Aural Null Procedure B

Flying Procedure B

The Aural Null Procedure B, can be completed in less time than Procedure A because it may take less flying. On the other hand it requires greater precision of navigation, and since it is based on a triangle it does not provide any indication of any accumulation of errors in the collected points. Again we have screen shots from SARMobileAirNav on the left, and geometry diagrams on the right. As before you may click on any of the images to see them in a larger size.

Procedure B begins the same way as Procedure A when the search aircraft first enters the signal area. The location is marked and the aircraft continues on course for some distance. It can be difficult to decide how far into the signal area to fly. The deeper the aircraft flies into the area, without going past the middle, the more accurate the pattern will be but the longer it will take. In this simulation the aircraft flew in about four nautical miles which, as we shall see, is only just enough. The aircraft then turns 90° left or right. The direction doesn't matter, because the procedure requires doubling back. A lucky guess may significantly reduce search time, but at this point there is no way to tell which direction that may be.

The aircraft continues on the new course until it flies out of the signal area. Again we are using the green square call around markers to help orient to the location where the aircraft left the signal area. In this case, because the simulated aircraft did not fly very deep into the signal area, it was quite close to the edge of the signal area all along the second leg. So when it turned around it actually flew quite far past the point where it left the signal area. To get back inside the signal area as quickly as possible the aircraft jogged right. This problem may have been avoided by turning left instead of right, but that may have resulted in entering the signal area in the turn, which has its own problems. But problem solved, the aircraft returns to the original course and continues until it again leaves the signal area. 

Again the course is reversed and the aircraft re-enters the signal area marking the third and final point. At this time the SARMobileAirNav software will construct perpendicular bisectors for each side of the triangle, find the location of the intersection and plot all that along with the circumcircle on the moving map display. The navigator performs the same task on the chart. The circumcircle need not be drawn, but we will see later that it is convenient to have the circumcircle visible.

With the plotting complete the aircraft can fly directly to the fix location and begin a visual search. This procedure took about forty five minutes to fly from signal detection until the aircraft was overhead the transmitter. This is just over half the amount of time the Procedure A flight took, and this was a 'worst case' flight since it would have been quicker to turn left instead of right after entry. But there is now way to know in advance which direction will be 'best case'. The Aural Null computation screen is on the right. There are no
 2dRMS values because with a triangle all three perpendicular bisectors will cross at one location, so the concept of an error value associated with the fix does not make any sense. The location of the simulated transmitter is the same as for Procedure A: N 45 10 00 W 76 20 00. So our fix is actually 1.12 nm from the transmitter, which is not bad, but until the transmitter is found there is no indication how good the fix is.

In the next section we will see what we at SARMobile are calling Procedure C.
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