Attempt to Calibrate MR

Now that we have the georeferencing sorted out, we can start plauyin a bit with the data to make some comparisons. There was a bit of trouble due to the radars being non concentrical. Therefore, only the line of sight at 46 degrees was imaged as a single ray by the two radars. In order to overcome this, I calculated the x-y locations of all the points imaged by RiverRad, and converted them to polar coordinates relative to the Marine Radar and extract them from the Marine Radar file. This introduces a certain error due to the different acquisition time of the points, but it is in general small (I am working on the numbers).

Once that is being done, I tried to convert the intensity signal from the Marine Radar to normalized cross sections, in a similar way to Gommenginger et al (2000). The transformation is of the form

sigma_0 (dB) =-C1*I+ 10 log(R^3) + C2

where C2 accounts for the radar characteristics (gain, pattern, beam width, wavelength, etc) and the R^3 accounts for range fall off and the target area. In the following I am using values I determined by visual inspection only (C1=0.2, C2=20 dB)

Once this is done, it is possible to compare the normalized cross sections at several look angles, for example:









where it can be seen for instance that:

• River Rad is more sensitive offshore. This was expected since we reduced the offset of the acquisition in the Marine Radar to avoid saturation of breaking events. We knew that the trade off of that decision would be that we would see less of the waves offshore.
• The pier has a larger effect on the MR data, where is casts a shadow zone from which only strong events are visible.
• Breaking events tend to be brighter in the Marine Radar than in RR. in fact, it seems that the RiverRad is saturating at about -dB (something we noticed in Duck already). However, the difference in values could be attributed to a poor choice of constants C1 and C2. Probably C1 has a larger influence on this.

  All times are epoch times of each sample, relative to a common reference time, so the data is synchronized. Distances are Ground Range relative to the MR.

Another way to compare the time series is to play a movie of the range evolution of the waves, as shown here (movie is 2.6 Mb)




The upper panel is the grayscale intensity from MR, and the lower panel shows the signal from RR (red) and the Marine Rad (cyan and blue). The blue line shows only the points were MR is larger than 10. Notice that there it seems to be a phase drift between the signals, which is simply due to the fact that I am comparing samples that are not exactly synchronous. Doing so would require to interpolate one signal into the other's time domain.

This interpolation was done to compare how good is the fit between the measured (RR) and the dB-converted, time-interpolated MR signal, considering the data collected during a full swath of the River Rad (about 90deg scan). The colorcode represents different RR runs (1818 to 1824). I am working on trying to quantify the agreement, to validate the conversion from Intensity to dB, or in other words, the calibration of the marine radar. This analysis includes some undesired effects such as

• the presence of the pier, that  is large values of MR whereas in RR are low because they were removed as non moving signals. This would explain data concentrated near the upper left corner 
  

• Areas where MR falls too low (near the sensitivity level) whilst RR has relatively large values. (lower right corner)
  

It seems, however, that I am going to take a look at my phase functions for a while.


Aditionally

Back to posting: Radar Overlays

I've been playing with the code I have from Bill, and also with the few data sets we brought back from Duck. I've been able to automate (well, sort of) an algorithm to identify which runs are synchronous. To do that I have converted the GPS times from RiverRad and the Marine radar to epoch times, to make them compatible with the ARGUS. Book keeping should be easier this way. That also enables me to figure out when the radar changed directions. By doing this I was able toidentify the slow scan used to estimate the zero azimuth of RiverRad, by targeting the pier. And obviously I went ahead and tried to find the angle.

So here is an overlay of synchronous data from both sensor, with the pier almost perfectly aligned. I found that a rotation pf 37.5 degrees was the best for RiverRad. I used the GCP coordinates put both radars in FRF space. Data from RiverRad uses horizontal range, our data currently uses only slant range. I think agreement is pretty good, but I am curious about the curvature shown by RiverRad.



Now I am going to try to see if I can overlap timestacks for the same (or at least similar) circular sectors.