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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:

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.

In case anyone needs reminding that resolution of an ELT signal is always needs to be done in a timely, efficient and correct manner; or the need to let people know when and where you are flying, and when to expect you back should read this press release from the US National Park Service.

This work is licenced under a Creative Commons Licence.

At this point in this series (which started here) I was going to ask you to apply your imagination to what can happen to an ELT during a crash. I thought this would be necessary because hard data on ELT crash survival has been difficult to come by. However, in going through Transportation Safety Board of Canada (TSBC) Aviation Reports over the past decade I’ve discovered that these reports have included steadily increasing content on the status of the ELT. The debate over ELT failure rates has been going on almost as long as ELTs have been mandated. In a NASA report published in 1990 the National Transportation Safety Board (NTSB) using 1983 through 1987 data found that in 75% of accidents the ELTs did not operate, the Air Force Rescue Coordination Center AFRCC using 1984 through 1987 data found that in 77.9%  of accidents the ELT did not operate. The NTSB further found that 88% of the failures were accident related.

Of the available TSBC Aviation Reports for the years 2008 and 2009 thirty six discuss the state and operation of the ELT. In nineteen (53%) of those reports the ELT could not provide alerting or search guidance because it was destroyed in the crash, destroyed by the post crash fire, submerged in water, unserviceable, or did not activate. Of the remaining seventeen accidents, in nine (25% of the total, 53% of the remaining), the ELT system was compromised by the ELT being separated from the antenna or antenna cable, or the antenna was damaged or destroyed. Of those nine reports, there were indications of a detectable or usable signal in four. Four of the reports indicate the ELT was capable of transmitting on 406MHz (TSO-C126), fourteen indicate the ELT was not able to transmit on 406MHz (TSO-C91 or C91a).

Of the four ELTs specifically identified as TSO-C126, one operated normally, one was submerged in water, and two of the ELTs had compromised antenna systems. Of these two, one was detectable by Low Earth Orbit (LEO) Search and Rescue Satellites (SARSATs) which were able to produce a position. The other was not detected by SARSAT or aircraft overflying the site, though there is no discussion in the report of the equipment, or operating procedures used by aircraft. Surprisingly similar to the numbers given in the 1990 report considering the TSO-C126 design was supposed to improve survivability. The Canadian Mission Control Centre (CMCC) is quoted in TSBC report A09Q0181:

…because the satellites are positioned so high above the Earth, if the antenna is damaged or blanketed by wreckage after the accident, the coded message cannot be captured. However, theLEO satellite can capture the coded message on 406MHz and fix the position of the signal, even if the antenna is damaged or poorly oriented. LEO satellites can sometimes receive a signal and identify its location, as was the case in this situation. [sic]

The corollary is of course that sometimes in these circumstances the LEO SARSAT can not receive the signal, as in TSBC report A09Q0111. So what does this tell us? In 1990 the best estimate was that in about 25% of air accidents the ELT was able to get a signal out to searchers. TSBC data for 2008 and 2009 confirm this except that in about 50% of accidents the ELT is not able to get a signal out, in about 25% it is, the remaining 25% is in a bit of a grey area. The ELT system has been compromised, but in in half of those there was evidence of a potentially usable signal. Would different techniques have allowed SAR crews to be able to detect and home a signal? My personal experience is that a compromised antenna system is often not a factor in airborne electronic searches when the receiving radio equipment is operated in accordance with best practices for the receipt of weak signals. For example, ELTs accidentally activated while being shipped to service depots do not have antennas attached. They are routinely located by SAR resources. I will have to keep looking.

This work is licenced under a Creative Commons Licence.

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:

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:

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 the pattern is circular the perpendicular lines will all intersect in the centre of the circle. If one or more of the points are not on the circumference then the perpendicular lines associated with those chord lines will be very unlikely to intersect at a signle point. We see this in the next diagram where the perpendiculars associated with point 1 are in red, and do not intersect in any single point; and those associated with the points which are on the circumference are in green and do intersect at a single point. This point is also the actual ELT location.

Using techniques from Aural Null pattern B to test the results.

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 and problematic points eliminated from the calculation, or more points 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.

I’ve been doing some data mining of published Transportation Safety Board of Canada aviation reports for an other article I’m writing. It is much more tedious than using the National Transportation Safety Board aviation accident database, but it gives me Canadian data. I was shocked to discover I had to create a data tag to capture the cases where, after a crash, the ELT was found to be turned off (In 36 reports for 2008 and 2009 mentioning ELTs I actuall found three such cases). It doesn’t really matter what you think about ELTs, if it is turned off it is cargo, not safety equipment. If you are the owner-operator-pilot, please make sure that you are complying with the ELT manufacturer’s instructions. If not, you may wish to talk to your Person Responsible for Maintenance (PRM) to see if the ELT configuration can be visually inspected prior to flight.

This work is licenced under a Creative Commons Licence.

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):

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.

This past Wednesday, February 9, two men were killed when their Cesna 150G collided with one of three other aircraft with which it was engaged in formation flying. Predictably a debate has started in the press between the perception of weak regulation and how safe the pilots involved were perceived to be. The Transportation Safety Board of Canada is investigating so we will have to wait for the report to know what ever can be known about this crash. Since the pilot of the other airplane involved in the collision, and the pilots of the other two airplanes in the flight all survived the TSBC should be able to get a fairly complete picture.

One thing that strikes me as troubling is that these pilots were practicing formation flying because they are involved in conducting formation flight over Vancouver’s Victory Square Cenotaph on Remembrance Day. I’m lead to wonder what purpose of flight of modern civilian light airplanes has in a Remembrance Day Ceremony. I wonder as well what the outcome would have been if this collision had happened on November 11th over the Cenotaph.

Safety is more than just a state of mind. Flight safety in particular requires development and adherence to a set of policies, procedures and protocols. It will be informative to see what policies, procedures and protocols these safety minded pilots developed for their activities.

I applied for my new format license in early April. It arrived while I was at the TigerEx this week.  Not bad, and it includes my GPL.

Just got back from assisting a Search and Rescue Exercise in Barrie. Had a blast, learnt a lot, flew in a CH-146 Griffin, met HCol Ed Robertson.

SAR Tech about to touch down.

Photos courtesy of the Canadian Forces Image Gallery, also know as CombatCamera.Ca

Did a SimSAR today. Here is the track reported in real time by my Blackberry. There are some wild values in the file, no doubt due to the fact that the Blackberry was on my belt and didn’t have a real good RF path to either the GPS constellation or cell towers.

Google Maps Track