"Bio Telemetry Tracking (Australia)"
BTMS Pty. Ltd. ABN 53 120 330 261 PO Box 739, St Agnes, South Australia, 5097. Phone 08 8264 5575 Bio Telemetry Tracking (Australia) Fax 08 8264 5576 www.btms.com.au General Information about Radio Tracking 1.0 Directional Radio Tracking The classical approach to radio tracking, which still applies in many situations, involves using a direction-finding antenna to locate an animal or object that has a small transmitter mounted on it. The method involves finding the direction of the strongest signal by rotating (left and right) a directional Yagi antenna. Most people can obtain a reasonably accurate fix with a good compass within 10 degrees using a hand-held 3 element Yagi. Depending upon the terrain, 5 degrees is also possible with hand-held 3 element Yagis for experienced operators. Fixed 4 or 7 element Yagis will provide more precise fixes (down to 2 degrees for a 7 element Yagi) but these require expensive infrastructure and may be overkill for the spatial scale of your study. Here are a few tips to increase your precision. • Don’t mount the compass on the antenna without testing for any declinations introduced by the antenna. This may vary across the bearing range of 0 to 359 degrees. • Purchase a good quality compass (say Silva). • Use a prismatic compass not an open (dial) compass. • Turn down the Radio Frequency (RF) gain (not the volume) on your receiver until you only just hear the signal near the peak. This will narrow the range of the perceived direction of the transmitter. • Another trick is to obtain a bearing on the strongest signal followed by bearings either side of the strongest signal. This is where the signal begins to drops off rapidly. These are called the Outer Lobe Bearings (OLBs). Often the mean of these is more precise than the centre reading. To be more thorough (and to ensure you don’t accidentally calculate the back- bearing behind you as the mean), you should record take all three readings. This also gives you some idea of the observation errors in your bearings (see Figure 1). • Following the last point, always check the signal is not stronger 180 degrees (behind you). All Yagis have a smaller back lobe. Left OLB. The left bearing at which the signal drops off rapidly The centre bearing. – the strongest signal Right OLB. The right bearing at which the signal drops off rapidly Figure 1 Taking three bearings to the animal Most studies need to be passive, meaning you do not want to track the animal too closely so that you disturb its natural behavior. This means that in order locate the animal’s position (as opposed to direction) you need to obtain directions to it from two or more locations. For reasonably mobile animals you would need two or more people to obtain the direction of the animal at precisely the same time. After correcting from the compass (magnetic) bearings you obtain to grid bearings (add 8 degrees near Adelaide), you can plot the two lines with a protractor on a map. Person 1 Animal Person 2 Figure 2 Two person fix to animal However, there are two problems with the two-person configuration shown in Figure 2. Firstly, because your bearings will have some degree of random error, if the angle at the animal’s position between the two observers becomes too acute (remember you don’t know where the animal is), the fix will become longitudinally erroneous (see error direction in Figure 3). Bear in mind that even with a 7 element Yagi having 2 degrees precision, this can still pose a major problem. In other words expensive antenna arrays don’t guarantee precise coordinates fixes. Person 1 Animal Random errors in bearings Error Margin Person 2 Figure 3 Two person fix to animal – a bad fix The second problem is that you have no check (or redundancy) built into your fix. In other words if either person makes an error, the position will be incorrect. We therefore recommend having at least three observers involved in triangulating the location of the animal. This both reduces the likelihood of the acute angle problem as well as reducing the chances of an incorrect position due to one person making a blunder. Surveyors using theodolites or sextants have understood this problem for hundreds of years Person 3 Person 1 Error triangle Person 2 Figure 4 Two person fix to animal The error triangle in Figure 4 now has the advantage of providing some indication of the likely error in the position of the animal. Bio-Telemetry Tracking provides ‘Triangulate’ and ‘Spatial Analysis’ software that is able to correct the magnetic bearings, carry out an error analysis of the triangles (or more complex figures for more than 3 observers), graphically portray this information in GIS Window with a background shape file and export the species locations to a chosen coordinate system. Email us for a copy. 2.0 Using the Yagi The fold-up Yagis (for example, a 3 element Yagi - 3EY) must have all of their elements rotated outward and 90 degrees to the main axis of the antenna. The handle and coaxial cable points towards you, which is the opposite direction of the signal you are attempting to find. ‘Line of sight’ electromagnetic radiation follows two distinct polarisation planes. These are vertical and horizontal planes. For this reason, when you hold the Yagi such that the elements are pointing perpendicular to the ground, the signal of a distant transmitter is usually stronger. However, in this orientation, you may loose directional precision. When the elements are parallel to the ground, the signal is generally weaker but you will be able to obtain a more precise direction. Generally, if the signal is emanating from a long distance away, you may only be able to detect it when the elements are perpendicular to the ground. However, as you become closer to the transmitter, a more precise direction is obtained when the elements are parallel to the ground. Nevertheless, if you cannot hear the signal, always try both orientations because very occasionally the signal is stronger when the elements are parallel to the ground (see later). If you draw nearer to the transmitter (usually under 300 metres in open country and under 100 metres in closed country), you may find it difficult to locate the strongest signal direction. To combat this reduce the sensitivity of the receiver with the RF Gain control. By adjusting this the signal can be made to appear weaker. An apparently weaker signal may enable you to obtain a more precise direction. 2.1 Range An important consideration in any study is the range limitation of the transmitter in relation to the environment. A signal passing through the air well away from the ground has potentially ten times the range of a line-of-sight signal traveling 2 metres parallel to the ground along a flat surface. Obviously the type of the terrain and the height of the user and species above the ground greatly affect the range of the transmitter. The frequency of the transmitter is also important. For example, ultra-high frequencies (UHF) need a pathway loser to line-of-sight than very lower frequencies. This is partly why VHF receivers are used in radio tracking. 2.3 Signal Strength and Polarisation The signal strength of your transmitter can be weaker when the antenna is bent over or low to the ground. When the antenna is bent 45 degrees from the ground, you may find the signal is stronger when the Yagi elements are 45 degrees to the ground. When the transmitter antenna is bent closer to horizontal, you may also find the signal is stronger when the Yagi elements are also horizontal. Nevertheless, a reasonably vertical transmitter antenna should improve the strength of the signal. The strength of the signal will also depend upon the following factors: • the height of the user – if you are on a hill, you will always improve the signal strength of weaker signals; • the height of the transmitter – likewise if the transmitter is high, the signal will be stronger; • obstacles between the user and transmitter – hilly terrain will inhibit the signal pathway and signal strength, as well as rocks, tree trunks and other animals; • the nature of the vegetation – thicker or wetter vegetation will inhibit the signal strength; • the power output of the transmitter – this is restrained by battery life and the Federal Government’s maximum legal power requirements; • the length of the transmitter antenna – if the antenna is broken off or shortened the signal will be weaker; • weather conditions – hot days or very wet weather will inhibit the signal; • interference from other electronic devices, electric fences or power lines. 2.4 Signal Pathway Depending upon the terrain, it is sometimes possible to find the signal reflecting off a cliff, hill or wet vegetation. This may indicate a false direction. While not guaranteed, the problem may be resolved by moving from your location. With practice you will be able to reduce these effects. 3.0 GPS Radio Tracking Even modern GPS chips still have high power demands. For this reason GPS collars must automatically switch themselves off between fixes. Because of their battery requirements, at this stage any species under 5kg may prohibit the use of GPS system. This will change over time and we expect in 5 years time, mice will have GPS collars. There are several optional configurations you need to consider with GPS systems. Not all manufacturers provide all of these options. 1. The GPS position is constantly transmitted – this would consume the battery in days. 2. The GPS data is logged ½ hourly, hourly or 2 hourly to a chip – this requires you to find the animal again to retrieve the data. 3. Option 2 but with a beacon transmission triggered when its memory is full (or a designated time) so that you can find it to retrieve the collar. 4. Option 2 but at designated times (once a week or once a month), the transmission sends the history of GPS transmissions for the previous period – this requires substantial redundancies to be built into the transmissions. 5. For larger animals or birds, either satellite transmissions of CDMA/GSM phone transmissions may be possible. Theses are expensive. 6. Option 4 but with a remote listening station that can hold multiple positions from one or more animals in its vicinity. 4.0 Selecting a Transmitter The size and behavior of the species will determine the appropriate transmitter. For example, a rat or mouse cannot carry a powerful transmitter or large enough battery. A transmitter under 1.5 grams will therefore be limited in range and battery cycle. In this case you would expected under 200 meters range and under 10 days of battery life. GPS tracking can not be fitted reliably to animals under 5 kg at this stage. At the other end of the spectrum, a Camel or Buffalo can carry large powerful transmitters with 2 to 5 years of battery life, depending upon whether a GPS is fitted or not. Because the power can be increased (to a legal limit), ranges on the ground of between 3km and 20 km are possible. In the air a range of up to 80km is possible for very large animals where the collar is more than 1 metre above the ground. For medium-sized mammals such as Foxes, Quolls, Bettongs, Koalas, Mallee Fowl, Wallabies and Possums tracked with a hand-held 3EY Yagi, 1 – 5 km can be expected depending upon the terrain. The battery life can be from 6 months to 3 years (for the larger of these). 4.1 Transmitter Construction The most common transmitter is a two-stage transmitter (e.g TX2-TP-4), mounted in resin on a leather or vinyl collar with a vertical antenna from 15 cm to 35 cm long. This is about the most powerful type of transmitter (within the legal limit). However, the output power will always be traded off with battery life. So the higher the output power, the shorter will be its battery life. This also depends upon the pulse width and quiet time between pulses (the pulse rate), or whether you are using a GPS collar. All of our TX2 transmitters now have a digital pulse and we have formulated some tricks in how these pulses are transmitted, which provide huge savings on battery life while still maximising power output. This was not possible with older ‘analog’ systems. Arboreal mammals and birds usually have a greater transmission range than ground-dwelling mammals because they are more often high above the ground. This means that a lower output power can be set to ensure a longer battery life. Generally, the antenna length directly influences the amount of signal radiated. This is called signal gain. The legal radio tracking frequency range is 150 to 152 Megahertz in the Very High Frequency (VHF) range. Normally a ¼ wave antenna at these frequencies should be 50 cm however, this is usually impractical and antenna lengths are usually between 20 and 37 cm. Any shorter and there will be a significant reduction in signal strength. 4.2 Loop Antenna Collars For small mammals the collar wrapped around the species neck is made of brass and coated in plastic. In this case, the collar itself is the antenna. This is important for species that use hollows in trees and logs because a wire antenna (say 20 cm long) may become caught up in the hollow. Unfortunately, this usually means the range of these collars is reduced to under 2 km. 4.3 Back Mounted Collars These transmitters are tailored into the shape of a slim line resin cell which can either be glued or attached by a body harness to the species. The species may need to be shaved over a short area of its back if glue is preferred. Most researcher use Superglue but find out from others first before you try it, in case there is an adverse reaction to the glue. Usually a 20-30cm trailing wire antenna is attached (similar to fine stainless steel fishing line) so that it faces to the rear of the species. 4.4 Implantable Transmitters These are used for species that either due to their physical shape or habitat type, it is impractical to either externally mount or neck collar a transmitter. These can also be used to monitor and transmit body temperature and heart rate without the use of additional probes. However, an external antenna passing through this skin is required and you need to consult with other experts, a veterinary surgeon or your Ethics Committee before planning this option. Unfortunately these systems are not ideal for long-range tracking. For example a Wombat implanted with a transmitter would have a reduced range of 1km to 100 metres (underground) whereas a collar transmitter would transit over four times that range. 4.5 Single stage Transmitters These transmitters are less powerful (approximately 30% less) than the two-stage transmitters and subsequently have a lower battery drain. This makes them ideal for collar or backpack packages on very small mammals or birds. 4.6 Specialty Transmitters (additional options) We offer other options including GPS logging and transmission (compressed and encrypted format requiring a specialised receiver system), temperature monitoring, heart rate (limited range), movement, mortality and posture. Usually these data are only available on the two-stage transmitters. The data are either encoded digitally in the transmission or the pulse rate can be varied according to changes detected by the monitoring sensor.