Never mind I got my answer.

 

My signals friend asked a signals friend who consulted his manual.... so I don't really think I can say how it's done in the Brit army.

 

My thinking two recievers were used was based on trigonometry and searching google, but that was concerning a frequency band rather than a release transmission point.

If you had Marconni antennas and simple recievers you'd probably need three. If you had circular antennas you could probably use two or even one if you can move and the transmitter is fixed. This is WWII stuff.

Knowing two angles (through knowing their locations and the direcion of the enemy), and knowing one side (being the distance between the two recievers), allows them to find the other two sides (and therefore distance to the enemy). Knowing where they are, and the direction and distance to the enemy, they can plot the enemies location.

Right?

Three antennas because redundacy is good?  

My guess is that there is a significant increase in accuracy and precision using three line-of-bearing and time-difference-ofd-arrival measurements rather than two.

Remembering my early Signal training, 1967, some radios used to DF got almost as strong a signal 180 degress and on target.  So two could be used with some accuracy in a normal battlefield situation.  In a guerilla warfare situation 3 were necessary for an accurate fix unless uperators were good and experienced.  I doubt if the situation is the same today.

 

Yimmy,

With GPS it only takes one. You take a fix, move, take a fix, bang.

 

Check Six

 

Rocky

Surely you don't need GPS to do that.  Just being able to read a ap would be fine.

But you still have a considerale amount of time to move from location to location, while radio transmissions are short for a reason.

 

You are right of course.

 

FYI when I took my private pilot training we practiced a DF steer with two stations, the third station must be good if you have it but I can't imagine it being strictly needed.

Check Six

 

Rocky

If DF'ing is done with some sort of direction antenna, like a hoop antenna, and turning it until you get the maximum strength signal, you can do it with just two separated measurements.  However, that's not how it's typically done when you're trying to get really accurate readings.  Doing it that way is likely to have errors of several degrees if you're doing well.

 

Accurate DF'ing is done using Time Difference Of Arrival (TDOA) because timing can be measured extremely accurately and can yield far more accurate results.  Each receiver post uses an omni-directional antenna, and by itself does not determine a bearing to the emitter.  Instead, each receiver post is linked together at a central station that synchronizes their operation.  The time it takes for the signal from the emitter in question to reach each receiver post is measured, and from those times the location of the emitter is calculated (given the speed of the signal is the speed of light).  The time to each receiver defines a sphere around that receiver.  One receiver only tells you the emitter is somewhere on the surface of that sphere, while two receivers tell you the emitter is somewhere on the isochrone (curved line) formed by the intersection of the two spheres.  A third receiver forms additional isochrones between each of the spheres, and where the isochrones cross is the location of the emitter.  Thus it takes three receiveing stations that are synchronized in operation to DF an emitter when using the TDOA technique; however, the results can be very accurate.