Support: 26/June/2008, Chris Hecker, WM06/06 + ET07/05

[mggr]

Chris contacted us with a lot of complex questions. Going to link full emails here rather than precis.

Hi Gary
I am currently working on the Eagle / Hawk data from the 2006 Campaign to Mojacar, Spain (WM06-06, PI: Teeuw, 2 June 2006), also as a testcase for the Ethiopia data.
I am using ATCOR-4 for airborne data for a model based atmospheric correction. while doing so I noticed artifacts that give troubles. I remember that the first year's data gave lots of headaches and was wondering if you knew about the symptoms I am getting and if you may have a workaround / solution for them. I put some screendumps in a *.ppt file for you.
 
Sensor Saturation
In some of the brighter areas of flightline 13, I notice some strange behaviour (it may be in other places as well, but I first noticed it in flightline 13, since our calibration sites are in that line). The Eagle spectra look OK as of about 0.7 micron. wavelengths shorter than that show a strong trough in their reflectance spectra. Slide 1 shows the spectra we recorded on the ground (in gree) and the Eagle spectra (in yellow and orange). It looks like the detector saturated and the data wrapped around when it reached the end of the ?sensitivit or long integer range. I checked the hdf file but couldnt really find any indication on which pixels had saturated during acquisition.
 
Here some questions:
- Was sensor saturation a problem that year?
- Can we get a mask that shows pixels that reached the saturation level (or close to saturation level) so we can mask them out. Or alternatively, the original DN values before they were changed to scaled radiance values.
 
Wavelength calilbration
The eagle and hawk hdf files come with central wavelength and width of each band. When I use that for the atmospheric correction, I get clear artifacts that show that there is a wavelength accuracy problem. slide2 shows a hawk spectrum with derivative looking artifact at 1.14 and also a general shift of the image spectrum towards longer wavelengths as compared to fieldspectra (green; for example alunite spectral feature at 2.2 looks shifted).
- are there any spectral response curves for individual detectors?
- is anything known about the shape of the response curves?
- can we assume that the widths of bands as listed in the hdf file is a fwhm for a gaussian shaped response curve (as an approximation)
- is there a newer wavelength calibration file than the one that was used then?
 
Spikes or steps
spectra often show a step around 0.688 microns and some spikes / noise at 0.860 (see slide 1).
- Is that related to an internal change to a different detector array?
- anything that can be done to correct for it?
 
 
Radiometric accuracy
radiance values in general look a bit lower than ground spectra with appropriate looking atmospheric correction. That's more an observation than a problem, since we can correct for that easily.
 
Actually, if I look at the figure 2c in the data_quality-2007.pdf , it looks to me like there is not only a radiometric shift between Eagle and Hawk but also a 50 nm wavelength shift. Sounds like a lot though, so not sure if that is possible.
 
Greets from Holland,
Chris