Friday, May 27, 2011

Aural Null Basis

Two weeks ago I had a conversation with a CASARA navigator about the basis of the Aural Null electronic search pattern. He insisted that the technique was based on the ELT radiation pattern. In other words the ELT radiates power in all directions, as the searchers move away from the ELT the radio energy is dissipated and absorbed by the process of propagating through the atmosphere. When enough of the energy is dissipated or absorbed so that there isn't enough left for the receiver to detect, the signal fades out. If the energy is radiated in the same strength in all directions, the radiation pattern is circular, then the signal will fade out at the same distance from the ELT regardless of which direction you are moving away from it.

On the other hand I know that the Aural Null is based on the radio horizon of the search platform. As pilots learn during basic training, VHF radio signals propagate in a straight line until they are blocked by terrain. (See: From the Ground Up Millenium Edition page 210; and AIP COM 3.5(b).) When the search platform moves past the radio horizon for the altitude it is at, the ELT signal can not propagate around the curve of the earth and the signal fades out. If the surface of the Earth in the area is reasonably flat relative to the search platform altitude, the radio horizon will be the same distance in each direction and the horizon will form a circle. I may have convinced him but I'm not sure.

Then earlier this week I received, from another CASARA navigator, a paper that discusses several search methods. The paper also states that the Aural Null is based on the ELT radiation pattern.

So let's have a look.

The National SAR Manual gives a table of detection ranges for ELTs from aircraft at various altitudes:
Altitude feetRange nm
100030
200045
300055
400067
500085
10000100
15000127
20000150
30000200

The National SAR Manual correctly warns that these distances are for an ELT operating at full power. However, prior to the 121.5 MHz SARSAT packages being switched off, ELTs in courier trucks and with their antennas removed, were routinely detected by the satellites in orbits with altitudes between 850 and 1000 km. So it takes a fair amount of degradation to prevent the signal from propagating to the radio horizon for search altitudes below 10,000 feet. With some simple trig one can plot the location on the Earth under the aircraft (in red) and the location of the aircraft in the atmosphere (in blue) (X and Y scales are in nm, the Y scale is exaggerated with respect to the X scale for clarity):

ELT Detection Range

As you can see the surface of the earth curves away as we move from the ELT. An aircraft maintaining a constant altitude above mean sea level would follow this curve. The position of the aircraft at different ranges form (except for rounding error) a straight line. This means that an airplane that can detect the ELT at 200 nm and 30,000 ft would be receiving the same "beam" from the ELT as the airplane that can only detect the ELT at 30 nm and 1,000 ft (assuming both aircraft are the same direction from the ELT). If the Aural Null was based on the radiation pattern of the ELT, and one airplane lost signal at 30 nm and 1,000 ft, then the airplane at 100 nm and 10,000 ft would not be able to hear it because the signal energy would have been dissipated or absorbed. However because the Aural Null is based on line of sight propagation and the radio horizon it is entirely consistent with the detection ranges provided by the Canadian Forces.

What is troubling is the implications of a CASARA navigator believing the former over the latter. Just as in the days before Copernicus when the misconception that the Sun orbited the Earth prompted intelligent and educated people of the time to derive complex (but wrong) models to explain their observations; labouring on the misconception that the Aural Null is based on radiation pattern can lead intelligent and educated people to incorrect conclusions about electronic searches.

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