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ISS predictions and variance over time

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#1 Mike Phillips

Mike Phillips

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Posted 05 March 2021 - 11:38 AM

I'm not sure this 100% belongs here, but am wondering about the accuracy of - https://transit-finder.com/results

 

I used to use calsky before it shutdown but really have yet to observe a transit of the ISS over the moon or sun.  I noticed it predicted a lunar transit for me some 3-4 weeks in the future, then it went away.  Is there some reason it would change position or forecast accuracy?  I recall the ISS gets a boost periodically to stabilize it's ever declining orbit, but am unsure if this would impact the predictions or not.

TIA,

 

Mike



#2 WarmWeatherGuy

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Posted 05 March 2021 - 12:11 PM

The boost will definitely change the prediction. Just the air drag will make predictions hard for a few weeks into the future. The typical error would be the timing but because the Earth is rotating you are not in the same place as originally predicted. Your eastward speed is around 1000 m.p.h. times the cosine of your latitude.


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#3 Tom Glenn

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Posted 05 March 2021 - 01:07 PM

Transit predictions are not as simple as many assume.  Low Earth orbits are unstable, and even in the absence of an orbital altitude boost, the predictions are associated with error.  The prediction is based upon a simplified ballistics calculation using a model called the SGP4 Propagator, which uses orbital element data that is supplied by NASA and NORAD, called two line elements (TLEs).  These are published every few hours for the ISS, and the prediction will change with each new set of data.  Predictions more than one week out from the event should be viewed with skepticism, and will often shift by significant amounts, enough to cause a local event to disappear.  This is especially true if an orbital boost event occurred during the intervening period.  It's best to continually check the prediction, and only set up your equipment based upon data that was published in the 24 hours preceding the transit.  And even in these cases, you will often notice an error of several hundred meters along the ground, up to 1km, depending on how stringent your target window is.  Some of this information is included in a few posts I've made in the past year.  

 

https://www.cloudyni...tember-14-2020/

https://www.cloudyni...vember-19-2020/

https://www.cloudyni...g-presentation/


Edited by Tom Glenn, 05 March 2021 - 01:25 PM.

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#4 Mike Phillips

Mike Phillips

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Posted 05 March 2021 - 01:39 PM

Wow, thanks guys!!!




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