz DDM Frequency (Obs. Spacing: 5990m)** \\ \hline Power Ratio & 500 m & 2.38 & 0.77 \\ \hline Power Ratio & 4600 m & 2.41 & 1.54 \\ \hline SNR & 500 m & 9.75 dB & 5.35 dB \\ \hline SNR & 4600 m & 6.63 dB & 5.23 dB \\ \hline Detection Prob. & 500 m & 91.67 \% & N/A \\ \hline Detection Prob. & 4600 m & 77.67 \% & N/A \\ \hline
**2 Hz DDM Frequency (Obs. Spacing: 3010m)** \\ \hline Power Ratio & 500 m & 2.43 & 0.87 \\ \hline Power Ratio & 2400 m & 2.46 & 1.13 \\ \hline SNR & 500 m & 12.91 dB & 6.48 dB \\ \hline SNR & 2400 m & 10.05 dB & 5.97 dB \\ \hline Detection Prob. & 500 m & 92.16 \% & N/A \\ \hline Detection Prob. & 2400 m & 87.12 \% & N/A \\ \hline \hline \end{tabular}
\end{table}
Table 4: Observation statistics and estimated LSQ fit of coherence detection threshold distances (4600 m at 1 Hz, and 2400 m at 2 Hz) for 1 Hz and 2 Hz DDM frequencies.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Courantyne River Region (Guyana/Suriname)** \\ \hline
**Parameter** & **Mean Value** & **Comment** \\ \hline River Width & 500 m & NARWidth [21] \\ \hline North Angle & 29 deg & Figure 8 \\ \hline
**Test Region** & **Min** & **Max** \\ \hline Latitude & 4.4 & 4.7 \\ \hline Longitude & \(-\)58.3 & \(-\)57.45 \\ \hline
**Track Direction** & **North Angle** & **River Angle** \\ \hline Ascending & 53 deg & 24 deg \\ \hline Desending & 127 deg & 97 deg \\ \hline \hline \end{tabular}
\end{table}
Table 3: Description of selected test region for river crossing coherency analysis.  
accompanied by an additional estimate as to the coherence in the level 0 observation. This approach assumes that individual users working on higher level geophysical parameter estimation will need to consider carefully how they apply the different surface observations in conjunction with the provided (or independently estimated) likelihood of signal coherency.  
We feel this initial study on the detectable surface resolution of small coherent surface features provides a useful first quantitative analysis of the impact of the non-coherent instrument integration and the resulting blurring of surface footprint and how this impacts the detection of small features within a larger integrated surface area. The surface resolution in the case of diffusively scattered land observations is a subject of further study and expected to be more on the order of the surface resolution of a typical ocean observation (on the order of 15 km or greater).