Radio Direction Finding and Taxi Radios

Hi all.

Not sure if this is the correct forum for this. But here goes. I am in the taxi business and we are trying to locate which cabs are sending offending radio traffic. So my thought was to use a radio direction finder. The problem though is that I live in a city with a lot of 20-30 story buildings. My questions are:
1. The taxi company has relay towers in the city and they are centered on roughly 463 and 468 MHz. Would this make it impossible to track where a taxi is broadcasting from, due to signals being repeated?

2. With all of the tall buildings, will reflections make it impossible to get a good direction to the taxi, if question 1 is not a problem?

3. Are there any other issues that I am not aware of?

Many thanks to you all!

It would be quite difficult to attempt direction finding on moving vehicles. If they are using a repeater as you say the taxi's may also be using a lower power.

The better way to do this depending on what type radio system you is to use ID's for each radio. This is real easy with newer Kenwood radios. If you have the right type radio system now you radio shop can set this up for you.

If you have an older radio system your cheapest solution would buy a totally new system that provides individual radio ID's.

rfradioconsult said:

If you have an older radio system your cheapest solution would buy a totally new system that provides individual radio ID's.

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Or install an add on ID board in each of the radios. Kind of a tossup on cost of the add on board for ID or buying a new radio with it built in. Knowing how many of the cab companies operate, any extra cost to their bottom line will probably never happen.

Most cab radios never see the service bench until they have a major problem.

I also have been told that there was a radio seller out in the Midwest that was selling old wide band radios to New York cabs and didn't care that they were operating the radios in wide band on UHF. Problem is these cab operators don't have a clue what is right or wrong in operating the radios. My guess is many of them probably don't even have an FCC license.

Yeah, I would second individual radio IDs.
MDC-1200, FleetSync, DTMF, there are a variety of options.

What model mobile radios are you using?

I went through this with a bus fleet.
Older radios were being used by some drivers to make rude/inappropriate comments about other drivers.
When we replaced the system, we made sure we used a PTT ID system, and made sure the radio ID's were showing on every radio.
When we trained them on the new radios, we specifically showed them how the radio ID's popped up on everyones display.

Issue mostly went away when they lost their secrecy. The occasional driver that forgets often gets taken to task by managers or ever other drivers. Works out pretty well.

On the other hand, if you are having this issue from radios that are not supposed to be on your system, or drivers that are purchasing and programming their own radios, then you will need to look at a way of instating some sort of subscriber access controls. This may not be possible with your current radio system.

Thanks everyone for your replies.

Yah, I agree about taxi companies and bottom line. Short version of the story is that manager wanted to install ID functionality to the radios because management will not buy new radios with that functionality. BUT...They also do not want to afford putting the chips or boards in which as it turned out was far cheaper.

So, the cab company is back to square one...No convenient method to catch problem drivers.

I thought if maybe an RDF set could give a general idea of where the signal is coming from, we could match that up with GPS (which they DO have), and narrow down the suspects to a much smaller group. Then knowing that we can weed that group down based on other factors, we could concentrate our search on the few remaining, by following those cabs one at a time with the RDF gear in a second car and provoking a response from them. Since we would then be in close range, the signal would hopefully be very strong.

Which radios are you using? Might be an easier way to do this....

I'd have to go back and check tomorrow. But they definitely do not have transmitter IDs.

treddie said:

I thought if maybe an RDF set could give a general idea of where the signal is coming from, we could match that up with GPS (which they DO have), and narrow down the suspects to a much smaller group.

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You can do that, but you would have to be up high with good line of sight in all directions, like on an apartment building rooftop. Not ideal and you may be up there for a while until the offender transmits.

Then knowing that we can weed that group down based on other factors, we could concentrate our search on the few remaining, by following those cabs one at a time with the RDF gear in a second car and provoking a response from them. Since we would then be in close range, the signal would hopefully be very strong.

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At that stage, you just need RSSI (a signal strength indicator); it will be very obvious when the RF is coming from a nearby vehicle.

RDF from a single point is just going to give you a direction. Most methods of RDF isn't very accurate. You'll get a few degree wide swath that the radio is -probably- in. Remember that in a city you'll get some multipath (reflection off buildings) that will greatly complicate things.

Having two RDF systems would give you two overlapping directions. It'll give you an area where the signal is likely coming from, but still, it's not going to be exact. Multipath is still going to be an issue.

Having two (or more) automated systems will allow you to triangulate, but a decent remotely controlled system isn't going to be cheap. If you try to do it with people, there is going to be errors and it's hard to get any sort of directional antenna pointed at a target fast enough. If the offenders are aware you are doing this, they'll just lay off for a while until you give up.

It doesn't work like it does in the movies, at least not with reasonably affordable equipment.

Upgrading your equipment to support some sort of radio ID's, or even going as far as to set up your mobiles to send out GPS location would probably be easier, maybe even cheaper.

There was a company many years ago that sold a product that could "fingerprint" transmitters. Supposedly it would look at the slight variations in the signal when the individual radios keyed up. They claimed that every radio had a unique pattern and it could be used to "fingerprint" individual radios.
Not sure how well it worked, and I can't remember the name of the company. Not sure if there is really a market for this stuff anymore since most radios will support some sort of radio ID's.

Radios

treddie said:

I'd have to go back and check tomorrow. But they definitely do not have transmitter IDs.

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Like McKenna says, the radios you have just may be able to be programmed to tx an id.

Sorry for taking so long to respond. I keep forgetting to get the radio make and model. Will try again tomorrow.

They look like a Kenwood TK-880-1. Perfect match cosmetically to that, anyway...Not sure about internally.

A true TK-880 will do Fleetsync and DTMF ID. That's all you really need. Just program each radio with a unique ID number (using either Fleetsync or DTMF) and set up the base radio (or all the radios) to display the caller ID.

Just knowing their radio is identifiable by the boss or everyone else is usually enough to stop this sort of stuff.

mmckenna said:

A true TK-880 will do Fleetsync and DTMF ID. That's all you really need. Just program each radio with a unique ID number (using either Fleetsync or DTMF) and set up the base radio (or all the radios) to display the caller ID.

Just knowing their radio is identifiable by the boss or everyone else is usually enough to stop this sort of stuff.

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Great advice Mike. That will work nicely.

So the TK-880-1 already has the function, just needs to be programmed?

treddie said:

So the TK-880-1 already has the function, just needs to be programmed?

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That is correct.

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Yeah, cheap and simple solution.
Assign each radio a unique ID. Use the vehicle/taxi number as a good way to match thing.

Like I said earlier, we had drivers in a bus fleet pulling this crap. We set up ALL the radios to show the radio ID's, and the issue stopped. Once the people that are abusing the radios realize they can be positively identified, the issues almost always stops.