... will most likely point to the ute's location, which is by essence variable, no ?
A mystery New Zealander has been hijacking police radio frequencies to sing verses from Old MacDonald Had A Farm and make oinking noises at the Old Bill. Reportedly operating for months, the unknown Kiwi apparently alternates between singing the pig-focused verse of the children's nursery rhyme, swapping the lyrics for …
75MHz is within feasibility for a loaded whip antenna on a vehicle. It'd look just like the antennas commonly seen for CB. If he can't be tracked, it's quite possible that is because he is constantly moving - either working with a driver, or playing pre-recorded insults.
Part of the problem with a single yagi or just one person on the direction finding is that one gets only one dimension. Also part of the problem, is that there are what is called mutli-path where someone uses skip to "bounce the signal" off another item to change the direction of the signal. Most likely the culprit will remove the antenna off the car while moving from place to place to be hidden. More than likely, they will be using a yagi themselves, so they "cannot be seen from the side".
We've had these kind of prats (and other interference problems) in the U.S., and hams have been asked to come out and "foxhunt" the source of the problem. They're generally pretty successful at it, too.
Blocking emergency frequencies can endanger people, and there's no reason or excuse for this sort of thing.
If the songster has a any technical prowess, it is likely going to be difficult to make him change his tune.
TETRA (Terrestrial Trunked Radio specification) or any radio system is vulnerable, especially with leading edge SDR (software defined radio) and other technologies available on the retail market. Nothing will really withstand penetration by dedicated amateur 'hackers' who get great satisfaction at breaking systems just for the technical challenge.
Trunked or digital systems often use 'control channels' - jam those and you 'own' the system.
DF (direction finding) can be thwarted, too. When I was constructing a hotel for my wife there was no cell service. I made a multi-element Yagi antenna to access a cell tower that was some 15-20 kilometres away, which was further than a 'local' cell tower whose signal was reduced to intermittentcy because of the 'luscious' jungle greenery. Using cell base data would obviously provide false information.
Using a cell handset in the immediate vicinity of a cell base only illuminates a single base as the handset signal level is cranked way down by the tower, depriving any other towers of a signal.
Even the US P25 system (multi-agency, trans-US) is regularly attacked with success.
Disclosure: I used to work on this stuff, in that area 25-30 years ago. I'm surprised it's still in service as it was supposed to have been gone a long time ago.
The frequencies involved are around 75MHz (AM) and due to the mountainous terrain there are a lot of shorthaul UHF and landline links tying hilltop repeaters together, so oinker in a valley in the back of Opiki could trivially be annoying cops as far away as Ohinganaiti whilst being bloody near impossible to foxhunt.
NZ police overlaid this with ~450MHz UHF repeaters about 25 years ago but coverage isn't very useful in hilly areas (surprise surprise) and they ended up having to retain the AM stuff as well as plonking down a bunch more UHF repeaters and links. If Oinker is using those repeaters he'll be even harder to trace.
At 75MHz the NZ police band is outside of standard (obsolete) NZ 100MHz landmobile transceiver bands, but ever since the Tait T500 and its PLL-tuned ilk came along, it's been trivial to have landmobile sets cover them too(*) so the culprit could be any one of thousands of innocent-looking landmobile rigs and it could be performing innocent tasks until the magic button gets pressed.
(*) After Tait sold a 40-channel T500 version, I modified it into a 2000-channel unit in the late 1980s which had sufficient frontend Rx bandwidth at 100MHz to tune from police up to aviation bands (12dB SINAD at -109dBm at the extremes and still kicking out 12W too), to prove a point. The local Radio Inspectors liked it so much they took 2
75MHz is still a line of sight frequency
Sure, which is why your FM radio doesn't work with a brick wall between you and the transmitter. Admitted, the FM band starts 13MHz higher and thus even less likely to get through solid objects but it's why you can never ever get a radio signal indoors.
In addition to the comments already made about DFing in mountainous regions, all of which are correct, the actual properties of radio waves and they way they 'propagate' also need to be considered. There are 3 main principles:
1. Reflection: wave changes direction by bouncing off other objects.
2. Refraction: wave changes direction due to variations in the air (or other medium). Examples are cold and hot spots where the density of the air differs, and the Foehn effect creates this in mountainous areas.
3. Diffraction: wave changes direction by passing over sharply defined edges — often called the “knife edge effect”.
That part of NZ is nowhere near flat, so all of the above means that DFing the culprit is going to be very difficult without an airborne RDF capability. There are signal path anlaysis tools, used by the likes of military and civil sigint organisations, that's going to give you an approximate area for the source but repeaters will confuse the analysis. If the guy is smart he's using a tight beam to a repeater, making him very difficult to localise.
At best you could locate the repeater, but you would have to go to he actual post and put equipmente to be able to pinpoint the culprit.
I suspect that if he has LOS to the mast he would stop...
Also, he could be transmitting for narrow beam to repeaters using a van, and no way you are going to find him.
Why all the boffinry? Which might I add is most fascinating as I had an interest in this subject in my youth.
If I was to hazard a guess I would say the perp owns a farm so if they check all the farms they should be able to find him based on the many different types of animal on said farm and the noises they make. There can't be that many farms in New Zealand?
"Is it not possible to detect where the signal first appears on the repeater network by comparing inputs across all and latency from each signal?"
Bearing in mind that a lot of this is still 1980s-era analogue radio linking(*) without any form of logging or sychronised timestamping: Not without a lot of work - far more than is worth expending to find this guy unless he actually interferes with operations - the cops have several channels in each area so that's unlikely.
(*) Or older. Some of the kit I worked on still had valves in it and as long as it continued to work, there were no plans to replace it.
Someone, somewhere, must be able to correlate which repeater was active at the time of the offending broadcast, which gives you an input frequency to listen on, and a rough location.
Having a rough location reduces the number of variables to manageable proportions...I guess:-) I have an amateur licence (G7, if anyone is interested), and have found (a while ago, I admit) that a little traffic analysis really helps with fox-hunts.
That said, I confess I don't fully understand their radio architecture...but the principles don't change:-)
Anecdotally, some fellow members of my local radio society were lucky enough to get hold of some ex-police Motorola UHF programmable handhelds around the time plod switched to Tetra....and one of them got
hammering from the local low-lifes who mistook them for el Fuzz:-)
What's the reason for still using analogue radio?
Years ago when I was a nipper - probably three decades ago in fact - I found an old Airband radio at my grandparents house. Twiddling the dial found some interesting talking, which turned out to be half of a police conversation. I'd hear reports of assaults, stolen vehicles and so on, but then a blank when I presume the other half of the conversation was taking place. Maybe they transmitted and received on different frequencies?
I'd happily sit listening, and even used to write out a newspaper-style log of what I heard for the enjoyment of the adults. The fun ended as my Grandmother used to play golf with someone fairly senior in the Police force, mentioned what fun I was having, and the frequency seemed to change. I tracked it down a few more times but I don't think it was too long before they turned off the analogue signal - mobile phones I seem to recall were on the brink of going digital at that point.
Airband (Aviation VHF) is Amplitude Modulated (AM). Police VHF (assuming analog) is almost universally Narrow Band Frequency Modulation (NBFM).
Old portable radios often covered many bands, including both "VHF-AM" (aviation) and "VHF-HI".
Plus, the old portable multiband radios were so crappy that they'd demodulate AM on the FM bands. Less so vice versa.
PS: Three decades ago is 1987. That's the modern era... :-) My Astronaut-8 radio is from something like 1974.
Possibly apocryphal, but I'd like to hope it's true:
Cold war Germany. It's raining. It's dark. The soldiers of the BAOR are bravely protecting Western democracy from the red menace just over the iron curtain. However the rapacious Russian hordes are not going to try anything because it's raining and it's dark and bugger that for a game of soldiers Comrade. Nothing is happening.
An unidentified voice comes over the radio. "I am a friendly bear. Are there any other friendly bears out there?"
A pause then a second voice is heard. "I am a friendly bear too." Then another, and another. Very quickly the radio net is flooded with friendly bears.
After some minutes of radio chaos an officer gets wind of what's going on and proceeds to read the riot act. A strongly worded lecture on radio discipline, operational security and a detailed description of the extremely unpleasant future of anyone who continues to abuse the radio net.
Silence falls for a moment.
Then an unidentified voice. "Well. He wasn't a very friendly bear was he?"
A batch of them. Networked. Providing apparent bearing back to a central server.
Back at HQ, the relative bearing lines are displayed on a map, in (effectively) real-time. Where they cross (perhaps several of them) is the location. If the point of intersection moves along a highway at 95 kmh, then (like automobile GPS systems do) you may assume that he's driving on that exact highway. A bit of intelligence can tighten the estimated location.
Sooner of later the perp will be caught on CCTV driving past an ATM or corner store security camera. Correlated with the time-stamped track (after the fact), you'll then have a grainy video of his vehicle driving by. Twice and it becomes a certainty.
Then, the next time he's on the air, you can close the net around that old truck, and catch him red-handed.
Let's say he's on the air twice a week. The mean time to catch him would be about two weeks.
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