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Empirical Answer Considered!

(see the end of the question for details)


Iridium satellite flares are a class of Satellite Flares that originate from satellites in the current Iridium constellation. Unfortunately, according to this helpful answer and as foretold by this one (though schedules have slipped) they are going away relatively soon.

Reflections come from fairly flat antenna panels on the satellites. The attitude of the spacecraft and especially the antennas are carefully controlled, and so the path of the footprint of the reflected light can be predicted with surprisingly good precision.

The web site Heavens Above will give you predicted visible flares if you give it your specific location. It estimates the brightness based on the your position relative to the position of the maximum brightness point of the flare. You can see it if you are tens of kilometers away, but it will be maximum if you are within only kilometers. Heavens Above has been around quite a while and I believe there are other prediction sites and services.

The simulation:

There will be an ephemeris for the position of the sun, satellite, Earth, and the topographic location that you specify.

There will also be an ephemeris or algorithm that handles the attitude of the spacecraft and/or the orientation of the antenna panels themselves.

However:

There has to be some kind of model for the shape of the illumination footprint from the panel in order for the prediction to estimate the brightness based on distance from the maximum.

Is there anything written about this? Is it just a fuzzy blob based on empirical or anecdotal reports, built up over time the way meteor counters build up data used to model debris trails in space? Or did someone actually take a specimen of an antenna panel outdoors and just do the experiment - hold it at different angles wrt the sun and an observer, and measure the brightness?

I'm wondering:

What goes into an Iridium Flare prediction model besides the ephemerides?

below "View of one of the Main Mission Antenna. The hinged base is on the right side." Cropped. From here.

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below cropped (and enlarged) image of a first generation Iridium satellite, showing a bumpy and diffusely reflecting metal antenna panel of the type responsible for producing the flares seen on Earth. From here.

enter image description here

enter image description here


Empirical Answer Considered:

One could conceivably just query a flare simulator a bunch of times, stepping by 1 or 2 km along an East-West line to build up a plot of the cross-track brightness as a function of lateral distance from the track.

Curious if it is different for each satellite, or each of the antenna panels, or depends on sun angle, or if its just a fixed width and the same every time.

A good one of those would definitely be worth +100!

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  • $\begingroup$ Predicting brightness seems to be the most difficult part. The various prediction websites and apps do not always agree on the magnitudes of the flares. And what you see in the sky does not always match the predicted brightness. Time predictions are very accurate, though. $\endgroup$ – eerie Apr 21 '17 at 19:25
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    $\begingroup$ @eerie that's good to know! Do you know if anybody has made some kind of systematic report of brightnesses of sightings? I know there would be no real science in that (compared to comet or meteor shower watchers who collect their observations on-line) but it would be fun to try. If the predictions differ, then they are all using different models. I wonder if any of them state publicly what they use. $\endgroup$ – uhoh Apr 21 '17 at 23:48
  • $\begingroup$ Unfortunately I am not aware of anything like that. Would be interesting though. $\endgroup$ – eerie Apr 22 '17 at 17:44
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    $\begingroup$ @JamesThorpe Thank you for catching that! There is some behavior I don't understand in the UI that occasionally eats my links during editing, I think when you use the imgur utility to import an image, unused links get eaten. Anyway, I've linked both what I'd originally intended, plus the one you've mentioned as well. $\endgroup$ – uhoh Apr 26 '17 at 13:30
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    $\begingroup$ @uhoh I have to admit, when I'm working with links I tend to use the [description](url) format due to laziness, it always seems (at least to me) to be more work to use footnotes! $\endgroup$ – James Thorpe Apr 26 '17 at 13:34
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I just happened to come across the answer to one part of your question - the footprint: There is an appendix on Iridium flares in Durrell Hillis' book "Creating Iridium":

"Flare track on Earth's surface is very narrow:

  • Solar "image" elliptical spot approximately 5 miles (N-S) by 9 miles (E-W)
  • Travels along roughly longitude lines at speed of space vehicle
  • Reflections make space vehicle visble for 5 to 20 seconds"
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    $\begingroup$ Excellent! That is certainly something that can be compared to various predictions, at least the E-W variation. Now I want to set up an East-West 1D array of weatherproof Raspberry Pi cameras and just let it run for a few months. Every 2km for 50km at least. GoPros if I was rich. Location could be anywhere, a place with clear skies would be ideal - the blue areas in the plot in this answer perhaps. $\endgroup$ – uhoh Apr 23 '17 at 1:00
  • $\begingroup$ I'm going to try to find a copy of that book, thanks for the reference. $\endgroup$ – uhoh Apr 23 '17 at 1:01
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    $\begingroup$ Wow, that's interesting. Pls keep us updated! $\endgroup$ – eerie Apr 23 '17 at 19:54
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    $\begingroup$ You might find it hard to get it outside the US (no overseas shipment, at least not to Europe). But there is another book I can highly recommend: Eccentric Orbits - The Iridium Story by John Bloom. $\endgroup$ – eerie Apr 23 '17 at 20:03

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