Very high frequency (VHF) radio telemetry was the first real-time technique used to track individual animals from a distance. A transmitter attached to the study animal broadcasts pulsed signals in the VHF portion of the electromagnetic spectrum (30 to 300 MHz). Study animals are given unique frequencies, so that individuals can be followed. Researchers use specialized antennas and receivers to track study animals.
Despite newer technologies that surpass the capabilities of VHF telemetry, the equipment continues to be a research staple. VHF is also frequently used in combination with satellite tags to download data. Moreover, a new application of VHF technology allows for automatic detection of tagged animals. See Automated Radio Telemetry section below.
VHF tracking systems consist of three primary components (transmitter, antenna, and receiver), which work together to provide information about moving animals.
VHF transmitters include four major parts:
- Power source (battery, or battery with solar cells)
- Electronics package (circuit board and crystal oscillator)
- Transmission antenna
- Attachment method
The weight of transmitters and the methods used to attach them to animals are among the most important considerations when designing a study. This is especially true for highly mobile animals that can be hindered by heavy or binding equipment. One of the reasons VHF radio telemetry remains popular is because the lightweight transmitters can remain active for extended periods.
Previous guidelines suggested that the weights of monitoring equipment should not exceed 3-5% of animal body weights. And for this reason, smaller and lighter transmitters extend the utility of radio telemetry based studies to smaller animals (0.6 gram transmitters now available). The transmission electronics are generally similar in most units, so weight differences are based on battery sizes. Larger and heavier batteries last longer, and some are designed to work for multiple years. Light transmitters with extremely small batteries have restricted operating lives of just a few weeks.
Transmitters come in a host of shapes and configurations, and there are multiple techniques for attachment to a variety of taxa. Some are designed to eventually drop of the study animal.
- Internal mounting techniques
- Glue-on attachments
- Suturing techniques
- Neck collars (mammals and some larger birds)
- Pennant-style harness (large ground birds)
- Complex backpack-style harnesses that minimize contact with the wings (large flying birds)
- Back harness with leg straps (smaller birds)
Antennas are used by the researcher to follow transmissions broadcasted from study animals. There are three general rules that apply to all receiver antennas.
- Antennas must be tuned to the range of frequencies being used in a particular study. Mismatched transmitters and receiving antennas can cause unreliable and poor performance. The reception range is greatly reduced when the wrong antenna sizes are used.
- Larger antennas are better at gathering signals over greater distances. Antennas can be made larger by having larger elements (the prong-like portion of the antenna) or by having multiple elements that work together to gather signals.
- Multi-pronged antennas also provide the benefit of directional sensitivity, which can be used by observers to discern the direction of the study animal.
Types of antennas
Like transmitters, receiving antennas range in type, size, and accuracy that each have benefits and drawbacks. The best antenna for a study is based on objectives, the directional accuracy needed, and the ability to access study areas by vehicle or foot.
- The most basic receiving antennas include single elements that can be mounted to a vehicle or to a stationary tower. These omni-directional antennas are useful for determining if radio-marked study animals are nearby, but they are not good for determining the precise location of a study animal.
- Multi-pronged and tuned loop antennas, are the most frequently used type because they provide directional information about the study subjects.
- The smallest directional antennas are designed to fold up and fit into a backpack so that observers can follow study subjects on foot.
- Larger multi-element antennas can be extremely accurate. They are also heavy, however, and are often mounted on vehicles or towers.
- For fast-moving or wide-ranging animals, receiving antennas can even be mounted to aircraft or unmanned aerial vehicles (UAVs) so that large areas can be covered very quickly.
VHF receivers range from simple hand-held radio receivers to complicated devices with integrated computerized and automated recording equipment.
As with receiving antennas, electronic receivers must be tuned to the correct range of frequencies. Receiving antennas are attached to the receiver via coaxial cable. Very basic receivers are similar to simple FM radios and require the user to tune it to a specific frequency. Modern receivers allow the user to enter the transmission frequency into a digital keypad. Some receivers are also constructed with scanning and logging capabilities or outputs for computers so that they can record the movements of many animals for extended periods.
In recent years, the increased use of personal wireless devices has caused interference in the range of the electromagnetic spectrum used for VHF radio telemetry. Problems are especially prominent in urban areas, where VHF telemetry signals can become difficult to detect. Manufacturers of VHF equipment have responded by releasing signal filtering devices that help remove some of the static that can come from other wireless devices.
Radio Tracking Techniques
1. Presence or absence
Simple presence and absence information inform investigators if animals are within reception range. For example, presence and absence data could be used to discern whether migrants are passing a fixed receiver system along a migratory pathway.
2. Homing locational data
Refined animal location information is the most common type of data sought by investigators, and there are two methods for determining precise animal locations. Field observers can use the equipment to guide them directly to the radio-marked study subject, i.e. observers home in on animals.
3. Triangulation locational data
By recording and mapping the bearing of an animal’s location from multiple angles, its position can be estimated. Triangulation can be done by hand on a map, or with software programs designed for this purpose. Errors can also be approximated. Triangulation software is available for mobile devices and phones so that animal locations can be estimated in the field. Some of the most sophisticated tracking systems include vehicle-mounted antennas, with integrated magnetic compasses and computers that display triangulation calculations and animal locations on geographic information system maps.
VHF signals can travel tremendous distances, but they can also be blocked when they encounter topography, water, or heavy vegetation. Thus, researchers often use the “line-of-sight” rule of thumb. The idea is that mountains, hills, or thick vegetation should not block an imaginary line connecting observers to study animals. In hilly landscapes, radio signals can bounce off hillsides, cliffs, or valleys. This can mislead observers away from study animals and render data unusable. Investigators sometimes avoid bounced signals by making observations from hilltops or observation towers. In extremely rugged terrain, small planes or helicopters can also be used.
4. Automated Radio Tracking
Recently developed radio transmitter and receiver technology now allow for automated tracking of animals at local, regional, and continental scales. Find out more about automated radio tracking on our dedicated page to this technology and learn about the Motus Wildlife Tracking system.
- After your receiver, antenna and cable, and headphones are set up and connected, tune the receiver to the frequency for the first transmitter you want to track.
- Set the receiver’s volume to a comfortable level. Set the gain control (receiver sensitivity) to the “full” gain position. As you track toward the animal, the gain should be reduced to the lowest level that allows you to hear the signal; adjust the receiver’s gain control as required. Avoid changing the volume level if possible.
- Test the antenna in both a vertical and horizontal plane, with a slow sweeping motion of the antenna around you 360 degrees. Use the speaker or headphones to listen for your transmitters.
- The ability to sense changes in signal volume is important. Try closing your eyes as you sweep the antenna in a circle and try to listen for the changes in volume.
- Try to determine in which direction the animal is located. As the volume of the beeping increases, you are getting closer. You’ll continue to move closer to the animal while continuing to slowly sweep the antenna in smaller “slices” of a circle.
- As the signal becomes stronger and directionality more difficult to discern, receiver gain can be reduced in order to decrease its sensitivity. If you need very low sensitivity, disconnect the receiver from the antenna, increase the gain, and then move back and forth in search of the transmitter. You’ll soon find your target.
- Have your receivers bench-checked before tracking season begins.
- Make sure receiver batteries are fully charged before leaving for the field.
- Check and test the equipment as a complete system, using a reference transmitter to verify all of the components are operating correctly.
Edited by Dylan Kesler, 2014.
Updated by Nathan Cooper (Smithsonian Migratory Bird Center), 2016.
Updated by Allison Huysman (Migratory Connectivity Project, firstname.lastname@example.org), 2020.
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