# A Real-Time On-Orbit Signal Tracking Algorithm for GNSS Surface Observations
## 5 Results: Algorithm Verification Using On-Orbit Data
### Open Loop Tracker Code Phase Predictions  
The algorithm delay predictions were calculated for both tracks as follows: the GNSS-R receiver position and velocity was estimated using the direct navigation signals. The GNSS transmitter positions and velocities were obtained from the published GPS ephemeris over the collection interval. The surface specular reflection point was then estimated using the algorithm detailed in Section 3. Subsequently the reflected signal code phase delay and Doppler frequency were estimated along the length of both tracks as described in Section 4. The resulting truth reference values for the direct and reflected signal code phase delays, as well as the open loop predicted reflected code phase delays are shown in Figure 4. Importantly, the code delay rate is used to update the open-loop tracker at a rate that is less than the total measurement integration time in order to prevent smearing of the measurement. Measurement distortion will occur if the open loop tracker predictions lag behind the configuration of the hardware processing, causing the instrument processing to lag behind the real signal dynamics introducing distortions into the power detected across the processed DDM. In Figure 4 the detected and predicted code phase delays align (and overlap) almost exactly. This figure is useful for observing how the difference in code phase delay due to the physical path difference changes as a function of time. The detected and predicted Doppler frequency across the example tracks are shown in Figure 5.  
Figure 3: (**a**) Geographic location of the two example tracks used to demonstrate the on-board open loop tracking algorithm. Detected using a 60 s raw sampled data collection from a Cyclone GNSS (CYGNSS) observatory. The two reflections shown are from pseudo random noise (PRN) 26 and 21, with PRN 21 crossing over to land at the end of the collection. (**b**) Detected signal to noise ratios for the two parallel surface reflections. Note that the signal to noise ratio (SNR) of the PRN 21 reflection changes significantly as the reflection transitions to land at approximately second 43. Finally, the errors in the predicted delay code phases and Doppler frequencies are plotted in Figure 6. The truth reference determination of the reflected signal represents only a rough estimate of the precise surface reflection location, due to integer processing bin uncertainties and other surface factors. However, the intent of this validation is to demonstrate that these tracking algorithm can successful predict the delay and Doppler to within the sub-bin level requirements of a typical GNSS-R delay Doppler map. These limits are generally; to within 0.25 chips accuracy in delay and within 500 Hz in Doppler, which is consistent with general GNSS-R observation requirements [28]. Figure 4: Examples of reflection Delay open loop tracking algorithm with CYGNSS raw samples. (**a**) Direct and reflected code phases for GPS PRN 26 reflection. (**b**) Direct and reflected code phases for parallel GPS PRN 21 reflection. The predicted and detected reflected code phases are very close in delay with the resulting error in the delay tracking algorithm shown in Figure 6a. Figure 5: Examples of reflection Doppler open loop tracking algorithm with CYGNSS raw samples. (**a**) Detected and predicted reflected Doppler frequencies for GPS pseudo random noise (PRN) 26 reflection. (**b**) Detected and predicted reflected Doppler frequencies for parallel GPS PRN 21 reflection. The detected Doppler is approximate based on the observed signal peak. The resulting error (difference between green and blue plots) is shown in Figure 6b. The errors in Figure 6 indicated that the duration along both tracks tested, the tracking accuracy is well within the requirements for DDM generation on-board the instrument. Notably, the transition from ocean to land for PRN 21, demonstrates the delay tracking performance with and without a land height correction.