Radio Location - An Introduction

By Peter Swart,January 2002

 

If you have ever seen a caver wondering over fields and dales, with an earphone in his ear, carrying a large hoop in his hand, and twiddling knobs on a box full of electronics, you are probably watching radio location in action. This article gives a brief introduction to what the caver is trying to do, and why it all works.

 

Firstly, what is the caver trying to do? Our intrepid traipser is trying to locate the point on the surface of the earth that is directly above a known location in a cave somewhere below. Our caver has sent a team of cavers under ground and asked them to set up a magnetic beacon in the cave at the point that needs to be located. Although a combination of an underground survey and a surface survey could achieve the same end, this method would usually require a lot more work, and if the cave is very complex, has the potential to introduce significant error. Radio location on the other hand is relatively quick, and potentially more accurate. As it is so easy, it can be used to check on the accuracy of traditional underground surveys.

 

How it works.

 

There are a number of variations on the theme, but the system that is described here is a good example of the general layout. (1) A transmitter beacon is placed inside the cave. This beacon consists of a coil of wire in the form or a loop, that is driven by an oscillator. Together, these components generate an alternating magnetic field that has a specific shape. If the coil is laid horizontal, the shape of the field is similar to a series of doughnuts of increasing size. From the side, the field will look like loops, while from the top, the field lines will appear like a star of lines radiating the centre of the transmitter loop. Close to the centre of the loop, the field lines are almost vertical. Because the field is a low frequency (approx 3KHz) magnetic field, it penetrates rock much better than a high frequency radio wave would be able to do.

 

 

Once the transmitter has been setup, and is generating the magnetic field, it is the turn of the person on the surface to find the field, then find those vertical lines coming out of the centre of the transmitter coil. Once they have found those, they will be directly above the transmitter.

 

The receiver has a similar configuration to the transmitter. It has a coil of wire, with an attached box of electronics. If the receiver coil is held vertically, (like a bicycle wheel when the bicycle is upright), somewhere above the transmitter, but off centre, then some of field lines from the transmitter will pass through the loop. The receiver generates a tone with a volume that is proportional to the amount of field passing through the receiver loop. If the coil is held vertically, and slowly rotated about an axis perpendicular to the ground, the tone will change as the amount of field passing through the loop changes. When the coil points directly towards transmitter, the amount of field that goes through the loop is minimal, so the tone generated will die away almost entirely, but will be at its loudest 90 ° further on. In figure 2, if the coil is at position a, fewer lines pass through the receiver coil than if the coil was at position b. In positions c and d, the receiver coil is lined up with the magnetic field, and very little, if any, field will pass through the coil, so the instrument will remain relatively quite. These positions are called ’Nulls’. As they always point towards the centre of the transmitter, they are exactly what the person on the surface is looking for.

 

 

The surface team will go the place that they guess, or have calculated, is the point directly above the transmitter. They will then wait until they detect the transmitter. In order to do this, they will place the receiver loop flat on the ground, in the hopes that as much of the magnetic field as possible will pass through the loop. This will ensure that they get the best signal possible, just in case their chosen starting point is very far from the transmitter. Once the transmitter has been detected, the surface team will pick up the coil and hold it in the vertical position, then rotate it until they find a Null. This first Null will indicate the line along which the tranmitter lies, but they will not know in which direction to walk.

At this time, the best direction to walk is perpendicular to the line of the coil. After about 15 to 20m, they will stop and try to locate another Null direction. The point at which these two Null lines cross is the point directly above the transmitter. This point is often referred to as Ground Zero, and is the point where a Null signal can be detected in all directions.

Now that our team knows where the transmitter is, it may be useful to know how deep the beacon is. There are a number of ways to do this, but the easiest is to lay the receiver coil flat on the ground at Ground Zero, and to note all of the settings on the receiver, and the output voltage generated by the loop. Once the experiment is over, and the caving team has returned to the surface, the receiver is set up in exactly the same way as noted at Ground Zero. The receiver coil and the transmitter coil are both held vertically, and parallel to each other. The receiver is them moved away until the same voltage is recorded as at ground zero. The distance between the coils will be approximately the depth of the cave beneath Ground Zero.

 

For further details, visit Brian Peases’s page on radio location. He has update information on the newest low frequency transmitters, and small ferrite cored loops.

 

 

References

 

1. Pease, Brian (June 1997) Cave Radio & Electronics Group Journal 28 (BCRA SIG)