Meteor detecting

[davebParticipant @daveb17630]

Posted by Rik Shaw on 18/06/2018 07:50:57:

Thirty something years ago we were camping on a site by Chesil beach. It was a clear but moonless night around two in the morn and black as your hat when I unzipped the tent flap to visit the site toilet. What I saw near took my breath away. I have never seen so many stars in the sky before or since. I saw two shooting stars before I had even reached the lav – an unforgettable memory! (the stars – not the lav)

Rik

Light pollution makes it difficult to see the night sky, mountains are probably the best place to view, if you haven't seen it before you will be amazed at the number of stars in the sky.

Dave

[Rik ShawParticipant @rikshaw]

Good point Dave and as you observe, light pollution is a problem, but back then and in such an isolated location with zero local lighting the sparkling magnificence was still pretty impressive with a milky way to die for!

Rik

[doubletopParticipant @doubletop]

Posted by Neil Wyatt on 17/06/2018 20:53:02:

Posted by Rik Shaw on 17/06/2018 20:44:51:

With that Sky dish pointing earthward you are unlikely to receive anything other than Mrs Dales Diary or Gardeners Question Time

Rik

Sadly the Sky dish is accurately aligned…

For OPTUS D1 and Home and Away and Neighbours?

[JourneymanParticipant @journeyman]

I have to ask: Why do you want or indeed need to detect meteors? It sounds about as useful as George Merryweather's Tempest Prognosticator. If the meteor is big enough you will definitely know if it has become a nearby meteorite

John

[Jon GibbsParticipant @jongibbs59756]

It sounds a fun thing to try.

I did some research into meteor burst communications a while back – trying to maximize the data throughput – because as the ionization trail dies away the reflectivity, and hence the link capacity, diminishes.

The problem with meteors as a means of communication is that they are not regular throughout the day. The earth mops up lots more meteors at 6am local time than it does at 6pm where only those travelling faster than the earth can graze the ionosphere.

Jon

[Geoff TheasbyParticipant @geofftheasby]

Bernard Lovell, using ex-radar equipment, at Jodrell Bank in 1946, was the first to discover and detect meteors during the day.

Geoff

[Neil WyattModerator @neilwyatt]

Posted by Journeyman on 19/06/2018 10:28:31:

I have to ask: Why do you want or indeed need to detect meteors? It sounds about as useful as George Merryweather's Tempest Prognosticator. If the meteor is big enough you will definitely know if it has become a nearby meteorite

John

Why sit in a workshop making useless objects when you could be out in the fresh air?

Edited By Neil Wyatt on 19/06/2018 17:34:20

[Neil WyattModerator @neilwyatt]

Posted by Jon Gibbs on 19/06/2018 10:45:05:

It sounds a fun thing to try.

I did some research into meteor burst communications a while back – trying to maximize the data throughput – because as the ionization trail dies away the reflectivity, and hence the link capacity, diminishes.

The problem with meteors as a means of communication is that they are not regular throughout the day. The earth mops up lots more meteors at 6am local time than it does at 6pm where only those travelling faster than the earth can graze the ionosphere.

Jon

Funnily enough although that's generally true, at the moment the current shower is the 'Daylight Arietids' which comes roughly from the direction of the sun and over the last half hour I was picking up one every minute or two.

[Neil WyattModerator @neilwyatt]

How about this for a biggy?

The long line is the meteor slowing down, the big 'flash' is the ionisation cloud. Probably…

[doubletopParticipant @doubletop]

Neil

Can you describe what we are looking at here please?

I've been wondering what it is you are receiving. From what I’ve read here I guess it is the signal from the source being reflected of the ionisation trail. But what are you capturing? The reflected broadcast FM signal, the doppler shifted CW component or both?

As your last note suggests ‘slowing down’ so I guess you are getting the doppler shift in the X axis but is Y amplitude and the ‘big flash’ being the stronger reflected signal from the ionisation cloud? As I also guess the display is aggregated over time how is that represented?

If you are getting the doppler shifted signal can you determine the velocity of the vector in the axis towards the receiver?

Pete

[Geoff TheasbyParticipant @geofftheasby]

I was going to let Neil explain, but I had this idea…

This is a HDSR display, generated in the computer. It is listening to Graves radar, on 143.0488 MHz, which it can not hear, but when a meteor passes, the Graves signal is reflected, leaving a trail on the 'Waterfall' display.

Imagine a loom weaving cloth, which slowly lengthens, as the shuttle passes back & forth through the 'shed' formed by the raising and lowering of the alternate threads. If something is trapped in there, a lump will form, and be incorporated into the cloth. This is like a waterfall display.

Geoff

[doubletopParticipant @doubletop]

Geoff

Thanks; a bit early in the day?

I get the concept of a waterfall display. So Y is time, with current time at the bottom and and X is frequency rising from left to right.

Pete

(edited  I've just realised I can zoom the image)

 

Edited By Doubletop on 20/06/2018 05:37:02

[doubletopParticipant @doubletop]

The main signal has a doppler shift of 900Hz and (using an online calculator) a velocity of ~950m/sec or 2125mph (towards Neils house)

Pete

( could well be wrong….)

[Geoff TheasbyParticipant @geofftheasby]

Early? I'm often conscious, if not compos mentis, earlye in the morning.

Geoff

[Neil WyattModerator @neilwyatt]

To try and answer multiple posts.

As Geoff says it's a 'waterfall' display but falling upwards, so the oldest data is at the top.

Frequency is the x-axis and brightness/colour shows signal strength.

The big blob is probably a stationary, or just moving with the atmosphere, cloud/trail of ionisation left behind by the meteor. I haven't fully calibrated the receiver, but it's about 200Hz out so that would move the column close to the radar frequency of 143.050 MHz showing little doppler shift*.

The streak is probably a return from the intense (moving) patch of ionisation at the location of the meteor itself.

It's a good detection as most tracks only show one or other of these features, and typically much smaller.

What confuses me is that if the blob represents a velocity near zero, the trace suggests the meteor moves towards and then away from the receiver.

But… the Arietid meteor radiant was just below the horizon a bit beyond north-west. This suggests it came in from the direction of the north Atlantic heading south east, and where the line crosses the blob would represent the point of its closest approach. When the ISS passes over you see exactly the same sort of frequency shift through zero, although it changes much more gradually.

The frequency drop is 1,450 Hz (distance between two ends of the trail).

This makes the line of sight velocity change about 3,143 metres per second (yes online calculator!)

3.1kms/sec seems fast, but the Arietids arrive with a velocity of about 39 km/s.

A bit of trig gives an angle of about five degrees, OK that's rough and ready and assumes we see the full trace but does suggest the meteor was observed nearly fromxwwuio the side, providing a sensible explanation for the doppler shift going through zero.

Neil

*the Graves frequency is 143,050,000. Tuning to 143,048,800 and listening to the upper sideband means that an unshifted signal will be at 143,050,00-143,048,800 = 1,200Hz.

The fact I'm getting it at about 930Hz suggests the tuning calibration is 270Hz out, but measurements of terrestrial staions suggests.

[doubletopParticipant @doubletop]

Neil

Thanks; I'm getting to understand this so some degree, helped when I found I could zoom the image and read the detail.

Surely you can't use the total frequency shift as the indication of velocity as it represents closing and receding velocities? You can only use the deviation from the fundamental +ve and -ve

Alternatively could it be the case that the range of the crossing is so great that the closing velocity is relatively minimal and the bright cluster, on the fundamental, is really the main signal with minimal doppler shift. As your array has a wide beamwidth the cluster exits for all the time it is within your beamwidth. Indicated by the height of the cluster (Y). Then the frequency shifts being some sort of spectral noise or harmonics that exist for the duration of the event being at the same time as main cluster? Basically, for any point in time during the event there is a large return on the fundamental plus a secondary signal that sweeps +ve to -ve as the event progresses. It would be good to see a plot with an expanded (Y) axis.

In the case of the ISS, as it is so close, there would be doppler shift so it would also be interesting to see a plot of the ISS to see what the ISS spectrum looks like for comparison.

Pete

[SillyOldDufferModerator @sillyoldduffer]

Posted by Neil Wyatt on 20/06/2018 09:34:20:.

…

What confuses me is that if the blob represents a velocity near zero, the trace suggests the meteor moves towards and then away from the receiver.

…

I gave this a quick and dirty try last night with an indoor whip and the only thing I received was spikes and noise from my laptop and switched mode PSU. Failed again, miserably!

Could the trace anomaly be caused by Graves altering the angle of its transmit lobe as it scans the sky? The blurb suggests the radar sweeps from horizon to horizon. Therefore the reflection angle could be affected by movement of both meteor and the ground angle of the transmission: could that explain the result?

Another influence might be alterations in the polarisation of the signal. The yagi is horizontally polarised when what bounces off a meteor trail is likely to be all over the place.

Dave

Edited By SillyOldDuffer on 20/06/2018 10:28:44

[Geoff TheasbyParticipant @geofftheasby]

The 'blob' is not necessarily a point of maximum signal, but the meteor exploding part way though its final descent. Yes, polarisation of received signals happens all the time, but is most noticeable in faint signals where it means the signal strength may reduce to below the noise.

To counter this, a 'turnstile' or crossed dipoles aerial may be made, which will pick up any polarisation whatever it is.

The doppler shift still applies, but the meteor isn't heading for 'you', the trail begins where the meteor hits atmosphere dense enough to be resistant, and stops when the meteor burns out.

I was listening this morning and didn't hear any either!

Geoff

[doubletopParticipant @doubletop]

Dave (SoD) got me looking at the spec for the Graves radar and the azimuth beam width is 7.5deg with 4 beams operating simultaneously and each beam doing 6 steps to provide 180deg coverage. Each step is 3.2 seconds. Looking at Neil’s plot the event is active for just under 3sec. Indicating that the duration of the event is a function of the time beam is illuminating the target, not necessarily the time the target is active.

Pete

[Martin KyteParticipant @martinkyte99762]

Posted by Neil Wyatt on 20/06/2018 09:34:20:.

…

What confuses me is that if the blob represents a velocity near zero, the trace suggests the meteor moves towards and then away from the receiver.

…

Just a quick observation,if I've read this correctly. If the object is falling throught the atmosphere albeit at an oblique angle when it is at high altitude there is a large velocity componant bringing it closer to the receiver and the transmitter. As it gets lower into the plane of the receiver transmitter the lateral motion will have an increasing effect. To put it simply down dominates initially and then away or towards becomes dominant at lower altitudes.

Also if the transit is crossing the line between the transmitter and receiver there is another velocity vector which moves towards and then away.

You could probablyreasonably assume that the atmosphere is 300miles thick and that most meterites burn up above about 50 miles.

Could that explain your traces?

regards Martin

Edited By Martin Kyte on 20/06/2018 11:22:38

[SillyOldDufferModerator @sillyoldduffer]

Posted by Geoff Theasby on 20/06/2018 10:55:48:

…

To counter this, a 'turnstile' or crossed dipoles aerial may be made, which will pick up any polarisation whatever it is.

…

Geoff

Good idea; I'll build one and put it out in the garden. But that raises another question, how high off the ground should it be?

I'm thinking a turnstile might work better at 2 or 3 metres than higher because ground reflections might reinforce high angle signals. Or is it better high up because increasing the horizon distance allows the aerial to see more sky. Except I want to focus on sky between here and Dijon. I'm beginning to think Neil's yagi, or a crossed yagi is a better answer. Why is nothing ever easy?

This is a terrible distraction from another slow-running project, which itself is already keeping me out of my workshop. I'm doomed.

Dave

[Geoff TheasbyParticipant @geofftheasby]

Height doesn't really matter because you want it to 'look' upwards, any other signals are unwanted. Some mount it in a foil-lined trough, but the jury is out on its effectiveness. Take care with the feed arrangements, one dipole is fed 90 degrees out of phase with the other, it is easy to do. Google 'turnstile antennas'.

Geoff

[Neil WyattModerator @neilwyatt]

Posted by Doubletop on 20/06/2018 10:23:53:

Surely you can't use the total frequency shift as the indication of velocity as it represents closing and receding velocities? You can only use the deviation from the fundamental +ve and -ve

Agreed, it gives the line of sight delta-v rather than actual velocity.

ISS see the AF window at bottom right where it is visible as a VERY faint line dropping from 1200 to 700Hz as it recedes (over the middle east which is why it's so faint!):

[Neil WyattModerator @neilwyatt]

Posted by Geoff Theasby on 20/06/2018 10:55:48:

The 'blob' is not necessarily a point of maximum signal, but the meteor exploding part way though its final descent. Yes, polarisation of received signals happens all the time, but is most noticeable in faint signals where it means the signal strength may reduce to below the noise.

To counter this, a 'turnstile' or crossed dipoles aerial may be made, which will pick up any polarisation whatever it is.

The doppler shift still applies, but the meteor isn't heading for 'you', the trail begins where the meteor hits atmosphere dense enough to be resistant, and stops when the meteor burns out.

I was listening this morning and didn't hear any either!

Geoff

The 'blobs' are what is most often see – they are known to represent the persistent ionisation trail left by the meteor and can be several seconds long (I've seen one that lasted a minute or more that was photographed by several people, including me). Two issues are (1) can you detect the 'meteor head' and (2) is it possible to detect it moving both towards and away from the observer?

I've been given a copy of a discussion paper (sadly without cover so we don't know where it's from) that states:

Does the RCS of a moving plate (plasma front) scatter enough signal to be observed? What
would a CW radar return look like from the situation described in this model?
A great deal will depend on the geometry of the observation ie the location of the radar
source, receiver and the direction of the trail. Without a mathematical description of the
geometry we cannot answer this question in detail. However it should be possible to make
clear some points.
Given sufficient illuminating power from a radar source at some general angle from the
meteor vector – and given sufficient plasma front RCS in the direction of the receiver, one
would expect to see a return signal with a changing Doppler shift with time as the LOS
resolved component changes with the geometry. If the meteor is slowing down, this
resolved velocity component would further decrease with time. (However many of the
plasma generation models in the literature assume a meteor particle has a near constant
velocity until its mass is exhausted).
Would there be a geometrical arrangement of radar source, receiver and meteor track
where the LOS velocity would decrease with time – as is sometimes observed?
If we take it that the initial return signal from the meteor would show some type of Doppler
shift depending on geometry, what might we see at later times? The moving plasma front
will at some point cease to develop (due to meteor mass being exhausted being too low in
the atmosphere) and a Doppler signal will disappear. However the stationary plasma trail
may still exist (lasting for 1 second or more) and a strong signal may be expected from this
large conducting object with a high RCS. It will though have little or no Doppler shift. High
altitude winds may impose some bulk movement on the trail, or it may break up due to
shear winds – and in such cases some small Doppler shift will be imposed on the signal.
The foregoing suggests that returned signals may sometime have two clear components
(ignoring wind effects for the moment) – a high Doppler weak signal component and a
stronger signal with little or no Doppler shift. In measurements made by the author of
meteor reflections from the Graves Radar, returns with these characteristics have
commonly been seen.

The emphasis is mine. The paper give this trace as an example of a meteor that approaches and recedes from the receiver:

[Neil WyattModerator @neilwyatt]

Posted by SillyOldDuffer on 20/06/2018 11:28:18:I'm beginning to think Neil's yagi, or a crossed yagi is a better answer. Why is nothing ever easy?

This is a terrible distraction from another slow-running project, which itself is already keeping me out of my workshop. I'm doomed.

Dave

People using 3 to 7 element Yagis consistently report the best results. Aim it about SSE.

Both horizontal and vertical polarisation are used, but it's not important as returns from meteors have random polarisation.

Some people point it up by 10 degrees. This bewilders me as I thought most astronomers accepted that the Earth is round and a horizontal antenna will be pointing high up in the atmosphere over Southern France.

[Author]

Posts