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very odd mobile antenna farm: DF setup with mixed antennas?? repo guys?

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tay0123

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#1
Hi all.

Twice I saw a car driving down my street (Twin Cities metro area) with a very odd arrangement of antennas on the roof, wondered if anyone here might be able to shed some light on what this is.

Car was a relatively new sedan, can't remember make or model but it wasn't anything typically used by government, it didn't scream 'unmarked cop car'. Perched on the roof were 4 antennas in a cross-style pattern like I've seen with DF (lo-jack) setups, but in this case the front/rear and left/right antennas weren't identical! The front/rear antennas appeared to be professionally installed NMO mount stubby things, maybe 1 in. wide and 3 in. tall, like you might see on a cop car or other government vehicle. Two of them, though. On the left and the right sides of the roof were what appeared to be patch antennas like we have for GPS. Those antennas had cables visibly running across the roof and into the rear door, with the cabling dangling down into the rear seat. My guess is that they were magmounts?

I saw this car (assuming it was the same car) driving past my location on two occasions (approx. three weeks ago in daylight and tonight). It seemed weird on the first observation. Tonight's observation really caught my eye as the car was driving on the wrong side of a relatively busy road; the car didn't swerve or act erratically, but still, wrong side of the double yellow.

Also, I should add that I didn't see anything that looked like a camera mounted outside the vehicle, and at night, didn't see any reddish glow like IR illuminators give off.

Okay, what's this person doing? A big outfit (Uncle Sam etc) would probably have all four antennas professionally installed, not a hodgepodge of clean installs and hastily arranged magmounts. If the antennas aren't identical, my understanding is that this sort of setup can't be used to DF something. My first guess would be some kind of repo operation? Only need one of those NMO mount antennas to access a commercial 800mhz system if the repo dudes used that and not cellphones. What good are two GPS antennas? I'm really confused, any ideas here?
 
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#2
I am guessing that someone is doing an RF coverage survey of your area. Driving on the wrong side, may have been an attempt to capture a "grid" that was on the wrong side of the road. I did these types of surveys and we would section a City or County into a statistically significant number of grids . Usually about 500 or so. Then drive into every grid and capture an average signal strength . If the measurement was above a certain value, it passed, if not it failed. The GPS was necessary to automate the process.
 
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#3
Oh and usually the antennas were temporarily mounted because we used rental cars, or cars from a pool of vehicles without radios.
 
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#4
It was not unusual to have two or more receivers capturing data. I once had five radio antennas, GPS, five VHF receivers, five laptop computers. It was tough keeping it all working in the heat.
 
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#5
Likely cellular carrier "drive testing".
They drive around and take measurements of signal strength.
The radios/antennas are listening to a particular cell site control channel to gather signal strength data. The GPS antennas are for precise location data. It all gets plotted out on a map.

Using the data they can add new sites or adjust existing to get the coverage they want.

Could also be used for LMR radio system testing, but more often it's the cellular carriers.
 
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