| Re: Calculate sensor response functions [message #69539 is a reply to message #69537] |
Fri, 22 January 2010 07:56   |
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Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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Crikey. Alternatively you can google "Hyperion spectral response" and you'll find this page:
http://www.eoc.csiro.au/hswww/oz_pi/specresp.htm
that contains the spectral response data for Hyperion (and ALI).
A much simpler approach.
:o)
cheers,
paulv
Paul van Delst wrote:
> Christian Haselwimmer wrote:
>> Hi,
>> I want to calculate sensor response functions (for E0-1 Hyperion)
>> based upon centre wavelength and bandwidth but am struggling to figure
>> out the best way to do this with IDL. I was wondering if anybody could
>> provide some advice or possibly existing code to do this.
>
> Well, sensor response functions are typically measured, not calculated, since the sensor
> response depends on a whole bunch of things (detector response, foreoptics transmission,
> the emissivities/reflectivities of the optical surfaces, etc.).
>
> Without the measured SRFs or those modeled by the instrument builder, a really simple
> approach is to use some sort of modified Gaussian curve:
>
> SRF = EXP( -a * |x - x0|^n )
>
> where a is a multiplier determined from the half power (HWHM) points,
>
> a = -1 * LN( 0.5 )
> ----------------
> n
> HWHM
>
> and x is the frequency in cm-1
> x0 is the central frequency in cm-1, and
> n is the exponent supplied by the user.
>
> Or, if you want a longer tail to your SRFs, you can use a modified Lorentzian shape:
>
> 1 HWHM
> SRF = ----- . -----------------------
> !PI ( x - x0 )^n + HWHM^n
>
> Or some combination of the two.
>
> But either way, the SRFs would be completely made up. Comparing calculations using made-up
> SRFs and actual instrument radiances will be biased because of that.
>
> Hyperion is a grating spectrometer, so you may be able to come up with a better instrument
> model based on the actual instrument specs, but you'd need to know a lot more info (I
> think). As an example, Lockheed-Martin provided the following SRF model for the EOS Aqua
> AIRS instrument (also a grating spectrometer, but ~3-15um) back in the 90's:
>
> SRF(x-x0) = exp( -a * (x-x0)^2 ) + $
> ( b * ( 1 - exp( -a * (x-x0)^2 ) ) * ( d + |x-x0| )^c )
>
> where a, b, c, and d are the SRF coefficients based on their analysis of the AIRS optical
> model.
>
> The above SRF model was great for testing, but we still needed the actual measured
> responses for modeling the instrument when it was launched.
>
> Maybe you should contact the NASA/GSFC and/or USGS instrument scientists to determine
> where that data is available?
>
> Anyway....
>
> cheers,
>
> paulv
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