Telonics Quarterly 1998V11N1
The Shift to Electronic Publishing
Nearly 10 years ago, Telonics jumped into the publishing business with the Telonics Quarterly. The goal was to provide customers with current information of practical help to the field. The first issue, published in the Fall of 1988, included articles on "real-time" and "time-delay" sensors, tips on avoiding static electricity problems with receivers, an overview of antennas, and notes on the proper care and exercising of transmitters.
Over the next 10 years, hundreds of other technical articles followed. A gratifying number of them found their way into college classrooms and professional field conferences around the world. By 1996, the mailing list for the newsletter had grown to include over 8,000 researchers. Since many of the articles we had published over the years were still being used as technical notes, we made another big decision that year - to build a website on the Internet that included all back issues of the Quarterly with abstracts and a key word search capability. After months of development, the new Telonics home page opened in Fall 1996.
It Doesn't Count If It's Not Line-of-Sight
Establishing a line-of-sight (LOS) bearing is fundamental to direction-finding when using conventional VHF telemetry. LOS implies that there is a direct view between the receiving antenna and the transmitting antenna located on the subject animal. In the simplest sense, the concept means you could stretch a string between those two antennas, with nothing blocking or reflecting the signal traveling along the course of this line. The presence of a hill or mountain between you and the animal is clearly a situation where LOS conditions do not exist. Radio waves at very high frequencies (VHF) between 100 and 300 Mhz do not propagate through large masses of earth. Thus, the direct-line signal is blocked (see figure 1) - any signals received at the antenna (RA1), are signals being reflected by the environment. They ultimately arrive at the receiving antenna via multiple pathways other than a straight line path.
In many conventional VHF telemetry applications, it is inherent that position determinations are obtained by establishing the intersections of several LOS bearing lines taken from several surveyed monitoring points or receiving sites by the research team. Often the ideal receiving sites are positioned so that the expected location of the animal will allow the angle of intersection of the bearing lines to be at or around 90 degrees and as close as possible to the actual position of the animals monitored. For accurate triangulation of the animal's position, the bearing angle from each receiving site must be as accurate as possible. If any of the bearings to the animal are incorrect, the triangulated position will be less accurate. Refer to figure 2 where five receiving sites have been established.
Since each receiving site has direct line-of-sight (LOS) to the instrumented animal, the radio wave can travel directly to each receiving site from the animal's position. The main LOS signals between the animal and the receiving site are sometimes called "the main bang" in radio vernacular because they are often the strongest signals imposed on the receiving antenna. Other reflected signals or 'multipath' signals exist in the environment because the transmission from the antenna on the animal is intended to be omnidirectional. For a further discussion on the effects of signal bounce on direction-finding, you may wish to refer to an earlier article entitled "The Phenomena of Signal Bounce and Phase Cancellation" (TQN Vol. 3, No. 4, Winter 1990).
Unfortunately, the ideal situation depicted in figure 2 seldom occurs in the real world unless you're working in a Kansas wheat field where no obstructions exist. Intervening objects, such as hills, interrupt LOS signal propagation, which means the real-world situation is often more accurately depicted as in figure 3. In this case, the "main bang" to RA1 and RA2 is blocked by an intervening topographical feature. Sometimes it is still possible to receive signals at sites one and two; however, the radio waves have "bounced" off other objects in the environment, thereby creating a false bearing direction. When standing at sites one or two, it is not possible to discern that the received signal is a "bounce." There is no difference in the characteristics of a radio wave that follow a reflected path than those coming directly or LOS from the source. If only receiving sites one and two were used for triangulation purposes, it is clear that we would expect the triangulation of the animal to be at point P2. It is only the use of multiple receiving sites that allow us to discern the true position of the animal at position P1.
How can a triangulated position obtained from a bounce signal be differentiated from position obtained from LOS signal?
It is generally done by the preponderance of the evidence. When monitoring a "bounce" or reflected signal at the receiving site, the resulting bearing is in the direction of the reflective surface, not the transmitter. Often a slight adjustment in the position of receiving site (a few hundred meters or more) results in a different signal path - and the new bearing angle suggests a very different position for the animal. If the animal's position is not LOS to any of your receiving sites, the triangulation of position which results from any two receiving sites will often not agree with the position from any other two receiving sites. The bearing lines will often cross at positions all over the map. This disagreement of positions obtained by analyzing the triangulated position from pairs of receiving sites often indicates that the animal's true position is not LOS. Therefore, some or all of the bearing determinations may be inaccurate. Under LOS conditions, bearings should converge on the "real" position. In short, it is the agreement between several receiving sites on the point of convergence (the triangulated position) that strongly suggests that the signal is LOS and the resulting position determination is reliable. In order to obtain bearing directions from multiple sites, it must be assumed that the animal is stationary during the time course over which the bearings were obtained. In a practical sense, this often means that bearings from multiple sites must be taken simultaneously because we cannot rely on the animal to remain stationary as a single observer moves from one site to the next.
It is important to reiterate that by listening at any given site, it is impossible to tell whether the signal received at a specific time is "bounce" or LOS. The signal bounce phenomena (multipathing) and the resulting lack of ability to obtain line-of-sight bearings represent the most significant difficulty associated with the use of conventional VHF telemetry for direction-finding.
When the animal's true position is LOS with a receiving site, many researchers with field experience will recognize that often the strongest signal is the direct signal and weaker signals represent weak bounces coming from other directions. While this is often the case, it is not always true. For example, the main signal may be traveling along a highly attenuated pathway where intervening vegetation may severely attenuate the signal. Under those conditions, even though the direct signal is present, it can be weak and cannot be differentiated from multipath signals. Therefore, the strongest signal is not necessarily the direct signal, and there are many instances where bounced signals may be stronger than the direct signal.
An even more complicated situation involves signal polarization.
A more complete discussion of polarization occurs in a TQN article entitled "Polarization: The Effect on Range Performance" (Vol. 3, No. 3, Fall 1990). When the radio wave is received at an antenna site and the wave is polarized in the same plane as the receiving antenna, a much stronger signal will be received than if the wave is received shifted 90 degrees in polarization. This is the reason that, sometimes, when standing with an "H"-antenna overhead (horizontal polarization), a weak signal may be heard. Upon tipping the antenna over to the side (vertical polarization), the received signal happens to be in the same polarization as the transmitting antenna and the signal strength is noticeably greater. It is simply the act of moving the receiving antenna into the same polarization as the radio wave that results in the higher signal level. There is no way to be absolutely certain of the polarization of the transmission being received from the animal because the transmitting antenna could be in any orientation when attached to an animal. Changes in polarization of the radio wave also occur when the wave encounters and reflects from objects in the environment. The reflected radio wave essentially rotates, thus changing the polarization of the wave as it propagates through space. In essence, it begins a process where the wave "corkscrews" through the environment. With the rotation of the radio wave moving through the environment, sometimes - just by chance - you receive a stronger signal from a bounce signal than an LOS signal from the animal because of the polarization of the wave.
Maintaining an LOS relationship between transmitter and receiver, and dealing with consequences of multipathing, represent important considerations in all forms of conventional VHF tracking. Understanding the physics of signal propagation is critical to successful ground tracking, to establishing a bearing from a fixed site (either permanently fixed or a mobile site where you move the antenna to fixed positions), and to aircraft tracking in topography with steep dissected terrain, canyons, or rugged mountains. To accurately determine the position of the animal, an LOS signal path must ultimately be established for an accurate bearing.
In situations where you are following the animal in relatively open cover, the position can often be confirmed by a visual sighting of the subject. In dense cover where sighting the animal is not possible, loss of LOS signal path means that the tracker is following "bounce" or reflected signal. Unfortunately, this often means that the tracker may travel in the wrong direction. As tracking proceeds, the signal often becomes weaker because the tracker is moving away from the source of the transmission. In addition, the received signal will appear to emanate from entirely different positions and bearing lines may appear to come from numerous places. To minimize the problems of tracking under these conditions, keep the gain setting of the receiver turned down as low as possible. Use a directional receiving antenna with minimum gain and take multiple bearings as the process of tracking continues to avoid being fooled by bounce readings that do not provide a consistent point of triangulation. Experience in the field will help you look at the terrain and determine when possible features may be causing the bounce, and perhaps suggest a reasonable solution that explains the observed bearings. Damp or wet hillsides or rocky outcroppings, curtains of wet canopy or vegetation, wet power lines, and large bodies of water can all reflect signals. A combination of experience and learning to study the environment will help you recognize when you are in a highly reflective environment and should expect to deal with signal bounce. After tracking an animal in such an environment, and experiencing the confusion resulting from reflected signal reception, it gets easier the next time around.
Multipathing also affects telemetry tracking from fixed-wing and rotary-wing aircraft.
Tracking animals from helicopters can resemble fixed-wing tracking when flying, or it may resemble ground tracking when hovering. In fixed-wing aircraft, there is constant forward motion. The signals received can come directly from the animal, from a sheer rock cliff in the distance, or from the reflective surface of large bodies of water. To some degree, the constant movement helps to discriminate the signals coming directly from the transmitter and those which are reflected from the environment. A transmitter with a rapid pulse rate also aids tracking when you are moving at relatively high speed in an aircraft. During the locating process, the observer often establishes a bearing from the aircraft (receiving site) to the transmitter on the animal. As with ground tracking, lack of convergence of the bearings suggests that multipathing is occurring. In some respects, tracking from aircraft helps to eliminate some of the signal bounce experienced on the ground, but it can also result in receiving signal bounces resulting from large terrain features such as distant mountain ranges or wet slopes at the other end of a valley. In general, LOS conditions are often better in an aircraft, so we can exercise the idea that the direct signal is often the strongest signal.
When tracking from aircraft, it is best to reduce the gain setting of the receiver by turning down the gain control as far as practical, and use small antennas properly mounted on the aircraft to reduce reception of weak signal bounces. It is also important that the antennas which are fixed to the aircraft provide a repeatable signal reception pattern to assure that the tracker can identify where the signal is coming from. Generally, two element antennas are superior in aircraft applications and, as a side benefit, they have less drag-making them safer. It is difficult enough to obtain a good bearing direction during the tracking process without having to deal with antennas which demonstrate inconsistent receiving patterns. For more information on fixed-wing aircraft tracking, please refer to a special issue of Telonics Quarterly (TQN Vol. 10, No. 1 Spring-Summer 1997).
Establishing LOS bearing from a fixed-ground station.
A reliable antenna system is again critically important. The problems associated with obtaining a reliable bearing should not be exacerbated by problems with the receiving antenna hardware, design, and implementation. Null direction-finding antenna arrays such as the RA-NS system have been in existence for almost two decades (see Telonics Technical Note XX). These null systems have solved virtually all of the problems associated with their predecessor - the old and now notoriously unreliable "peak/ null system." Repeatable bearing accuracy can be established and maintained using these systems throughout the course of the study. As with aircraft, establishing accurate bearings is difficult enough without having to put up with unreliable antenna arrays and equipment. Similarly, state-of-the-art telemetry receivers have adequate sensitivity (gain) control so that there is no need for a switching attenuator or an external attenuator in situations where strong signals are present. Receivers such as the Telonics TR-2 and TR-4 connect the output of a transmitter directly into the input of a receiver and still turn down the signal gain adequately in order to fully attenuate the signal. Good receivers do this, poor receivers don't.
As we suggested early in this paper, the requirement for establishing an LOS bearing applies to handheld ground tracking (following), aircraft tracking, and fixed-site monitoring. For most fixed-site monitoring, the best antenna for the job is one with a minimum number of elements. Systems with fewer elements generally have less gain and a simplified, well-defined receiving pattern which tends to help eliminate the reception of weak signals bounced off features in the environment. In the very early days of telemetry, fixed sites where high accuracy bearings were required usually required five-, eight-, or even 14-element high gain antenna arrays to establish a "pencil beam" or a narrow pattern of reception. Often bearing directions from these complicated and large single antenna arrays were confounded by the reception of signals coming in on back lobes of the antenna. Today the use of the RA-NS Null Direction Finding Systems allow the use of antennas of any gain to be used without sacrificing bearing accuracy. Thus, it is recommended that the smallest (lowest gain possible) antennas be used to minimize reception of unwanted and confusing weak signals (often "bounce signals").
Irrespective of the advances made in providing the telemetry field with reliable antennas and receivers, the requirement remains for LOS to establish accurate bearing locations. By definition, a fixed site doesn't move, so it is impossible to determine if the strongest signal is a bounce or the LOS signal. Assuming good judgment is used in selection of a receiving site on higher elevations within the study area, and assuming the site is located as far away from reflective surfaces as is possible, all that remains is to obtain bearings from multiple sites to establish the triangulated position. The more sites the better. If the position is determined from two receiving sites, the validity of the position cannot be confirmed. One or both bearings received could be LOS bearings or they could be "bounces." Even with three sites, the triangulated position as indicated by using bearings from sites one and two could be different from the position indicated from sites one and three, or sites two and three. Most studies which have carefully considered the problem end up with five or more sites for monitoring directional bearings. In addition, ground truthing the area from which signals are received with a test transmitter is strongly recommended to delineate regions of LOS reception. The technique will indicate regions where signal bounce presents difficulties for a given receiving site. When the animal is in a problem area, the site which has a problem is not used. If it sounds complicated - it is, but the alternative is a poor data set.
In the earlier days of telemetry, many of us were so enthralled to obtain a position we really didn't dwell on the accuracy. Today, VHF telemetry tracking has become refined and many of the more subtle problems associated with this technique are well understood. VHF telemetry can be used to establish highly accurate positions which must be correlated to highly accurate GIS data sets. Reliable equipment is a must and an understanding of radio wave propagation characteristics is essential to success. In the final analysis, if you are not LOS you are indeed SOL.
- Stan Tomkiewicz