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Radio Direction Finding Forum - Discussions regarding direction finding and transmitter location

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Old 01-02-2016, 9:29 PM
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Default Radio Triangulation

How does Radio Triangulation work. like if your trying to find someone who is transmitting on an illegally on a channel. can someone please explain how are they caught.
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Old 01-02-2016, 9:54 PM
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long story short...

(assuming they are in a static position), you need a reciever with a directional antenna, and a compass.

using the directional antenna, you take a compass reading of the offending signal. You then travel a couple of miles in one direction, and take a compass reading of the offending signal again. you plot the directions recieved on a map, from each position you observed from...and you see where the lines intersect. More bearings, higher resolution.

If your readings come back to a repeater site, well... there's half your answer, then you look at the input frequency.

This is all oversimplified, but it's a generalization of what I've done in the past.

Last edited by thebigphish; 01-02-2016 at 9:55 PM.. Reason: I'm an idiot, and I can't spell.
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Old 01-03-2016, 11:22 AM
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Another method uses the "Doppler" effect to determine the direction instead of a directional antenna. The rest is basically the same for both methods.
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Old 01-04-2016, 2:15 PM
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Quote:
Originally Posted by TheWarlock View Post
How does Radio Triangulation work. like if your trying to find someone who is transmitting on an illegally on a channel. can someone please explain how are they caught.
It has already been explained, but the term triangulation comes from triangle, as you need (at least) three readings to have any reliability. These need to be taken from different sides transmitter, forming a triangle around the area.

Starting further out is better, to help pinpoint the area of closer inspection. As you move closer to the transmitter, you need to have the receiver shielded, and a way of reducing the strength of the signal being fed to the receiver.

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Old 01-04-2016, 2:31 PM
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Remember all of this is possible for a stationary signal. Having one that is mobile and on the move is much harder to pinpoint. Granted it's not impossible by any means, but depending on what the offending signal is disrupting sometimes it's not worth the trouble to go out and do something about it.
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Old 01-04-2016, 2:42 PM
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With a modern integrated direction finder/antenna the received signal is displayed on a compass and you can basically drive where the compass points you right to the source. They can get a little confused in a dense urban environment with lots of tall buildings, otherwise some people have reported they found the offending transmitter in about the same time it would have taken to drive there if they already knew the address.
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Old 01-05-2016, 5:56 PM
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What has been described here so far are "classic" methods, tried and true using relatively simple equipment. Some of them have been used for almost 100 years by both pros and amateurs alike.

But the question is, who is looking for the offender? Because who, and how deep their pockets are, can change things.

Another example of how it might be done, TDOA, Time Difference of Arrival. Multiple stations, three or more, with equipment that can time tag an event in the transmission, either automatically or manually. These receive locations do not require directional antennas. Each receive location will receive the signal at a slightly different time. These time differences equate to propagation time deltas between the source and each receive location. Once you know these time deltas (and the location of your receive stations) a little math plots a hyperbola based on each pair of receive stations. The point where these hyperbola intersect is the location of the source transmitter.

The above is really not difficult at all to do.

Another example uses a slightly modified TDOA or a PDOA technique on an airborne platform. The aircraft has multiple non-directional receive antennas. Either Time or Phase of the received signal is measured as it is received at each antenna. A little math results in a bearing, azimuth and elevation, from the aircraft. Now received energy at a single source cannot yield range, so the target is just someplace along that line of bearing. But assuming a ground based target that bearing (remember, both azimuth and elevation) intersects with the earth someplace, and if you have a detailed 3D map you can plot that place. Where the beam intersects the ground is the approximate location of the source of the signal using a single receive platform on a single transmission, however short that transmission may be down to and including a single sub microsecond pulse transmission.


T!
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Old 01-05-2016, 7:34 PM
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There are many commercial systems that use the time or phase method with electrically scanned fixed antennas to give an instant bearing on the offending signal and most public service DF people use equipment made by OAR. I have a DDF2020 DF system by Global TSM Group that takes it a step further and instantly displays a line or "fix" from your location to the signal overlaid on a laptop running Google Maps.

Once this system is set up and running you just wait for the offending signal, press the space bar on the laptop and it displays an instant compass heading on a map from your location. You then drive a ways, repeat and it draws another fix line until you have several that intersect and that's where your target is.

I think it cost me around $350 and it interfaces to most any receiver or scanner, See GLOBAL TSCM GROUP, INC. - Radio DF DDF2020T
I've only played with it a few times but it works well and is easy to use.

prcguy


Quote:
Originally Posted by Token View Post
What has been described here so far are "classic" methods, tried and true using relatively simple equipment. Some of them have been used for almost 100 years by both pros and amateurs alike.

But the question is, who is looking for the offender? Because who, and how deep their pockets are, can change things.

Another example of how it might be done, TDOA, Time Difference of Arrival. Multiple stations, three or more, with equipment that can time tag an event in the transmission, either automatically or manually. These receive locations do not require directional antennas. Each receive location will receive the signal at a slightly different time. These time differences equate to propagation time deltas between the source and each receive location. Once you know these time deltas (and the location of your receive stations) a little math plots a hyperbola based on each pair of receive stations. The point where these hyperbola intersect is the location of the source transmitter.

The above is really not difficult at all to do.

Another example uses a slightly modified TDOA or a PDOA technique on an airborne platform. The aircraft has multiple non-directional receive antennas. Either Time or Phase of the received signal is measured as it is received at each antenna. A little math results in a bearing, azimuth and elevation, from the aircraft. Now received energy at a single source cannot yield range, so the target is just someplace along that line of bearing. But assuming a ground based target that bearing (remember, both azimuth and elevation) intersects with the earth someplace, and if you have a detailed 3D map you can plot that place. Where the beam intersects the ground is the approximate location of the source of the signal using a single receive platform on a single transmission, however short that transmission may be down to and including a single sub microsecond pulse transmission.


T!
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Old 01-05-2016, 10:20 PM
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For more than you'll ever what to know on this topic. Transmitter Hunting: Radio Direction Finding Simplified: Joseph D. Moell, Thomas N. Curlee: 9780830627011: Amazon.com: Books
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Old 01-06-2016, 2:56 PM
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Quote:
Originally Posted by prcguy View Post
There are many commercial systems that use the time or phase method with electrically scanned fixed antennas to give an instant bearing on the offending signal and most public service DF people use equipment made by OAR. I have a DDF2020 DF system by Global TSM Group that takes it a step further and instantly displays a line or "fix" from your location to the signal overlaid on a laptop running Google Maps.
The systems you describe donít use the time or the phase of the RF to find direction. It can be argued they use phase, but it is not phase of the RF, and so is not the phase of arrival (those were the techniques I was discussing), but rather it is phase of the audio.

(prcguy, I assume you know this, I am adding it for others) These systems are called either Doppler Direction Finding (the source of the DDF part number you mention) or Pseudo-Doppler Direction Finding. For most modern systems the Pseudo is the more correct name. The original systems (and years ago I used some of them) had a physically spinning antenna, instead of multiple electronic switched and virtually spinning antennas.

Regardless, yes, these systems work pretty darned well and pretty simply. And several of them have map modules to automatically plot the DFed bearing. It has never been cheaper and easier than it is today to get good results.

For those not familiar with the technique, picture a single antenna mounted on the outer edge of a spinning platter, something like a high speed record player. As the platter turns the antenna goes around and around. As the antenna moves in the direction of the transmitter the received frequency is shifted upwards, as the antenna moves in a direction away from the transmitter the frequency is shifted downwards. By using an FM receiver on this spinning antenna the Doppler shift (induced by antenna motion) can be detected in the audio of the receiver, this works even when the transmitted signal is NOT an FM signal. By processing the receiver audio you can pretty easily get a direction from the center of the spinning table.

Modern systems, like the DDF2020 mentioned, replace the complex and expensive mechanically spinning antenna mount with simple and cheap multiple fixed antennas that are virtually spun by rapid switching between the antennas, but the rest of the approach is the same.

T!
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