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[aprssig] Catching a Jammer with APRS!

Bob Bruninga bruninga at usna.edu
Sun Sep 30 03:33:27 UTC 2007


If you have heard me speak about APRS you have heard me lament how APRS is not being used for DFing...  and few other clients even implemented the most powerful technique of Signal Strength signal finding..

Here is a simple automatic DF signal reporting system your club can throw together easily that might work for your jammer problem:

1) Take a handful of old TNC's (or new APRS trackers)...
2) Take a handful of Receivers tuned to your repeater input
3) Connect external CD input of TNC to Carrier Squelch of monitor receiver
4) configure TNC with very high rate beacon timing
5) connect TNC to a transmitter on a local unused simplex digital channel.
6) set each TNC to a different DWAIT value
7) Spread these blackbox receivers around the coverage area of your repeater.

Now, everytime ANYONE keys up your repeater, any of these monitor receivers that hear the input will REMOVE external carrier detect from teh TNC and so the TNC will send a packet.  All you have to do is watch the packet channel when the jammer is heard on the repeater and the packets will show you which receivers heard the input and which didnt.

(it doesnt matter what the packets say, they are only triggers)

Now you know the neighborhood or area to focus your continued DFing.  Move the black boxes closer to that area.  Again, next time he transmits, you will still be able to better localize the signal. etc..

Yes, this is a kludge, using only existing hardware, and no special firmware and no special software and not even APRS.  THis is just a simple way to do the original APRS [plan for automatic DFing networks which we never could implement because only APRSdos had it built in and without other players, it is one hand clapping.

But it is easy to see, that Opentrack, Tiger track or Byonics or anyone that is making pic trackers could easily modify an input pin on their devices so that not just the presence of the squelch on the moiitor can be transmitted, but now also the SIGNAL STRENGTH.  Doing this, gives us the automatied DF signal strength network APRS was originaly supposed to do...

But instead of waiting for "someone else to do it", this technique can work perfectly in some cases where you "suspect" a jammer.  Just put ONE of these black boxes in the neighborhood of the suspected jammer, with no antenna or enought attenuation so that only the "suspect"'s radio will trigger it.

Now then if you hear one of these packets when you hear the jammer, then you KNOW it was his transmitter....  or at least you now know it is within very close range of that black box...

Again, many times it is better to just do the best with what you have NOW rather than dream and wait for what you "could do" if... if... if...

Just an idea if you have a particualr problem that this might fix.  Oh, the DWAIT paramaters are needed so that all N of these packets from the N different monitor boxes do not collide.  And being on a simplex freq, there should not be any collisions with anything else.

Notice, even if you are not monitroing packet channel LIVE when it happens, that your TNC can time stamp all packets and HYPERTERM can store them for days, and so you can go back after the fact to see at exactly the time the jammer was on, what the packets were saying about what receivers heard it!

APRS is not just vehicle tracking, it is supposed to be much much more.
Bob, WB4APR




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