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SGAT for Sensitivity Analysis of Twilight Error

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Can anybody recommend a "how to" or sample code for using SGAT to describe twilight error, specifically twilight error and its effect on the calculation of an appropriate sun elevation angle?  GeoLight's 'getElevation( )' function mentions that SGAT can be used to sensitivity analysis of twilight error.  But, I haven't found any documentation for SGAT that is approachable for a newbie like myself.

 

I'm working with tags that were deployed on Mountain Bluebirds and I'm seeing a slight eastward bias in the coordinates calculated by GeoLight, so I'm wondering how much of this bias is due to twilight error or a poor estimate of the sun elevation angle.  Where else in the GeoLight workflow could this bias be creeping in?

 

Thanks.

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One of the the things SGAT does is essentially take an estimate of twilight error (derived from calibration data) and translate it into location error for the entirety of a tag deployment. So if the same bias you mention is in your calibration data, SGAT might help eliminate it.

 

If the point estimates move ever farther eastward over the course of the deployment period, there may be a clock drift issue. You could also have a "mountainside effect" where one twilight experiences shading but not the other.

 

I suggest you look closely at your calibration data and plot the difference between known and estimated twilights. Do this separately for sunrise and sunset and for before and after calibration periods (if you have both).

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Eli, thanks for the tips.  

 

Can you possibly elaborate on how to estimate and plot the twilight error?  I do not have a set calibration period, but rather a stationary breeding period (2-3mos.) for which the BBs are known to have stayed in the same area.  Would I plot the known vs. estimated twilights from this period, then extrapolate that to the rest of the data?  Does SGAT provide known twilights for some areas and dates, or can this info be pulled from something like the NOAA page?

 

Much appreciated.  

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