# Geolocation, Calibration and Surface Resolution of CYGNSS GNSS-R Land Observations
## 4 Analysis of Coherent Surface Resolution Using Ancillary River Width Data
However, there will be cases when a river surface is rough relative to the reflecting L-band wavelength (19 cm) and will not generate coherent forward reflections. In these circumstances, the area of surface scattering will be significantly larger than an integrated Fresnel zone and not result in coherent detection of the river crossing. 3. After (approximately) July 2019, the CYGNSS output data rate was increased from 1 Hz to 2 Hz Level 0 observations. The 2 Hz data output rate corresponds to a 0.5 s instrument integration interval, which reduces the length that the single look surface footprint is integrated across the surface at the specular point velocity. Generally, CYGNSS 1 Hz observations result in a surface integration on the order of 6 km in the along track direction due to satellite motion, while 2 Hz observations will span roughly half of that distance. Note that short intervals of coherent reflection (such as from a small water bodies) within the larger integration footprint often dominate the received power, resulting in detections of small water bodies within the longer integrated surface footprint. 4. Horizontal errors in the surface geolocation of reflections near the Courantyne river will potentially introduce small errors (less than \(\approx 1\) km) in the river width estimates. The upper bound of the constellation's ability to resolve features on the surface, under a coherent reflection regime, is expected to be governed by the incoherent integration time. The reduction of the integration interval reduces the along track smearing of L1 DDMs, thereby improving the overall spatial resolution. To illustrate this, CYGNSS measurements from over a (mean) 500 m wide section of the Courantyne River were analyzed, illustrated in Figure 8, over 8 month periods when the  
Figure 7: Analysis deriving from the NARWidth data set illustrating effects of varied river widths on the CYGNSS Level-1 coherence metric and probability of detection. (**Top**) Across a sixth month period from 1 September 2018 to 28 February 2019, exclusively comprising 1 Hz data. (**Bottom**) Across a sixth month period from 1 September 2019 to 2 February2020, exclusively comprising 2 Hz data. constellation operated exclusively in 1 Hz and 2 Hz modes. Details of the selected test region and CYGNSS track crossings are described in Table 3.  
As illustrated in Figure 9, it can be observed that the 1 Hz CYGNSS data mean power ratios remain above the detection threshold for distances up to 5 km, slightly less than the \(\approx\)6 km distances traveled during the 1 s integration time. Within this 5 km radius, the \(PR\) is maintained above the detection threshold as the result of dominant coherent reflections arising from the (mean) 500 m wide river. The contributions of the coherent reflector (the river) get'smeared' along track thereby degrading the surface resolution of the individual observation. This is contrasted with the behavior of measurements made after the switch to 2 Hz sampling over the same region where it is observed that due to the halving of the incoherent integration time, the along track smearing is reduced to approximately the distance traveled along track between two consecutive specular points of \(\approx\)3 km. As a result, the power ratios are estimated as 'dominantly coherent' up to 2.4 km away from the river center resulting in an average improvement in spatial resolution of \(\approx\)48% brought about by the reduction in integration time. a summary of the observation statistics based on the data shown in Figure 9, is included in Table 4.  
Figure 8: Test region latitude/longitude used in CYGNSS integration time analysis. Test region was centered around a 35 km long section of the Courantyne River between Guyana and Suriname. Figure 9: Effects of varying the CYGNSS mission’s Level-1 product integration time for observations in the vicinity of the Courantyne River on the CYGNSS coherence metric and Signal-to-Noise Ratio (SNR) at varying distances from the water body.