Sunday, February 13, 2011

Localizing ELT Signals Without Special Equipment - Aural Null

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Update

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

I have worked radio signals (receiving, transmitting, analyzing, systems design, etc) for over thirty years. I've been a private pilot since 1977. For the past decade I have worked as a volunteer in Search and Rescue. Over this decade I have notice the disturbing trend of other SAR volunteers making some uninformed assumptions about the behaviour of radio signals, antennas and the radios themselves. At first these assumptions were fairly benign, but recently a number of assumptions have come together in a group of people and have lead to experimentation and use of a technique which, I find, is extremely troubling. I hope to be able to explain how troubling, and why, but first we have to start with an introduction to Emergency Locator Transmitters and how to search for them. If nothing else this, if you have never encountered this material before it may help you understand how a search for an ELT proceeds. You may also wish to review some material on how VHF radio propagation affects searching for ELTs.

Emergency Locator Transmitters

There are two main types of Emergency Locator Transmitters (ELTs) in use in Canada today. The older style TSO-C91 units, often called 121.5 ELTs. These have been in widespread use in aircraft since the 1970s. They are triggered by the forces of a crash (or hard landing) or may be turned on manually. They transmit primarily on 121.5 MHz, the aviation emergency communications frequency (though some may also transmit on 243 MHz), a 100 mW signal that has very distinctive swept tone modulation that sounds much like an emergency vehicle siren. The more modern, and recommended style TSO-C126, often called 406 ELTs,  transmit a primary digital signal on 406 MHz at 5 W, but also transmit a similar signal as a TSO-C91 ELT on 121.5 MHz. The 406 MHz signal is designed specifically for SAR satellites to receive and locate automatically. The traditional 121.5 MHz signal helps searchers narrow down the location from a few miles to the exact location without needing new, special equipment to process the digital signal. TSO-C126 ELTs may also be equipped with GPS receivers and can then transmit the GPS coordinates of the airplane to the SAR satellite.

In the Air Search and Rescue world it is often necessary to use the information available to pilots of aircraft that have no specialized equipment on board to try to locate an Emergency Locator Transmitter (ELT). This is most often the case when the signal has been detected by an airplane transiting the area on other business unrelated to the aircraft with the activated ELT and maintaining a listening watch on 121.5 MHz. The pilots of such aircraft are requested to file a report with Air Traffic Services. In Canada these reports will find their way to a Joint Rescue Coordination Centre (JRCC) where they will be analyzed and collated to help give a picture of where the ELT (and perhaps some survivors needing assistance) is located. There are a number of immutable physical properties of the radio signal generated by the ELT, and the universe through which it travels that allow us to do some remarkable things with very simple equipment. There are also some engineering features of that simple equipment that can aid or hinder us in our efforts to find the ELT, and hopefully the survivors, in time. So let's look at some of those physical properties and engineering features, then we can look at what JRCC does with pilot reports of ELT signals, and what remarkable things search crews can do with that very simple equipment, the same equipment any General Aviation pilot is likely to have at his or her disposal.

Line of Sight

One of the immutable physical properties of the ELT signal is that because, like television and FM radio broadcast, the frequency is in the band know as Very High Frequency (VHF) the signal travels in straight lines, or what is know as line of sight. This means that the signal is only detectable by aircraft that are closer to the ELT than the Radio Horizon. There are many mathematical formulae available for different units of measurement, and for use by different groups. For our purposes we need to know that an ELT that is performing properly, has a serviceable battery, and an intact antenna that is not blocked by material that obstructs VHF frequencies should be detectable by an air band receiver as far away as the radio horizon, but no further. In fact, even if some of these confounding factors are present, the signal is still able to be detected to the radio horizon, but I will get to that in a later article. We also need to know that the distance from the ELT to the radio horizon is dependent on the height of the ELT and the height of the airplane listening to it.

Squelch

The squelch is one of the design properties of a standard air band receiver that is convenient for the crew, but makes detection of ELT signals, and determination of the ELT location a bit more difficult. This feature is provided by the Squelch Circuit. The purpose of the Squelch Circuit is to turn the audio output of the receiver off when there is no valid signal being received. This eliminates the need of the crew to listen to the constant white noise that receivers produce in the absence of a strong enough signal. With the Squelch Circuit in operation, it is possible that an airplane could be in range, within the radio horizon, of a transmitting ELT but the crew will not hear the signal because the Squelch Circuit has 'decided' the signal is too weak. For this reason I have always recommended that search crews turn the Squelch Circuit off, though I don't recommend this procedure for routine flights maintaining a listening watch due to the increased fatigue caused by listening to the noise produced.

 Automatic Gain Control

Another property of a standard air band receiver that is convenient for the crew is the Automatic Gain Control circuit (AGC). Unlike the Squelch Circuit, the AGC does not make the job of locating the ELT more difficult when using the techniques described here. In fact the AGC helps by ensuring that the receiver gain (the amplification of the radio signal so that it is strong enough to be heard) is always at an optimum level for the signal received. The AGC will make a return visit to in future articles however. Unlike the Squelch, there is almost never a way to disable the Automatic Gain Control on an air band transceiver.

Analyzing ELT Reports

So, believe it or not, you now have all the information you need to take ELT detection reports from pilots, and compute a very good probable location. For example, let us suppose that a trans-continental jet transport is maintaining a listening watch on 121.5MHz while cruising at 30,000 feet. The crew hears the ELT signal, notes their location and reports this to air traffic services (ATS). Some time later they loose the ELT signal and again note their location and make a report. Alerted to the activated ELT, ATS ask aircraft in the area to listen on 121.5 MHz and report any signals detected. A light airplane flying at 3,000 feet tunes to 121.5MHz and immediately hears the ELT signal. The pilot reports to ATS and continues to listen to the signal until it is lost and reports the location to ATS as well. All of this information is sent on to JRCC where it is used to estimate a location of the ELT as in this diagram (click on the diagrams to see them full size):

ELT Position Estimate

The position of each airplane when they fly into or out of the reception range of the ELT is plotted. Then a circle of the appropriate size for the airplane altitude (and therefore the distance to the radio horizon) is drawn centred on each point. Where the three circles intersect is the estimated location of the ELT.

Aural Null

With this estimated location, appropriate resources may be dispatched to deal with the situation. If those resources are aircraft equipped with Radio Direction Finding (DF) equipment, locating the actual ELT location is quite straight forward. If those resources are aircraft without DF equipment then there are established procedures called Aural Null that will allow those aircraft to establish an accurate location for the ELT. There are two types of Aural Null, Procedure A and Procedure B.

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.



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.


Conclusion

So, this is how, with the application of some physics, mathematics and geometry, the relatively mundane information of where an ELT can be heard, and where it can not be heard can assist Search and Rescue in locating the ELT location quickly and accurately. In my next post I on this topic will cover what hapens when the area where the ELT signal may be detected is not circular. In the mean time if you have any questions, just leave them as a comment and I will try to answer them as I go along.

Footnote

Some people have taken to calling these search patterns Aural Fades in the erroneous belief that the term Aural Null is incorrectly applied. You may see that term in some literature, particularly from CASARA. According to the Oxford English Dictionary, in Radio and Electronics the term null is defined as: A condition of no signal; a direction in which no (or minimum) signal is detected or emitted. Also: a point, state, or region in which no effect occurs, or in which effects cancel each other out. This is a much better description of what is going on than the term fade would imply: Of sound: to die away or out. Also, with in, up, to increase gradually in loudness from a low or inaudible level. This may seem like a rather small nit to pick, but I believe it was the shift in thinking from looking for a condition of no signal to looking for the signal to die away or out or to increase gradually in loudness has lead to misguided conclusions about what the reception of an ELT signal can, and cannot tell you.

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