Could an MDT Tx freq be used like a mobile extender?

[inigo88]

Hey guys, I'm just asking theoretically here, I was talking with a friend of mine about the feasability of this. My area uses an APCO25 digital motorola smartzone trunking system, and from what I can understand the portables transmit directly to the towers and not through a mobile extender in the car. My local police departments have very nice new MDTs, and I was scanning around recently and found their MDT Base freq. Using this site www.kb9ukd.com/digital, I was able to determine that they're using an RD-LAP 19.2 kbps signaling protocol, and when I researched that on google I found out their input is 45 MHz below the output frequency. I researched this all out of curiosity, I understand it's illegal to decode MDT transmissions (and with digital I'm sure it's extremely difficult too).

My question is: Many scannists use mobile extender frequencies to determine if a police car is close by (for instance CHP's in california is 154.905 almost totally statewide, so it's an easy one). Obviously the unit has to be transmitting from their portable for you to be alerted that they're within 1 or 2 miles of you. I'm just wondering if the input freq of an MDT would work for the same purpose? Example: You hear a burst of digital noise on the input freq and you're alerted that an officer is using their MDT and close by. Does a mobile MDT give some kind of idle signal to let the base station know they're logged on, or do they have to be sending a message on the MDT for the input freq to have any traffic?

I was just curious if this would be a feasable idea, considering most police officers don't transmit on voice while doing a speed trap, but might be chatting away on their MDT.

I was also wondering if I could determine what voice traffic was close by by scanning the input freqs to the trunking system (since it's APCO25 I assume the inputs would also be digital). It's a Motorola smartzone TRS on T-Band (480 Mhz), and I've heard the inputs are 5 MHz up above the output freqs. Is this accurate?

Please don't mistake my questions for any malicious intent. I'm just intrigued by, and trying to understand the technology, and see if there are any realistic ways I could find out if there was law enforcement activity nearby (it's really fun on road trips or as a passenger in a friends vehicle :wink: ), in a county that doesn't use mobile extenders.

Thanks in advance for any assistance,

 

[nd5y]

I thought of that too a wile back. You would have an even better chance of
finding them if they have GPS/AVL location data on the MDT channel
becuase the location data is updated every few minutes regardless of
whether or not they are sending text messages.

It might get out of hand on some systems like the city of Dallas
where the MDTs are on a multi-channel multi-site system or be impossible
if they are using wireless data on on a cellular/PCS carrier.

 

[WayneH]

Obviously the unit has to be transmitting from their portable for you to be alerted that they're within 1 or 2 miles of you.

And the extender retransmits the base radio also. Since CHP is pretty active it's easy to know when a unit is nearby.

I'm just wondering if the input freq of an MDT would work for the same purpose?

It won't work...well. (Unless they do use AVL like nd5y suggested) Most mobile data transmissions are short and infrequent. Not enough data to judge a nearby source and the mobile data is often small enough that if you're scanning you'll miss it. Also, most departments frown on private chit-chat over the MDT system.

I was also wondering if I could determine what voice traffic was close by by scanning the input freqs to the trunking system (since it's APCO25 I assume the inputs would also be digital). It's a Motorola smartzone TRS on T-Band (480 Mhz), and I've heard the inputs are 5 MHz up above the output freqs. Is this accurate?

This is your best choice though not terrific, and the mobile freqs are 3MHz higher. 5MHz is for 460 and below.

Please don't mistake my questions for any malicious intent.

Ahhh, reverse psychology huh? I've notified the proper authorities!

-Wayne

 

[inigo88]

Please don't mistake my questions for any malicious intent.

Ahhh, reverse psychology huh? I've notified the proper authorities!

-Wayne

You caught me. :wink: :lol: :lol: :lol:

Thank you for letting me know about the 3 MHz larger steps for the inputs, I'll plug those in to a new bank and give them a try. So far no luck on the MDT inputs, but I could have been out of range. My friend used to be an explorer for the local PD and told me they do use GPS/AVL in the units, and that they use their MDTs constantly to coordinate where they're going to meet up to chat, to have a coffee, or just to write dirty jokes to each other and screw around with the dispatcher.

Thanks for the input,
-Inigo

 

[cdenton]

PoPo Proximity

I've been trying something like that here in Michigan. I put the input freq's to my local towers in to see if I ever heard anything on them. I had them in as MOT (same as the Control Channels) and heard nothing. I just switched them over to FM about thirty minutes ago (oddly enough) to see of that makes any difference.
Should I just keep trying on MOT?
BTW, I work with the wives of a couple of State Troopers, and they confirm that they are typing on those MDTs all the time (both troops work the night shift and have partners - one types, one drives!).

 

[inigo88]

You can't scan the MDTs in Motorola trunking mode as all they do is transmit data back and forth. My question was whether I could listen on the channel that sends the data back from the car to the station and check signal strength to see how close they were. This can be done in FM mode. You have a Valentine 1... you have nothing to worry about. :lol: :lol: :lol:

-Inigo

 

[nd5y]

The problem is the MDT transmissions are usually very short so it is hard to tell how strong the signal is.

If you lived someplace with Open Sky, Pro Voice or full time encryption it might be handy to monitor all the inputs with no antenna and the attenuator on. That way if you start hearing stuff you would know they are pretty close to you.

 

[cdenton]

MDT Input

Well, the Valentine 1 offers some protection against the State Police, but I get a lot of info from the scanner, too. In my county (Kent), the local post generally has one 610 ("city") car on at night during the week, one 610 car plus a road car on week-ends, and between two and three cars out and about during the day and evening. It's pretty easy to just keep track of where they are and use that to figure out where they aren't.
Out on the open highway, away from town and all the CB monkeys with their constant inane chatter, the CB is a powerful enforcement-avoidance tool - especially if I'm travelling through an area where I don't have the homefield advantage of being familiar with patrol patterns.
I think all three devices are needed to have the best chance of keeping my money.
Of course, they still pop up from "nowhere" once in a while ... paying close attention and keeping a sharp eye out is still the best protection, I think.

 

[flyingwolf]

And of course obeying the posted speed limit will keep your money in your pocket too.

 

[cdenton]

MDT Freq

Well, I suppose that might work ...

 

[inigo88]

I live in California... we don't do speed limits... or stop signs...

 

[cdenton]

MDT

Retired State Trooper told me a good one about rolling through stop signs:

A guy in a Porsche rolls through a stop and gets pulled over. He gives the cop a bunch of static; "what's the big deal, I slowed down. What's the difference?" and the like. Cop drags him out of his car and starts beating him with his night stick. Knocks him half senseless. After a minute or so, he asks the guy: "Now, would you like me to slow down, or come to a complete stop?"
Tell you what, Troops love that joke.

 

[nh7cy]

living in California.. you'd want to stop at a stop sign to avoid getting killed by all the people who dont.. as for downtown san francisco.. you'd even want to be cautious crossing the street on a green light..

if you've really gotta get somewhere and you for some reason or another either want to or have to ignore the speed limit.. get a radar detector.. otherwise slow down and keep the others of us out there from having accidents.. plus it'll save you a whole lot of money and car insurance.. anyways.. getting a little too far off topic..

anyways.. if you're trying to monitor the MDT's in marin county then good luck.. like one of the other posts stated.. even though they can or may be used often.. the data bursts are so short and seldom that your scanner most likely wouldnt pick it up unless you already know the channel.

 

[bsavery]

living in California.. you'd want to stop at a stop sign to avoid getting killed by all the people who dont.. as for downtown san francisco.. you'd even want to be cautious crossing the street on a green light..

And if your stopped at a red light, watch your rear view mirror! That car comming up behind you just might not stop! :shock:

Bob