# Passive Radar



## geosman (Sep 15, 2007)

OK I know what I am suggesting is a bit wild but that is why I am asking the question(s) below.
I have scanned a number of articles on passive radar i.e. using FM broadcast spectrum to bounce signals off of targets and capturing (as per Frank Lind at U of W at Manastash ridge) the returns directly and via the target.
Frank Lind uses several receivers and expensive (has to be he has a grant) equipment.
My thought is don't use multiple receivers use multiple transmitters. Here is how I see it working: 
1) the direct signal is captured simultaneously with an indirect signal to a second (very directional antenna)
2) At a point produce a marking spike on both and record thru 2 sound cards (1 signal to each).
3) After a specified time, compare the shifted left direct signal to the delayed signal. Initially would attempt to use a best fit cross correlation function to find a match point.
4) the separation in time is used to calculate distance to target.
5) quickly shift frequency (using dollar store FM receivers (modified) ) to do it again with a different station.
6) now one should have the true direction and distance to the target.
Now the questions are:
A) Can this be done fast enough? We are not intending to capture UFO overflights so I think that this can be done within 1-2 seconds probably less per antenna motion.
B) Can I really move a 2 ft antenna 10 degrees at a time ? Think I can get a 12 degree antenna beam built.
I calculate 162 antenna positionings or 2.7 min per 1/2 the sky.
C) Can I really employ 2 sound cards simultaneously or do I need to buy a single card with dual inputs?
Please this is a serious operation I am considering. One primary objective is mapping the ionosphere height as balloons and radiosonde cannot provide the data needed.
Thanks for your comments.


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## octaneman (May 13, 2009)

Hi geosman 


That is one serious project you got going there, have you considered the power needs to run such a set-up ? Tossing a signal in the atmosphere is one thing, but having an antennae large enough to capture and decode the signal is something else. Instead of building the system from scratch, there is equipment out there like radio telescopes that can be retrofitted to do what you want. The equipment will have high end filters built into the system with software that will give you a clear picture of the atmosphere. But unless you want to build your own system visit the amature astronomers web sites, they can show you how to build a radio scope with a minimal cost you can then modify it to suit your needs. 


http://www.radiosky.com/booksra.html#lonc

Hope this helps.


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## geosman (Sep 15, 2007)

First of all I won't be sending out any signal. I will be letting a number of FM broadcasters do that. It is their signal I will be capturing. I will then get 2 signals from each station. The direct signal will be what is called the ground signal and the second is the one reflected from a target. The second signal will be bounced off the target. The "gotcha" I am trying to work out is use of 2 sound cards simultaneously (one from each antenna) or a single sound card with dual input. The latter being used for dual microphones in garage bands. 
Then the 2 unfiltered (at first cut) signals (digitized via the sound cards) will be exposed to what is literally a shifting back and forth until I obtain the best fit. The difference in time between the simultaneous spike I will add while receiving will be the time difference and at the (near) speed of light will indicate the extra distance. Then using a $1.00 FM receiver from the dollar store (because they scan, I can modify to force specific scanning of 2-4 stations. I can calculate the bouncing target height and direction, since I will know the station's tower location and distance to me it is a simple trigonometry problem.
I agree filtering will help but as proof of product I will leave that till later. One last point is that I will be looking at the modulated wave's digital values as received.
Now I am asking if U or anyone can pick apart this hypothesis. As I said initially, Frank Lind at UW has done something like this from Manastash Ridge but he had grad students and a bag of cash to do it. I am trying to do this on the cheap but as I see it it should be possible with just the equipment I mentioned.
Oh, as per the antenna, I am intending to use a very light weight parabolic reflector with a motor gearing I have designed to pick about 200 points in the sky to aim at and use 2+ FM stations. I believe if I use a parabolic with a designed 12 degree beam and my sky points being at 10 degrees I will get a little overlap.
This is going to be tryingly slow at first but I am sure there are formulas to obtain high speed signal matching.
So what does everyone think?


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## octaneman (May 13, 2009)

I did a more in depth research into what you are attempting to build. So far from the articles Ive read on the manatash project, the only piece of hardware was used was the Universal Software Radio Peripheral to convert the incoming signal. If I understand you correctly, you want to use the 2 sound cards ( or 1 with duel inputs) to take the place of the USRP. The modification of your $1.00 receiver by removing the mu pin will cause it to sweep the frequencies on both AM and FM, ( very clever) and in doing so you are hoping it will lock onto the frequency in the direction that your parabolic antennae will be pointed to. The problem of using sound cards that I see is that while the signal is being bounced from one card to another, it has to be demodulated for your receiver to actually "see" it. Since your modified receiver is constantly sweeping, I'm not sure that a sound card's crystal oscillator will be fast enough. 
Thats my take on it.


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## geosman (Sep 15, 2007)

Thanks a bunch Octaneman for your efforts they are really appreciated.
Let me correct one misinterpreted (perhaps on my part, mis-stated) point. The radio(s) will not be set to scanning willy nilly. The idea is to control the scanning so the direct and indirect signals will arrive thru their separate radios which will both be tuned to the same station. My supposition is that this can be done either via a reed switch replacement for the momentary switch on the radio or thru something as simple as setting a tuned circuit e.g. cr.
Your point of the "mu" pin is intriguing as I am unfamiliar with the term or its purpose. Can you explain?
Now with regard to the sound card(s). Here is a problem I have yet to resolve. I learned there are cards with dual ports which I presume are capable of resolving a signal (for simplicity presume from the radio speaker). This presumption comes from the fact they are used with dual mics to capture music from a band and therefore would not be 2 ports squeezed thru a single digitizing circuit.
Hence, the signals are captured entirely separately until they are compared. At which time they are digitized 0-255 (or better) at 44K / sec I think these will work successfully.
I can flip the 1 sec buffer to a file for each signal to do the compare while the sound care happily continues to capture the signal into another buffer.
With regard to 2 sound cards. This I have yet to investigate fully. It seems 2 can be on a platform but in some (if not all hence the question) but must be selected one over the other when used. This according to my investigation so far is not always the case. My knowledge of the sound card does not get to the hardware level yet. So, I ran into a couple of sites which are suggesting they can be used simultaneously as I think they should be capable of. As long as its buffer gets emptied properly. I might need to write a (gag, chock!#@$%) interrupt routine for that.
Have I now been able to make it cleaner as to the approach? If not or if you see any holes here please flag them so I can stumble further into this morass.
Understand I am so convinced of this now that I can easily overlook obvious problems.
Late addition re Universal Software Radio card not intended at all. See below:

Direct:
xmitter --> Rcv Directional Antenna --> $1.00 Radio --> Sound card --> buffer for compare

-----spark or dirac function marker applied ---|

Indirect:
xmitter --> Rcv Parabolic Antenna -->#1.00 Radio--> Sound card--> buffer for compare

Since Indirect is slower in arriving, the same music will occur later as will be evidenced by the marking function sent to the radio's circuit. I see the graphic are gross so let me just say the marker function will be sent to the radios simultaneously


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## octaneman (May 13, 2009)

On every digital radio there is a "mu" pin or trace to which is written on the board and is refereed to on the schematic as the mute pin. The function corresponds to the tuner side of any digital radio which allows you to tune in to stations one-by one manually. By cutting off the mute function on the tuner, what happens is that when you press the up/down selector on the tuner, it will begin to constantly sweep going through the entire spectrum of both Am and Fm bands. When you mentioned that you were going to modify a cheap $1.00 radio for your project and said that you new how to do it, I automatically thought of that particular modification and considered it a brilliant radical approach to your design. I concluded that as the tuner was constantly sweeping automatically, the oscillators within the sound cards ( including whatever software) will lock on to the frequency you wanted to capture. The analogue signal would be automatically digitized through the sound cards analogue to digital converters giving you a crisp clear signal, that can be compared to a frequency generator or seen through any PC audio oscilloscope. 


I'll do further research and consult with a few guru's along the way and get back to you. Yours is a fascinating project and It would be pretty amazing if you post your completed project here on TSF along with step-by step pics.



Note : The radio in question to do the mod is a radio shack model 12-587


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## geosman (Sep 15, 2007)

My $1.00 radio has just a momentary switch and NO schematic. So the thinking was to somehow count pulses to switch the two alike. OR if I get smart enough to somehow install a bank of resistors (or whatever electrical contrivence needed) to flip them always to the same station.
Thanks whatever you can contribute is great to me. I am currently starting to come up with a parabolic (circular) reflector which is light enough to be rotated for digital (oh! gag me!) tv using a biquad. If that works with about a 2 ft reflector I'll see how it looks with the local FM stations. From there I will go forward.


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