Backscatter Radar

Things that do not fit in the above categories

Re: Backscatter Radar

Postby VK4XA » Wed Jan 27, 2010 8:52 pm

VK7JG wrote:Can you teach an OLD dog new tricks ? :wink


Whilst I dont really qualify as 'old' (I'm only 51) I am certainly learning new tricks......this is bloody interesting! :D
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Re: Backscatter Radar

Postby VK3OP » Wed Jan 27, 2010 9:30 pm

David,

It's great that you have isolated the source of the spurious, and that it is not the 706. Hopefully we can arrange another test
when you Andrew and I are all available for more comparison. Andrew let me know when you wish to run another test.

Cheers

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Re: Backscatter Radar

Postby VK3OE » Wed Jan 27, 2010 9:47 pm

Hello Frank,

The 7500km is a spurious, but it seems to be something to do with the hardware/software. I have run a local test using the 756 and 7000 and it is evident that although at a much lower levels than David sees. Could be in the Spectrum Lab or soundcard or transceivers, more testing is required. It is internal to the system.

The great thing is that it is the only spurious we see, otherwise ????? So return signals look to be te real thing. Always a problem with a new system such as this, what is real and what is spurious.

Thanks again for your help.

Andrew
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Re: Backscatter Radar

Postby VK3OE » Sat Jan 30, 2010 2:46 pm

Hello all,
This is version 4 of this post, I have corrected some first errors, included the antenna patterns, included a better estimate of the path loss VK3AUU to VK3OE on 50MHz, added the radar equation and lastly proposed some propagation effects.

Some further checking is in order, particularly for the very high amplitude return signal shown in the second picture of Jan 26, 7:43 pm which shows the probable return signal from the West Coast of the USA at 13500km being equal to the direct signal. This still seems surprising to me so a few calculations are in order. The direct signal between David VK3AUU and me with antennas at 60 degrees is about S5 which is about -105dBm. For the return signal to be about the same level as the direct, the total path loss to the reflector must also be the same as the path loss VK3OE to VK3AUU. The transmit level is +50dBm so the total path loss is 155dB. The antenna patterns result in a loss of 26dB on the direct path VK3AUU to VK3OE ( from RMD), the antenna gains combined are 22dB so that the difference is an increase in gain between the direct and reflected signals of 48dB. This reduces the required return path loss in total to 155-48 = 107dB. The one way distance to VK3AUU is 60km and the ONE WAY distance to the West coast is 13500km. The ratio of the distances is 225 times further which is roughly 2^8 times so that the free space path loss is 8*6 = 48dB more than the loss between VK3AUU and VK3OE. Assuming the worst case requirement for reflector gain at the distant end, assumes free space loss so that the reflector gain at the other end is then required to be 107-(2*48)=11dB. (The 2 * 48dB come about because of the radar equation).

I used the calculation used for large passive reflectors at microwave frequencies to calculate the area for a passive reflector required to give 20dB gain at 6m using gain= 20*log10(4*pi*area/(wavelength^2)). It turns out that a passive reflector at 6m for 20dB gain (being very conservative) is roughly 30 square m or 0.00003square km. Assuming that each point of the reflection in the returned signal trace is about 10km deep, and if we assume a width of 10km is involved in the reflection then an area of 100 square km is illuminated for each point in the trace and just 0.0000003% of the illuminated area is required to reflect to get the measured signal to reflect. Given the nature of the mountains of the US West coast, then this result seems entirely within the realms of of a probable explanation for the result. A higher path loss requires even less reflection gain to get the levels received.

If the signal travels via the f layer it will be constrained in the vertical direction so it may be better to assume 4dB for every double of the distance so that the additional one way path loss (above the loss VK3AUU to VK3OE) to the US West Coast is 32dB. This then gives a reflector gain requirement of 107-(2*32)=43dB for the reflection gain and not 11dB if 6dB for every double distance as previously used. This requires a reflector of 400 square meters, still just 0.000004% of the estimated reflection area of 100square km.

The signal at between 25000km and 30000km is twice as far and is 30dB lower in level than the signal at 13500km. Given that the free space path loss increases by 6dB for a doubling of the distance and that a similar area could be illuminated in South Africa (long path) then it is probable that a similar explanation to that for the 13500km return above would explain the result at 27000km, although with more path and/or reflection loss.

The checking continues and it seems to be reducing any doubt about the performance of this backscatter radar system. Checking for moon echo signals is a further possibility but I am not sure that the timing and frequency stability we currently have is sufficient to achieve a good result as averaging is required over several minutes.

Maybe I have it correct this time, third attempt and put in the radar effect as well, fourth attempt!!!

Seems too good to be true!!!

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Re: Backscatter Radar

Postby VK3OE » Sun Feb 28, 2010 10:40 am

Good News on progress for an application.....

Wolf, DL4YHF of Spectrum Lab fame is incorporating the chirp technology into Spectrum Lab, some of the modules are already there in the black-box section. Chirps can be generated, received and displayed in the time domain scope. There are several extra bits still required to make it completely functional such as range instead of time for the x axis, timing inputs from the computer clock and converting the data to dB to get sufficient dynamic range. Early testing looks very good as once set up it is easy to operate. Just watch the scope for radar returns.

An updated version of Spectrum Lab should be available soon (weeks) so keep an eye on the developments.

A full description of the chirp backscatter system together with initial results will be published in DUBUS 2/2010.

73,
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A backscatter sounder

Postby VK5PJ » Sun Feb 28, 2010 8:35 pm

Andrew,
would it be possible to make this function from a single site, given that you do not want to Tx and Rx at the same time?

(I had deleted my previous post as had reconsidered what it contained) :oops:

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Re: Backscatter Radar

Postby VK4WDM » Thu Mar 04, 2010 9:38 pm

Ok, let me get this straight without the technobabble. You fire a signal NE across the Pacific. It hits the coast of California and is reflected back to another receiver thus showing that the path is open. The time taken for the return gives you the distance and that confirms it is California. Has this old guy got it right? If so WOW! :D :D

Some questions:

1. How narrow is the beam? Can you tell if it is the coast of Mexico, California or Alaska?
2. How big does the target have to be? What about Hawaii of some of the smaller Pacific Islands?
3. I presume that the signal gets up so high that intervening land masses don't get in the way so you could, say, check a path from VK4 to ZS?
4.Will it work on 2m? I have often thought that VK to KH6 on 2m is possible. This might tell us when it might be possible. Also other paths such as: VK6 to Reunion or even Africa, or VK7 to Vk0.

This might seem nerdy but an explanation for the technology-challenged would be useful.

73

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Re: Backscatter Radar

Postby VK3OE » Fri Mar 05, 2010 6:34 am

Hello Wayne and Peter,

I will post a detailed reply on the weekend.

Thanks for your questions.

73
Andrew
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Re: Backscatter Radar

Postby VK4EKA » Fri Mar 05, 2010 11:35 am

I am only watching the developments, and not participating, so don't take my comments as gospel.
It is not necessarily land the signal reflects off.
On 1280MHz the radar shows backscatter from rain.
On Six metres I am sure you have worked backscatter from Sporadic E patches.
From here the classic is Brisbane to Sydney by beaming at a Sporadic E patch out near Norfolk Island.
No land reflection needed.
Similar happens with the Aurora on 6 and 2 metres.
I look forward to the explanation on the weekend.
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Re: Backscatter Radar

Postby VK4TU » Fri Mar 05, 2010 1:09 pm

I would say that the reflections are off everything. By knowing the expected distances of the hops, if it's mid-point then it's off the ionosphere, if it's full-distance it's from whatever's at the end. The important point isn't knowing "hey there's propagation to XYZ and it's X Km wide" but rather we have 2 hops open towards the North East, let's start calling. It should also let us know the geometry of the path so we can tell whether they are Es or F2 hops. At least that's the way I understand it. It's certainly more informative than waiting for beacons or activity from a part of the world and very exciting work in its possibilities
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Re: Backscatter Radar

Postby VK3OE » Sat Mar 06, 2010 9:33 am

Peter VK5PJ asked: "would it be possible to make this function from a single site"?

This is a rather bigger question, as operating from one site changes the system from a bistatic radar into a monostatic radar. This is because at a single site the receiver will be overloaded by the transmitter so the receiver must be blanked off during transmit. The effect of this is that the minimum range is determined by the length of the transmit pulse plus the receiver recovery time. For a 20ms transmit chirp and a 10ms recovery time for the receiver the minimum range is about 4500km. If the receiver listening time is 170ms (assuming a 200ms transmit-receive cycle) then the maximum range is 25500km. This means that most VHF tropo, Es would not be seen as it is within the minimum range. However it could be very effective for looking at long distance propagation on 6m and HF.

The processing gain would be reduced to 16dB or 19dB for a complex receiver (from the 33 or 36dB for the one second bistatic system) but that is probably not too much loss. The transmit power is also reduced by 17dB because of the short transmit pulse when compared to the one second transmit pulse. So, the overall loss of gain when comparing the systems is 36dB. This starting to get serious and may limit the capability of the system on 6m but it will most probably still be OK on HF.
The range resolution remains the same as the one second system at 75km provided that a 2000Hz wide chirp is used.

The big trick is to get the receiver to recover quickly enough so that the minimum range is not compromised, this will require some diode RF switching for the transmit receive function as well as an input clamp for the receiver to minimise the overload from the transmitter as the receiver will need to be running all the time. The receiver AGC will also need to be turned off. Spectrum Lab can certainly generate the required chirp but getting a transceiver to run the required transmit-receive cycle is a big ask. A separate receiver is most probably the way to go with a custom T/R switch and receiver input limiter.



Wayne VK4WDM asked; distance and that confirms it is California, Has this old guy got it right?

You certainly have it right Wayne, the system gives the distance and you see the echo therefore it is open to California!!!

1. How narrow is the beam? Can you tell if it is the coast of Mexico, California or Alaska?
This is a bit more problematic as the beam-width is set by the combined beam-widths of the transmitter and receiver antennas. You would be lucky to have a beam-width much smaller than 20 degrees so this limits the guess as to where the signal is returning from. Rotating the antennas automatically and plotting on a "A" scope plot would also help... The future...

2. How big does the target have to be? What about Hawaii of some of the smaller Pacific Islands?
The target does not have to be land as the sea surface will also provide a good backscatter signal but is somewhat dependent of the state of the sea surface. Land appeared to provide a better signal in the tests that we did.
3. I presume that the signal gets up so high that intervening land masses don't get in the way so you could, say, check a path from VK4 to ZS?
This is correct for Es and F, in fact the test we did showed a long path return from ZS. For tropo the land intervention will still be an issue, you will see the tropo terminate abruptly and a big signal from the blocking land mass.

4.Will it work on 2m? I have often thought that VK to KH6 on 2m is possible. This might tell us when it might be possible. Also other paths such as: VK6 to Reunion or even Africa, or VK7 to Vk0.
This is also correct. the system has the ability to look in any direction and make an idea tool for checking paths to anywhere. It is not limited by frequency, it can be used in any amateur band to check the propagation in any direction so that the reliance on beacons or TV transmissions etc. can be finally broken.

The system will also directly measure the backscatter as well as the surface reflections, this will be a developing art to determine which effect is being seen. For instance, for tropo there should be a continuous return as the elevated duct will leak along its whole length. Rain scatter on VHF and microwave bands will be interesting to try out. Aurora on 6 and 2 can also be checked assuming that the Doppler shift does not cause too much problem for the chirp receiver!!

The idea of this new system is to break the bounds that amateurs have been previously limited to by always needing to have a signal at the other end. This is no longer necessary. The possibilities are substantial and it will be easy for anyone to have a go. I am looking forward to having the first release software so that testing can begin in earnest and everyone can try it out. The results will generate much discussion and provide a great tool for finding new paths. There is also the possibility of placing internet connected remote stations at good locations but that is another forum post.

Thanks for your questions, 73
Andrew VK3OE
PS. Keep checking the latest release of Spectrum for further developments.
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Re: Backscatter Radar

Postby VK3OE » Tue Mar 09, 2010 10:08 am

I have just received an email from Wolf DL4YHF advising that a new version of Spectrum Lab is available for testing the chirp Backscatter Radar, see below for the links.
Good luck in testing, I have not yet had time to try it out but please let me know how you get on and we can pass comment on to Wolf.

"Hello Andrew,

I have just uploaded a new SL version (V2.75 b05) which has the Chirp
Radar configuration built inside the "Quick Settings" menu (under 'Other
amateur radio modes' .. 'Chirp Radar Experiment'). The second
programmable button (which shows 'Radar: Transmitting' or 'Radar:
Receiving') can be used to switch between RX and TX. The RX display
still uses the old time-domain scope, but the trigger now uses a
sample-counter based time (it doesn't need the received signal for
triggering). Averaging is off by default, but it can easily be turned on
in the TD-scope window.

Some experiments with additional windowing are still pending, but I
thought you (or other users) would like to test this early beta version.

The installer can be downloaded from here:

http://www.qsl.net/dl4yhf/spectra1.html

And the preliminary documentation of the Chirp Radar config is here:

http://dl4yhf.ssl7.com/speclab/chirp_fi ... hirp_radar

(Note: the old pages at freenet.de were discarded when freenet suddenly
decided to charge money for hosting a few MByte of webspace)

Cheers,
Wolf . "

73,
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Re: Backscatter Radar

Postby VK4ABW » Tue Mar 09, 2010 10:48 am

Hello Andrew. I have been following this topic and after your last post realised that i may be able to contribute.

My current setup consists of 6 totally separate radios and antenna's on 6 mtrs. Off this, 4 towers each have a 13el long boom yagi (60ft) with full rotation and are fed from my study via 5-50 or 4-50 LDF hardline. These 4 towers are separated roughly 80mtrs from each other which allows me to easily have several receivers parked on 50.110mhz whilst transmitting on the band. There is some de-sense between my northern tower and southern towers but it is very minimal.
The point i'm getting at here is that i have no problems transmitting 1,500watts on my southern tower whilst simultaneously listening on my northern tower. I will have to do some checking to see what the minimal recovery times are, but i am pretty sure that if i turn off the agc on the Icoms, it would be extremely quick.

Do you think it is worth while for me to install and test the latest version utilising my qro permit for testing purposes :?:

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Re: Backscatter Radar

Postby VK3OE » Tue Mar 09, 2010 7:43 pm

Hello Garry,

Absolutely, give it a go!!! The critical issue is that you may not be able to see "in close" (see answers to questions above). However if you can get the receivers to recover quickly then the arrangement should enable you to see from about 4500km out to around 27000km, from your own station if the total transmit and receiver recovery time are about 30ms. It may also help your receiver if you put some limiter diodes (low threshold reversed in parallel RF diodes with a 200ohm resistor also) across the RF input to the receiver so as to ensure that the receivers have the best chance of recovery. If your antennas have enough decoupling to give an S5 to S7 level then the close in will also work but given the distance that is probably not achievable.

Good luck, let me know how you get on.

73
Andrew
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Re: Backscatter Radar-Monostatic Mode

Postby VK3OE » Sun Apr 04, 2010 8:22 pm

Hello,

I have recently modified the latest version of Spectrum Lab to work as a Chirp Radar using one transceiver, monostatic mode, so that a short chirp is sent and when the receiver recovers return signals can be seen. I used a 756PIII with VOX, and AGC set to fast. The receivers recovers fast enough so that returns past 5700km are seen. An example of a return on 21 MHZ is attached, together with a no return trace. I used 64 x averaging to stabilise the trace. The direct signal was obtained through the monitor function, but if this is not available, then a small amount of the sound card output should be coupled back to the input to get a direct signal reference. The distance is calculated by multiplying the time from the direct signal to the return by 150,000 km to get that actual distance. If your receiver can recover more quickly then the first signals will be received from closer in. There is about 33dB of gain lost running in this mode but given that I was using just 50 watts and a dipole on 21 MHZ then that does not seem to be a problem. This should also be viable on 6m but there has not been any propagation to try it out. Of course if you want close in reflections then you will have to run in bistatic mode, with a distant receiver so as not to overload the receiver.
21MHZRadar.png
Monostatic Amateur Radar, 21 MHZ, Dipole, 50 Watts IC756PIII


If anyone wishes to get the Spectrum Lab config file for the maonstatic radar application please email me VK3OE@bigpond.com

73

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Re: Backscatter Radar

Postby VK4GHZ » Sun Apr 04, 2010 11:44 pm

Andrew, this may be a dumb question (?), but why is there a discrepancy with the direct signal time in the two graphs above?

With the same test equipment/procedure, shouldn't the direct signal back time in ms be constant?
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Re: Backscatter Radar

Postby VK3OE » Mon Apr 05, 2010 10:27 am

Hello Adam,

Yes, not a dumb question at all, in fact I have been pondering this for some time and it appears that there are two issues.

Firstly there is drift is due to differences in the sound card input and output clocks. This can be compensated for In Spectrum Lab and shows as a slowly moving (in time ) pulse in the time domain scope when the system is running.

The second issue is that the chirp receiver is not yet synchronised to the chirp generator when the system is started so that the location of the direct signal pulse is arbitary at the moment. There is a big issue of how to time everything so as to get the correct location of the impulse from the chirp matched filter receiver. Ongoing development !!

Many thanks,
Andrew,
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Backscatter Radar - Frequency error

Postby VK5PJ » Sat Apr 10, 2010 11:20 pm

Hello Andrew et all,
while pondering other matters I had one of those eurka moments, I think that any possible frequency error between stations in Bistatic mode can be easily adjusted out of the equation . If the transmitting station runs a fixed tone of, well lets say 1000Hz for this example, then the Rx station can simply measure it in Spectrum Lab and then adjust their Rx freq to make it read 1000Hz, once this simple process is done, they can move on to running sweeps safe in the knowledge they are on the same freq. In this way even an old valve rig with a propper VFO could be used, as long as it is stable over time.

This does not help with drift over time but if this is suspected, then they can periodicially re-calibrate using this simple procedure.

Any thoughts, have I missed a clanger :?:
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Re: Backscatter Radar

Postby VK3OE » Sun Apr 11, 2010 9:34 am

Hello Peter,

Yes, running a 1000Hz tone will work very well as one would assume that in bistatic mode (separate TX and RX) the direct signal is strong enough.

73,
Andrew
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PS Wolf is working at improving the Spectrum Lab implementation as it still has a few issues which limit the system capability, but it is basically working.
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Re: Backscatter Radar

Postby VK4ABW » Mon Apr 12, 2010 11:08 am

Hello again Andrew.

Question: is the ms time to distance (x axis) being looked at? as a user set option it would be great.

This would alleviate using a calculator to work out my return distances. I did get a few returns on the w/end around 90ms & 195ms using a single 13el yagi and 200w pointing NE. I'm using a USB interface for all keying plus tx/rx audio, so the system is operating damn quick. Only using the IC7700 barefoot at present as the qro amp will introduce too much delay for monostatic operation.

Another possibility is to hook up one of my TE amps in FM mode (fast switching), which will allow me to chirp 400w.

I am unable to use the monostatic radar during TEP to the NW due to chinese crud occupying the whole band :(

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