Off-Tuning - Cardinal Pass - What's in a Name?

This is the third in a series of articles that begins with CASARA, Arctic First Response and Electronic Searches.

When I first became aware of CASARA Ottawa experimenting with electronic search techniques, back when I was the unit training officer, the techniques didn't have a name; but as the discussions that ensued progressed the term Cardinal Pass was coined and stuck. The descriptions that I was getting of the procedures were varied depending on who I talked to, but as I was preparing to write my paper analyzing the technique, and no longer a member of CASARA, I requested a full description from CASARA Ottawa. That request was refused so I wrote my paper based on what information they had provided to me previously. Later I received a paper written by three members of CASARA Ottawa (Anne Barr, Mike Case and Langley Muir) in which the off-tuning augmentation and Cardinal Pass techniques were treated as separate entities. I wrote about the off-tuning augmentation in my last post, this time I will be writing about the Cardinal Pass.

The description from the paper written by Anne Barr, Mike Casey and Langley Muir the Cardinal Pass for an ELT (for which a GPS is required is):

We will discuss its use in the case of an ELT first. We assume that we have already used all of the standard methods and have narrowed the location of the ELT to within fairly narrow limits, say +/- 2 nm, are flying at a suitable search altitude, and have off-tuned the receiver to drop the received signal strength. We also assume that that we have dropped the volume of the receiver to approximately half or below and then have left it untouched, which will allow hearing the signal volume rise and fall as one approaches and departs from the vicinity of the ELT. It is important to avoid changing the receiver frequency, the altitude of the aircraft, or the volume control throughout the rest of the procedure.

Once we have a good idea of the approximate location of the ELT but do not necessarily have a visual sighting of it we can still determine its position quite accurately with two passes near to the target while reading the location display on the GPS. Passing close by the target on a true track North or South, the location of maximum signal strength, determined aurally, will give the latitude of the target and passing on a true track East or West the aural maximum will give a longitude.

If one cannot accurately hear the maximum in the signal, for whatever reason, one should be able to hear when the signal is acquired and when it is lost and, in that case, we could revert to the geometry as described in the CASARA publications. However, we can avoid the graphical techniques (which would be difficult at such a small scale on one’s lap in a light aircraft) and can simply use the average of the GPS positions of the acquisition and loss of the signal. These will be quite close together since we have already increased the sensitivity of the whole operation so that we can only hear the signal for a very short distance. Again, considerable experience can be quickly gained by using a practice ELT in a football field and a hand-held radio and GPS.

This certainly seems quite a simple procedure. Of course it is still hampered by the action of the Automatic Gain Control (AGC) which I described in my last article. In order to be able to hear a change in signal amplitude on the two passes the receiver would have to be off-tuned enough to drop the signal strength to the point that the AGC has reached the maximum available gain. This can be difficult to achieve with any great facility. They don't specify a procedure to ensure that is the case, but they do provide a fall back procedure if the operator is unable to hear the amplitude change. The following table provides receiver sensitivity, minimum signal level for the AGC and the amplitude range in dB:

Receiver	Sensitivity	AGC Minimum Level	No AGC Range
MX 300	1.5μV	2μV	2.5 dB
ATR 720	1.5μV	5μV	10.5 dB
Apollo SL40	1μV	5μV	14 dB
Val Com 760	2μV	10μV	14 dB
Garmin 530	2μV	5μV	8 dB

Given this difficulty, the equal loudness determination problem (given the sweeping tone of an ELT signal) described by Fletcher and Munson covered in my paper I would recommend that they forgo the complexity of amplitude discrimination and simply divide the cardinal transects half way between the detection and loss points.

There is also the question of knowing when you have narrowed the location of the ELT to within +/-2nn; there is no procedure provided to measure this value. A standard Aural Null search will not provide this information, so they must mean us to use a non-standard method.

While I was still CASARA Ottawa training officer I asked the three people involved to describe the Cardinal Pass technique and its use. I got three different answers. Langley Muir gave me essentially what finally made it to their paper. Anne Barr told me that the Cardinal Pass allowed search crews to eliminate the long flying legs of the Aural Null and narrow the ELT position more quickly; a task their paper now assigns to the off-tuning augmentation to the Aural Null. During discussions I sent an email reply to Muir and copied to the other two in which I said:

The cardinal pass technique is interesting, and deserves more investigation but at the moment still presents some difficulties for widespread use.

In reply Mike Casey gave me the most detailed description:

If I may …

Further to Langley’s email we have two other occasions where the technique is documented and the timeline for signal resolution is relatively short. There was the JRCC tasking for Hawkesbury last July and the evaluation ELT this past October. (There was one other event where Mo Egan flew as the navigator but I do not believe that we have the GPS data for that event). I believe that Langley can speak to another event at last September’s SAREX.

Hawkesbury

The JRCC tasking was to fly an expanding square with 10 mile track spacing as high as practical. The signal was acquired at 0020Z on 121.50 as the flight passed to the north of the town of Alfred at 4500 feet ASL. There was an audible signal when off-tuned by 0.25 (squelch open) which led the navigator to deduce the signal was in the vicinity and to abandon the original tasking and to home the signal in a more focused manner. If memory serves, the CSP for the expanding square was Aberdeen (453007440).

The CASARA Ottawa crew knew that a SEREBEC air asset had been tasked to work the same signal. Before descending to a lower altitude, the Ottawa flight orbited between Vankleek Hill and Hawkesbury (at altitude) until it was resolved that there was no conflict with the SEREBEC asset. In the time that it took to confirm this the navigator observed that the signal was detectable when off-tuned to a greater degree, possibly  0.50 to 0.75 (squelch open) when close to the town of Hawkesbury.  

There was (by fluke) one track to the north and one track to the west only because the Ottawa river was the ‘base line’ and the town of Hawkesbury (the bridge) was the primary visual reference. As the flight flew over the town the off tuning was more pronounced with a rapid drop in signal strength as the flight continued north of the river. The flight turned to the east and then to the south in order to follow the river westbound. When flying to the west it was (in part) to resolve whether the signal was coming from the larger Hawkesbury airport NV4 (453707439). The audible signal did increase but began to decrease as the flight approached the bridge. The flight continued past the bridge with the audible signal rapidly decreasing effectively eliminating NV4 as the source.

The flight turned to the north and then turned to the south-east to begin what was to be the start of a sector search. Bets on the on the flight at the time were on a structure/barn on the Quebec side of the river. The flight continued on its heading with the audible signal increasing after passing the structure.  With greater and greater off-tuning the audible signal increased as the flight entered Ontario. When the audible signal began to decrease, and with prior knowledge of Hawkesbury East PG5 (453507433), the focus moved from the structure to the airport.

Several passes were made over the field with the signal off-tuned substantially, as high as 123.30 (squelch open), resulting in a rapid increase in audible signal peaking as the flight passed over the airport and an equally rapid decrease in audible signal as the flight continued along its heading.

I will return to the content of this email in a later article, but for now it is clear that the procedure used during the Hawkesbury SAR tasking has little in common with the Cardinal Pass as described in their paper; but much in common with the off-tuning augmentation to the aural null. At the time I, perhaps naively, assumed that the name applied to a combined use of the two techniques serially; and this is, after all, what they seem to be proposing in their paper: two signal amplitude driven electronic search techniques, one course, one fine, both using a sensor which is not able to reliably provide signal amplitude data. I did try to warn CASARA and the RCAF of this possibility in my paper:

In addition to the failings of the technique itself, belief that the Cardinal Pass is viable requires the belief in radio propagation, receiver performance, and human hearing abilities that are not supported by scientific study or principles. As these beliefs become common, they will have a further detrimental effect on operational capability by confusing members about the foundation and practice of official techniques.

More importantly this agile nomenclature may have allowed CASARA leadership to make statements to the RCAF that they felt were factually accurate, but lead the Air Force to the wrong conclusion. Mr David Elias, the public affairs officer for 1 Canadian Air Division told me on March 16, 2011:

Noting trials of the Cardinal Pass have concluded this technique to have limited merit, we have been given explicit assurance by CASARA that they have not, nor will they, employ the Cardinal Pass in real life search and rescue operations.

Considering the statements in the Barr, Casey and Muir paper published December 2, 2010, sent to me by Muir on May 24, 2011 and Mike Daniels' statements made on June 17, 2011 (which I covered in my second post of this series) it is difficult to accept that CASARA had concluded the technique had limited merit in March. And while by December 2, 2010 Barr, Casey and Muir may have decided that the technique describe by Mike Case (included above) was not a Cardinal Pass, it was definitely not an authorized technique and is even more likely to place airplane crash survivors in jeopardy than the Cardinal Pass. Not authorized? One of the other things Mr Elias said to me was:

The Department of National Defence’s current Contribution Agreement requires that all new "projects" CASARA may wish to initiate must be vetted through the Air Force for their approval. ... The Cardinal Pass has not been approved, nor is it accepted as a search procedure for CASARA.

I am deeply concerned that after over 10 years as a volunteer member of CASARA, I had to learn of that requirement from the public affairs officer of 1CAD. How much are you willing to bet that the off-tining augmentation to the Aural Null was not approved either? But I will also get into this in more detail later in the series.

In my next article I will look at what role scientific principles played in these events.

Happy Robert Burns Day

Is there for honest poverty
That hangs his head, an' a' that
The coward slave, we pass him by
We dare be poor for a' that
For a' that, an' a' that
Our toil's obscure and a' that
The rank is but the guinea's stamp
The man's the gowd for a' that

What though on hamely fare we dine
Wear hoddin grey, an' a' that
Gie fools their silks, and knaves their wine
A man's a man, for a' that
For a' that, an' a' that
Their tinsel show an' a' that
The honest man, though e'er sae poor
Is king o' men for a' that

Ye see yon birkie ca'd a lord
Wha struts an' stares an' a' that
Tho' hundreds worship at his word
He's but a coof for a' that
For a' that, an' a' that
His ribband, star and a' that
The man o' independent mind
He looks an' laughs at a' that

A prince can mak' a belted knight
A marquise, duke, an' a' that
But an honest man's aboon his might
Gude faith, he maunna fa' that
For a' that an' a' that
Their dignities an' a' that
The pith o' sense an' pride o' worth
Are higher rank that a' that

Then let us pray that come it may
(as come it will for a' that)
That Sense and Worth, o'er a' the earth
Shall bear the gree an' a' that
For a' that an' a' that
It's coming yet for a' that
That man to man, the world o'er
Shall brithers be for a' that

Vans RV-7A In-Flight Break Up

Prior to the inception of Safety Management System (shorted to SMS) safety was improved primarily by investigating accidents, determining a cause and enacting regulations, procedures or policies in an attempt to prevent the cause from resulting in more accidents. Safety Management Systems is designed to provide policies and procedures that help us act in ways that prevent accidents by identifying risk and acting in advance. This holds the promise of continuing to reduce preventable accidents; but SMS must be used in order to work. We also must still pay attention to the lessons of accidents past.

One very expensive lesson was learned at the cost of fourteen souls on board Continental Express 2574 September 11, 1991. Due to a maintenance error, the leading edge of the left horizontal stabilizer was not properly secured. During descent for landing the leading edge broke off the stabilizer rendering the Embraer 120RT uncontrollable. The crew lost control, the airplane entered a steep nose down attitude causing negative G loads that exceeded the structural limits of the airframe which broke apart and crashed in a farmer's field.

That such a small part could play such an important role in the control of an airplane may seem unimaginable to the those unfamiliar with aerodynamics; there are few extraneous parts on an airplane.

As all the other pilots of my generation I learned that airplanes have a designed maneuvering speed. This is the maximum flying speed at which the full deflection of controls will not over stress the airframe. While not a conscious conclusion, I'm sure I would have made the same assumption that the co-pilot of American Airlines 587 did on November 12, 2001. The flight encountered wake turbulence from a heavier airplane. During recovery from the encounter the co-pilot's cyclic rudder reversals, using rudder control in one direction followed quickly with rudder control in the other direction, over stressed the vertical stabilizer (or fin) of the flight 587 Airbus A300 resulting in separation. Without the fin and rudder the airplane could not be controlled. The cost of this lesson was 265 killed, five of them on the ground.

This brings us to January 23, 2010. Three  pilots fly their home built airplanes to a small airport near Linsay Ontario. This airport is a popular spot to fly to. A well travelled highway passes very close to the airport. So in addition to a vibrant local flying community, the airport boasts a full service restaurant popular with motorists as well as fliers. The three departed for Smiths Falls together, but on the way one broke off from the group headed to Bancroft.

The other two continued towards Smiths Falls in tandem. Along the way the lead airplane conducted a series of aerobatic manoeuvres, the tandem airplane, equipped with a video camera, was to follow and record the lead aircraft. During this phase of flight, the lead pilot lost contact with the accident airplane. A search was conducted which located the crash site. The fin and rudder were found to have separated in-flight landing 0.6 nm from the rest of the airplane.

The accident airplane was equipped with an electronic flight information system (EFIS) which recorded information from flight and engine instruments at 5 second intervals. This data, and the video recorded during the flight were recovered during the accident investigation. During the aerobatic sequence, following a rapid descent, the video recorded the onset of vibration around the longitudinal axis, followed by yaw, roll and ground impact. This is consistent with structural failure and separation of the fin. Without the fin, the pilot would be unable to control the airplane.

From the TSBC report for the accident airplane:

The maximum manoeuvring speed (Va) of 124 knots is the maximum permissible speed at which full and abrupt controls can be applied. Any speed in excess of Va with full control application could result in g-loads in excess of design limits.

 And:

The never exceed speed (Vne) of 200 knots is the maximum permissible speed under any condition. Any speed in excess of this could result in structural damage. Full control application at Vne would produce a load of approximately +15.0 g.

The EFIS record of the flight contained some spectacular numbers. Prior to the aerobatic sequence the airplane was level at 2650 feet above the ground travelling at 168 knots. The maximum recorded flight parameters were: vertical acceleration 3.5g; roll 115°; pitch up 19°; pitch down 45°;descent rate 12,000 feet per minute; airspeed 234 knots. The airplane was flying far faster than the maximum manoeuvring speed prior to entering the aerobatic sequence, and surpassed the never exceed speed during the sequence.

Much of the accident report focuses on the airplane paint scheme which was added after the first flight. Adding weight to a control surface can change the balance of the control adversely affecting protection from flutter:

Flutter is the rapid and uncontrolled oscillation of a flight control resulting from an unbalanced surface. Flutter normally leads to catastrophic failure of the structure. Due to the high frequency of oscillation, even when flutter is on the verge of becoming catastrophic, it can still be very hard to detect. Factors that can contribute to the onset of flutter include high speed, a reduction in stiffness and a change in mass distribution.

Even if the control balance did not bring on the flutter, deflection of the rudder too much when operating above manoeuvring speed, or even small amounts when above the never exceed speed could have stressed parts of the airplane to the point that structural integrity was compromised. In the end it does not matter if the fin separated because of progressive failure, resulting in vibration; or the early onset of flutter cause by imbalance caused structural failure, the results would be the same.

The cost of this lesson is, by some measures, less than the other two. The pilot and sole occupant was the only casualty. The lessons are just as valuable. Flight safety is often a mater of establishing margins between the known safe envelope, and catastrophic failure. Quite often we can cross into those margins and return safely. However, each action that narrows the margin takes us closer to catastrophe.

Fly safely.

Using Singleton Classes in BlackBerry JDE

This work is licenced under a Creative Commons Licence.

BlackBerry smartphone applications support multiple entry points. This is very handy for programers who want to provide a GUI applicatin that also has a background processing component that listens for push data or some other useful function. This situation is complicated by the fact that each entry point is treated as its own process so classes get instantiated in separate address space. This is usually what the programmer wants, but occasionally we want to share the same object(s) between the two (or more) processes that make up a program. RIM recommends that you create a singleton class using the RuntimeStore

First the singleton class (borrowing from the RIM article on the subject, and the StackOverflow question that prompted this post):

import net.rim.device.api.system.*;

class MySingleton {
   private static MySingleton _instance;
   private static final long GUID = 0xabad1deaL;
private int i;

   // constructor
   MySingleton() {
      i = 0;
   }

   public static MySingleton getInstance() {
      if (_instance == null) {
         _instance = (MySingleton)RuntimeStore.getRuntimeStore().get(GUID);
      if (_instance == null) {

         MySingleton singleton = new MySingleton();

         RuntimeStore.getRuntimeStore().put(GUID, singleton);
         _instance = singleton;
         }
      }

      return _instance;

   }

   public synchronized int getI() {
     return i;
   }

 

   public synchronized void setI(int newI) {
      i = newI;
   }
 
   public synchronized int incI() {
      i++;
      return i;
   }
}

Next we define the main classes for the two entry points, the UI class defines the main, but it could be defined in the non-UI class:

import net.rim.device.api.system.*;

class MyUiClass extends UiApplication {

   public static void main(String argv[]) {
      // A simple way to differentiate between entry points is to call one with an argument and the other without
      // Auto-starting applications can have some other requirements that have to be followed, such as waiting for
      // the OS to complete booting so that networks, the UI and other needed services are available.

      if (argv.length > 0) {
         MyUiClass c = new MyUiClass();
         c.enterEventDispatcher();
      } else {
         MyClass c = new MyClass();
         c.enterEventDispatcher();
      }
   }

   private MySingleton _singleton;

   public MyUiClass() {
      _singleton = MySingleton.getInstance();
      // ToDo: initialize class to to the real work
   }

   public doSomthingToI() {
      _singleton.incI();
   }
}

and...

import net.rim.device.api.system.*;

class MyClass extends Application {

   private MySingleton _singleton;

   public MyClass() {
      _singleton = MySingleton.getInstance();
      // ToDo: initialize class to to the real work
   }

   public int getValueOfI() {
      return _singleton.getI();
   }
}

Hands On BlackBerry PlayBook 2.0 at CES

A YouTube video uploaded by MobileSyrup shows what we have in store from the up-coming PlayBook Tablet OS 2.0 upgrade scheduled for next month.

Highlights are:

Virtual Keyboard. A very intriguing feature has been added: contextual next word prediction. I can't wait to try that out myself. Auto double space end of sentences, correct and auto contractions are finally here!

Integrated Messaging. Finally a native messaging application, but it does much more than Email, integrating social media with the email application. Rich text features for mail composition are supported.

Native Contacts Application. Like messaging, the contacts application integrates contacts from all configured accounts and does a "intelligent" merge.


Native Calendar Application. The new calendar application can manage multiple calendars, continuing the integration from messaging and contacts. Also continuing the social integration (unfortunately not shown due to technical issues) the calendar application shows you who you will be interacting with during the day and allow you to get status information from social media in the calendar.


Improvements to BlackBerry Bridge. Remote control allows the BlackBerry smart phone to control the PlayBook for presentations, or any other time you want to have the PlayBook do something having it in your hands. Open on PlayBook allows you to select media or documents on the BlackBerry smart phone to open on the PlayBook.


Documents to Go. Improved editing features. Presentations can be edited now.

At almost 18 minutes long this is a good look at what is coming down the line for PlayBook owners. Go have a look.

BlackBerry Simple Sorting Vector

This work is licenced under a Creative Commons Licence.

Someone requested an example of using a SimpleSortingVector with the Facebook API. I don't know what a FriendsRequestObject looks like so I have assumed it has a getName() method that returns the name as a String.

import net.rim.device.api.util.Comparator;

/**
* A comparitor class to compare the class that defines the objects you want to sort
*/
public class MyComparator implements Comparitor {
  public int compare(Object o1, Object o2) {
    FriendsRequestObject f1 = (FriendsRequestObject)o1;
    FriendsRequestObject f2 = (FriendsRequestObject)o2;

    return f1.getName().compareTo(f2.getName());
  }

  public boolean equals(Object obj) {
    return compare(this, obj) == 0;
  }
}

import net.rim.device.api.util.SimpleSortingVector;

SimpleSortingVector ssv = new SimpleSortingVector();
ssv.setSortComparator(new MyComparator());

Profile[] f=user.getFriends();
for(int i=0;i < f.length; i++){
  String id=f[i].getId();
  String name=f[i].getName();
  vector.addElement(new FriendsRequestObject(id,name));
}

ssv.reSort();

CASARA, Electronic Searches - A Radio Tutorial

This post continues a series I began in my post on CASARA, Arctic First Response and Electronic Searches.

I have sent individual invitations to the three authors of the paper I'm reviewing (Anne Barr, Mike Casey and Langley Muir) to read what I am posting, and to submit their own comments. So far the only response I've gotten was from Anne Barr who referred to an email sent out by Mike Daniels (CASARA Ontario President) in which he said:

There is an issue here of two very knowledgeable people at odds regarding the Theory of the Aural Null Procedure. I doubt whether either of you will ever agree on this issue. Therefore the best resolution is to agree to disagree and move on. Quite frankly we are all far too busy to carry this discussion further, I have instructed CASARA Ottawa to close their book on this issue once and for all. By doing so they may get on with their business on hand.
...

As stated I have requested CASARA Ottawa close their book on this issue of Aural Null/Cardinal Pass and not to initiate or respond to further correspondence on this issue unless it originates from CASARA National or from one of the above mentioned agencies. Further, I have suggested to John Davidson, President, CASARA National and the Executive that they do the same.

The business on hand is, of course, searching for lost aviators. To do that they will on occasion need to use aircraft without Direction Finding equipment to perform electronic searches. This will require that they use the Aural Null technique. If, as Mr Daniels asserts, "There is an issue here of two very knowledgeable people at odds regarding the Theory of the Aural Null Procedure", then the other knowledgeable person should be able to provide valid scientific, engineering and technical data substantiating that the techniques documented, and that have been used on actual search and training missions are sound, effective and safe. Alternatively they could acknowledge that the techniques are unsound, ineffective and unsafe. I would also appreciate a full explanation of what instructing CASARA Ottawa to "close their book on this issue" really means. So I extended the same invitation to John Davidson, President of CASARA National. Until they respond, I can only assume that because they consider "two very knowledgeable people to be at odds" they consider the material in the Barr, Casey and Muir paper to be correct and it will continue to influence CASARA operations. So I will continue with my review.

A good portion of the Barr, Casey and Muir paper is given over to Off-Tuning methods, so it seems appropriate to go into off-tuning in some depth. Off-tuning is the practice of tuning away from the actual frequency on which a transmitter is sending a signal. Due to a number of factors a transmitter will always emit some of its energy on frequencies other than the one intended. One mechanism for this is harmonic radiation. Harmonics are frequencies that are a multiple of some fundamental frequency. So an ELT designed to transmit on 121.5 MHz will also transmit on harmonics at twice, three, four, etc times that frequency or 243 MHz, 364.5 MHz, 486 MHz, etc. However since modern transmitters are designed to transmit on many evenly spaced frequencies, or channels, (even ELTs) their design and construction gives rise to spurious transmissions on those channels adjacent in frequency to the intended channel. So an ELT transmitting on 121.5 MHz may also transmit on 121.525 MHz, 121.55 MHz, 121.575 MHz, 121.6 MHz, etc. It may also transmit on 121.475 MHz, 121.45 MHz, 121.425 MHz, 121.4 MHz, etc. Or on some subset of these frequencies depending on the transmitter design. These spurious transmissions are so common that they may cause interference to users of adjacent channels so regulatory agencies such as the FCC enact regulations to control them. For ELTs these are:

   (h) For ELTs operating on 121.500 MHz, 243.000 MHz and 406.0-406.1 MHz
   the mean power of any emission must be attenuated below the mean power
   of the transmitter (pY) as follows:

   (1) When the frequency is moved from the assigned frequency by more
   than 50 percent up to and including 100 percent of the authorized
   bandwidth the attenuation must be at least 25 dB;

   (2) When the frequency is removed from the assigned frequency by more
   than 100 percent of the authorized bandwidth the attenuation must be at
   least 30 dB.

These regulations are much less restrictive than normal aviation band communications transmitters intended for routine communications. An ELT, as emergency equipment is given a regulatory "break" in the operating parameters it has to meet. Normally the amount of power on each spurious emission will continue to be reduced as the spurious frequency is farther from the intended frequency. Even though the regulations would allow all spurious emissions removed more than 100% of the bandwidth to be 30 dB lower than the emission on the intended frequency, there would be some reduction of power for each step away in frequency.

This property allows people hunting transmitters to narrow down the location by tuning off the intended frequency when they are very close. Using this technique may, for example, allow a CASARA ground team to identify which airplane of several located at an airport has a transmitting ELT. The technique also works in the air, but there are some other factors that make the situation not quite that simple. If you read my previous post on the subject, these other factors are some of the complications that Barr, Casey and Muir chose to leave aside. I will address them in due course. If you have not already read that post I suggest you do so before continuing with this one.

In their paper Barr, Casey and Muir state:

There are essentially four factors that will determine the audible volume of the ELT signal; the relative orientation of the two antennae, the aircraft altitude, the receiver frequency, and the volume setting on the receiver. When prosecuting a search it is important that one changes only one variable at a time, observes the effect, and only then change one of the other variables. After one is in the near vicinity of the ELT, using the standard techniques, one can then refine the search further using the other techniques described in this paper.

Unfortunately this completely ignores the action of the automatic gain control, and propagation effects. It is also unclear where they came up with a relation between the aircraft altitude and the audible volume. The effect of the volume setting needs no explanation. Changing the receiver frequency is off-tuning; tuning to a spurious emission with more or less power will have an effect on the amount of power received. Changing the orientation of the receiver antenna with respect to the orientation of the transmitting antenna will cause polarization effects and change the amount of power received. However the orientation of the ELT antenna, in the case of an actual search, is unknown. The orientation of the receiving antenna is determined by the attitude of the airplane as it maneuvers. So there is little a search crew can do about or with polarisation effects other than to be aware of their potential. Propagation effects, especially multi-path induced fading, will also change the amount of power received. The automatic gain control is built into the receiver to compensate, as much as possible, for what ever affects the amount of power received.

It may help to think of the automatic gain control as a robot that has its own volume (gain) control that works on the signal before it gets to the pilot's volume control. The robot never shows the pilot what setting it is using. The robot turns its volume control up or down as the amount of received power decreases or increases trying to keep the signal amplitude it sends on at the same constant level. The pilot's control then increases or decreases the output volume from the one maintained by the robot. This is why a receiver is able to deal with a transmission from the control tower only a mile distant that may have a 50 or 100 watt transmitter, and the reply from an airplane with a 10 watt transmitter 25 miles away without the pilot changing the volume control. Of course there are limits to the amount of control the robot has. A very weak signal may be so weak the robot turns its volume control up to the maximum. Any reduction in power beyond that point could result in a reduction of output volume, or the loss of signal reception. A very strong signal may be so strong the robot turns its volume control down to the minimum. Any increase in power beyond that point will result in distortion of the output signal. Except at these two extremes, it is not possible to determine with any degree of certainty what the strength of the signal was before the robot applied its adjustment. This also underscores a problem with the technique proposed by Bar, Casey and Muir. How can the crew only change one variable at a time, when an automated system is constantly changing one of the variables behind the scenes? Clearly a technique that does not have this limitation is needed.

Without knowing the location, power output and the transmit antenna radiation pattern of the ELT, there is no way to operate the search airplane, other than by chance, to put the receiver in the position of dealing with a signal that forces the automatic gain to the minimum. In fact, the ELT output may not be enough to force the automatic gain to a minimum regardless of how close the search airplane gets to the ELT. It is possible to operate the airplane to put the receiver in the position of dealing with a signal that forces the automatic gain to the maximum. Off-tuning far enough, flying far enough away that path loss attenuates the signal enough, or flying at the cusp of the radio horizon so the signal is attenuated can all put the receiver into a condition where it can not apply any more amplification through the automatic gain. Tuning further off frequency, flying further away or continuing over the horizon respectively will result in the loss of the signal. In fact crossing the radio horizon from beyond to within the horizon and noting the position of the cusp as the signal is detected is the basis of the Aural Null technique. The Aural Null procedure that Barr, Casey and Muir propose as an augmentation to the standard procedure is a combination of off-tuning and using distance to reduce received signal strength until the ELT signal is not detectable.

If we assume the received signal strength decreases monotonically with increasing distance from the ELT, on the intended frequency and all spurious emissions, then their procedure does sound logical. Succinctly it is: once the ELT signal is first detected, the crew begins to off-tune the receiver. At first off-tuning may result in a loss of signal if the spurious emission is not strong enough. Eventually though, as the airplane continues in a direction that reduces the distance to the ELT the signal will be detected on the off-tuned frequency. As the searchers continue to close the range to the ELT, they will be able to off-tune the receiver by greater amounts and still detect the signal. Once the signal is lost on an off-tuned frequency where it was previously heard, the crew can conclude that they have passed abeam the ELT, change direction by 90 degrees and continue the procedure. But can we reasonably make that initial assumption?

One of the things that Barr, Casey and Muir leave aside from consideration are propagation factors. As a radio signal propagates from transmitter to receiver it is affected by the environment. This will have an effect on the signal strength at the receiver that is called fading. The dominant form of fading is multi-path induced fading, often shortened to multi-path. Multi-path affects consumer reception of broadcast radio and television as well as cellular telephone so it has be studied extensively. Radio engineers are primarily interested in fade depth which is the amount that received signal strength is reduced by propagation under various conditions. Fade depth is factored into the link budget equation to determine how much power and gain is needed to ensure reliable communication. Most of the work in the field has been done for terrestrial or satellite communications but S. Loyka, A. Kouki and F. Gagnon published a paper in the proceedings of the IEEE on the computation of fade depth for ground to air communications links. Their results show that, in the presence of specular reflection, for clearance angles (the angle the search aircraft is above the horizon of the ELT) between 0.1° and 10° fade depth in dB is well estimated by:

-20 × log(θ) + 25

where θ is the clearance angle in degrees. For clearance angles of about 5° fade depth can be 10dB, 1° gives 25dB and 0.1° 45dB. Search aircraft at an altitude of 500 feet above the ELT will have a clearance angle less than 1° when further than approximately 5 nm away; at 1000 feet the same clearance angle is achieved beyond approximately 10 nm. Fading, and in particular multi-path induced fading, does not affect received signal strength uniformly. Since the fade depth depends on the relative lengths of multiple propagation paths from the ELT to the search aircraft it will vary as the aircraft moves about or changes altitude. If we consult the received signal strength charts from my previous article:

it is easy to see that the 25dB attenuation possible at 1°, or even 45dB at 0.1° will have little effect for the 20dBm budget curve at ranges less than 70km. However when combined with the Barr, Casey and Muir off-tuning aural null method, the total effect of off-tuning and fading can take the signal below detectable strength at large distances from the ELT. But since the fading is not uniform the received signal strength does not decrease monotonically as distance from the ELT increases. Since a deep fade would be indistinguishable from passing abeam the ELT; following their procedure a search aircraft encountering a deep fade while off-tuning would erroneously conclude they had passed the ELT and turn. They do not provide any procedure for detecting or reacting to this misguidance due to fading.

The graphic on the left (from www.sarmobile.ca which has a more in-depth description of ELT propagation effects) one can see how fading in combination with off-tuning can create areas of local signal peaks that can quite easily seduce a search aircraft away from the ELT. During my time as a member of CASARA Ottawa I observed this happen on at least three separate training flights, two while I manned a training beacon on the ground, one as aircraft pilot following guidance from a navigator that, as it turns out, was using this technique. In each case I reported the situation during the exercise debrief. It wasn't until I was appointed unit training officer that I learned Barr, Casey and Muir were purposefully experimenting with this technique and using it when dispatched on actual search missions.

When designing critical systems, especially if health or safety depend on the critical systems, one strives to design in a fail safe or at least fault tolerance or graceful degradation. The standard aural null is fault tolerant. If the ELT performance is degraded to the point that the transmitted signal is not detectable at the radio horizon, and the radiation pattern is non-circular for that or some other reason, there will be errors in the geometry. Those errors are detectable, correctable and are small when compared to the total search area. The augmented aural null procedure proposed by Barr, Casey and Muir is not fault tolerant. As shown above, errors introduced by fading are more likely to be large than small because the greatest fading occurs at lower clearance angles and therefore greater distances. No procedure for detecting and correcting large guidance errors is provided. Clearly a technique which instead of guiding searchers to the vicinity of the ELT, guides them to a location 10 nm or more away from the actual location presents a safety problem to any survivors awaiting rescue. Rather than augmenting the operation of the standard aural null, this procedure destroys the fault tolerance of the standard procedure, and introduces unnecessary risk to survivors.

Also in their paper Barr, Casey and Muir state:

At the time of writing the methods described below have not yet been extensively tested, but both logic and preliminary testing would indicate that considerable improvements in time required and the precision of the location are possible. More practice and more experimentation is, of course, desirable and reasoned criticisms and discussion of potential pitfalls are always welcomed. Again, these methods are meant to augment and not to replace the usual CASARA procedures, which are still necessary and fundamental.

Well here are some the reasoned criticisms I have been trying to get them and the rest of CASARA to understand from the time before they wrote their paper, from the time when I was unit training officer, in fact. They appeal to logic to justify their procedure, but their starting assumptions are not true, so their logical deductions fail. They have not done extensive testing, even though the technique was used on actual search and training missions. Neither, it seems, have they examined the math or physics already well established in the field. In fact they claim:

The overall topics of wave propagation and the mathematics of the various aspects of finding maximums and minimums are the subject of a vast literature in the fields of mathematics, physics, and engineering. Nevertheless, in the case of CASARA actually looking for an ELT we have little need to understand either advanced mathematics or complex physics.

I would concede that when following an established search procedure there is little need to understand the math and physics. But when modifying existing procedures, or developing new ones, the need to understand the science is crucial.

It is possible that I have made a mistake along the way, but the number of scientists, engineers, technicians and signals analysts who have reviewed and agreed with my work and conclusions is very long. Surely, as I said at the start, if this is a difference of opinion between two very knowledgeable people, then the other person should be able and willing, if not eager, to present a valid scientific rebuttal.

One of the cornerstones of flight safety is the public acknowledgement of mistakes. As one of my high school teachers was fond of saying "learn from the mistakes of others, you can't possibly live long enough to make them all yourself". That would serve well as the motto of any safety program. It is not possible to learn from the mistakes of others if they hide them; neither is it possible to learn from your own mistakes if you don't acknowledge them. How then can we rely on an organization that refuses to accept valid criticism from qualified professionals? Aren't the lives of people who may go missing on a flight in Canada worth what ever time it takes to get this right? What are they doing that makes them too busy to critically examine search techniques used in training and on actual missions which have come under professional criticism?

The next post is this series is Off-Tuning - Cardinal Pass - What's in a Name?