Research into Salar de Uyuni Reflectance Values October 9, 2014

Time series with updated Uyuni calibrations With Day Reflectance updates

Our work: Reflectance Averages over Source Zenith Zenith Angle 2012 day 2013 day 2014 day 2012 night 2013 night 2014 night 10-15 62.2 70.3 15-20 66.3 60.3 71.1 75.4 20-25 69.8 62.9 73.3 71.6 25-30 70.5 62.4 54.6 73.0 62.1 30-35 69.1 60.0 74.8 71.0 57.1 35-40 68.2 65.0 60.1 69.2 40-45 69.4 65.4 61.0 68.8 66.4 45-50 69.7 66.0 60.9 72.1 70.1 64.79 50-55 67.5 60.5 65.5 55-60 59.0 59.9 68.6 67.6 This is to give you an idea of what values we’re getting. Night values use new Uyuni Calibration. Day uses updated RF for 2013 and 2014.

On-line References PDFs on ftp\Combs\Miller\Research Laparelli,R.A.C., F.J. Ponzoni, J. Zullo,Jr, G.Q. Pellegrino, and Y. Arnaud, 2003: Characterization of the Salar de Uyuni for In-orbit Satellite Calibration, IEEE Trans. Geosci.Remote Sens. 41(6): 1461-1468. Bouvet, Marc, 2006: Intercomparsion of imaging spectrometers over the Salar De Uyuni (Bolivia), Proceedings of the Second Working Meeting on MERIS and AATSR Calibration and Geophysical Validation(MAVT-2006) ESRIN, Frascati, Italy - I have both a paper and slides. Both these references are for studies over Uyuni. In fact, they both look at the area approximately where our location B is. (We’ve used location D which is on the western ‘arm’ of the salt flats)

SPECTRAL BANDS OF CE313-2/CIMEL RADIOMETER The Lamarelli et al paper The Lamarelli paper (SdU.pdf) basically looks at whether Salar de Uyuni is a good test site by checking the Bidirectional reflectance factor (BRF) using two portable radiometers with five spectral bands similar to the Landsat Thematic Mapper. It looks like B1 and B2 (TM4 and TM3) are the closest to DNB (see Table V). TABLE V SPECTRAL BANDS OF CE313-2/CIMEL RADIOMETER

I think the part most relevant to our work is Table IX I think the part most relevant to our work is Table IX. While it is BRF and if I’m reading the table right, the average values for B1 is 76.09,72.6, and 68.47, and for B2 75.6,70.83, and 68.96. Compared to our daytime tables, the 2012 day values are within the low end of the two bands, while 2013/2014 are lower. TABLE IX STATISTICAL PARAMETERS (IN PERCENT) RELATED TO THE EXPERIMENTAL DATA COLLECTED AROUND EACH TEST POINT

The Bouvet paper I believe this paper is probably the closest to our work. He is comparing TOA reflectance from five satellites over Uyuni for intercalibration purposes. I believe the two items important to us are Figures 4 and Table 3. Figure 4 (next slide) is a comparison at 865nm for the various satellites. While I’m not sure what’s happening at last 300 days (I couldn’t find an explanation in the text or in the slide show), the first 900 days looks very similar to our time series for 2012. Since DNB is at 700 nm, I suspect ours to be slightly lower, which it is. However, none of the points drop below 60 like our time series for 2013/2014 Note: his conclusions are that SeaWifs(the green dots) reflectance tends to be 10% lower than the rest. Which makes our results look even lower.

Figure 4:The time series of TOA reflectances at 865 nm from the A-MODIS (blue cross), AATSR (orange triangles), MERIS (red diamonds), POLDER-3 (purple crosses) and SeaWiFS (green stars) over the Salar de Uyuni.

In the same paper, Bouvet also simulates the differences at the TOA reflectance level between sensors by generating a synthetic spectrum using MODTRAN. The results are in Table 3 below Band MODIS AATSR MERIS POLDER SeaWiFS 412 nm 0.7550 0.7556 442 nm 0.7655 0.7658 0.7659 490 nm 0.7673 0.7662 510 nm 0.7567 0.7574 560 nm 0.7287 0.7253 0.7241 0.7184 0.7281 670 nm 0.7549 0.7622 0.7664 0.7643 860 nm 0.7951 0.7972 0.7980 0.7976 0.7948 Table 3: band average reflectance simulated with the instruments Relative Spectral Responses and a MODTRAN simulation for a nadir looking sensor observing a surface with a spectrally constant reflectance of 0.8, located at 3600 meters of altitude, beneath a US 1976 standard atmosphere, in aerosol free conditions, with a sun zenith angle of 50 degrees For 50-55 degrees, our results were 2012 had an average value of 67.5, 2013 = 60.5, and 2014=60.5. These are significantly lower than above.

Conclusions While neither of the above studies are exact matches for what we’re doing, I do believe they indicate that our day results are lower than they should be, especially for 2013/2014.