I've been kicking the idea around of setting up a net with sdr dongles and a raspberry pi's to try and accurately pinpoint or triangulate trucked radios operating on p25 phase one and overlaying the information on to a google maps. Does anyone think that this is possible or perhaps know of an easier way?
Radio Triangulation Questions
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Ubbe
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It is possible as users with SDR dongles that receives ADS-B transmissions from airplains can pinpont them when they do not send out any position information, like milair and airforceone and whatever. So I suggest you look into how that are done and you can perhaps use the software without any major modifications, just a change of frequency.
/Ubbe
/Ubbe
prcguy
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You would need directional antennas at each receiver location and preferably with a null function to give a more precise bearing.
Ubbe
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You can use two vertical omni antennas connected to a receiver through an electronic switch. If the two antennas receive a signal at the exact same time you hear nothing special in the audio. If one antenna would receive the signal slightly before the other, like when the signal source comes from an angle, the switching of the antenna would generate the same effect as frequency modulation and create a tone in the recieved audio that can be used to convert the audio frequency to an angle. Two such receivers at different locations can give you a precise position at a very low cost and are not as complicated as MLAT which needs to see the start of a transmission or preferably a pulsed signal like those in phase2 systems. You only need the two sites to send you the frequency of the audio, or the angle if it computes that, and you plot them manually on a map.
Any method used will have huge problems when trying to position land based transmissions as a direct signal are almost impossible to get and only reflected signals that bounce off buildings and terrain will be received with a delay that will give false results.
/Ubbe
Any method used will have huge problems when trying to position land based transmissions as a direct signal are almost impossible to get and only reflected signals that bounce off buildings and terrain will be received with a delay that will give false results.
/Ubbe
Ubbe
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Our FCC authority in my town have one site with a high rise mast south of Stockholm and another one to the west and they have a $25.000 Rhode&Swartz doppler system on each site and cannot pinpoint anything with accuracy. They have to drive much closer to the object with their mobile direction finder and search for the transmitter.
If you are able to monitor over a longer time like 30 sec and the object is moving then you probably will have the majority of the results pointing at the general direction. If your receivers are placed so that there are buildings or terrain close by then it will be a lot of reflections that disturbs the measurements. You really need to have the equipment high over ground and no nearby buildings and the transmitting object should also be out in the open. Using a simple system you could probably tell if a transmitter where east of town and another where north-west and at best 1-2 mile accuracy, but that might be enough for you.
/Ubbe
If you are able to monitor over a longer time like 30 sec and the object is moving then you probably will have the majority of the results pointing at the general direction. If your receivers are placed so that there are buildings or terrain close by then it will be a lot of reflections that disturbs the measurements. You really need to have the equipment high over ground and no nearby buildings and the transmitting object should also be out in the open. Using a simple system you could probably tell if a transmitter where east of town and another where north-west and at best 1-2 mile accuracy, but that might be enough for you.
/Ubbe
Project25_MASTR
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Time Difference of Arrival. It’s the same principle GPS receivers utilize.
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It is VERY doable using only simple vertical antennas spaced evenly apart from each other. As noted it uses TDOA (Time Difference of Arrival) for the signal hitting each antenna to calculate the direction. Same principal as used with LoJack systems. This is completely different from "legacy" RDF where you use the null from a directional antenna at multiple sites then calculate the possible area for the emitter using "least squares" math. There are many articles regarding this and other techniques for RDF on the RTL-SDR website. Here's the most recent -
direction finding
direction finding
prcguy
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Nets in my area have been trying to use beam antennas for years and have never located anyone. You might get a rough idea of what town the problem is in but that's about it. Most 2m or 70cm beam antennas have a beam width wider than 45 degrees, so they are not very precise plus it takes time to turn the rotor, etc.
An antenna with a null function can have a very narrow beam width and you can probably get less than 5 degree accuracy, but these are usually used at close range to get the final bearing due to their poor sensitivity.
The only way to locate someone reasonably close in less than 30 seconds is with off the shelf DF equipment and I use one of these: GLOBAL TSCM GROUP, INC. - Radio DF DDF2020T
With the kn2c DF system you connect to a laptop running google earth and a receiver. When the offending signal is on the air you press the space bar on the computer and it instantly draws a line from your location to the offending transmitter. With several of these spread across town and others doing the same you can have several lines pointing at the source from different directions and where the lines intersect is your guy.
Or with time a single person can get a bearing on a hilltop, drive to another hill then another and the screen on the laptop will fill up with lines that can point right to the problem. With any of these methods you can have multipath from signals bouncing off buildings, water towers, etc, giving false readings. Its best to be on hill tops when starting to DF someone to get an honest bearing, then go into the slums and neighborhoods once you have a solid fix from several hill tops.
I recently got a call about an interfering signal on C band satellite in So Cal that affected potentially thousands of users and agreed to take on the challenge of finding it. With a spectrum analyzer and C band horn antenna I quickly found a mountain range that was the source but could not pinpoint which of a dozen hills it might be. Looking from part way up some of the hills I would get a solid reading then when I went to the suspected location there was no signal anywhere at all.
I finally went to the very top of the tallest hill in the area and then got a bearing to a distant hill with visible towers. I drove across town to the other side of that hill and from another hill my equipment pointed to the exact same towers. I then got within about a half mile of the towers but could not get closer due to locked gates on any road that led to the site. Even from a half mile away I could identify which of the several towers had the offending signal due to the very narrow beam width of my antenna.
The person that hired me then got in contact with the site owner and the next day I was able to determine down to a couple of dishes where the problem was but could not climb the tower with my equipment. I could not locate the offending transmitter from inside the building because most modern tower mounted microwave dishes have the transmitter and receiver in the dish and only a low frequency exciter in the building feeding the dish with RG-6 type cable or Ethernet.
With the promise of an FCC complaint and the offending frequency in hand, the building owner quickly contacted all users at the particular level on the tower and the next day the offending transmitter was shut down and C band satellite was saved until next time.
A couple of important things should stand out from my incredibly long story. My problem transmitter was on continuously making it much easier to find it. Ham jammers are not as cooperative in letting you find them. Second point is I could not get a usable fix on the offending transmitter until I went to the highest hill in the area where I had a direct shot at the transmitter and not a multipath bounce and scattered signal to waste time following.
Want to beat the jammers at their game? Get the proper equipment and use lots of volunteers and good practices and you will then have a chance.
An antenna with a null function can have a very narrow beam width and you can probably get less than 5 degree accuracy, but these are usually used at close range to get the final bearing due to their poor sensitivity.
The only way to locate someone reasonably close in less than 30 seconds is with off the shelf DF equipment and I use one of these: GLOBAL TSCM GROUP, INC. - Radio DF DDF2020T
With the kn2c DF system you connect to a laptop running google earth and a receiver. When the offending signal is on the air you press the space bar on the computer and it instantly draws a line from your location to the offending transmitter. With several of these spread across town and others doing the same you can have several lines pointing at the source from different directions and where the lines intersect is your guy.
Or with time a single person can get a bearing on a hilltop, drive to another hill then another and the screen on the laptop will fill up with lines that can point right to the problem. With any of these methods you can have multipath from signals bouncing off buildings, water towers, etc, giving false readings. Its best to be on hill tops when starting to DF someone to get an honest bearing, then go into the slums and neighborhoods once you have a solid fix from several hill tops.
I recently got a call about an interfering signal on C band satellite in So Cal that affected potentially thousands of users and agreed to take on the challenge of finding it. With a spectrum analyzer and C band horn antenna I quickly found a mountain range that was the source but could not pinpoint which of a dozen hills it might be. Looking from part way up some of the hills I would get a solid reading then when I went to the suspected location there was no signal anywhere at all.
I finally went to the very top of the tallest hill in the area and then got a bearing to a distant hill with visible towers. I drove across town to the other side of that hill and from another hill my equipment pointed to the exact same towers. I then got within about a half mile of the towers but could not get closer due to locked gates on any road that led to the site. Even from a half mile away I could identify which of the several towers had the offending signal due to the very narrow beam width of my antenna.
The person that hired me then got in contact with the site owner and the next day I was able to determine down to a couple of dishes where the problem was but could not climb the tower with my equipment. I could not locate the offending transmitter from inside the building because most modern tower mounted microwave dishes have the transmitter and receiver in the dish and only a low frequency exciter in the building feeding the dish with RG-6 type cable or Ethernet.
With the promise of an FCC complaint and the offending frequency in hand, the building owner quickly contacted all users at the particular level on the tower and the next day the offending transmitter was shut down and C band satellite was saved until next time.
A couple of important things should stand out from my incredibly long story. My problem transmitter was on continuously making it much easier to find it. Ham jammers are not as cooperative in letting you find them. Second point is I could not get a usable fix on the offending transmitter until I went to the highest hill in the area where I had a direct shot at the transmitter and not a multipath bounce and scattered signal to waste time following.
Want to beat the jammers at their game? Get the proper equipment and use lots of volunteers and good practices and you will then have a chance.
Nice job df'ing that thing! I've had to df microwave signals before and it can be particularly difficult. Signals will give strong bounces of relatively small objects, so it's not always clear that you're following a reflection.
prcguy
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In my case I forget the name of the hill but its in La Habra Heights less than a mile east of what I believe is the KOCE transmitter tower. The site has some CHP microwave and other antennas on another tower.
There's plenty of clutter out that way to make tracking it very challenging. I'm curious what would have been transmitting on c band. It's still licensable for terrestrial point to point, but I'm not aware of anyone actually doing it since AT&T shut down TD2.
prcguy
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It was a 3GHz point to point circuit and looked like an LO went out of lock and out of band with a sweeping signal in the lower portion of C band satellite. It was trashing a specific C band transponder freq from quite a distance and I'm sure many people were affected.
slicerwizard
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Depends on what sort of accuracy you need. Take a few of rtl-sdr.com's new RDF offering and set them up on high rooftops and you'll get locations down to a half mile radius or so - probably.
Project25_MASTR
TX-COMU
They don’t have to be evenly spaced. As long as the location of each antenna (along with VF of feedline and length) and all sites are synced to the same reference, you can triangulate the location of the source.
NARDA actually sells a commercial solution for this. Uses GPS to sync the receivers.
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In reference to "evenly spaced" I meant the short vertical antennas used on top of the vehicle or antenna plate at a single DF site.... NOT even spacing of the sites over many miles of terrain.
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