Thread: police laptop\radio frequency

Not really, because you'd have no way of knowing whether the radio or data signal is coming from teh car, the police station or bouncing off a repeater.

Originally Posted by DarquePervert

Not really, because you'd have no way of knowing whether the radio or data signal is coming from teh car, the police station or bouncing off a repeater.

Hey DP, you didn't even tell him to search. Just so the OP knows, there was a thread VERY similar to this a while back. You should search and see what happened.

Michael

see, i was thinking about that, but using the GPS unit in each squad car. But hten only certain jurisdictions have that and each is different, so...it wouldnt work outside of one particular area.

Originally Posted by Genesisfactor

see, i was thinking about that, but using the GPS unit in each squad car. But hten only certain jurisdictions have that and each is different, so...it wouldnt work outside of one particular area.

You idea about Jurisdiction difference is true, but really, you can;t get that far into the analyses, because there is no way for you to access the GPS data.

Remember that GPS units DO NOT TRANSMIT. They only recieve. The GPS information is sent on the services DATA radio network. Unlike the old motorola systems, in particular the MDT systems, RD LAP and data on new systems (such as the ones with newly installed GPS, which is a VERY new feature) are all encoded. Soo, breaking it is very difficult, if not near impossible, and it is also illegal to break the encryption.

Radio DF'ing is almost impossible for many reasons. First, Most PDs operate io-n a repeater system. So, the signals "comes" from many places. Even if you listen to the repeaters "input", sometimes the units transmit on a portable with very little power, and sometimes with a mobile, with much higher power. This will make DF'ing very diffuclt. The next thing has to do with traingulation. IN order to get a somewhat accurate location, you need to triangulate the signal. That means you will need at least 3 receivers that receive the signal at the same time. That means all three must be within receiving distance of the transmitter. This would either take 3 fixed locations, with antennas mounted High, like 20 or 30 stories high, and don't forget, for a city or town, you will have to have overlapping coverage of the antennas. In a large city, you will need many receivers. Or, if you want to make them mobile, while possible, the problem will be having at least three receivers hear the transmitter. The next issue will be, to triangulate, you have to have the exact location of the receivers that are doing the trinagulation, and they all have to be on the exact smae time. This is usually occomplished with a connection to the atomic clock, or from a GPS signal. That is not to hard to accomplish, but it must be done cause even a difference of a second will mean VERY innacurate results because the triangulation requires calculation in the nano and milliseconds range.

Then, once you overcome the trinagulation issues, the information MUST be sent somewhere, to accomplish the calculations to determine the transmitters location. Then, to top it off, police cars and officers are mobile, and they transmit often. In an hour, there will be so many transmissions from so many locations all over the city, that it will be impossible to know who is where and when. While most agencies do have som sort of ID on the radio signal, it is not easy to decode. There is no SW only decode of MDC ID, though there is some hardware out there that does allow serial output, and I have never seen a aftermarket decoder for MODAT.

Then, to top it off, the FED is requiring agencies to migrate to Apco 25. Apco 25 is almost impossible to monitor, and many of the agencies are adding encryption to the voice channels to make monitoring impossible.

At this point, it just aint worth it or possible.

Michael

Originally Posted by spmclaugh

Wow, nice post. Well first off, like I was saying, triangulation would be a luxury feature; I don't feel it's necessary. After all, I know of only ONE radar detector which can give the direction that the radar is coming from (forget the model/brand at the moment, I researched these a few years back).

But as far as the rest: If you know the locations (via GPS, assuming you're driving around in a town you live in and have already mapped out) and relative output power of each repeater, you can "filter out" their portion of the signal, to some degree. Assume, for a moment, that there was some way that a cruiser could park next to a repeater, plug into it via a physical wire, and transmit/receive directly. If he did this, the repeater(s) would output in a certain pattern (intensity falling off like some constant * 1/r^2 from each repeater, ignoring reflections and other complications). With multiple repeaters, the intensity at one point would be a superposition of contributions from multiple repeaters.

Now, in reality, the cruiser doesn't do this, it has its own radio which transmits/receives directly (and is THEN amplified by repeaters, etc). So if you here ANY radio activity on the police frequency, and you know the locations and relative strengths of the repeaters, you can generate (using a computer) the intensity you should measure if the cruiser was FAR away, so that its contribution was negligible. Now compare that to what your detector REALLY measures, and you can determine if the cruiser is somewhat near you.

If you record the "cruiser portion of the intensity" at a few points, and save the location of the points by stamping them with a GPS coordinate, you can get a vague idea of the cruiser's direction by watching whether the intensity increases or decreases as you drive around. You're just using discrete points to get an idea of the gradient of the intensity (or, cruiser-portion of the intensity). This is assuming his signal is still being transmitted as you drive around, of course. But if you latch onto the data frequency, there should at least be a steady stream of ACK's. Like I said though, that's probably not a feature that is critical.

Does that sound plausible?

I think you may not understand how radio systems work. Basic explanation is this,

A repeater listens on one frequency (called the input frequency) and whatever it hears, it automatically transmits on a different frequency, called the output frequency. The officers radios actually transmit and receive on the exact opposite of the repeater. So, the repeater listens on "channel A" and transmits on "channel B", but the officers radios transmit on "channel A" so the repeater hears it, and listens on "channel B" so the officers can hear the repeater on "channel B"

Also, the PD has MANY repeaters, too cover the area as completely as possible. This means there is overlap of the signal. Also, it will be IMPOSSIBLE for a receiver to know which repeater it is hearing, so it can't tell if repeater A is weak, or repeater B is weak. While that is a problem of some sorts, you could always listen to officers signal, on "channel A", however, if the officer is using a protable radio, they are not very strong while mobile are much more powerfull.

Now, remember, a mobile/portable transmits in ALL directions. ANd there will be multiple transmitters. You will never be able to accurately track the signals, and you will have no idea where they are cause they will all be moving, and you will be moving, Assuming a signal is 5 miles away (just assuming it can be determined to be 5 miles away), it is 5 miles in any direction. It just makes no sense to try. As you move, another signal will be 5 miles away, while the original might be 3 miles away. what about the original being 5 miles away, and then someone else tranmits 3 miles away, then the original transmits 4 miles away, so where are they now? ALso remember, that they can be 3 and 5 miles IN ANY DIRECTION. No amount of calculation power will be able to determine who is where when, and going where.


Michael