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BiggarDigger

Meteor detection: path from Scotland to GRAVES

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I've been pondering a question about the path geometries from my site on the northern edge of the Southern Uplands of Scotland and the GRAVES transmitter site.

I suspect I'm seeing a lot of non-meteoric detections and indeed a number of long period reflections from what I believe are high flying aircraft in addition to meteor trails, but this leads me to question the radiation patter on the GRAVES transmitter site.

Some initial assumptions:

  1. GRAVES is radiating in an arc centred approximately in a southerly direction
  2. The transmitter antennae have a decent front to back ratio, and,
  3. The frontal lobes are elevated skywards, meaning the rear lobes are actually pointed in to the ground

(all of which can be inferred by photos of the site: interestingly, Google Maps etc now have the area blurred, but there are plenty of photos of the site online) 

Based on these assumptions, it follows that the reflections we detect should be back scatter from incoming meteors over Southern France or beyond. 

The conundrum I've been pondering is that those locations in Southern France, Iberia, Italy and the Mediterranean are well out of my 100km height radio horizon, so I must be receiving forward scatter from signals radiated to the North from GRAVES.  My 100km height radio horizon is just about into Northern France, so the reflecting medium is either much higher and I'm detecting meteors south of Graves, or I'm seeing forward scatter radiated in a Northerly direction.

So something is amiss, either in my assumptions, or the heights of the incoming meteors that I detect.

My suspicion is there is a significant rear lobe radiating from the site. 

The primary reason for this suspicion is that I see long period reflections of the order of 15 to 20 seconds (or more) which do not exhibit the typical Doppler shift and which are modest in strength.  Placing a high flying jet liner over the Southern half of the UK at 12km altitude, gives a path which would see both GRAVES and my site, allowing for a little over the horizon tropospheric propogation.  Such aircraft would, I think, be flying slow enough to give long period reflections and minimal Doppler shift.  If there is a notable northerly radiation from the site, I may be lucky enough during periods of significant tropshperic enhancement to hear the groundwave from GRAVES.  However, much more likely that sites in Southern England would hear it  under such conditions.  It would be very interesting to know if anyone has heard GRAVES on a direct ground wave in the UK.

Here's a few examples:

1. what I believe are aircraft reflections and 2, what appear to be meteor reflections:

1. Aircraft?

Meteor20180901101504.jpg.615055f37f63052c8f0df396004e73d7.jpg

 

2. Meteor

Meteor20180916212955.jpg.87cc2c0cba91ad8bbd1df1e5a3dd7600.jpg

(ignore the heterodyne around 1420Hz as this is caused by electrical noise from my PC monitor)

Richard

Edited by BiggarDigger
Typos

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Your assumptions about Graves are correct. The beam is essentially confined to within an elevation of 15° to 40°, in a Southerly direction. According to Ref 1, the 180° in azimuth is covered by 4 switched arrays, each of which steps its azimuthal beams. This is better described in the ref. Interestingly, a bit of geometry shows that it is possible, in the South of England at least, to have line of sight of scattered Grave signals well into France. I always find it intriguing that when you see the ISS heading for the horizon it is by then well over the Med! But then it's altitude is significantly greater than that of meteors. According to Ref 2, the  height distribution of meteors is centred around 85-90km, with only very few above 110km. As you say in your other post, perhaps other propagation mechanisms are in play here, rather than line of sight.

I concur that your picture 2 is of a meteor, but I couldn't be sure what is shown in picture 1. Certainly aircraft are detected, but I can't say I've seen any.

Below I've also given a couple of useful(?) references I've picked up along the way, should you not have come across them. IIRC there is another but I'm having trouble locating that.

Ref 1  The 143.050MHz Graves Radar a VHF Beacon-1.pdf

Ref 2  Detection_of_meteors_by_RADAR.pdf

Ref 3  Radar Echoes from Space DK5EC.pdf

Ian

 

Edited by The Admiral
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The second capture certainly looks like a meteor to me, as it has a short head echo at the left (narrow line sloping left to right and down) followed by the ionised trail reflection. Care is needed, as aliasing artefacts will often be seen as vertical lines along any long-duration signal. but they tend to be completely vertical and symmetrical above/below the signal itself, whereas head echos tend to be usually above only (very occasionally below) and sloping.

Hard to say on the first capture what it is. It could just as easily be an ionised trail reflection, with the head echo not visible, e.g. too small, sub-optimal angle or simply that the FFT settings didn't pull it out. For the latter problem you need to increase the % overlap (Options -> Spectrum display settings -> Watefall Scroll Interval -> Tick "automatic" and set to (say) 75% overlap). This will ensure that short duration signals aren't lost if they fall near the beginning or end of an FFT window.

Regarding the possibility of forward scatter from some kind of back lobe on the radar transmitter, I'm pretty sure I read a post on here about someone detecting the ISS whilst it was over Southern Ireland, which is obviously well behind the (assumed) arc of the radar.

Whilst we're on the subject of head echos, I found this very interesting paper:

http://adsabs.harvard.edu/full/1998JIMO...26..117R

It explains how to make some useful measurements of meteors. No free lunch though, with a single detector you either have to make an assumption about the true velocity of the meteor (possible for meteor showers where mean velocities are known) or the distance to the point of closest approach to the observer.

What I found most interesting was that it explains how to interpret the head echo. I had naively assumed that the changing Doppler shift of the head echo was purely down to the meteor decelerating as it enters the atmosphere, but that isn't the case at all. The shift is actually due to the change in radial velocity of the meteor as it moves across the line of sight of the detector.

Any meteor moving on a straight path must have an apparent motion towards the observer to a point of closest approach and then away from the observer again. (For an meteor to remain at a constant distance from the observer, it would have to follow a circular path with the observer at the centre of the circle - plainly impossible). This motion towards and then away from the observer is responsible for the doppler shift in the reflected radar waves of the head echo.

Assuming a horizontal, right to left waterfall display: The slower the radial velocity, the more horizontal the slope of the head echo. Bear in mind that a slow meteor moving directly from left to right of the observer and a faster meteor which is moving from left to right but also towards the observer (so the path appears foreshortened) would exhibit similar radial velocities, hence you have to make an assumption about the true velocity of the meteor to calculate anything useful.

If the meteor is decelerating, then the slope of the head echo will become more horizontal as the meteor slows and the radial velocity reduces. So if you have a head echo which is curved then that is good evidence of deceleration, but the effect seems fairly subtle.

I need think about it a bit more, but the fact that we are a long way behind the transmitter may be why we mostly see head echos that slope down and right, whereas perhaps if you were nearer to the transmitter or in front of it, you might see head echos sloping the other way?

Edited by IanL
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1 hour ago, The Admiral said:

IIRC there is another but I'm having trouble locating that.

Found it!

Speculation on Meteor Echoes 2012.pdf

From the material I've read I get the impression that head echoes would not be able to be detected by amateur equipment, but I'm not convinced of that. I do believe that the 'initial strike' trace is indeed that, though exactly how that reflected signal is created I'm uncertain about. There is much talk of the movement of the specularly reflected zone as the column of ionised air is developed (see ref above). I have also read that a meteoroid doesn't slow down on atmospheric entry, but just ablates as it goes, creating its own associated ionised shell of gas around it. Thus as you say Ian, the reducing Doppler shift of a head echo won't be the change in its velocity. The almost static trail of ionisation it leaves behind is what constitutes the body of the received signal. Also, it's worth remembering that with any moving reflector, the Doppler shift of the received signal will be a combination of Doppler shifts, the one from Graves to the (moving) ionising trail, and a second shift from the trail to the detector.

1 hour ago, IanL said:

Regarding the possibility of forward scatter from some kind of back lobe on the radar transmitter, I'm pretty sure I read a post on here about someone detecting the ISS whilst it was over Southern Ireland, which is obviously well behind the (assumed) arc of the radar.

Indeed that is puzzling. I guess it might just be possible to detect the ISS when (well) off the coast of Ireland, which I have heard of. I think I'd want some confirmation of this sighting so that the location of the ISS can be pinpointed. I suppose it may be that the emissions from Graves might overspill outside the 180° south-pointing sector into the North.

That link looks interesting! Thanks for posting.

Ian

Edited by The Admiral
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9 minutes ago, The Admiral said:

Also, it's worth remembering that with any moving reflector, the Doppler shift of the received signal will be a combination of Doppler shifts, the one from Graves to the (moving) ionising trail, and a second shift from the trail to the detector.

Yes, the paper I linked discusses that fact also. That last link you supplied is very interesting, brings together a lot of information though it's going to need a few more readings to understand half of it!

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Thanks both, those look very interesting references.  I'll need to review them this evening after work.

I still lean towards the majority of my meteor detections being due to forward scatter of a rear lobe from the array, if only due to the geometries involved.  Very intrigued by a report of an ISS detection over Ireland.  I've got a record at home of an ISS detection which I'll dig out later and see if I can correlate the date and time to a location.

In my data, I see quite a few minor reflections with head "echoes" but yes, the stronger bursts can cause aliasing within the FFT algorithm, particularly when the GRAVES array switches antennae. However, these do tend to be noticeably different to the bland trace in my first screen shot.

Good job the skies are cloudy tonight as I foresee an evening of reviewing papers and articles ahead!

Richard

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I didn't get time last night to review the documents and papers, but will try tonight to look through them.

However, I did stumble across a Google site: https://sites.google.com/site/radioastronomydm/observations/meteors which describes my suspicion about forward scatter from rear lobes.

If this is correct (and the geometry suggests so), then there are further implications for antenna arrangements.  People located in the southern areas of the UK may be better served by having low frontal lobes and an antenna pointing close to the horizon in order to receive back scatter from the forward radiating lobes at the transmitter.

People, like me, in the North of the UK would likewise be best served by antennas pointing at the horizon with low frontal lobes to be able to receive forward scatter from the rear lobes of the array.  This will be sub-optimal as radiation from the rear lobes should be significantly lower than the frontal lobes of the transmitter, but there's not a lot we can do about than other than move South!

The interesting area is in the central part of the UK (central England, Wales etc), plus most of the Republic of Ireland.  The geometry suggests these locations may be better served by elevating the antenna to try pick up the forward scatter from reflections over the English Channel or Northern France.  Low angle lobes and horizontal antennae would be looking for back scatter over Southern France or beyond, which may be at the maximum range for 100km height reflections.  There should be a balance point somewhere depending on latitude, front lobe pattern and gain of the receiving antenna whether it would be better to go for rear lobe radiation or frontal lobe radiation from GRAVES.

I looked up my detection of the ISS, which was at 09:25 on the 2nd September this year.  Historical tracking puts the ISS over the Bay of Biscay at the time:

1471824490_ISSandMeteor.png.50b5530410b8a2e0ff83526396de4dc2.png

This which would have been a back scatter from the frontal lobes at GRAVES, with a potential meteor off the rear lobes too (again ignore the heterdoyne fro my PC monitor).

I'll see if I can find a suitable prediction for a pass over northern France or southern UK, detection of which would confirm radiation off the rear lobes.

Richard

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If there is someone in that central area it would be possible to test using two SDRs and antennae at different elevation angles (probably switching the angles and retesting to average out any difference in sensitivity). Could be done with one antenna if conducted over a longer period taking care to avoid showers.

The main issue is the change in lobe pattern  at different elevation angles. The modelling I did for zero and ten degrees up made little difference to the pattern. I haven't tried higher elevations but would make sense to model before tests.

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On 18/09/2018 at 14:00, BiggarDigger said:

I looked up my detection of the ISS, which was at 09:25 on the 2nd September this year.  Historical tracking puts the ISS over the Bay of Biscay at the time:

1471824490_ISSandMeteor.png.50b5530410b8a2e0ff83526396de4dc2.png

This which would have been a back scatter from the frontal lobes at GRAVES, with a potential meteor off the rear lobes too (again ignore the heterdoyne fro my PC monitor).

  • Richard

I've had a look at Heavens Above and is this 09:25 UTC (10:25 BST)? HA shows the ground track to be

PassGTrackLargeGraphic_aspx.jpg.597b5dcc8142d23a2e549a6c4b399cf6.jpg

At that time the ISS is over the Med! So this example can't be using back fire radiation. Also, the Doppler shift is negative at this point, so the ISS must be receding, not approaching, which rather confirms its location. The position when the Doppler shift is zero is between 10:25:35 and 10:25:40 (BST), assuming the other 'blip' is a meteor, at which point the ISS would, by this prediction, appear to be approaching Sardinia. That's rather further on than I'd expect I must admit. I don't know just how precise these ISS transits are predicted.

However, this may be a way to check on whether you are using the back-fire or main beams. For traverses like this:

PassGTrackLargeGraphic_aspx.jpg.defcfbd16a906761f680254138bf7fb4.jpg

any reception of scatter must surely come from back-fire. Perhaps it'd be worth searching out such transits and seeing if you can pick anything up. Unless of course the back-firing beams are at low angle and would not intercept the ISS at its high altitude. Still, if you do pick them up that would be a positive indication.

Ian

Edited by The Admiral
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That's pretty much the orbital track that I saw Ian.  It was using an historical ISS tracker, but it placed the ISS just off the coast of the Basque Country in the Bay of Biscay heading South towards the Pyrenees.  Perhaps there's a minor discrepancy in the timings.

Nonetheless, both the Mediterranean and Biscay would be backscatter off the frontal lobes (or perhaps sidescatter in the Biscay case).

I looked up a few passes in the coming weeks and noticed two interesting passes on Friday evening this week.  I'm working from my phone, so don't have the detail to hand, but if I recall, there's a pass at a little after 8pm South of Graves and a second later in the evening North of Graves.  I'm hoping to book some quiet time on Friday evening to watch the reflections and if I can capture both, it may give further information on the backfire radiation and hence antenna alignment.

Watch this space!

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A quick update on this: I haven't been able to capture any of the ISS passes since the middle of last week.  On face value, most evening's there's been two suitable passes, one to the north of GRAVES, the other either passing roughly overhead, or slightly south of the site.  None of these have resulted in a positive identification, despite opening up the IF bandwidth on the SDR, looking for large Doppler shifts......

As I was typing this, explaining why I think my antenna geometries may be difficult to detect the ISS, I got a trace: but according to Heavens Above, it's not the ISS!

148417022_Satellite24thSept20181820UTC.png.9d2f03d1f129c3c2d60d2049b0303df5.png

 

Any thoughts about which object that is?  It passed through zero Doppler shift (i.e. a straight line to GRAVES?) at 18:20:42.  I'm at approx 55.6N, 3.5W (happy to share by PM more accurate lat and lon) if anyone can do the maths and/or look up an ephemeris.

From the Doppler shift, the velocity should be obtainable I think, which should in turn lead to orbital altitude... and, if the zero Doppler shift position is indeed an indication of crossing a straight line between plotted between my site and GRAVES, it should give a triangulation point.

If this object can be identified, I'd be very interested to know its track and location relative to GRAVES and whether it was North or South of the site as that will illuminate the reflection path.

Richard

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1 hour ago, BiggarDigger said:

I got a trace: but according to Heavens Above, it's not the ISS!

Richard, it looks like it to me, but it all depends on whether your times are UTC or BST! Heavens Above is in local time, so as long as the location is set for somewhere in the UK, as for the screen shot below, it will be BST.

image.png.6c0bcc052ba02c52dd865709f36f64ed.png

Your "zero" (around 1200Hz IIRC) is at around 19.20.35 on the waterfall, which would make sense from the map as it's where the ISS changes from approach to depart.

Edit. I should have read on! Wouldn't the 18.20 be UTC? On my system both the waterfall and 'Capture' button give the same, UTC, time. I see yours are 1hr apart. Which is which?

Ian

Edited by The Admiral
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25 minutes ago, The Admiral said:

Richard, it looks like it to me, but it all depends on whether your times are UTC or BST!

image.png.6c0bcc052ba02c52dd865709f36f64ed.png

Your "zero" (around 1200Hz) is at around 19.20.35, which would make sense from the map as it's where the ISS changes from approach to depart.

Ian

But remember Ian, I'm at least 450km North of you.  That pass just clips your horizon, but it should have been below my horizon for the ISS...see the ISS footprint for Biggar with the upcoming 21:56 pass annotated:

PassGTrackLargeGraphic.jpeg.d5abf285d75986362ba10ca48d1fcb04.jpeg

Actually, this illustrates my overall line of thought: if the ISS, orbiting at more than twice the height of meteoric reflections, is only just South of GRAVES for me, it follows the edge of any meteor footprint must be North of the site for my location.

The timing in the screen grab was in real time, i.e. local BST and the 1200Hz response (zero Doppler shift) was at 19:20:40 local; 18:20:40 UTC.

From what I can tell, the Doppler shift maths is not straightforward if you don't know the vector through which the reflector is travelling (i.e. the incident angle).

I'm scratching my head at the moment.  

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Well, if it's 19.20 BST then it fits with the track I posted. I'd forgotten about the possible effect of you being much further North, and so that particular pass doesn't seem to be listed for your location. But don't forget that the circle represents 10° above the horizon, so I'm guessing that there is still a possibility that you'd still pick it up? Well, I'd go further than that, I'd say that you had. Interesting to see if you picked up the 21:56. In fact, I generally give the location as Dijon as then it's easy to locate and it should give all the trajectories south of Graves, even if they are too far south for you to detect.

Ian

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2 minutes ago, The Admiral said:

Well, if it's 19.20 BST then it fits with the track I posted. I'd forgotten about the possible effect of you being much further North, and so that particular pass doesn't seem to be listed for your location. But don't forget that the circle represents 10° above the horizon, so I'm guessing that there is still a possibility that you'd still pick it up? Well, I'd go further than that, I'd say that you had. Interesting to see if you picked up the 21:56. In fact, I generally give the location as Dijon as then it's easy to locate and it should give all the trajectories south of Graves, even if they are too far south for you to detect.

Ian

Ahh... now that makes sense Ian: Heavens Above only reporting passes that are 10 Degrees above the horizon, will bring the 19:20 pass it into my range, by 5 or 6 degrees and it agrees with the timing of the zero Doppler shift observation.  So yes, I concur in that case the detection is probably correct.

I didn't get a peep out of the 21:56 pass, but I'll try centering on Dijon and see what that gives me.

I think my antenna geometries and physical location are limiting factors here too.  To minimise the length of coax, the antenna is on the South East facing wall of the house, so I'm blocked until the object is almost South West.  As a result, I miss a lot of extreme range Western passes of the ISS.  That leads me think my ideal pass for these purposes (that is, to investigate forward scatter from the rear lobes of the radar) is a North West -> South East pass similar to the one you posted from 2nd September but shifted East by a few hundred km to cross the UK, Belgium, Germany, etc as shown in red below:

662124391_Idealpass.png.44190cbf48bd13efbf788b8578486ad0.png

 

A pass of that nature would give me a reasonable opportunity to detect on the main lobe of my antenna while the ISS is North of the radar site....but I don't think the ISS will give that trajectory very often ?

Richard

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38 minutes ago, BiggarDigger said:

but I don't think the ISS will give that trajectory very often

No I don't think it does, if ever? Looking at ISS Tracker it seems that the most northerly track crosses the UK in the Midlands latitudes, so it tends to be rather West-East during those traverses.

Worth remembering that if you do set HA to Dijon that the time zone is BST+1 ! Also, as the ISS moves towards the horizon in the trace you've shown above, the Doppler shift will approach -6kHz, so you'll go off the bottom of the scale on your waterfall trace. I've sometimes noticed the image showing up on my trace, with the frequency trend being reversed.

Ian

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That's right Ian, I believe the maximum latitude the ISS ever gets to is around 51.5 North, which makes it very difficult.

It's late and I need to look at it a bit more closely, but I watched the pass at 00:09 earlier and may have a trace on the ISS while it was above the English Channel.  It's very weak and tenuous however...1041320838_25thSeptISS0009.png.0c597c1fcbf64e357cd79daaafe04021.png

Ignore the heterodyne at 1450Hz caused by my monitor.  I can see a very weak signal reducing in frequency from 00:09:38 to 00:09:57, which was while the ISS was North of Graves:

970036170_IStrack000925thSept.PNG.b09ac7ee1fe476ffa9f17faf1eae676e.PNG

Checking the data tomorrow may provide more info.  For example, the zero Doppler shift point appears to be roughly correct at 00:0950.

If it is the ISS, it would have been at very high elevation from Dijon and even at reasonably high elevation for me too, so I'm not sure how useful it is. Nonetheless, if this is correct, there appears to be sufficient radiation off the rear lobes to provide reception of forward scatter which would intercept incoming meteors at a similar latitude.  Such reflections might possibly (probably?) be more effective and result in higher signal strengths than this candidate due to lower radiation angles, particularly receiving the reflections at my site.

Perhaps this recording is a bit academic as the trace is so weak, but it's interesting nonetheless.  Bear in mind this is all driven by wanting to understand how I'm receiving meteor reflections that on face value appear to be north of the radar site and are limited by my 100km altitude horizon.  By inference, it suggests possible alternative antenna mounting and direction arrangements for those at mid latitudes.

More data and analysis is required, but it's a start....

Richard

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Interesting Richard. It certainly looks like the ISS. Certainly with such a rapid change in Doppler shift it must be moving fast. Difficult to know if it's West firing 'spill-over' or genuine back lobes. You do lose it though when it goes more North of Dijon.

It might be worth seeing if there are other large satellites moving to the South of Dijon.

Ian

Edited by The Admiral

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Hi BiggarDigger,

Thank you for this thread.

My apologies for not becoming involved earlier. Cheers Ian for the heads up! The above reference to a detection of the ISS from off the coast of Ireland was mine and I attach an excerpt from my astronomy log for further information. Note my immediate thoughts as to how the ISS could have been detected initially well outside of the East-South-West radiation direction of GRAVES. I will have to search my computer for the original detection screen shots which I will post when I find them.

Excerpt from Astronomy Log 2015.docx

My detection site is located about 3 miles to the NE of Nottingham and the aerial is a home made one following the Sky at night magazine design which is fixed high up on the side of a garden shed (around 12 feet high) and facing toward Dijon which I have a completely clear horizon of living on a ridge overlooking the Trent Valley about 69 meters above sea level. The aerial is horizontally polarised and lifted about 10 degrees above the horizon. I have a FunCube Pro+ dongle and use Spectrum Labs software. I attach another astronomy log excerpt concerning its build.

Excerpt Astronomy Log 2014.docx

I have not specifically attempted to follow the passage of the ISS when its path is unsuited to GRAVES but have many times followed it on other occasions with perhaps the furthest location South being above Sardinia. I am always amazed at the distance objects can be detected. At times the passage of the ISS is very loud in the speakers.

Best Regards,
Steve

 

Edited by SteveNickolls
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Thanks Steve for joining in. Of course, and this is not to belittle what you have found, I am mindful that our conclusions are only as good as the data we have, in particular, the exact location of the ISS at any given time. I do wonder how precise these prediction sites are, though in the case you've given, position would have to be several minutes adrift in order for the reflection to have taken place within the accepted beam. All grist to the mill, as they say!

Ian

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Just recently there was a pass over by the ISS and I detected a short reflection placing the ISS some short distance away from Dijon but to the NE direction. I attach the screen shots.

ISS Passover 25.9.2018.docx

Thanks Ian for your post, it came in when I was typing this. Yes I agree to some approximation on time/location  but an interesting subject nonetheless.

Cheers,
Steve

P.S.

There will be another pass in just around 90 minutes which will be well to the north of Dijon, I will see if anything can be detected, I suspect not.

P.P.S.

I came across this article about an upgrade to GRAVES from 12.12.2016- https://www.defensenews.com/space/2016/12/12/deal-breathes-new-life-into-france-s-space-surveillance-radar/

Edited by SteveNickolls

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Yes I caught that one as well, but only just! Just one blip beginning at 19:03:55 UTC.

1.thumb.jpg.105a46b3ff1f960460d9e14a5aef085b.jpg

The ISS was purported to be here:

image.png.b640d6a0134cc2d06a0855a8a2dc304b.png

I think the problem here might be that it is above the purported beam for much of the time. I can't really see where it is on the ISS Tracker, but if you use the zoom button you can get the surrounding land in much greater proportion than the ISS.

I didn't see anything on the previous pass, too far for me I think. I'll look at the next one, though it is well and truly North of Dijon.

Ian

Edited by The Admiral
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That's fascinating Steve and thanks for posting the data.  I missed the earlier pass this evening, but might be able to make the pass at around 21:40, though I will need to go out shortly after as I'm on "Dad's Taxi" duty tonight!  Even so, the weather here is lousy with heavy rain and gales, so my propagation path will be very disturbed and may add significant extra attenuation.

I looked very briefly at the possible detection from last night again and although I didn't record raw IF data from the RTL-SDR for further analysis, I'm fairly sure it is the ISS north of GRAVES.  The Doppler shift appears to be a good fit for what I would expect from the orbital path, in particular the zero Doppler shift point being on a direct line between my site and the radar.

I also re-ran a quick calculation again at lunchtime and confirmed within reasonable tolerances that my range to the ionosphere where meteors should produce reflections is just a little over 1000km:

  • Assume the radius of the Earth is 6371 km (not precise due to oblate spheroid distortions, but good enough I think for these purposes).
  • Meteor reflections occur at ~ 90km altitude. Let's call that altitude h.
  • We can draw a right angle triangle (my antenna pointing at the horizon forming a right angle to the radius of the Earth) with r=6371 on one side and r+h = 6461 on the hypotenuse.
  • By Pythagoras, the short side of the triangle (from my antenna to the reflection medium) is 1074km, which is probably a good approximation to the ground range.
  • To allow for a slight distortion we can call that 1100km ground range. 

That  puts the reflection medium for meteors over north and central France from my site and well away from the target area of the radar.  I should get very few meteor reflections, unless there is a significant high angle back fire off the array.

The probable detection last night of the ISS north of GRAVES strengthens the case for radiation to the north and forward scatter to receiver sites in Scotland.  Given the number of decent meteor candidates I record with short but significant head Doppler shift, I can't think of another explanation.

That may mean for sites such as yours Steve in Nottingham, you may get reflections at higher angles, or if you orient the antenna to the horizon, you may detect reflections South of the radar.

Richard

Edited by BiggarDigger
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For what it's worth here is a map that I produced and posted some time ago.

1460837845_Gravestransmitterzones2.thumb.jpg.8dc600b50abcc5c0d04ff515553a4304.jpg

The purple tinted area is where I reckon the purported Grave beams will intersect meteors at 100km altitude. I reckon it's between 120km and 375km from Graves. The red lines R1 and R2 represent my range going from the horizon up to 5° above the horizon. So, fortunately for me, I should be able to see them. For you Richard I guess they'd be shifted roughly to the NE, so I can see why in theory at least, you shouldn't be able to detect them.

Given that the antenna gain profile is essentially dictated by constructive and destructive interference of the incoming wave caused by the presence of the ground, I saw no virtue in angling the antenna at all. I tried some EZNEC calculations with different angles and the profile changed little, at low angles at least.

Your results certainly are intriguing. I'm going to try the next pass (21.41). Can't imagine I'll see anything, but you never know!

Ian

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Nothing detected for the 21.41 pass over.

Cheers,
Steve

BTW Ian, can you wok out what the area GRAVES covers at the height of the ISS around 410km up?

Edited by SteveNickolls
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