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Aural Null

The Aural Null is the simplest type of electronic search. The only equipment required is a standard airband communications receiver (or transciver). Because of this, almost any general aviation aircraft is a useable search platform. The succesfully performance of an Aural Null search does require good navigation skills and some knowledge of how airband communications receivers operate, and how Very High Frequency (VHF) radio waves propagate.

There are two generally accepted methods of performing the Aural Null known as Procedure A, and Procedure B.

Procedure A

In procedure A the search aircraft approaches the estimated ELT location with a receiver tuned to 121.5 MHz. When the signal is first heard (point 1) the crew notes the location and continues to fly a constant track and altitude until the ELT signal is lost (point 2). The crew computes the centre of the track between points 1 and 2 (point 3), turns around and flies back to point 3. There they turn 90 degrees left or right and fly until the signal is again lost (point 4). The crew turns about and tracks the reciprocal course from point 3 to 4 until the signal is again lost (point 5). The crew calculates the centre of the track between points 4 and 5 (point 6), turns around and flies back to point 6 to begin a visual search. The diagram below shows the tracks as East-West and North-South. This is not a requirement of the technique but may make plotting the points and calculating the centres easier. Click on the image to see the drawing full size. More information on how to effectively use aural search patters is available in our Practical Guide to Aural Searches.

Aural Null - Procedure A

Procedure B

Procedure B is somewhat more complex, but can take less time to determine the ELT location. The procedure starts the same way as procedure A with the search aircraft flying towards the estimated ELT location listening to 121.5 MHz. When the ELT is first heard, the crew notes the location but only continues on that track, at a constant altitude, for a short time then turns left or right by 90 degrees. Maintaining altitude the aircraft flies on the new track until the signal is lost. The aircraft is turned around and flown on a reciprocal track until the signal is again heard. This location is noted and the aircraft is flown until the signal is lost again. The aircraft is turned around and flown until the signal is heard again and the location noted. These three points where the ELT is first heard are three points on the circle formed by the Radio Horizon of the ELT. This is why the search aircraft must maintain a constant altitude. This procedure is slightly more sensitive to navigation accuracy than procedure A so I prefer to use only points where the signal is heard rather than mixing points where the signal is heard with points where the signal is lost. This eliminates any asymmetries in the search aircraft antenna placement and navigator reaction time when plotting the position. With three points, the crew can draw three cord lines. These chord lines are bisected. Lines drawn perpendicular to the chord lines from the centre of the chord lines will intersect at the centre of the circle and give the actual ELT location. The search aircraft flies to this point and begins a visual search. Click on the image to see the drawing full size.

Aural Null - Procedure B

Procedure C

We came up with a third, and in our opinion better, way to do Aural Null searches while developing our Practical Guide to Aural Searches. We call it Procedure C. You should take a look at that procedure, and the entire guide.

Try It Out

If you are interested in trying your hand at an Aural Null search, but don't want to go through the complexity and expense of actually flying an airplane, we have a simple Aural Null simulator Java Applet available at java.sarmobile.ca/eltsim.php.
Subpages (1): Radio Propagation
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