The idea of the SE950 was that ships in W.W.II could locate submarines by using RDF. The target signal might be a transmission or from the Local Oscillator (LO) from a receiver leaking out to the antenna.
One use for RDF is part of a navigation system where the transmitting beacon location is known. This might be a low frequency, high frequency, VHF or UHF. The ARN-89 is an example of a LF-HF RDF system and the Light Weight Beacon is an example of an LF beacon that can be backpacked to a known location.
This was made by Bendix.1892151 Direction finding system, Hyland Lawrence A, Wired Radio Inc, Dec 27, 1932, 342/445, 343/728 - loops & sense whip, switchable unidirectional or bidirectional
1981884 System for detecting objects by radio, Hyland Lawrence A, Taylor Albert H, Young Leo C, Nov 27, 1934, 342/27, 367/128, 342/453, 340/991, 342/407 -
bi-static RADAR, depends on re-radiation rather than reflection since the frequency is much lower than current RADAR systems (i.e. probably HF).
2144310 Radio apparatus and method of manufacture, Hyland Lawrence A, Bendix Radio Corp, Jan 17, 1939, 343/842, 343/866, 29/605, 29/602.1, 343/872, 403/270
2159379 Radio antenna, Hyland Lawrence A, Bendix Radio Corp, May 23, 1939, 343/705, 343/872, 123/41.56, 244/1.00A, 343/866 - shielded loop
Another use might be when an aircraft is involved in Search And Rescue (Wiki: SAR) where the beacon transmitter is in an unknown location. Survival Radios such as the PRC-90 transmit on known frequencies so the aircraft direction finder system only needs to operate on a limited number of frequencies.
Modern aircraft ejection seats contain a beacon transmitter, similar to the Vietnam era URT-33. Also built into aircraft there are locator beacons that are activated by a crash. Locating a downed aircraft is a problem that's not been solved as of 2015. The case of MH370 (Wiki) is troubling. The crash type beacon would be ineffective if a plane crashes into water because it will be under water in a very short amount of time. Although an improved crash beacon could be made (or probably already exists) that would eject from the aircraft and float to the surface. The ultrasonic Underwater Locator Beacon (Wiki: ULB) only has enough battery life to work for a month. So in the case of MH370 where the crash location could be in a huge area the battery had gone dead by the time the search location was narrowed. There needs to be improvement in the ULB. One area is so that standard Sonobuoys can detect it's signal. The existing signal was designed for optimum range when a very specialized ULB receiver is used. But in the case of MH370 where no one knew where to look that's of no value. But there are a huge number of sonobuoys all over the globe and the related equipment and personnel trained to use them.
There has been talk about live streaming aircraft data so that a plane in trouble would be recognized before it crashed, but that's not practical because if every plane aloft or moving on the ground was streaming data there would need to be a system that could (1) have enough bandwidth and (2) process that large volume of data in real time. A more practical system would be to put a box between the airplane and Flight Data Recorder (Wiki: Black Box) that would look for signs that a crash was likely. The problem is if the algorithm is too sensitive there will be a lot of false alarms and if too rigid it may not transmit enough information prior to the crash. Note this system would probably be put out of action the instant of a crash.
Here's an idea I sent to the FAA:
"A simple solution might be to make an ADS-B transponder with internal rechargeable battery in an enclosure that's strong enough to contain an internal explosion and or fire caused by it's own battery. This would allow the crew to turn off it's external power, but not turn off the transponder function. The battery would be charged from the aircraft electrical system and there would be signal from the transponder if it's battery state was low. That signal can be monitored both by the crew and anyone receiving it's signal. The battery needs to have enough capacity so that the beacon can operate for more time than the plane it's in. Rather than just using the internal battery to replace the normal external power when the external power is off the transponder would go into a mode that depends on internal sensors. Part of this battery mode would be a signal saying it's on battery power. If a combination of sensors all say the plane is stopped on the ground then the transponder would turn off it's internal battery, but even then would run if external power was applied."
During W.W.II there were HFDF systems called Wullenweber (Wiki), Circularly Disposed Antenna Array or AN/FRD-10 (Wiki). In these systems which started off using a motor driven high speed scan and later upgraded to solid state beam rotation a very short transmission was all that was needed to get a good bearing to the source. This eliminated the cover that was provided by burst transmission like the GRA-71 that transmitted a message at 300 Words Per Minute instead of maybe 15 WPM for a manual Morse transmission.
The Army has a number of DF systems to locate enemy ground radio transmitters, for example the PRD-1 HF Direction Finding Radio used in Korea and Vietnam, Radio Receiving Set AN/TRQ-23 which uses the OE-4/GR antenna group. This system uses a motor drive to rotate the DF antenna and a CRT to provide bearings.
The Drop Zone Assembly Aid System (DZAAS) is a directional receiver worn on the wrist the points to a package on the ground that's arrived by parachute.Key, Object & Pet Location Tags are a new class of devices. Most work in conjunction with a smart phone and can help in locating something. Most of them do not depend on direction finding, but rather proximity between the tag and smart phone.
I have a couple of Black Boxes that are in machined aluminum boxes with water proof sealing that probably good to very great depths, i.e. they are the most indestructible electronic products I've ever seen. Not only could a tank drive over them but also they are immune to high levels of shock and vibration. They transmit a narrow pulse that all my receivers will not receive because a matched detector would require a much wider bandwidth than commercial or military receivers use, so the signal is very stealthy. I'm sure there's a directional receiver that works with these to help someone find whatever is attached.
They can have systems that are similar to the Wullenweber only at much higher frequencies. This could be used to locate other ocean going vessels or land based radio transmitters.
Anti Radiation Missiles (Wiki: ARM) are designed to fly into a source of radiation like would be used on a ground to air weapon system. For example the Shrike and HARM.
Many law enforcement agencies use LoJack (Wiki)
receivers on their cars. The cars use 4 roof mounted
antennas that work in a pseudo Doppler mode to provide a
There are also vehicle beacons that allow a tracking vehicle to follow without being in a direct line of sight. But I expect these have been replaced by tracking devices that combine a GPS receiver with a cell phone.
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An Evaluation of Short-Based Radio Direction Finding 1978 - for VHF EPRIB beacons with analysis of Doppler and other systems
page created 5 Oct 2009.