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Radio Direction Finding Forum - Discussions regarding direction finding and transmitter location

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  #21 (permalink)  
Old 03-03-2015, 2:03 PM
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Umpteen years ago when I was an RO in the merchant navy, DFing was done with crossed loops (Bellini-Tosi Loops) up on the bridge deck which fed a set of four coils in a screened box in the receiver and a rotating coil picked up the reconstituted signal. The pickup coil was rotated by knob and pointer over a compass card. This gave two possible directions to the source signal. Adding a signal from an omni-directional antenna turned the figure-of-eight pattern into a cardiod (heart-shaped) pattern so you could determine the real direction.
A yagi antenna works fine for VHF, bit too large for the beacons on 300kHz!

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  #22 (permalink)  
Old 03-03-2015, 3:06 PM
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Originally Posted by AC7NT View Post
I have heard that Phased Array antennas are the quickest way to locate and FIX locations of transmitting equipment. Also - the military has been working on RDF systems since RADAR came about, so the capability of quick analysis and reporting of an enemy signal should not be thrown out as "Hollywood" only option.
Since I am the only one who mentioned Hollywood, I assume you are responding to me?

You should probably look a bit closer at what I posted. The only thing I said was “There is Hollywood, and then there is real life.” Hollywood often invents technology, or enhances its latent ability, for the sake of a good story. I also then went on to mention several systems that have been in use in the past and are in use today to…wait for it…quickly report and analyze signals. At least one of which would absolutely be on that ship, and possibly two of them (definitely AN/SLQ-32 as you can clearly see the antennas on the ship, and possibly AN/WLR-1H). I did not mention things like SSQ-108, SSQ-120, SSQ-137, SRS-1, or SLR-25, of course they could also be present, but that doesn’t change the outcome in this case.

A single platform phased array antenna on receive still only gives an angle or direction of arrival on a received signal, it gives no distance for a non-cooperative transmitter with an unknown or asynchronous transmit time. From a surface receive location (such as a ship) this generally does not provide a fix on position or location. From an airborne receiver this can result in a position fix, although that is not a trivial matter. With motion, either from the target or the sensing platform, a plot can be developed over time. However on the show the US ship was at anchor in a port, and the Russian ship was outside the harbor, either moving very slowly or not at all.

The military has been working on and using RDF (Radio Direction Finding) much longer than radar has been around. RDF was a valid tool decades before the first operational radar. However that does not mean there are not limitations to the instantaneous data available to such systems.

Under the conditions of the show, and without another platform to provide a cross cut or more data, the scenario depicted, an exact geolocation plotted on a scope (without the use of radar or IROS3) for a target that was behind a mountain and had no line of sight, was not possible. A bearing cut without range data, generated by some passive sensor and inserted into the Tactical Data System could absolutely be displayed on the bridge, but that would have been a bit more pedestrian than what was shown.

IF the ship was data linked to another platform giving a cross cut for bearing, such as a drone or helicopter with the required RDF capability, then such a plot might be possible. There was no indication of that in the show. IF the ship had real time take from intel or tracking satellites, then such a plot might be possible. There was no indication of that in the show, and they probably would have been aware of the Russian ship earlier.

When I saw these scenes on the show I commented to my wife that the presentation of the exact location was a pipe dream. She is used to my critiques of these kinds of things, and pretty much ignores them. But would a lifelong roofing contractor fail to notice if a TV show depicted a “professional” roofer using a ball peen hammer as a shingler’s hammer?

T!
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Old 04-21-2016, 9:54 AM
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This is something I can speak on with a bit of authority since while in the navy I maintained one of these systems.

The system I worked on was called Combat DF AN/SRS-1. Combat DF is still in use on many Arleigh Burke class destroyers and Wasp class amphibs. There are other systems also, Outboard which is older than CDF and COBLU which is the newer system going on ships now.

As far as CDF goes, when a signal is found, the operator shoots a line of bearing and then also has the option of sending out a system message that goes out to other ships and shore sites. When the other ships and shore sites receive the message, the systems automatically shoot a line of bearing as well, assuming they can hear the signal, then send back another message giving their location and the line of bearing from their location. As the messages come in an elipse displays on the monitor that changes shape and size getting more and more accurate with every response. The entire process takes less than a minute when being done by an operator.

CDF also has an add-on system called Automatic Digital Acquisition Subsystem (ADAS). This is basically a bunch of receivers, each assigned a small portion of bandwidth. Combined they watch entire spectrum for signals that meet certain parameters. When a signal is heard it is recorded, DFed and a message is sent out. All of the data is then stored and an alert is sent for review by an operator. ADAS works almost instantaneously since some signals it is finding are very short burst digital signals that an operator spinning a radio dial may miss.



As far as the hardware goes, there are no movable parts. The system either has 16 or 32 HF antennas all the way around the ship. The system takes measurements off of every antenna and uses signal strength from each antenna to determine the direction of the signal. There is a little more to it than that, but that the gist.

For VHF+ there is a mast mounted antenna that has an octagon type frame that surrounds it and each side of the octagon acts like the HF antennas do.


The white disks on the front of the LHD shown and going along the sides are all covers for the HF antennas.The antenna at the top of the forward mast, hard to see details in this photo is the VHF antenna. There are also too whip antennas for receive only towards the rear that can't bee seen on this photo. If you find a detailed rear shot, the four very rear antennas aren't covered.

https://upload.wikimedia.org/wikiped...onsibility.jpg

Last edited by MikeOxlong; 04-21-2016 at 10:00 AM..
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Old 04-21-2016, 12:53 PM
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Quote:
Originally Posted by sacluded View Post
The system I worked on was called Combat DF AN/SRS-1.
You will find I mentioned SRS-1 in my post

Quote:
Originally Posted by sacluded View Post
As far as CDF goes, when a signal is found, the operator shoots a line of bearing and then also has the option of sending out a system message that goes out to other ships and shore sites. When the other ships and shore sites receive the message, the systems automatically shoot a line of bearing as well, assuming they can hear the signal, then send back another message giving their location and the line of bearing from their location. As the messages come in an elipse displays on the monitor that changes shape and size getting more and more accurate with every response. The entire process takes less than a minute when being done by an operator.
And the above is why I said:
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Originally Posted by Token View Post
Under the conditions of the show, and without another platform to provide a cross cut or more data, the scenario depicted, an exact geolocation plotted on a scope (without the use of radar or IROS3) for a target that was behind a mountain and had no line of sight, was not possible. A bearing cut without range data, generated by some passive sensor and inserted into the Tactical Data System could absolutely be displayed on the bridge, but that would have been a bit more pedestrian than what was shown.

IF the ship was data linked to another platform giving a cross cut for bearing, such as a drone or helicopter with the required RDF capability, then such a plot might be possible. There was no indication of that in the show.
The question that started the thread was how was the ship in the show “The Last Ship” able to plot the position, a location and not just a bearing, of a target using passive techniques under the specific conditions they were in. And the answer was, they can’t, but when has fact ever stopped Hollywood from telling a story?

T!
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Old 04-21-2016, 1:41 PM
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Originally Posted by Token View Post
You will find I mentioned SRS-1 in my post



And the above is why I said:


The question that started the thread was how was the ship in the show “The Last Ship” able to plot the position, a location and not just a bearing, of a target using passive techniques under the specific conditions they were in. And the answer was, they can’t, but when has fact ever stopped Hollywood from telling a story?

T!
With the way CDF works, and I can only assume the other systems since I didn't work on them, there wouldn't be an indication of the other platforms or shore sites without going into the message log. The operator hits a single button and the rest is automated. The CDF operator then can send the plot to CIC through another system. Somebody in CIC or on the bridge would never see any of the "magic", they would just have a target displayed that was passed from SSES (Ships Signal Exploitation Space).

So that's how it could be done with a passive system. Without seeing the show, I don't know what conditions they were in, but many of the remote systems that return LOBs to the requesting ship are automated. If this is some kind of post apocalyptic show, it could be possible those unmanned systems are still operational.
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Old 04-21-2016, 2:46 PM
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So that's how it could be done with a passive system. Without seeing the show, I don't know what conditions they were in, but many of the remote systems that return LOBs to the requesting ship are automated. If this is some kind of post apocalyptic show, it could be possible those unmanned systems are still operational.
In the show there was only the single ship. No helo in the air, no UAVs, no other surface platform to provide a cross cut.

T!
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Old 04-21-2016, 3:33 PM
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In the show there was only the single ship. No helo in the air, no UAVs, no other surface platform to provide a cross cut.

T!
What happened to the shore sites?
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Old 04-21-2016, 6:43 PM
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What happened to the shore sites?
The show is set post apocalyptic, a virus has killed a large portion of the Earths population, nations effectively no longer exist, most people have died and there has been no power for months. The ship has not been able to make electronic contact, voice or data, with any other US forces or facilities anyplace in the World. In this specific scene they have pulled into Gitmo to try and find fuel, and were unable to make any electronic contact with the base prior to pulling into port. They find that the entire base, except for a very small contingent, has succumbed to the virus, just like the rest of the World. While in port a rogue Russian ship that is pursuing them approaches the port and threatens the ship while still outside the port and BVR / direct terrain masked, no line of sight to the Russian ship from where the US ship was piered up (a fact used to the US ships advantage later). They had no idea the Russian ship was approaching or present until it called them on the radio.

Given the description and conditions shore site support is not a factor. The ship does have a Seahawk, but it was not up at the time.

T!
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