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The following paper is copyright 1987 by L-Tronics. All rights reserved. Permission is granted for local reproduction for non-commercial, educational purposes provided credit is given to the source. Printed copies are available from L-Tronics as LHTM-2. Locating Non-distress ELTs and EPIRBs By E. L. Dartanner More than 95 percent of searches for Emergency Locator Transmitters will be concluded as a non-distress condition-often at airports, harbors, or homes. These "false alarm" ELTs and EPIRBs must be located and deactivated rapidly to avoid their interfering with bonafide emergency signals or overloading the search and rescue satellite system. Some of these urban searches can be just as challenging as those for a crash in mountain wilderness. This article will address the equipment and techniques that will enable you to locate these transmitters quickly and efficiently. ELT location requires a receiver to hear the signal and a way to measure either signal strength or direction (preferably both). Direction can be measured using left-right homing, beam antennas, or body shielding; strength is measured by meter or by sound. The L-Tronics Little L-Per is used as an example in this article because it can do all of these things. If something is unclear, or for more information on the mechanics of using a system, refer to the section at the end of this article which summarizes the procedures, and to your equipment's operating manual for more details.
Figure 1. Not only should you understand the mechanics of how your equipment works, you should also know how the ELT signal behaves so you can interpret the information your equipment gives you. Figure 1 shows an ELT on an airport ramp, which represents one of the simplest conditions you'll find; however, these principles can be applied to all ELT location situations.
LOCATING AN E.L.T. ON AN AIRPORT RAMP Use your receiver with an external antenna on the vehicle as shown below and drive completely around the airport, or as nearly so as you can. Determine where your left-right homer indicates the location of the ELT or the area of highest signal strength so you can begin your foot search close to the problem. If your mission is at night or in bad weather, particularly at an airport without an operating control tower, don't rule out the possibility of an actual crash on or very near the airfield-it HAS happened! To begin your foot search, walk out on the ramp away from buildings, planes, cars, etc., where you have a good view of the airport. Find the direction to the ELT using the left-right DF mode, maximum signal strength in the RECeive mode, or by body shielding. (Details on using the three techniques are shown below). Walk to another clear area about 50 yards away while watching the meter as you move. In DF mode, left to right needle swing is normal; just keep swings about equal. Needle movement can also be expected in the RECeive mode. The DF will point to an area of hangars. buildings, or aircraft. Walk in the indicated direction while listening to the ELT. It is important to continue to decrease the sensitivity control of your receiver (or detune a tunable receiver) as the volume or strength of the E LT increases. DON'T adjust the volume control; this is particularly critical during that "last 100 yards" to the ELT. DON'T remove the antenna; with no antenna, the volume of many radios depends more on how they are held than the signal direction. If the ELT is in an aircraft (or vehicle) parked out in the open on the ramp, you should be able to walk right to it with whatever equipment you're using. With a left-right DF, the sensitivity will be at minimum and you can walk completely around the plane and the DF will keep pointing to it. In RECeive mode, sensitivity will be at minimum and the signal will quickly fade away if you walk away from the plane. With a tunable radio, you'll be able to hear the ELT while tuning over almost the entire band. Look for the ELT antenna on top of the aircraft fuselage aft of the cabin. To double check your finding, select 121.6 MHz on your receiver, touch the DF antenna to the suspected ELT antenna. Adjust the sensitivity (not volume) until the signal is weakly audible. If the ELT signal disappears when the antennas are separated by only a few inches, you have the right one. If your DF is convinced you've found the right airplane, but you don't find an external antenna, there may be a portable ELT located inside. This is common for home-built or experimental aircraft and small helicopters. E.L.T.s IN HANGARS AND BUILDINGS If your search takes you to an area of hangars or buildings, note the receiver control settings and strength of the ELT as you walk. You may need to return to areas of strongest signal later. When you reach the building, circle it while 200 feet or so away (or as far away as open space permits) before entering to make sure you have isolated the right building. A left-right or beam type DF will point to the building with the ELT in it on ALL sides, even though the signal is leaking out at very odd places. Signal strength should fade out as you walk away from the building in any direction. Once you have isolated the ELT to a single building, go inside and look around. Ask occupants if there's an ELT stored nearby (radio shop, store room, parachute, personal locker, etc.). When you're inside a building, many reflective objects are so close that DF is not possible, even with averaging. In a small, confined area, it becomes practical to select RECeive mode and look for the strongest signal; you can fold the elements of the antenna for ease of handling inside, but do not use any receiver without an antenna. Check the ELT antennas of each airplane in the hangar with your receiver on 121.6 MHz as described earlier. If you are unable to find the ELT, return to areas where you had strong signals and work them. Remember, when you're close, the sensitivity control will be near minimum (the signal will be very loud). In a very tough problem, get back into the open and take a few more bearings from clear areas. IN SUMMARY
TWO SIMULTANEOUS E.L.T.s Two simultaneous ELTs will produce confused DF bearings and variable signal strength, much the same as strong reflections, but will be noticed over a much wider area than building reflections. Listen to the signal you are receiving; you can almost always hear two unsynchronized tones. How to resolve the problem? The first step is to isolate one of the ELTs. You may not be able to completely block out one signal, but you should be able to find a location where you hear one better and therefore get a bearing or heading. Use metal buildings, hills-any large object-to try to block out one of the signals. Then work the other until you have located it and turned it off. The second one should be "duck soup." WHAT TO DO AFTER FINDING THE E.L.T. The primary task now is to shut off the ELT or, if not possible, to reduce its range. This will prevent interference with reception of an ELT signal from a crash site and reduce the number of "hits" being received by the search and rescue satellites. If the ELT is in a locked airplane, or in a locked hangar, try to locate the owner. NEVER break into an aircraft, vehicle, or building to turn off an ELT; contact local law enforcement for assistance. Once you have someone who can open the airplane, check the ELT. (Turn your receiver on so you can monitor the signal.) There will probably be a switch on the instrument panel; a few planes have a switch outside on the fuselage below the antenna. If the panel switch is in the ON or ARM position, turn it to OFF. If this doesn't silence the ELT, the switch may be defective, which is not uncommon. Be particularly suspicious of a defective switch if the ELT signal sounds "wobbly," erratic, or has no tone at all. If the switch is in the OFF position, turn it ON. Don't be surprised if it increases in strength and changes in pitch; it's almost surely a bad switch. In this case, you can either disconnect the batteries or the antenna to silence it. This will often necessitate removing an access panel on the aircraft to reach the ELT. Keep monitoring your receiver to verify the ELT goes off the air. Be SURE the person who has responded understands that the ELT has been disabled and must be repaired before the aircraft is flown. If you are unsuccessful in reaching someone who can open the aircraft, or if a considerable delay will be necessary, the signal can be greatly reduced by wrapping the external antenna with aluminum foil, as shown in Figure 2. Figure 2. Take a piece of foil 12" wide and about five feet long. Place the tip of the ELT antenna in the center of the foil, being careful not to punch a hole in it. Fold the foil down on both sides of the antenna and let the ends lay flat on the fuselage. Tape the foil to the fuselage and fold the two sides together to completely enclose the antenna. It is critical that the foil extend AT LEAST 18" over the skin of the aircraft beyond the antenna and that it be taped or otherwise secured to the skin. Without these flaps, the foil wrapping will be ineffective. Leave at least one note in a prominent location (the door above the lock is good) for the owner, because the batteries may run down completely before he or she arrives. Also contact the fixed base operator, airport manager, or FAA for assistance in notifying the owner. In any event, write down the time the ELT was disabled; the aircraft type and N-number; the ELT make, model, and serial number; the owner's name; and the circumstances causing the activation, if known. Relay this information to your mission coordinator, the RCC, the FAA, or other agency as applicable to your chain of command. DFing IN BUSINESS & RESIDENTIAL DISTRICTS One of the best things you can do to help you locate transmitters in these areas is to have selected a number of sites where good DF bearings can be taken. These sites should be as high as practical (to keep obstacles from blocking the direct signal) and at least 40-50 feet clear of objects (to avoid nearby reflections). Even a 50 to 100 foot rise over surrounding buildings, like a freeway overpass, can greatly increase your chances of hearing the ELT. Many search hours have been spent driving around city streets just trying to hear a satellite-reported signal. Even one bearing will establish a positive search direction, confirming a city search for a single team or suggesting back-country potential for a full search effort. Don't be concerned if bearings from three or more sites don't converge exactly when you plot them. They will still show an area of high probability in which the signal can be heard at street level. With the Little L-Per in the left-right homing or DF mode, you can use magnetic or fixed antennas on the vehicle to continuously monitor the direction to the ELT. This is particularly useful for DFing in areas of buildings, trees, and other reflective objects. Mount these antennas on the roof so the left one is to the front and the right one to the back (see Figure 3). The handheld antenna placed out the passenger's window with the arrows facing forward will accomplish the same result (Figure 4). As long as the vehicle is moving, the signal will come from one direction, so in the DF mode you will have relatively consistent readings to the left as you travel toward the ELT. As you start to pass by it, the readings will go both left and right, then go predominantly to the right as you drive away. Turn a corner to redetermine direction. Don't worry about the needle "flickering;" this is perfectly normal. Simply note on which side the needle stays most of the time. You'll find this technique will make DFing much easier and faster than working with signal strength or bearing-taking alone.
Figure 3 Figure 4 Proceed into the area using an external antenna on the vehicle (or out the window), which should get you within a block of the ELT before you have to DF on foot. If you don't have this capability, at least use an external whip so you can monitor the strength of the ELT signal as you drive. Keep notes of the locations where you have very strong signals so you can return to them if necessary. As the E LT signal gets louder and stronger, decrease the sensitivity (not volume) control to keep it just audible. Canyons of concrete and steel can cause problems similar to their natural cousins. Probably the biggest mistake you can make is to distrust your initial bearing information (taken from clear areas) as you get down into business or residential districts. You will be in an area where many things can disturb the natural characteristics of the ELT signal. However, with a fore-aft DF antenna, as long as you keep moving and average the needle movement, you will have reliable DF information. If you have a strong signal but don't seem to be making much progress, start a grid search near an area of highest signal strength or positive DF indication and follow it until you locate an area or building. Once you've isolated a suspect building by DF from all sides, enter and ask around first. Countless hours have been saved when someone says he knows a pilot who works there, or that nearby is a radio repair shop, mail handling facility, or a boat or plane in a backyard. If an electronic search is required, use the RECeive mode and search for the area of strongest signal. You can fold the antenna for convenience.. Keep moving in the direction that causes the ELT to get louder. Remember: Decrease the sensitivity (not volume) control as the signal gets stronger. E.P.I.R.B.s AT HARBORS AND MARINAS The EPIRB (Emergency Position Indicating Radio Beacon) is the marine version of the ELT. Older units operate on the same frequencies with the same power output and has the same swept tone sound. Newer "406" beacons operate only on 121.5 MHz with somewhat lower power and often with an upward sweep. Therefore, even if you started out looking for an E LT, you may end up at the harbor instead of the airport. As it becomes clear you are likely to have an EPI RB and you haven't already done so, find several areas at least a quarter mile from the harbor, preferably a few hundred feet high, where you can take bearings. Either plot them on a map or visually sight a vessel or landmark for a reference. This will help you begin your search in the most likely area, as it is not uncommon for the signal to get weaker as you get into the harbor. This seems like a contradiction of our earlier statement that the signal will get stronger as you get closer. But if the EPI RB is below water line in a boat, the water will block the signal horizontally, much the same as a hill would, although the vertical range may be great. This can account for search aircraft easily locating the correct harbor while those on the ground labor to hear the signal when they arrive there. Also, the signal may "leak out" of a hatch or other opening of the boat's structure, causing a strong signal in one particular direction and weak signals in others. Use the same equipment and techniques of ELT locating when working EPIRBs. If you're in a small harbor or marina, you should have no more trouble locating it than you would on an airport. While ELTs are rarely found in larger aircraft, EPIRBs are often carried on larger vessels as part of the ship's life rafts and other survival gear, so don't automatically rule them out as sources. If you're at a harbor with freighters being loaded, consider the possibility of an ELT in the cargo. It wouldn't be the first time an ELT was found in a partially disassembled plane packed for export. With left-right DF, average the swings of the needle as you walk to get true direction. In the RECeive mode, look for the direction of the strongest signal, averaging the "ups and downs" of strength. Decrease the sensitivity (not volume) control as the EPIRB gets louder, as you did at the airport. Don't be surprised at the number of reflections that can be caused by the masts and rigging of the vessels. They don't look large enough to be troublesome, but you will notice the effect when you get around them. This is another reason why it is helpful to take a few bearings before you get to the harbor. As with ELTs, after securing the EPI RB, get the manufacturer's name, model number, and serial number; vessel identification and location; and cause of activation and time of deactivation for authorities. MOVING TARGETS Do you know how to determine if an ELT is moving? Many valuable hours of search time has been spent-with the resultant frustration-by teams who didn't realize they were chasing a moving target. If you have a Little L-Per, this question can be easily answered in minutes. Select the DF mode and center the needle. Hold the antenna stable for at least a minute and watch the needle. If it moves, you have either a moving target or a moving reflection. A stationary ELT will not change direction or strength when you hold the antenna still. If the ELT is in a moving airplane, the changes in the signal will be slow (5-20 seconds) and relatively smooth. An ELT in a moving vehicle will almost always result in faster (1/2 to 4 second) jerky needle movements. A moving boat or ship will be harder to diagnose because it moves slower, but become suspicious if your bearings that lead out to sea begin to show a shift in direction. If the reflection is moving, you have a stationary ELT and something near it is moving. For instance, if the ELT is in an automobile parked on a busy street, cars passing by will cause a reflection and a resultant movement of the DF needle-usually a slight "jumping" of the meter needle, which will return to center. In this case, the bearing will not change. A different type of moving reflector is when a stationary ELT's signal is bounced off a distant moving object. For example, if an E LT is on the opposite side of a mountain from you so you can't hear it and an airliner flies within line of sight of both you and the ELT, you would hear the reflected signal and the DF would point to the airliner. This phenomenon will not last very long, but you may be able to hear or see the airliner. The Goodyear Blimp also makes a great moving reflector All these examples are taken from actual field experiences. OPERATION OF E.L.T. LOCATORS ELT location requires a receiver to hear the signal and either a way to sense the signal's strength or its direction-or both. The whole process of location is commonly called direction finding or DF because direction is usually more useful than strength at long range. In contrast, strength is often best inside a building or hangar. Equipment operating on the signal strength principle is a combination of a receiver and directional antenna (it hears the signal loudest when pointed toward the ELT). Strength may be shown on a meter or judged by sound. An example of this system is the Little L-Per in the "RECeive" mode, or a receiver with a yagi or beam antenna. A left-right homing system such as the Little LPer in the "DF" mode senses the signal's direction of arrival and displays it on a meter. Another way to sense direction is to use your body to shield the incoming signal. Almost any radio capable of hearing the signal and having a non-directional antenna such as a built-in whip or a "rubber ducky" can be used this way to sense direction. There are four things you must be able to do with your equipment to locate an ELT with reasonable efficiency.
We will explain briefly the mechanics of using various equipment types with the Little L-Per as an example. Keep in mind the four principles mentioned above as you read the following paragraphs. Left-Right Homing With the left-right homing system, the left antenna receives signals better to its left, while the right one prefers those to the right. In the DF mode, the antennas are electrically switched back and forth rapidly (producing a tone or hum), causing the meter to point to the side having the strongest signal. If the ELT is to the right, the right antenna will have the strongest signal and the needle goes to the right. When you are facing the ELT, each side of the antenna sees an equally strong signal so the DF needle centers and the tone nearly disappears. To operate the Little L-Per in the DF mode, select the frequency (be sure you have the correct antenna), turn the mode switch to DF, the SENSitivity control to minimum, and volume to about 12 o'clock position. Turn up the sensitivity control until the ELT signal is just audible and the needle goes left or right of center. Hold the antenna as high as practical to get the most accurate readings. Turn toward the needle until it centers. You are facing the ELT. Walk toward the ELT by keeping the needle roughly centered. Left to right needle swing is normal when you are moving; just keep swings about equal to average the effects of reflections. If the direction seems erratic, periodically stop and make a 360 degree turn to determine direction quality as described below. As the volume increases and/or the needle gets too sensitive, decrease the sensitivity (rather than volume) control to maintain about half scale left-right needle swing and a weak but audible signal. A higher sensitivity control setting may DESTROY DF bearing even though a lot of meter swing remains. The closer you get to the ELT, the more frequently you must reduce the sensitivity knob setting. To take a bearing and check its reliability, get into an area away from buildings, cars, etc., and center the needle so you are facing the ELT. Stand in place and turn a full circle to the right. The needle will go to the left, center when your back is to the ELT, then go to the right until it centers again when you're facing the ELT. The fact that the meter centered twice approximately 180 degrees apart shows that this is a good location, free of reflections, and a reliable bearing or heading can be taken. If the needle centers more than twice, you are in an area of reflections and your bearing will not be reliable; move to another location. Signal Strength This method uses a receiver with a field strength meter and a directional antenna. We will give instructions for the use of the Little L-Per in the RECeive mode, although the same principles can be used with other equipment. In this mode, the meter reads signal strength; needle positions to the left show a weak signal and move upscale to the right as the signal gets stronger. The antenna receives best when the signal comes from the left side as indicated by the arrows on the antenna. Be sure to use the antenna for the frequency range you are using. Select the frequency, RECeive mode, SENSitivity at minimum, and volume at about the 12 o'clock position. Turn the sensitivity up until the meter goes one-third to one-half upscale and the signal is just audible. Hold the antenna as high as practical for the most accurate readings. Stand in place and turn a circle while watching the meter and notice where you are when the needle goes furthest upscale. Return to the position of maximum meter reading. In this position, the arrows on the left arm of the antenna point to the ELT. Notice that you are not facing the ELT as you were when using the left-right homing method. Also, you will NOT have a null or low reading directly opposite your maximum reading. If multiple maximums are found, the location is unsuitable for reliable DF. Walk in the direction the arrows point. As volume increases and/or the needle nears the right-hand stop, decrease the sensitivity, not volume, to bring the needle downscale. The closer to the ELT, the more rapidly the volume and meter reading increase. If the direction seems erratic, periodically stop and make a 360 degree turn to determine bearing quality. Body Shielding Another method to measure direction to the ELT is by body shielding. You can use a tunable aircraft band receiver or scanner on 121.5 MHz, or an FM band radio set to 100 MHz. Cheap, pocket FM radios work best. The secret is to set the volume to maximum and then tune away from 121.5 (or 100) MHz so that the ELT signal is audible but very weak or noisy. This makes the volume go up or down with changes of signal strength (defeats the automatic volume control in the radio). Hold the radio a few inches from your body at belt level with the antenna pointed up. Turn a full circle and listen to the signal. It will be loudest when you are facing the ELT and weakest when your body blocks or shields the signal's path. Since the area of loudest signal will be relatively wide, you will get a better idea of the direction to the ELT by listening for when the signal is weakest (your back is to the ELT). Walk in the direction indicated while listening to the ELT. As it gets louder, tune the receiver further off frequency (don't adjust volume) until the ELT signal is again weak. Turn a circle periodically to verify your direction and continue the process. Remember, the stronger the signal, the closer you are to the transmitter. When you are very near the ELT, you can tune nearly the whole band of an inexpensive FM or aircraft band radio and still hear the ELT. A better made scanner or receiver will be tuned about 4 MHz off frequency (117 or 125 MHz). While this "tummy thumper" method is the least efficient system, it will locate most ELTs on an airport ramp if they are not too far away and if the operator is patient. Body shielding or other methods that depend on hearing strength change are nearly impossible to use when the ELT is broadcasting a carrier but not the swept tone. |
