Author Topic: Satellite signals intelligence - recent  (Read 9844 times)

Offline zubenelgenubi

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Re: Satellite signals intelligence - recent
« Reply #40 on: 02/14/2018 08:15 PM »
I never believed the 100-meter diameter estimates. They seemed to be extrapolations based upon assumed manufacturing capabilities, but they were never attached to an assessment of technical requirements. Simply put: did any antenna need to be that big? Nobody answered that.

Question re-post:
Has anyone ever used the observed magnitudes of Orion satellites to estimate the diameter of the primary dish?

I've read that they are a few magnitudes brighter than "typical" GEO satellites: typically +8.
« Last Edit: 02/14/2018 08:17 PM by zubenelgenubi »
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Offline gosnold

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Re: Satellite signals intelligence - recent
« Reply #41 on: 02/14/2018 08:37 PM »
You could only estimate the diameter if you knew the material and the rough pointing of the reflector. That's a lot of assumptions.

Offline zubenelgenubi

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Re: Satellite signals intelligence - recent
« Reply #42 on: 02/14/2018 09:19 PM »
You could only estimate the diameter if you knew the material and the rough pointing of the reflector. That's a lot of assumptions.

True.

What if the observer is near or on the same longitude as the satellite, and takes magnitude estimates over a period of hours?  If you observe a consistent brightest magnitude, could one assume that the main dish is face-on to the observer at the time of maximum brightness?  It's still calculations based on an assumption of the satellite's attitude with respect to the observer, but it could at least provide a lower limit on dish diameter?

Has anyone in the amateur astronomer/satellite observer community ever tried to determine the reflective albedos of likely dish surface materials?

Could magnitude observations of commercial geosynchronous communications satellites with large, deployed dishes of known diameter and orientation with respect to the observer, then be reliably extrapolated to estimate the diameter of an unknown, presumably larger dish?

Another, more exotic idea: Observe a stellar occultation.  Determining the occultation chord length would place a lower bound on the dish diameter.

Well, just some thinking aloud, thinking like an observational astronomer.  I wonder if anyone has ever tried anything like this.
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Offline hoku

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Re: Satellite signals intelligence - recent
« Reply #43 on: 02/15/2018 12:24 PM »
You could only estimate the diameter if you knew the material and the rough pointing of the reflector. That's a lot of assumptions.
...
Another, more exotic idea: Observe a stellar occultation.  Determining the occultation chord length would place a lower bound on the dish diameter.
...
Ok, I'll "bite". From hearsay etc., we "know" that satellites with large antennas have been constructed/launched for the past 20+ years, but there is no official indication on the antenna diameters (i.e. if they measure several 10s of meters, or the alleged 100 m).

Geostationary satellites move at 15 arcsec/second against the stellar background (360 degree in 24 h). At 36 000 km distance, 100 m corresponds to an angle of 0.6 arcsec. Thus a stellar occultation by a 100m diameter antenna in GSO will last about 40 milli-seconds, hence you need recording equipment with a time resolution of the order of 10ms (frame rate 100Hz).

There are relatively inexpensive CMOS cameras available, which are suitable for night time observing. Still, a decent size telescope would be required to detect a (faint) star at sufficient signal-to-noise in individual 10ms exposures to get a good light curve of such an occultation event. You'd probably want to go down to stars with visual magnitudes around 12 in the visual, and use a telescope with at least 15(?) inch diameter to have a decent chance of detecting one event in the course of an observing night.

If you know where to look in the sky, it might be worth a try - but also be prepared to store and process a couple of terabyte of data  8)

Offline gosnold

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Re: Satellite signals intelligence - recent
« Reply #44 on: 02/15/2018 06:31 PM »

Ok, I'll "bite". From hearsay etc., we "know" that satellites with large antennas have been constructed/launched for the past 20+ years, but there is no official indication on the antenna diameters (i.e. if they measure several 10s of meters, or the alleged 100 m).


Interesting. If you can beat the resolution limit, and if the satellite attitude is not fixed relative to its velocity vector, you can get different profiles. A reconstruction like those used in medical CT scanners could then be used to reconstruct an image. I can't find any literature on that though.

Offline zubenelgenubi

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Re: Satellite signals intelligence - recent
« Reply #45 on: 02/15/2018 09:28 PM »
re: timing accuracy/data capture
Some of the amateur asteroid occultation chasers have (IMHO) impressive gear, and it's mobile.
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Offline satwatcher

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Re: Satellite signals intelligence - recent
« Reply #46 on: 02/19/2018 08:40 AM »
There are relatively inexpensive CMOS cameras available, which are suitable for night time observing. Still, a decent size telescope would be required to detect a (faint) star at sufficient signal-to-noise in individual 10ms exposures to get a good light curve of such an occultation event. You'd probably want to go down to stars with visual magnitudes around 12 in the visual, and use a telescope with at least 15(?) inch diameter to have a decent chance of detecting one event in the course of an observing night.

Good analysis! Note though that you'd want the occultation to occur when the satellite is in the Earth's shadow, as otherwise the 8 mag satellite will outshine a 12th mag star by about a factor of 40.

Another option to study the size would be during crossings of the satellite across the solar or lunar disk. You then are looking at its shadow, and with lucky imaging you may be able to use the frames with the best seeing.

Offline gosnold

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Re: Satellite signals intelligence - recent
« Reply #47 on: 03/25/2018 04:30 PM »
Good analysis! Note though that you'd want the occultation to occur when the satellite is in the Earth's shadow, as otherwise the 8 mag satellite will outshine a 12th mag star by about a factor of 40.

I found some articles about shadow imaging of GEO satellites:
https://amostech.com/TechnicalPapers/2017/Poster/Douglas.pdf
http://sciencedocbox.com/Space_and_Astronomy/67404496-Shadow-imaging-of-geosynchronous-satellites.html

Turns out apparently you can beat the resolution limit and get details that would otherwise require a 100m telescope.

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