Friday, January 27, 2012

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:

ReceiverSensitivityAGC Minimum LevelNo AGC Range
MX 3001.5μV2μV2.5 dB
ATR 7201.5μV5μV10.5 dB
Apollo SL401μV5μV14 dB
Val Com 7602μV10μV14 dB
Garmin 5302μV5μV8 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.


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.

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