OBSERVATIONS FROM SCOTLAND & NORWAY       24 October 2007                   GM1SXX & LA2QAA

Dedicated to the memory of John Branegan GM4IHJ SK.

The 'FuzzyGreyWedge' and Earth Satellites.

Greyline propagation is a well-exploited means of communications, especially among radio amateurs.    The term 'Greyline' suggests a narrow line along which propagation can occur on the day-night boundary or to give it a somewhat more 'official' name, the Greyline Terminator. Radio hams like to use Greyline while Astronomers prefer terminator.  'Big Arnie' may beg to differ!  A quick search on Google shows several Greyline Calculators that can be used as propagation aids.

John, LA2QAA posted a recent comment on the AMSAT-BB which resulted in a few interesting responses.  As aficionados will be aware, the ancient but semi-operational Oscar7 satellite frequently exhibits rather strange behaviour as it crosses the greyline /terminator with mode switches and unexplained increases in downlink strength. These effects will probably never be properly understood thanks to a lack of useful telemetry from this ancient but fascinating satellite.  A recent suggestion on the BB suggested that some unusual effects when passing through the greyline might be a contributor to this seemingly odd behaviour.

For this OBS, we will put Oscar7 aside for a few moments and take a closer look at the greyline.

The 'Greyline'  .... and a few notes.

It's fun to watch the explanations of this strange satellite behaviour on the AMSAT-BB.  John Branegan would have loved this stuff.  If there is 'another side', maybe he's watching from there. I hope so.  GM4IHJ's 'SATGENS' are archived on this and other sites.
 
re the BB>....
Aldo IZ1ANT said....
>Hi John, and other.
>I was wondering if the satellite has some solar array looking toward the Earth
and if the albedo can be strong enough to allow the solar panel to produce some
more power.
May be not all the solar panels are in good conditions.
Just my penny
Aldo IZ1ANT HB9EFT

Some very interesting points.
1) Albedo is that fraction/proportion of light reflected back (backscattered) from a body.  The oceans absorb, but ice and certain other surfaces reflect,  all to varying degrees.

The Earth certainly does back-scatter light back into space.   The percentage of reflected light (albedo) varies with seasons /latitude/ vegetation and ice/snow cover.  When a LEO satellite is still in 'darkness' as it approaches the greyline, it in fact is NOT in complete darkness at all.  This point is very easily proven and/or visually observed using suitable craft as targets.  Aldo is one of the few people I've noted on the reflector to even consider albedo effects.  Contrary to what some people believe, there is no sudden night-to-day transition and there's no sudden day-to-night one either.  That's not how it is in the real world. The backscattered light from the illuminated side illuminates the atmosphere on the 'dark' side of the terminator. The Earth's atmosphere does not just end suddenly, it just gets thinner with increasing altitude and even at the altitude where LEO satellites operate, there is of course some residual thin atmosphere. This residual atmosphere gets a little mention in your Keplerian Orbital  Elements as the Drag figure.  These gas molecules can and do reflect and refract light.  The illuminated atmospheric gas molecules  on the 'lit' side of the terminator injects light into the 'dark' side while the light being back-scattered from the surface of the planet on the 'lit' side contributes even more illumination to the 'dark' side.   

2)AO7's systems including it's solar panels seriously degraded, so albedo effects  *may* not be a major contributor ... but,  and its a big but,  for other birds, the Earth's albedo will certainly have a contributing  effect as the satellite goes from 'darkness'  to sunlight. The solar arrays will start to produce power BEFORE the bird crosses the terminator... from night to day, or at least the terminator's predicted location in space.

3) Maybe not all the panels work... this seems perfectly reasonable given the age of the bird... think micrometeorite / space junk damage. 
.
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Here's another good un... this time a proper 'Observation'... from KC8FN
 
In the three minutes before AO-7 switched to Mode A there was a marked
increase in the downlink signal. The switch occured at ~0141z. Prior to
this timeframe I heard some CW just over the noise but could not copy.
QSO with WA6DIR was interupted with the mode change.
I can't determine if the increased signal was due to a shift in relative
polarization or an AO-7 internal process, but it's orbital path was more
or less parallel to the sun line at about 48 degrees latitude. It might be
of interest to gather  more observations just before mode changes.
KC8ZFN
John

Interesting for a number of reasons. It's a real observation. The unexplained increase in signal is indeed unexplained but the facts are rationally presented. Yet again, its difficult to be sure of the real reasons for the signal increase.  Oh, and don't forget the possible effects of BMEWS RADARS which can get into the transponder's passband.  Albedo effects  *could* be involved to some degree.... but I don't think it's the main factor at work here. The mode change... well that's of interest also.

'So many interesting things to follow up.... so little time'.

OK.  I thieved that one from G3RWL and 'tweaked' is a little... GM1SXX  

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Another really good un from John G7HIA.  

>BTW - you can see the grey line in space. Watch a visual of the ISS going
West to East in the evening. The ISS is very bright near TCA and as it gets
into the East the brightness diminishes, goes through a dull orange colour,
then disapears into the darkness.
73 John G7HIA

You can indeed SEE the 'greyline'.  I (GM1SXX) must confess to not having watched the ISS fly through the greyline of late but I've certainly watched it in the past.  MIR flew at the same sort of altitude in a similar inclination as ISS. I did often watch MIR cross the terminator it and also a few Shuttle flights launched to 57 degrees inclination. Some other LEO satellites make excellent visual targets.  There's even a dedicated website for visual Observing at http://www.satobs.org/seesat/seesatindex.html 

 From a dark site, it's very easy to see the spacecraft change colour as they fly through the terminator (that's what astronomers call the night/day transition... AKA greyline).   A cheap pair of binos is more than adequate for this job. You must get away from streetlight though.  LEO's move quickly, so telescopes are usually not a great way to do this... even at low magnifications, too difficult to track.  I can confirm that what John says is true from personal observation of MIR, ISS, the Space Shuttle and other bright satellites.

Now were the greyline/terminator actually a LINE, or anything remotely like a line, the slow changes in colouration would simply NOT happen.  The craft would either snap into view or disappear just as suddenly. The transition would be abrupt. This simply never happens. John's is a particularly good observation because fits the observed FACTS.
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Personal notes by GM1SXX.

The 'greyline' is NOT a line in the conventional sense of the word. In fact it's no more a line than chalk is cheese!
 
In high-ish LEO it is affected by the Earth's albedo and at lower LEO altitudes, by the Earth's atmosphere scattering the light into the unlighted areas plus the effects of albedo.   No wonder I find the term greyline rather a strange one. 
 
So, viewed from space, it might appear as a line at extreme altitudes (1), but still with backscattered light from Earth messing it up,  and spreading out into a broad 'fuzzy wedge' at lower altitudes mainly because of atmospheric scattering.
 
(1)Actually, you'd have to be far enough away from the Earth for Albedo effects to be near zero, for it to resemble a line.
 
At the altitudes that radiowaves would be reflected, it could most definitely NOT be a line. Scattering by atmospheric gases would see to that... and thats BEFORE the Earth's Albedo backscatter from the 'lit' side of the 'line' is taken into account.
At 600Kms, it is definitely NOT a line.
At 1000Kms it's STILL definitely not a line. Still *way*  too close to the earth's surface) the atmosphere is very thin and the Earth's albedo effects would probably predominate in providing illumination.
For AO7, it would come out of darkness into a slowly brightening situation that would increase over time and then fairly quickly escalate into full sunlight.  I doubt if AO7 is observable visually (too small, too dark and too high).

John HIA's description is very typical of the observed behaviour of a LEO bird from a reasonably dark site.   It shows that the concept  of a 'greyline' fundamentally flawed since John G7HIA's observations , and those of astronomers and skywatchers worldwide, fit with the 'observed facts'.  The greyline is a fairly wide swathe of sky... not a line nor anything like a line.  If one can easily view the transition of a craft through the greyline  smeared over a considerable period of time... a minute or so for ISS  (at around 5 miles/sec remember) on a space vehicle that's is teetering on the edge of the atmosphere, it's VERY OBVIOUSLY NOT A LINE.  Were the transition from bright object in 'full sunlight'  to 'extinguished' to take one minute that would represent  a line about  300 MILES wide !  

 The last time I travelled 300 miles, I did it with the help of an aeroplane and it took well over an hour! .... GM1SXX

(Does that sound like a line to you?... LA2QAA.)

SXX doesn't like the term Greyline because it very definitely doesn't fit the observed facts.   GREYWEDGE sounds so much more honest to me.  Fuzzygreywedge fits the observed facts even better. How radiowave propagation behaves along a fuzzy grey wedge shaped transition is another matter altogether for the academics. For sure it is likely to be far less consistent than greyline computer models predict. For the moment at least, those simple greyline computer models are all we have.

It can be easy to confuse theory with reality. Don't be fooled!  Terms like 'greyline' have slipped into the public 'understanding' without too many questions being asked of it  because in a broad and general sense, it sorta works  . Greyline like its counterpart 'terminator'  is an idea, not a reality. It's a model of reality and not an actual reality.  That's not always a particularly bad thing since it's something that can be exploited ... greyline propagation.   That aside, it remains as only a shallow model to explain the observed facts.  If we have a bit of advice to pass on to readers, it is this..... Always ask your own questions.  The answers could surprise you.  What's more, it's a great deal of fun. 

'Fuzzygreywedge propagation'.  You read it here first.

It's said that a picture is worth a thousand words, so here's one.

This beautiful image, shot from aboard the ISS clearly shows the graduated nature of the 'FuzzyGreyWedge'. The thin illuminated edge near the top right hand side is the Earth's atmosphere.  Thin, isn't it!  The terminator runs diagonally across the view. You can see that there is no sudden transition from dark to light. Instead, there's a gradual brightening.  

It's not only not a line, hell it's not even grey!

Greyline, Terminator?  Show us where?

Yup, it's all a bit of a 'grey area'... LA2QAA.

Some links.

http://www.eoearth.org/article/Albedo

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/albedo.html

http://www.satobs.org/seesat/seesatindex.html

Greyline program from PA3CQR

Greyline program

World Time - interactive world atlas shows daylight, twilight and night zones

73 Al.
GM1SXX
 

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