Sunday, 26 May 2013

Is 'Highly Variable' or 'Seems to Work to Some Extent' Good Enough?

In our last article we saw how a simple misunderstanding in executing an electronic search technique can cause delays in locating a crash site. In this article we will look at how misunderstanding the technology involved can result in the widespread use of ineffective techniques. In the last article both search aircraft dispatched by the Joint Rescue Control Centre were equipped with Radio Frequency Direction Finding (RFDF) equipment. In this article we will see an attempt to make deployment of RFDF equipment to volunteer search aircraft more convenient that actually sacrifices effectiveness.

First let's look at the RFDF equipment used by the Civil Air Search and Rescue Association (CASARA), the volunteer group. In November of 2011 the Canadian Armed Forces announced that it was expanding search and rescue coverage in the arctic by using private contractors and civilian volunteers (CASARA). Effective radio direction finding capability will be important when conducting searches is the large and mostly empty Canadian Arctic.

According to statements we received from the National President of CASARA the primary electronic homing method used by CASARA is the L‑Tronics Little L‑Per portable direction finder. This device is designed to operate in a hand held mode on the ground. The operator scans the device in azimuth while monitoring a display that indicates if the radio signal is being received from the left, from the right or from directly ahead or behind. Using this equipment and simple ground navigation techniques a search team is quickly able to locate an Emergency Locator Transmitter (ELT) once they are within reception range. The Little L‑Per works by using two dipole antennas spaced at a distance close to one quarter of the wavelength of the radio signal. The electronics inside the Little L‑Per alternately switches each dipole from an antenna to a reflector forming a two‑element Yagi antenna that is pointed alternatively left and right. A Yagi antenna is directional, receiving more signal from one direction, less signal from all other directions. With the antenna switched in this manner the Little L‑Per can measure the strength of the radio signal coming from the left, then from the right. If the signal from one side is stronger the ELT is presumed to be on that side. If the signal is of equal strength on both sides the ELT is presumed to be directly in front or behind. If you are feeling a little lost by this description don't be concerned. There is an excellent demonstration of how a Yagi antenna works given by Diana Eng KC2UHB hosted by Make Magazine. We suggest you go watch this video before continuing with this article.

So you're back. Wasn't that well done?

L‑Tronics also provides antennas to mount on a vehicle so that search crews may drive and hunt for the ELT location at the same time. They also make available antennas and plans to install directional antenna arrays on aircraft so search crews may take to the skies. This additional flexibility can significantly reduce the time to find an emergency beacon, especially one associated with an airplane crash which may have a large search area. There is a small problem though. With a ground vehicle antennas may be simply installed in the correct locations with magnetic bases that hold the antennas firmly in place at the relatively low speeds that ground vehicles reach. Airplanes are normally made out of aluminium alloys which are non‑magnetic and have few places where an antenna may be clamped onto the structure. To have the antennas located at the proper spacing to form a two‑element Yagi they usually must be permanently attached. To the left are diagrams of a typical antenna placement (red) viewed from the front and side. This requires holes be drilled through the airplane skin. Not all aircraft owners want to have extra holes drilled in their airplanes, so it is not possible for all volunteer search units to always have access airplanes with the antennas installed. It is the solution to this dilemma that give rise to the problem we will discuss in this article.

As a quick aside, you may have noticed that the antennas, viewed from the side, are rather curiously bent. Normally whip antennas are used, but there is not sufficient ground clearance beneath the airplane for the size of whip antenna that would be needed. The engineers at L‑Tronics recognize this and provide instructions for converting the whip antennas into bent whip antennas. There are also commercial versions may be used. Bent whips function in very much the same way as whips. Of course normal whip antennas could be used and mounted to the top of the wing where it crosses the fuselage, but this is the usual mounting location for the aircraft communications antennas. Mounting the direction finding antennas near the communications antennas would cause each to interfere with the operation of the other.

So a solution was sought to temporarily mount the direction finding antennas so that they may be removed leaving no permanent changes to the aircraft structure. We are not certain where the idea of mounting the antennas on the struts came from, but we think it likely that it was adapted from a common practice in wildlife tracking. Here we see a picture of a 3‑element Yagi antenna mounted to the strut of a Cessna aircraft. This particular installation was used to track cranes, but the practice is common world wide for various fauna. A Yagi of the correct size to receive the emergency beacon signal on 121.5 MHz would be too large to safely mount on a strut. Even the drag of the two whip antennas mounted on one side could unbalance the airplane and make it uncontrollable in some flight regimes. So a decision was made to mount one antenna on each strut, far enough out from the fuselage for adequate ground clearance.

This diagram on the left is what was finally arrived at. Clearly the antennas are spaced much further apart than in the belly mount example. If you were paying attention to what Diana Eng was saying in her video you know that the size and spacing are not correct to form a Yagi antenna. The theory of this approach as explained to one of our team is this: Since the fuselage is between the two antennas the left antenna receives signals from the left side of the airplane louder that signals from the right side of the airplane, and visa verse. The switching performed by the L‑Per then compares the signal strength from the left antenna with that from the right antenna (instead of comparing the strength from directional antennas pointed left and right). That sounds plausible unless you know, as our team mate did, the wave length of the emergency beacon signal, about 2.5 meters, is longer than the fuselage is wide, about 1 meter (just over 3 feet). Radio signals quite handily propagate around obstacles that are smaller than their wavelength. Think of waves passing under a pier. As the wave passes each pile supporting the pier you may observe a small drop in the wave size, but by the time the wave is a very short distance away all evidence of the pile acting on the wave will be gone. There will be some loss of signal strength as the radio wave passes the fuselage but will it be enough to determine an accurate direction? All the time? When your life, or the life of someone you love depends on it? A spokesperson for L‑Tronics had this to say: "We have experimented with configurations like this and have heard of results from others as well. The results are best described as highly variable." One has to wonder why CASARA would use such an installation. Why does convenience trump effectiveness in search and rescue?

Unfortunately this is not the only questionable technique in use. We were recently reviewing a CASARA training presentation that contains images of antenna installations similar to ours above. The presenter's notes for the slide with those images includes the following statements:

Antennas mounted on struts ... Not recommended by Ltronics, however it seems to work for us to some extent. 
The best system is permanently mounted belly antennae which are much more effective and accurate due to proper spacing and better ground plane. 
Also used are internal antennas which work; but must be handled with some care as they are blocked by the airframe.

As you may imagine to us as scientists, engineers and technologists it is horrifying that a search and rescue group, even a volunteer group, would use and train their members in the use of techniques which seem to work to some extent. We asked CASARA why they were using this technique given the significant technical problems with them. Their only response was that CASARA members only use techniques that have been reviewed and approved by the Royal Canadian Air Force. So we asked the Royal Canadian Air Force for the documentation of their review and approval. After an exhaustive search they were unable to find any such documentation. We are left with many questions and no answers.

On a final note we went back to the L-Tronics website to see what we could learn about the internal antennas described in the training presentation. They have this to say:

Internally mounted antennas, such as wires taped to windows of a metal aircraft, generally give unsatisfactory results. The major problems are ambiguity and false courses, particularly to the rear of the aircraft, and sensitivity to the presence and movement of cabin occupants. Thus, such an antenna may seem to work on a limited test but have major problems on a real search.
We know of no temporary or "inside the cabin" antennas that are satisfactory for regular use. A portable DF set with its antennas inside the cabin of a light plane is worse than using the plane’s own communications receiver and wing shadowing.

This is interesting because the next technique we are going to be discussing is wing shadowing. It seems that L-Tronics does not think very much of that technique either.

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