Gestión del proceso de influencia

Figure 2.14 shows an example of DDMs from four different UK-DMC datasets. All the maps are normalized with respect to their maximum value. The DDMs have been

(a) (b)

(c) (d)

Figure 2.14: Normalized DDMs obtained from the four UK-DMC datasets (R21, R12, R20, R29).

obtained using a fixed 1 ms coherent integration time, and a 1 s accumulation time. The DD resolution of the maps is 0.18 chip and 100 Hz, and the delay and Doppler axes are expressed relative to the delay-Doppler value at the SP. A visual comparison between the maps reveals that most of the power is confined within the horseshoe shape, and around

Chapter 2 Fundamentals of GPS-Reflectometry 29 the specular point (zero delay/zero Doppler), for the calm sea case (R21), whereas for the rougher sea case there is more scattered power both between the branches of the horseshoe shape, and along them, for larger delays and Doppler values (R12, R20, R29).

The analysis is here restricted to normalized maps, meaning that we cannot exploit any magnitude information, but only the shape of the DDMs. Such limitation is due to the lack of a calibration of the incident GPS signal, as well as unrecorded effects within the receiver (amplifiers, automatic gain control), affecting the amplitude of the received signals. This actually limits also our ability to discriminate different sea states, as the decrease in scattered power values at the SP for rougher sea is neglected when both the maps are normalized. We now focus on DDMs from the R21 and R12 datasets, which are the two datasets with the strongest difference in sea state. Co-located buoy wind and wave measurements indicate a condition of low wind and waves for R21, and medium wind and waves for R12. The wind speed and Significant Wave Height (SWH) registered at the time R21 was collected were respectively 4.5 m/s and 1.98 m, whereas a wind speed of 8.3 m/s and SWH of 2.8 m were registered at the time R12 was collected (see table 2.1). Unfortunately, datasets corresponding to more different sea states (i.e.

low wind and waves vs high wind and waves) were not available at the time the GNSS-R processing was performed to obtain DDMs. Nevertheless, the relative DDMs of these two datasets already show some interesting differences. The general behaviour of the DDMs for calm and rougher sea can be predicted based on our knowledge of the scattering for the two cases. A condition of relatively calm sea corresponds to a scattering closer to a quasi-specular regime because the surface is almost flat, and the DDM would be characterized by a strong and sharp peak at the Specular Point (SP), and a rapidly dropping scattered power for points away from the SP. Instead, a certain degree of roughness of the sea surface causes the amount of power scattered from the SP to be lower, and more power being scattered from other points of the GZ, far from the SP. The described power distribution for calmer and rougher sea can be seen in the contour plots of the two DDMs, in figure 2.15. A more pronounced skewness for the calm sea DDM can be also noticed in both figure 2.14(a) and 2.15(b), and when such skewness occurs it is usually due to the antenna pattern. The antenna gain is usually not symmetric over the GZ, and in this particular case the area of the GZ corresponding to positive Doppler

(a) (b)

Figure 2.15: Contour plots of two of the normalized DDMs shown in figure 2.14.

shifts was characterized by a larger antenna gain than the rest of the GZ, causing an asymmetry of the power distribution of the DDM. The effect of the antenna pattern over a DDM will be explored in chapter 3. Figures 2.16 and 2.17 show slices of the R21 and

Figure 2.16: Delay waveforms obtained as vertical slices (along the delay) of the DDMs for R21 (top) and R12 (bottom) datasets, corresponding a calmer and rougher

sea state respectively.

R12 DDMs through the delays and Dopplers respectively, where lines of the same colour indicate a slice of the DDM at the same delay or Doppler frequency. Both the delay and the Doppler waveforms for R21/calm sea case show a more rapid drop-off with respect to the corresponding ones of the R12/rougher sea case, thus confirming once again that the scattered power along the horseshoe shape drops faster in calm sea conditions than rougher seas. The skewness of the R21 DDM translates into a stronger asymmetry of the Doppler waveforms with respect to the R12 case. Finally, a closer look at the DWs

Chapter 2 Fundamentals of GPS-Reflectometry 31

Figure 2.17: Doppler waveforms obtained as horizontal slices (along the Doppler) of the DDMs for R21 (left) and R12 (right) datasets, corresponding a calmer and rougher

sea state respectively.

at the SP, along with the DWs integrated over all the Doppler values has been carried out for the two DDMs, and they are illustrated in figure 2.18. The DW at the specular point for the R21 case (blue) in figure 2.18a shows a narrower peak with respect to the R12 case (red), as expected from a quasi-specular scattering regime due to a calm sea state. A more rapidly dropping tail of the R21 DWs can also be noticed, although this effect is much stronger when the integrated DWs in figure 2.18b are considered. The integrated DW for R12/rougher sea case shows an interesting shift in the peak towards larger delays, and the reason for that still needs to be investigated.