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.

 

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.

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.

I've recently read a little more explanation since my last post, and I think I may still have some details wrong, but I think what I wrote is close to the way at least some systems work to generate immediate DF results.  It appears to me there are two primary methods, one using angle of arrival of the signal to each receiver antenna, the other using time difference of arrival of the signal to pairs of receiver antennas.  But I've read that some systems used for long-range detection (of aircraft, for example) need three receivers just to get a two-dimensional fix in azimuth and range, and actually require four if you also want to determine altitude of the emitter.  Furthermore, while some systems can get quite precise in azimuth and range to the emitter (as in within a degree and a few thousand feet when it's two hundred miles away), altitutde is more difficult to pin down since it can also be off by many thousands of feet (which is probably only going to varying over a range of maybe 50,000 feet to begin with).