Aural Null Procedure A

Flying Procedure A

Flying an Aural Null Procedure A is simple, but does require skill from both the pilot and the navigator. To help explain the procedure we have included images of screen shots from the SARMobileAirNav application on the left, and the equivalent image from the Geometry page on the right. You may click on the images to see them in a larger size.

The procedure starts with the search aircraft flying on a search task. The task may be for the specific purpose of finding an emergency transmitter that has been reported by an over flight, SARSAT or other entity with appropriate receiving equipment, or the task may be a visual search on which the aircraft encounters the signal. The navigator takes note of the position of the aircraft and the flight continues either on the established track, or on any convenient heading. It may be easier, for example, to fly the legs due North, South, East and West. On the other hand it may be more convenient to follow ground features that provide guidance on perpendicular directions. Eventually the aircraft will fly out of the signal area. The navigator should take note of the aircraft position when the signal is lost, but this position is not used for the technique. As we have seen radio propagation may cause the signal to be lost and regained several times before it is finally lost. Noting the positions where the signal is lost will help in manoeuvring the aircraft back to the signal near where it was lost. In the SARMobileAirNav screen pictures the points used in the Aural Null technique are represented by magenta circles, the convenience positions are green squares. 

The aircraft enters the signal area, the navigator marks the point with a magenta circle. The current heading of 270° true is suitable so the aircraft continues until the signal is finally lost, marking that location with a green square. The aircraft continues to ensure they are well clear of the signal area. This is important to ensure fading does not affect the accuracy of the procedure and the aircraft has completed the turn before re-entering the signal area. This will ensure that the attitude of the aircraft is as close to the same each time an aural null point is marked. It also ensures that the aircraft is moving more or less towards the centre of the signal area. This means that the inevitable delays cause by reaction time will displace all aural null points inwards by nearly the same amount, preserving the shape of the signal area (hopefully circular) and improving accuracy. With two points acquired the navigator can draw the first chord on his chart and bisect it.

Unfortunately the aircraft can not proceed directly to one end of the perpendicular bisector. So the course is reversed from 
270° to 90°. When reaching the middle of the first chord, the aircraft turns 90° left or right. At this point there is no way to be sure which direction will be optimum. In this example I have used all right turns so that the navigator has the best view of the ground for navigation. The procedure will work just as well with all left turns, or mixing left and right turns if needed. After the 90° turn the aircraft continues until it is completely and finally outside of the signal area. The heading is reversed again, and the aural null point marked as soon as the aircraft re-enters the signal area. The search aircraft proceeds along the perpendicular bisector through the full diameter of the signal area and repeats the aural null point marking procedure one final time.

Now the navigator can construct the final bisector and direct the aircraft to the centre of the circle where a visual search, a sector search or expanding square is performed to locate the source of the signal.

This simulated flight took about one hour twenty minutes from first signal detection until the location of the emergency transmitter could be computed. This is quite a bit of time, but if the closest available aircraft with more advanced equipment (such as a direction finding receiver) was a two hour transit away this will have reduced the time until the vicinity of the emergency transmitter may be searched by 40 minutes. 

Our final pictures are images of the Aural Null computation from SARMobileAirNav. The simulated ELT in this case
was located at exactly N 45 10 00 W 76 20 00. The position calculated by the software is also N 45 10 00 W 76 20 00 with a 2dRMS distance of 0.11 nm shown on the right. The software uses the points to construct a simple, convex, cyclic polygon, then finds the circumcircle and circumcentre as describe in the section on polygon methods. This is displayed on the navigation screen, on the left. The error is propagated back to the points which were used to form the polygon sides and respective perpendicular bisectors than provided the intersection point. This allows an assessment of the quality of each point, the poorer quality points will have a higher individual 2dRMS value. Remember that Procedure A does not provide any method of computing or estimating 2dRMS values even though the information needed to compute the value is contained in the data collected. The procedure simply does not make use of this information.

Next we will look at Procedure B.