Over the next few weeks I will post chapters from a set of notes that Jock Donaldson has thoughtfully written, and would like to share. I think he has found a beautiful balance between technical detail and readability, no mean feat for an engineer! We look forward to your comments and feedback. At the end of the series if you would like the complete document please feel free to mail me at Jason@awetelemetry.co.za for a pdf version. We start at the beginning, with a short Introduction to the guide and a chapter on VHF Transmitters…
There have been many advancements in wildlife tracking over the last decade or so, and some of the old faithful technologies are also still around. This guide aims to provide some insight into the different technologies out there with a specific focus on suitability for different applications as well as some analysis on the total cost of operating each technology.
Using this guide
Each short chapter focuses on describing a specific tracking technology in detail whilst not delving too deeply into technical discussions. The final chapter provides a comparison between each technology for a variety of use cases. If you don’t have time to read the entire guide, each chapter starts with summary of the technology discussed and the final chapter contains a table comparing the technologies for different use cases.
Tried and trusted basic animal tracking
VHF transmitters have been around for decades, and for good reason. Transmitters are available in small sizes with good battery life. The technology offers the operator an immersive experience in locating animals in the wild. VHF transmitters are best used when animals are to be found and visually monitored very regularly. The total cost per location acquisition from a VHF transmitter is very high due to the manual nature of tracking using VHF technology. Tracking via VHF transmitter is also more intrusive to the animal than some of the other technologies and may affect behaviour. Many wildlife tracking solution providers combine VHF transmitters with their other tracking technologies to provide a good combination of rich data acquisition and the ability to physically find the animal in the field when required.
How do VHF transmitters work?
VHF transmitters work by transmitting short interspaced radio pulses of a Carrier Wave (CW) signal in the Very High Frequency (VHF) radio frequency band. A CW is an unmodulated radio signal that carries no information. Typically the transmit frequency is set somewhere between 148.000 MHz to 152.000 MHz and there should normally be at least 10 kHz of spacing between different transmitters in order to be able to differentiate between them. The pulses of CW signal are typically sent at a rate of anything between 30 and 120 pulses per minute. Some transmitters have activity (or mortality) sensors which detect the movement of the animal to which they are affixed. These transmitters change the rate of pulsing based on how recently the animal has moved. Some transmitters may change pulse patterns such as performing a ‘double pulse’ to indicate the animal’s demise – inferred based on the sensed lack of movement for a prolonged period.
How do you track an animal fitted with a VHF transmitter?
Tracking a VHF transmitter is very manual, but can be fun if you have enough time to go searching for signals. VHF transmitters are traditionally fairly basic in how they operate. The transmitter transmits its pulses, and and a person tracks it with a receiver. The person operating the receiver needs to get a feel for the equipment and the procedure and it takes an outing or two to ‘get the hang of it’.
Tracking a VHF transmitter
A narrowband receiver in CW detection mode is tuned to the frequency of a transmitter (you need to know the frequency upfront) to detect its pulses. When a narrowband receiver is in CW detection mode it produces a tone when a CW signal is received at its tuned frequency. Since the VHF transmitter only sends short pulses of the CW signal, they will be translated to a series of short tones or beeps by the receiver. The volume or clarity of the beep represents the magnitude of the received radio signal. A directional antenna (such as a Yagi antenna) is connected to the receiver to amplify the transmitted radio pulses from the direction in which it is ‘pointed’. By turning the antenna slowly to sweep the horizon, the operator listens carefully for beeps that are generated by the receiver when the radio signal is received. The beep volume or clarity will slowly increase as the antenna tends in the direction of the transmitter, and will then fade as the antenna tends away from the direction of the transmitter. The operator can then estimate the direction from which the signal is the strongest. It is useful for the operator to have a topographic map handy to draw a line from their location in the direction of the transmitter. To get an estimate of the location of the transmitter the operator can move to another vantage point perpendicular to the direction of the transmitter and perform the same procedure. When plotting a second line from the operator’s second location through the direction from which the signal is strongest, the transmitter can be estimated to be located at the intersection of the two lines. For a more precise determination of the position of the transmitter, the operator can move in the direction from the where the signal is the strongest until the animal is sighted. As the operator gets closer to the transmitter, the beeps generated as a result of the received signal will become strong at an ever increasing angle and it will be more difficult to estimate the direction from which the signal is clearest or loudest. A few methods that can be used to overcome this issue is to
- Detune (move slightly off the centre frequency) the receiver,
- use the receiver’s attenuation function if available, or
- preferably use the receiver’s squelching function if available. Squelching silences the receiver for all CW signals below a certain signal magnitude and is quite useful in close proximity.
VHF transmitter range
Good quality VHF transmitters have the potential to be received over vast ranges. A top VHF transmitter can be picked up over 20 km or even further in the right conditions.
There are however some factors that have an adverse effect on the practical range you are likely to receive a signal in real conditions. It is worth noting that range is determined by the receiver’s sensitivity, the receiving antenna’s gain, the transmitter’s transmit power and efficiency and the attenuation caused by the obstacles between the receiver and the transmitter.
A receiver’s sensitivity is a measure on how easily it can discern radio transmissions above static noise. If you are using a low quality or a broad band receiver you will not get as much range out of your transmitters as you would when using a high quality narrowband receiver.
Antennas on both the transmitter and the receiver are critical components in a radio link. Generally for animal tracking collars the transmitter will have an omnidirectional 1/4 wave dipole antenna. The length of the antenna has to be 1/4 of the wavelength of the signal. For a 150 MHz transmitter that works out to be about 50 cm long. Generally there isn’t enough collar to conceal 50 cm of antenna for all but the biggest animals which necessitates some antenna to either protrude out of the collar, or for the antenna to be clipped and tuned – resulting in loss of efficiency. The close proximity of the antenna to the animal’s body once fitted also results in the loss of some radiating efficiency.
On the receiver side the operator has the option of coiled antenna elements or un-coiled elements. The length of un-coiled elements is around 100 cm (each element is a 1/2 wavelength dipole antenna). The elements of coiled antennas for operation at 150 MHz are also 100 cm but they are coiled to produce a shorter antenna that is more practical to transport and wield. Coiled antennas do however lose some efficiency as a trade-off for their practicality. A good balance between practicality and efficiency is using an antenna with spring cable elements – which easily slide into a tube for transport, and spring out when removed for use. The number of elements in a yagi antenna have an effect on gain. The more elements, the higher the gain, but the narrower the receive angle.
Another critical part of the antenna setup is the co-axial cable that runs from the antenna to the receiver. Low quality or damaged cables with faulty connectors are a common issue that can cause high attenuation of the signal and thereby reduce the practical range of the system.
Different configurations of the transmitter in the collar can also impact performance. The more of the antenna that is in very close proximity to the animals’s body the worse the efficiency of the antenna. For very small animals the size and type of battery used plays a role in how much power can be delivered by the transmitter on each pulse. It is necessary to reduce the transmit power on transmitters where very small cells are used.
The best scenario for a good range between receiver and transmitter is a clear day with low humidity and no obstructions between the transmitter and receiver – both being in an elevated position. Any obstruction between, or even near the direct line of site between transmitter and receiver will attenuate the signal and cause a reduction in practical range.
Common obstacles that adversely effect range are: vegetation, precipitation, hills and ravines. The height of an animal from the ground also has quite an effect on range.
VHF transmitter power consumption
VHF transmitters can be very power efficient depending on the brand and how they are configured. The less power consumed by a VHF transmitter, the longer it will last in the field before it needs to be replaced. Alternatively, if less power is consumed, a smaller battery can be used – which in turn allows for smaller and lighter tracking devices for the tracking of smaller animals.
Whilst some brands of VHF transmitters are more power efficient than others, once you have settled on your choice of brand there are only three things that can be changed on a modern standard VHF transmitter to improve battery life:
Reduction in the transmit power of a VHF transmitter reduces power consumption at the cost of the range at which the transmitter can be received.
The length of each pulse of CW signal sent can be shortened to reduce power consumption but the trade-off is that shorter pulses may be harder to hear and indeed some receivers may not be able to detect very short pulses at all. Typically pulses are in the range of 10 to 30 ms.
The pulse rate (pulses per minute) can be lowered to reduce power consumption. The trade-off in this case is that it becomes increasingly difficult to sweep for the strongest signal when a transmitter transmits at lower pulse rates. This is because it is more likely to sweep past the direction from which the signal is strongest whilst the transmitter is not transmitting, and it also becomes more difficult to compare one beep with another if the time between the beeps is longer. If your transmitter is transmitting at a low pulse rate it is important to turn the antenna very slowly when sweeping the horizon for signal. Transmitters with pulse rates of less than 30 pulses per minute are challenging to track.
Suitability of VHF transmitters for different applications
VHF transmitters can be quite small and can be used for tracking very small to very large animals with varying range. For very small animals such as rodents, birds, small reptiles or even insects, specialised VHF transmitters remain one of the only options available.
For the purpose of comparison with the other technologies discussed in this guide we will consider a few applications disregarding size constraints.
Physically finding an animal
When it is required to visually monitor an animal or a group of animals, VHF transmitters are a very good, tried and tested option. If your search area is very large and the animal has only a VHF transmitter fitted, it may take some time to search for and get in range of a signal initially, and then some more time to work your way to the animal. The field personnel in charge of monitoring animals regularly will generally become accustomed to the terrain, animal home range and behaviour. And since they are likely to have knowledge of the last sighting – they quickly become good at knowing where to start looking for transmitter signals.
Collecting location data
VHF transmitters do not capture or store any location data and it is up to field personnel to manually determine an animals location by tracking it physically in the field. This presents a few challenges if the intention is to use the data for behavioural studies:
- The act of physically tracking the animal regularly may alter its behaviour due to the field personnel’s proximity to the animal.
- Each location point is time consuming to capture and it is therefore difficult to get a rich data set.
- The collection of location information consumes a lot of costly resources – e.g. field personnel, vehicles and fuel.
- Getting animal locations at night time is particularly challenging logistically as access to the terrain may be restricted. In addition it can be dangerous navigating the required terrain at night.
Collecting other data
If more information such as activity level or temperature is needed, standard VHF transmitters are a poor option as they do not collect or store information.
VHF transmitter collar costs
VHF transmitters are the cheapest of the tracking technologies discussed in this guide. It is however not necessarily the most cost effective technology to use – depending of course for what purpose you are required to track an animal.
Cost of Equipment – Low
The cost of a VHF transmitter collar is typically lower than that of a collar containing any of the other technologies in this guide. You do however need to purchase a narrowband receiver capable of detecting CW signals and a directional antenna if you don’t already have these.
Cost of Deployment – High
Fitting and removing tracking collars to wildlife is very expensive. Depending on the animal being collared, various techniques are used to capture and often tranquillise the animal before fitting the collar. It is sometimes necessary to charter spotter airplanes, helicopters and specialised wildlife veterinarians to get the job done. The total cost in resources to locate, tranquillise and fit tracking devices to animals can sometimes exceed the costs of the tracking equipment.
Service Fees – None
VHF transmitter collars are generally a once-off purchase with no recurring service fees from the collar provider.
Cost of Use – Extremely High
Acquiring location data from VHF transmitters is very resource intensive. As an example it could take many person-hours to determine a single confirmed location. In addition to personnel costs, further costly resources such as vehicles and fuel are consumed during the activity.
Total Cost – Very High
All things considered, a pure VHF transmitter collar is undoubtedly the most expensive option in this guide if you were to compare the technologies on a cost per location basis. A pure VHF transmitter collar is however still a good option if you require daily visual monitoring of the animal because the human resources are therefore already committed to the cause. When used in combination with other technologies, a VHF transmitter is a very useful feature on a wildlife tracking collar.VHF Technology rating
Reliability – Excellent
VHF transmitters are generally very reliable. Because they are simple, there are less things that can go wrong with them. Regular use of the narrowband receiver and a fairly awkward yagi antenna can however cause some disruptions due to antenna damage, cable faults and flat receiver batteries. Fortunately these can be addressed without re-fitting a collar.
Ease of Use – Poor
Finding an animal once a good signal has been found is quite easy, although manual and time consuming. It can however take a lot of searching to find a signal in a large search area or challenging terrain.
Data granularity – Very Poor
VHF transmitters do not collect any data – it is up to field personnel to manually determine locations. The time-consuming nature of determining locations also inhibits the amount of data that can be collected within a study period.
Battery Life – Good
VHF transmitters can be very power efficient and can last very long on batteries. If however you consider that your achievable data granularity is poor, you could conceivably get comparable battery life out of the other technologies if they were configured to acquire data as infrequently as is the case with a VHF transmitter.
Things to consider when ordering a VHF transmitter
It is important to keep track of the frequencies of transmitters being used in your area of work and to make sure that any new transmitter being ordered operates at a frequency well clear of any other transmitters.