# Geolocation, Calibration and Surface Resolution of CYGNSS GNSS-R Land Observations
## 2 Geolocating GNSS Reflections from Land
### Land Geolocation Example  
An example land surface reflection track is shown in Figure 2. The track starts in South Texas (the western most point of the track) and extends toward the east to approximately Lake Pontchartrain in Louisiana. Five example observation locations are shown along the track from west to east at relative seconds; 11, 33, 50, 60 and 120, respectively, indicated as consecutive green markers. The corresponding 5 DDMs at these seconds are shown below the track map. These DDMs start as predominantly non-coherent reflections to the west at higher altitude and more varied topography and proceed down in altitude toward sea level and a region with a significant surface water and wetlands coverage. The geolocation results of each of the 5 example DDMs can be briefly summarized as follows:  
1. Second 11: Relatively high terrain with a varied surface topography. The reflected power appears incoherent, with high confidence in the geolocation. 2. Second 33: a \"double peaked\" DDM, which complicates geolocation determination. Delay and Doppler location checks are based on the highest of the DDM power peaks, which in this case is ambiguous. This \"double peak\" DDM (with one peak at delay bin 6, and another at delay bin 10) is due to the irregular topography of the surface reflection at second 33, where two distinct and spatially separated surface locations are meeting the geometric conditions suitable for directing power toward the receiver. Attempts at surface retrievals using this DDM would be problematic. 3. Seconds 50 and 60: Surface reflections transition from rougher more varied topography to the flatter terrain of western Louisiana. The corresponding DDMs transition from noncoherent at second 50 to largely coherent at second 60. 4. Second 120: An area containing multiple surface water bodies within and near the estimated geolocation region. The DDM itself shows a very strong coherent reflection within a limited region around the estimated specular point. The local surface contour maps used in the geolocation at Second 11 are shown in Figure 3. The top-left plot shows how the delay difference of the Second 11 DDM (peak roughly in bin 8 in Figure 2 (bottom-left)) maps across the 205 km \(\times\) 205 km local surface grid. The dark area around the initial prediction center clearly indicates that the surface region that corresponds to the peak delay is near the estimated center. The Doppler difference map shown on the top-right further narrows the surface region of interest. Finally, the reflection geometry difference angle (bottom-left) isolates the likely scattering region. These three criteria combined result in the estimated surface geolocation \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Criteria** & **Threshold** & **Verification** \\ \hline \(|\delta\tau|\) & 2.5 C/A Code Chips & Maximum delay \\ \(|\delta D|\) & 200 Hz & Maximum Doppler \\ \(\delta\Psi\) & 2 Degrees & Reflection geometry \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of land Geo-location criteria parameters and thresholds used during performance analysis. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**SNR Thold (2 dB)** & **Delay, Doppler, Snell** & **Confidence** & **Land Percentage** & **Comment** \\ \hline Above & Invalid & 0 & 1.3 (0 to 1) & Most likely incorrect \\ Below & Invalid & 1 & 21.5 (1 to 2) & Probably incorrect \\ Below & Valid & 2 & 28.4 (2 to 2.5) & Likely correct \\ Above & Valid & 3 & 48.8 (2.5 to 3) & High probability correct \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of land geolocation confidence flag logic. region shown in the bottom-right, where the estimated geolocation for this DDM is predicted to extend over several 1 km surface pixels at and around the center of the local area grid. This geolocation estimate provides the center location of a larger diffuse scattering region (10 km or more).