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To define relative spectral response, I referred to EFFECT OF RELATIVE SPECTRAL RESPONSE ON MULTI-SPECTRAL MEASUREMENTS AND NDVI FROM DIFFERENT REMOTE SENSING SYSTEMS by David James Fleming, 2006.

One factor that is often overlooked is the effect of a sensor’s relative spectral response (RSR), or spectral response function (SRF), on broadband spectral measurements. The RSR describes the quantum efficiency of a sensor at specific wavelengths over the range of a spectral band. Currently, general descriptors, such as bandwidth and average bandpass, are often the only spectral characteristics considered in analysis of sensor spectral measurements. However, cross-sensor wavelength variations in RSR can lead to measurement discrepancies between sensor measurements that make them not directly comparable (Teillet et al., 1997). In order to provide consistent quantitative spectral measurements of vegetation land cover and derived metrics, such as spectral vegetation indices, the effect of a sensor’s SRF must be considered and understood.

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Spectral bands are often generalized (Pagnutti et al., 2003) in terms of full width at half maximum bandwidth and central wavelength corresponding to the maximum value of the response function (Liang, 2004) as shown in Figure 11.

enter image description here

In view of this, I was a little bit surprise that, for example the bandwidth of MODIS band 7 (dotted line) as defined here, seems to do not match with the relative spectral response in the way defined above. enter image description here

Is there something that I did not correctly understand in those concepts?

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I don't follow--where does the red line in your second image come from? –  dmahr Oct 9 '13 at 13:28
    
From here mcst.gsfc.nasa.gov/calibration/parameters but with the shut down, I think that the server is down. –  DJack Oct 10 '13 at 9:07
    
The server is up now. There are discrepancies across the entire spectrum, not just in band 7. The raw files ask you to contact Jack Xiong <Xiaoxiong.Xiong.1@gsfc.nasa.gov> or Vincent Chiang <vincent_chiang@ssaihq.com> for more information. –  whuber Feb 25 at 21:33
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@whuber I contacted them the 27/01/14 but the mail of Vincent Chiang does not exist anymore and I got no answer from Xiaoxiong. I did not push further the investigation. I ll try to find an updated mail for M. Chiang. –  DJack Feb 27 at 8:42
    
I have just sent a new mail to <vincent.chiang@sigmaspace.com> (seems to be his current mail). –  DJack Feb 27 at 9:52

2 Answers 2

up vote 3 down vote accepted
+100

So MODIS is a complex instrument and it has been a while since I worked with the data. So bear that in mind.

If we look at band 1, the data is pretty good:

enter image description here

But as you point out with band 7 the data does not line up. I had to check it myself because I was certain it had to do with the different channels per band (which is why the raw RSR file have from 20-40 RSR responses) but that was not the case.

But if we look at the other file provided by NASA: ftp://mcst.hbsss-sigma.com/pub/permanent/MCST/PFM_L1B_LUT_4-30-99/L1B_RSR_LUT/pfm-in-band-rsr.pdf

And specifically at band 7. We see that the actual RAW data (the bottom graph on each page) does line up with the values provided on: http://modis.gsfc.nasa.gov/about/specifications.php

It is just the processed in band response which does not line up.

That means that the basic values provided by NASA for band 7 (7 2105 - 2155 1.0 110) are incorrect (by about 50 nm), but because they have released the RSR data we can use that to determine the real wavelength for each band. Although does that really matter? No unless you are comparing multiple sensors to each other where the wavelength and not the band is crucial, but at that point you should be using the RSR data anyway.

But interesting question.

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Thx for your answer. Actually for my analysis it is of critical importance because I am interested in a particular absorption trough located at 2100 nm, what is almost at the top of the RSR curve. I would have thought that the instrument "design" specs would be updated with the measurements done in the lab. I am now aware that I have to use the RSR systematically in the future. Can you precise what is the difference between the top and bottom of pfm-in-band-rsr.pdf that you mentioned in your answer because I am not sure I ve got it? Smthg related to the channels ? –  DJack Mar 4 at 7:51
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You should be using the RSR data. As we can see from the .doc file in: mcst.hbsss-sigma.com/pub/permanent/MCST/PFM_L1B_LUT_4-30-99 The results in the PDF relate to the RAW RSR and the the corrected RSR. So they seem to have done a number of corrections to the original RSR data to achieve a more accurate result (atmospheric correction, Along Track Wavelength Aberration Correction, and Grating Correction, see the .ppt file). –  Vesanto Mar 4 at 8:50
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Relating to channels, if you look in the mcst.hbsss-sigma.com/pub/permanent/MCST/PFM_L1B_LUT_4-30-99/… file. You can see that this is the RSR response of a single band, band 1. But it is made up of 40 channels. I'm a bit rusty as to why it was split into channels. But to get the actual RSR for the band, the different channels need to be averaged. I did this about 4 years ago during my Masters, but have lost the files. –  Vesanto Mar 4 at 8:57

Answer given by Kwofu (Vincent) Chiang, one of the named contact in the raw files :

If I understand your question correctly, you are wondering why MODIS spectral specification bandwidths listed on NASA web site are different than those computed from the RSR provided at the FTP sites. The MODIS specifications page on NASA web site provides the instrument "design" specifications in general; considered it as defined specs to build the sensor. The actual spectral responses of each instrument (there are two MODIS -- PFM on Terra and FM1 on Aqua) at its sensor level were measured carefully in the lab and their detector(channel)-dependent RSRs for each band are listed at MCST's FTP sites. You should use the actual RSRs for your analysis.

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