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Locate Tetra radios/transmitters - help

Posted: Mon Dec 30, 2019 4:28 pm
by jompa

First off I want to say i have never used radios or anything like that before, so I have zero experience with this.
And this might be the wrong forum to post this, or there are similar topics here that explains it that I have not found, but I'll try anyways.

But I have an idea and have searched my way to this forum, and maybe there is an answer here.
I have a cabin far in the mountains, that we access in the winter by using snowmobiles.
I was wondering if it is possible to use this direction finder from KerberosSDR to locate / pinpoint the location of tetra radios/transmitter in a area of 5-10 km? If I get the right antennas and such.

I have several reasons for this, but mainly to locate emergency services that might be in the mountains, they all uses radios with Tetra net (380-385mhz).

The problems I can think of is the range and accuracy because of mountain tops and such, pinpointing each radio transmitter when there can be several transmitters in range at the same time, and I need to see 5-10km in 360* from the antennas.

Again I have no idea of what am talking about, this might not be able at all and sound very stupid, but it would be very helpful if someone could answer if this is possible or not.

Re: Locate Tetra radios/transmitters - help

Posted: Mon Dec 30, 2019 5:22 pm
In Germany it is possible to locate Tetra basestations using the "Bundesnetzagentur EMF-Database" (Germany's authority for frequency management) where all transmission sites are listed. Maybe something similar is available in your country, too.


Re: Locate Tetra radios/transmitters - help

Posted: Mon Dec 30, 2019 5:28 pm
by jompa
Alright thank you, I'll check into that.

but maybe I was a bit unclear, what I meant was to locate handheld radios / transmitters used by law enforcement out in the mountains and such. I've read that the radios they use transmit data back to the base stations all the time for gps and things like that, so I figured that I should also be able to pick up their signals and pinpoint their location with the use of the right tools.

Re: Locate Tetra radios/transmitters - help

Posted: Tue Dec 31, 2019 12:11 am
by ckoval7
This is very possible. You will need multiple geographically diverse receivers, at least two, but three or four would be better. If you just have one at your cabin you'll be able to know which direction a signal is coming from, but won't be able to to pinpoint it.

Direction finding vs. locating transmitters

Posted: Tue Dec 31, 2019 2:24 am
by snn47
Hello Jompa,
How accurate dou you want to be? I'll try to explain the different options as I know them

Direction finding (DF)requires at minimum 1 receiver and a (highly) rotateable directional Antenna, and provide the direction towards the transmitter from the antenna direction, but no distance, unless you know accurately enough the difference between when the signal was transmitted and received. Some HAM radio operator practise portable DF with just a HB9CV antenna and a receiver. Bearing accuracy is influenced not only by the antenna gain/beam width, but also by propagation, e.g. if you receive multiple signals via reflections, in addition to the direct path signal.

Doubler direction finder do not require directional antenna but 2 receiver and a DF antenna array. A center antenna provides the reference for 4 or more antenna around it. If you search HAM radio again you'll finde HAM's have built their own Doppler antenna basis using an umbrella for portable DF contest use.

cross-bearing direction finding requires 2 receiver sites either with (highly) directional antenna or Doppler DF. Optimal would be if both receive should sites are about 90° to the receiver site, which is seldom the case. Depending on antenna gain/beamwidth and distance to the transmitter smaller angles work. E.g. we (several HAM's) pinpointed an interferer in ~6 km distance to within a few 100 m uncertainty with our base station antennas (~16 dBi ) using cross-bearing direction . However locating the transmitter location with portable DF equipment (HB9CV antenna) tokk an hour due to multipath on larger houses in the heavily built area.

Multilateration (MLAT) using triangulation and the time difference of arrival of the signal, works also without use of directional antenna , but requires at minimum 3 receiver locations .Receiver have to be located around the transmitter and be within angles of ~±(20° to 160°).

Flight tracking is not multilateration, because it decodes the aircrafts location and other data contained in SSR Mode S 1090 MHz extended squitter and do not perform multilateration like MLAT sensors do.
MLAT sensors are used to provide an additional means of aircraft detection to existing SSR-Radar sites (~27 dbi antenna gain and monopulse processing achieving ~1.2° angle accuracy) using 1090 MHz ADSB extended squitter transmissions of Mode S ADSB out equipped aircraft which transmit at a rate of <6.2 Hz. Multilateration uses time difference of arrival of the signal received at the MLAT sites to calculate the aircrafts location. One difficuilty is the accuracy of the time of reception at the receiver sites. are used e.g. for providing alternative means in additionto existing SSR-Radar sensors to detection aircrafts location using ADSB transmission (or SSR Mode S 1090 MHz extended squitter). Data contained in the extended squitter mayalso are also by MLAT sensor.

The reverse use is multilaterion using ground sites for the navigation of aircraft. Currently used is slant range distance derived from DME/TACAN known ground stations with aknown location. This requires again at least 3 DME/TACAN ground stations. The accuracy or the location depends, within the geographical angle limitations stated above, and the accuracy of the slant range distance. If you are interested search for APNT (Alternatice Precission Navigation and Timing).

Re: Locate Tetra radios/transmitters - help

Posted: Thu Jan 02, 2020 8:36 pm
by jompa
Thank you very much for the detailed option overview. Very much appreciated.

If I can pinpoint a signal to around 100m that would be very good.
I think maybe the doppler DF is best / easiest for me with no knowledge.

If i got the KerberosSDR package and follow their guide, would that work for my purpose with their DF with 4 antennas? If so what kind of antennas should I use? I want to mount them outside on my roof, so they are stationary. I have looked at some 380-400 mhz antennas, but they vary very much in price.

if I get 4 antennas with 3 dBi would that be sufficient?

Sorry for all the bad questions, but I'm very excited about this and have alot of future plans with this.