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Getting a fix

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Getting a fix (theory)

The directional aerial or Doppler Shift system gives an exact bearing.  Only two bearings are required.  The bearing lines drawn on a map will cross at the exact location of the target transmitter.  A single bearing is a line indicating the direction of the target from where the bearing was taken.  The target could be anywhere (in the world?) along this line.  More than one bearing will "fix" the exact location, hence the expression.  To be pedantic, the target could be either where the lines cross or on the other side of the world where they also cross.

In the diagram, a car stops at the side of the road at location A and takes a bearing.  It then drives to location B and takes another bearing.  The lines cross at only one location, so the target is on the road shown at location T.  If the target doesn't move, its location is known within a few feet.  If only it was so simple in real life.  There again, it's no fun if things are so simple.

Getting a fix (Reality)

The directional aerial or Doppler System will have a beam width.  Above, it was assumed that the beam width was zero.  In  reality, the signal strength will be a maximum (or a minimum) over a range of degrees.  Rotating the aerial slightly on either side of the bearing won't change the signal strength much or at all.  Instead of the target being proved to a fixed point, it is now proved to a fixed area. The target is anywhere inside the blue area.

Note that the further away the target is, the more the bearing lines open up.  The target is therefore somewhere in a larger area.  The method is to take bearings to get the initial area, then to move closer to the target and take more sets of bearings.  The target area will get smaller as you get closer.

The process is reduced in accuracy if the aerial has a skew.  If you hold the aerial on the right (or the left) of your body to take a bearing, your body may affect the polar diagram of the aerial and skew the bearing left or right.  Close to your car, bearings may also be skewed.

Reflections from buildings or hillsides can complicate things.  This is only usually a problem if the reflected signal is stronger than the direct signal.  Taking bearings from different locations shows up which bearing is in error.

Another major problem is when you get quite close to the target.  The signal will be so strong that it swamps your receiver.  You can't get a peak and you can't get a null.  The signal seems to come from all directions.  One common method to overcome this is to change to a different receiver.  Instead of using a normal radio which has been made as sensitive as possible, to receive signals of one millionth of a Volt, make a really cheap and nasty one transistor signal sniffer.

The target station may be using a hand portable transmitter and sitting on a bench outside a village pub.  He is usually very amused as he recognises the cars which pass by, then return again and again.  Eventually he sees people walking up and down, holding strange contraptions in their hands.  If he is kind, he will keep talking on his radio.  If he is feeling sneaky, he will pass transmission back to the other target station for a while.

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Page updated on 09 January 2017

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