Sunday, July 8, 2012

Wing Blanking or Wing Shadowing ELT Homing Technique

As I wrote in a previous article, Wing Blanking has been described and used by a number of air search and rescue organizations. But what is wing blanking? Also know as wing shadowing, the theory is that the all metal wing of a modern monoplane is opaque to radio signals. Maneuvering the airplane to place the wing between a radio transmitter and the airplane receiving antenna would therefore cause a momentary dip in reception of the radio signal. One could then infer that the radio transmitter is on a line extended from the antenna through the wing. Quite an elegant solution when it works. But like everything in the real world it is not quite that simple. The Civil Air Patrol has put quite a large effort into documenting the procedure, in their Mission Aircrew Reference Text they describe the technique:
The signal null or wing shadow method is based on the assumption that the metal skin of the search aircraft’s wing and fuselage will block incoming ELT signals from the receiving antenna during steep-banked turns. The observer can make simple estimates of the magnetic bearing to the transmitter by checking the aircraft heading when the signal is blocked.
As it turns out though, the signal isn't blocked, it is only attenuated. As a result, when the search aircraft approaches the location of the ELT the technique does not work as well:
If you are very close to the ELT, you can expect to hear no null, due to the higher signal strength near the transmitter and the inability of the wing to block the signal.
CASARA Ottawa has a different procedure calling for off-tuning the receiver and continuing wing blanking turns as the aircraft nears the ELT:
Once the signal is detected with the radio the aircraft is turned through 360° in a relatively steep bank (30°). As the aircraft goes through the turn the signal will fade when the wing is between the ELT and the aircraft antenna. ... As the signal strength increases the radio should be tuned off-frequency to reduce its effective sensitivity (while you can’t change the actual sensitivity of the radio, this technique has a similar effect).
So, does the wing block the signal or not? If not, what is really happening here? The whip antennas that are used for aircraft communications radios are more formally know as monopole antennas. All monopoles need a ground plane. An all metal fuselage or wing makes a very good ground plane. An ideal monopole will have a ground plane that continues an infinite distance in all directions. It would not be very practical to build such an antenna, but it is important to know this. An ideal monopole transmits or receives most signal energy parallel to the ground plane dropping off to near zero in the direction along the antenna length, and none at all below the ground plane. Any monopole with less than an infinite ground plane will transmit or receive most energy at an angle slightly above the ground plane, and a finite amount of energy below the ground plane. Here is an 3D plot of an ideal monopole transmit or receive pattern:
Notice the 'hole' in the top of the pattern. This is were the energy has dropped of to near zero. A non-ideal monopole would have a similar hole. So, rather than the wing 'blocking' the signal, wing blanking is actually the result of maneuvering the plane to point the weakest part of the antenna pattern at the transmitter. The dip in signal is a natural result of the transmitter being in a direction that the receiver is not very efficient receiving from. So why is the difference important? Well, for one, we can now start discussing the phenomenon as a property of the antenna system (the antenna and the wing-ground plane). Two, if you think about the wing blocking the signal, why does a high wing aircraft need to bank at all, never mind steeply. If the wing could block the signal, then just having the ELT directly port or directly starboard would be enough to cause a loss of signal. We know from the Cardinal Pass proponents that doesn't happen. In fact they claim the signal increases as the plane passes abeam the ELT, not that there is a signal dip. So what is really going on?

Well the wing doesn't really block the signal. The antenna system has a radiation pattern, that is also the reception pattern. The system ground plane (the wing or fuselage) is not infinite so some of the energy radiated or received comes from below the ground plane. This is very convenient in airplanes because most of the stations pilots want to talk to will most often be below their wings. Imagine the chaos that would ensue if pilots lost communications with air traffic control every time some random part of the airplane happened to come between the two antennas. But just like the pattern on top of the wing, the pattern below will have a hole in the center, and an area of relatively weak reception around it. The airplane must bank in order to aim this area of weak reception towards the horizon. The 360° turn scans this weak area around the horizon until it is pointed at the ELT causing the signal drop. This doesn't work as well if the plane is too close to the ELT because there is enough signal for the receiver AGC to compensate for the dip in signal strength.

This also explains why wing blanking works on some airplanes, but not others. It doesn't depend on the antenna being in the radio shadow of the wing, It depends on how effective and how close to infinite the ground plane is. It will work in all planes, but in some the size of the weak area will be too small to get pointed close enough to the right direction to cause a signal dip. NASA has published a paper which describes experiments on monopoles mounted on top of aircraft. Their measurements of a λ/4 monopole on top of a 737 (page 20, figure 10) provide a base line radiation pattern that I have reproduced in simulation:
Cross-section of Airborne Whip Antenna Radiation Pattern
Cross-section of Airborne Whip Antenna Radiation Pattern
 A larger and more effective ground plane might result in a radiation pattern shown below. This pattern would provide more effective 'Wing Blanking' because the attenuation at 30° below horizontal is 10 dB more than in the pattern above. An aircraft with the pattern below would be easier to use in 'Wing Blanking' than one with the pattern above.
Cross-section of Airborne Whip Antenna Radiation Pattern
Cross-section of Airborne Whip Antenna Radiation Pattern
With an appropriate radiation pattern for the airplane it is a simple matter to simulate the transmission and reception of the ELT signal as the search airplane turns. There are several factors that are considered in the simulation. First, the angle of bank the search aircraft is using. This determines which part of the radiation pattern (above) is pointed at the ELT. Next the elevation of search aircraft above the radio horizon of the ELT. This determines which part of ELT radiation pattern is pointed at the search aircraft. These two factors give us the receiver and transmitter gain parameters. We also have to consider the transmitter power output. I will be using the specified output for an ELT of 100 mW. Finally we will have to consider the distance from the ELT to the search aircraft and, since the aircraft is maneuvering, the relative polarity of the transmit and receive antennas. To start with I use a distance of 16 nm and a vertical ELT antenna.

As the search aircraft turns, the gain provided by the receiving antenna radiation pattern changes. When the lowest gain part of the pattern is nearly pointed at the ELT the received signal strength will be lowest. If the signal strength is low enough that the receiver Automatic Gain Control (AGC) is driven to its maximum setting, the audio output will dip. The next plot is the received signal strength seen by the search aircraft receiver. The lowest signal level, -102 dBm, is probably enough to provide a detectable dip in output volume, depending on the actual receiver in use.
Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees
Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees
This is a nice symmetrical plot. The symmetry comes from the vertical ELT antenna. As the heading of the aircraft changes during the turn, the relative angle between the receive and transmit antennas remains the same. So the signal attenuation due to polarity differences remains the same. That is not true if the ELT antenna is not vertical. The next plot shows what happens if the ELT antenna is 20° from vertical.
Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees - ELT 20 degrees
Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees - ELT 20 degrees
It is no longer symmetrical, but the asymmetry will not affect the ability of the crew to determine a direction. The situation gets worse as the relative polarity angles approach 90° as in this next plot. With the ELT antenna 40° from the vertical the relative polarity will interfere with direction accuracy. The direction to the ELT is not as clear as it was in the first turn plot.

Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees - ELT 40 degrees

Any time the angle of the ELT antenna from vertical, added to the bank angle of the search aircraft is greater than 90° it becomes virtually impossible to get a good direction, as in these final two plots with the ELT antenna 40° and 50° from vertical respectively but on an azimuth nearly opposite of the true direction to the ELT.
Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees - ELT 40 degrees
Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees - ELT 40 degrees
Notice that not only does the polarity cause a significant dip at a heading of 60°, but it also keeps the 'Wing Blanking' effect dip from going below -100 dBm. In this case it is possible that the receiver AGC will be able to compensate for this reduction in signal and 'hide' the dip from the crew. If this happens the crew won't be confused by receiving two bearings, they will only receive an incorrect bearing.

Wing Blank/Wing Shadow ELT Homing Turn - 16 nm - Bank 45 degrees - ELT 50 degrees
Quite a difference from the first plot. The lessons here is that if your team is using wing blanking, or wing shadowing to home ELT signals, you should be training them to recognize when ELT antenna orientation will affect the accuracy of this technique. If you aren't you are setting them up to deal with this situation for the first time on a search when the safety of any survivors may be at stake. You don't really want that do you?

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