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

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Old 12-17-2015, 1:39 PM
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Question Direction Finding with Trunked Systems.

Hello all. I've thought about this for quite some time now. So lets say there is an 800MHz trunking system, and someone (unauthorized) keys-up on it. How hard would that be to triangulate? Because if the frequency changes every time then do you have to go through every frequency just to get closer to the transmitter? Thanks.
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Old 12-17-2015, 1:46 PM
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Quote:
Originally Posted by MyMotoROLA View Post
Hello all. I've thought about this for quite some time now. So lets say there is an 800MHz trunking system, and someone (unauthorized) keys-up on it. How hard would that be to triangulate? Because if the frequency changes every time then do you have to go through every frequency just to get closer to the transmitter? Thanks.
You would have to monitor the control channel, to know which voice channel the offender is accessing, then check the input frequency for that repeater. Obviously, that can only be done when the offender is transmitting.

Most likely the system administrators will just block that radio.

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Old 12-17-2015, 3:39 PM
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Generally there is no real need to use triangulation to locate a rogue radio on a trunked radio system. If it's truly a rogue radio, especially if it's causing serious issues, they would simply use their system console to send out a "kill" command to the radio in question. This would "brick" the radio, putting it in a state where it won't transmit, won't receive (normally at least), and this state can't be reversed by simply reprogramming it. The radio can be revived in only two ways (perhaps a third if the user has the right blend of software, hardware, and saved pretty much everything from the radio prior to the kill), sending it to Motorola for revival (and they'll ask many pointed questions on how and why it was killed in the first place), or by sending an "unkill" command from the exact same system console that sent the original "kill" command.

There are ways to triangulate on the radio if it is a problem, but not a big enough one for them to simply kill it. Basically where it's more important to catch the person and recover the radio than simply remove the problem. They can send various commands to the radio so it sends signals that they can trace without disturbing the general radio traffic that much. Think things like an I-Call (call to an individual radio, not over a talkgroup), and the like.
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Old 12-17-2015, 3:57 PM
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Alright! Thank you both!
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Old 01-27-2016, 1:45 PM
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As a result of the frequency division of trunking, success of DF against unauthorized operators is directly correlated to the length of their transmissions. The longer the transmission, the better the fix that can be gained.

Along with this, the most appropriate way to combat an unauthorized source of interference like that (that is likely to only make short transmissions) is with dispersed simultaneous-reception triangulation... rather than the traditional one-man lojack type progressive triangulation. Two stations capturing the same transmission at the same time will have a much easier time determining a triangulation point.
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Old 01-27-2016, 3:02 PM
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Yeah I heard some weird chatter on my cities trunking system, I suppose it was an officer, I think I made a post about it a while back. I heard laughing on it 2 times on 2 different talk groups. Random laughter. Thanks


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Old 01-28-2016, 10:05 AM
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As a result of the frequency division of trunking, success of DF against unauthorized operators is directly correlated to the length of their transmissions. The longer the transmission, the better the fix that can be gained.
The same applies to non-trunking channels. Your DF equipment should be capable of following any frequency changes just as rapidly as regular subscriber radios. If it isn't, well there's your problem.
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Old 01-28-2016, 9:09 PM
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On the issue of triangulation, it depends on the system type. If it's a multi-site system, it is relatively easy to triangulate it. If a single-site system, it's back to having to DF from several sites where you can hear the offending signal.

True the radio can be disabled IF it's on the system and that option is enabled. But, there are many cases where that is not possible and DF'ing is required. That said, it's no more difficult than DFing any other signal, and depending on the system type it might be much easier.
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Old 02-10-2016, 7:32 AM
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Quote:
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The same applies to non-trunking channels. Your DF equipment should be capable of following any frequency changes just as rapidly as regular subscriber radios. If it isn't, well there's your problem.
Good point. I did realize that recently once I got my new Adcock array set up, and felt a little silly. Figured out that I could decode the P25 just fine in the receiver while tapping the IF-out and getting an even clearer doppler tone than audio-out.
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