| Re: Calculate sensor response functions [message #69537 is a reply to message #69535] |
Fri, 22 January 2010 08:25   |
Christian Haselwimmer
Messages: 3
Registered: February 2009
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Junior Member |
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Paulv,
thanks for your detailed response. I am aware of the Hyperion spectral
response curves at the link you mentioned but for some irritating
reason the zip files appear to be corrupt, which is why I am going
down this route. I would ideally want to use the measured Hyperion
SRFs but I haven't been able to find these on the web other than the
from the link above.
I have seen in a number of papers that the SRFs for Hyperion have been
modelled using gaussian curves based upon central wavelength and FWHM;
this is all I really want to do but do not have sufficient experience
of IDL to implement this, hence my post.
cheers,
Christian
On 22 Jan, 15:56, Paul van Delst <Paul.vanDe...@noaa.gov> wrote:
> 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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