Wildlife Radiotelemetry Overview

Fundamentals of Wildlife Radiotelemetry

• Wildlife radiotelemetry arose during the 1960s following the development of microelectronics for the space program.

• It serves as a technique to increase the efficiency of collecting data, though it can be expensive and time-consuming.

• Primary applications include studying animal behavior, habitat use, home range size, survival, cause of mortality, reproductive success, and physiological processes.

• Key components of a telemetry project comprise study design (clear objectives), equipment, field methods (sample size, trapping, monitoring), and data analysis (models and statistical methods).

Transmitters and Attachment Methods

• Transmitter weight should not exceed $3\%$ of the animal's total body weight.

• Transmitters produce a frequency-specific signal via a stable quartz crystal oscillator, measured in Hertz ($\text{Hz}$), Kilohertz ($\text{KHz}$), Megahertz ($\text{MHz}$), or Gigahertz ($\text{GHz}$).

• Wildlife radiotelemetry typically utilizes frequencies between $150 - 220\,\text{MHz}$. The Very High Frequency (VHF) band spans $30 - 300\,\text{MHz}$ and the Ultra High Frequency (UHF) band spans $300\,\text{MHz} - 3\,\text{GHz}$.

• Frequency regulation requires licensing: the Federal Communications Commission (FCC) regulates state/local government and private sector frequencies, while the National Telecommunications and Information Administration regulates federal government frequencies.

• Signal parameters include pulse width (milliseconds), pulse interval (milliseconds between pulses), and duty cycle (pulse width + pulse interval), which dictates battery longevity.

• Lithium cells store twice the energy capacity of other battery types. Exercising stored lithium batteries $1\,\text{day}$ per month prevents the buildup of a passivation layer that inhibits electrical current. Transmitters may also be solar powered.

• The antenna is the weakest link of the transmitter system and is reinforced using rubber sleeves, springs, or leather. Broken antennas decrease transmitter range.

• Attachment methods vary by species and include collars (e.g., leather), harnesses (leather or elastic), necklaces, leg bands, tail clips/mounts, glue, sutures, prongs, subcutaneous attachments, ear tags, and surgically implanted devices.

Trapsite Devices and Specialized Tracking Systems

• Trapsite Transmitters: Report whether a trap is triggered or not triggered, minimizing disturbance to the trap site and enabling prompt animal removal.

• Light-Level Geolocators: Archival tracking devices that record photoperiod data (daily cycles of light and dark) using light sensors, a clock, memory, and a computer. Data are synchronized with local time to compute geographic coordinates, requiring physical recovery of the device to retrieve data.

• Motus Wildlife Tracking System: An international collaborative research network using automated radiotelemetry to track small flying organisms (birds, bats, and insects) fitted with nanotags weighing as little as $0.2\,\text{g}$ ($0.007\,\text{oz}$). Receiving station towers detect tagged animals within a range of $12.4\,\text{miles}$.

Receiving Systems and Antennas

• Receiving systems function to recover, amplify, and convert signals into an audible range.

• Transmission lines (co-axial cable from antenna to receiver and cable from receiver to earphones) are the weakest parts of the receiving system.

• Receivers feature programmable scanning, attenuators, and automatic gain control switches (which automatically lower gain as distance to the transmitter decreases).

• Antenna Types:

  • Omni-Directional (Whip): Non-directional, detects equal signals through $360^\circ$, typically mounted on vehicles.

  • Yagi: Directional antenna with $2$ or more elements; a $3$-element Yagi is accurate to $\pm 5^\circ$, a $12$-element Yagi to $\pm 3^\circ$, and dual Yagi antennas in tandem to $\pm 1^\circ$.

  • H-Antenna: Directional $2$-element antenna that is smaller and more portable than a Yagi, accurate to $\pm 3^\circ$.

  • Loop Antenna: Small, highly portable $1$-element directional antenna with very short range, accurate to $\pm 5^\circ$.

Radio Wave Propagation and Reception Range

• Radio waves are electromagnetic waves emitted by the transmitter antenna that pass through certain visual barriers.

• Initial wave orientation is either vertical (perpendicular to Earth) or horizontal (parallel to Earth), but can shift as waves pass through vegetation, rocks, or precipitation.

• Suspected shifts in polarization are resolved by rotating the receiving antenna elements through both horizontal and vertical planes to locate maximum signal strength and directionality.

• Reception range weakens as waves spread out and encounter obstacles such as animal bodies, fences, wires, terrain, vegetation, and precipitation.

• Antenna elevation ("getting high" for line-of-sight) and physical placement are the most critical factors influencing reception range.

Sources of Error and Mitigation

• Primary sources of telemetry error include unknown receiver locations, poor map reading, mis-plotting locations, misreading UTMs, poor compass bearings, operator error, equipment misuse, signal reflection/bounce, and field/weather conditions.

• Error is minimized through rigorous equipment training, routine system checks (coaxial cables, batteries, earphones, antennas), strong map and compass skills, proper understanding of weather effects, and maintaining line-of-sight positioning.