Sunday, February 20, 2011

Localizing an ELT Without Special Equipment - Checking the Aural Null

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Update

There is a very good and more detailed explanation of the Aural Null hosted at SARMobile.Ca

Some of my astute readers will have realized the Aural Null techniques, described in the previous article, require relatively level terrain to work. Some will also have realized that the ELT performance, the amount of power it is able to actually get 'on the air' must be enough to be received at the radio horizon for an Aural Null to work. But let's take this one step at a time. First let's look at how the crew can tell if the radio reception patten is not circular, and what can be done about that.

Let's assume that our nice circular radio horizon is disrupted by an inconveniently placed hill that casts a shadow which is manifest as a bite taken out of the nice circular reception pattern. This is, of course, not a very realistic scenario, but it will help keep the diagrams from getting overly complex and that will help demonstrate the point. So we end up with a radio reception pattern below, and as our crew flies an Aural Null Pattern A as described earlier they will end up with the situation shown in this diagram:

Aural Null - Noncircular
An aural null performed on a non-circular pattern.
The beauty of the Aural Null techniques though, is they can be checked. Since the Aural Null depends on a circular radio horizon with the ELT at the centre and the points where the signal becomes detectable or undetectable are on the circumference, geometry allows us to check the search pattern for accuracy. There are two methods of testing. The first uses the property of a circle that requires points on the circumference (1, 2, 4 and 5) to be equidistant from the center (the crew has calculated to be at point 6). This is easily checked by using a ruler, or if you don't mind poking yourself with sharp objects, a set of dividers:
Aural Null - Noncircular test 1
The points on the circumference should be equidistant from the centre.
An other test, taken from Aural Null procedure B involves bisecting the chord lines formed by sets of points (1,4), (1,2), (1,5), (2,4), (2,5), and (4,5) and drawing perpendicular lines. If all the points are on a single circle, then all the bisectors will intersect at the centre. On the other hand if only one is not on the circle, the bisectors will intersect in up to four locations.

Bisectors intersect at four different locations, only one is the true centre.

So if the radio horizon pattern of the ELT is mostly, but not entirely circular the Aural Null patterns may give erroneous results, but the accuracy of the results may be tested by the crew. More points may be acquired and tested until enough good ones are know to find the location of the ELT. In addition this shows that procedure A has a slight advantage in that if it doesn't check out, a group of three points may still exist that provide an accurate position using procedure B.

Unfortunately it is often not that simple. Next time I will show you some properties of how radio signals work that can make life very difficult for searchers, particularly if something is limiting the amount of power the ELT is able to radiate.

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