RADAR TECHNIQUES FOR WILDLIFE 1 Ronald P. Larkin 2 and Robert H. Diehl 3

1. RADAR TECHNIQUES FOR WILDLIFE1 Ronald P. Larkin2 and Robert H. Diehl3 2Illinois Natural History Survey 3Department of Biological Sciences, University of Southern Mississippi 1Power Point by Nova J. Silvy

2. INTRODUCTION During World War II, English ornithologists found the new secret weapon known as RADAR (RAdio Detection And Ranging), while looking for ships and aircraft, was receiving echoes from birds. These pioneers immediately recognized that radar was a powerful tool for monitoring and studying movements of flying animals. Radar directs a high-energy beam and some of that energy is reflected back from objects, in this case flying animals. Most large radars have the power and sensitivity to detect birds at great distances when the birds are in the open and can be reached by the radar beam. However, radars are limited in the distance they can observe flying animals and are unable to detect low-flying animals because the earth, and anything bound to it, curves from under the radar beam or topography prevents a clear view of the animals.

3. Radar Display A radar display does not reveal which kind of animal produces a radar echo and, without specialized research to relate animals to echoes (ground truth), radar does not directly allow a wildlife scientist to know how many animals are responsible for an echo. However, radar allows following animals through the blackest night, inside clouds, and occasionally at great distances contributing to knowledge of animal movements. With minimal computing resources, one can download radar data about every 5 to 10 minutes from much of the continental United States almost as soon as the radars record the data. The data are free or available at negligible cost. Meteorologists, aviation agencies, maritime users, and the military operate radars useful for observing wildlife.

4. Top: The radar beam detects migrating birds (and almost certainly some insects and bats), then at greater range it passes completely over the layer of animals. Brighter colors represent stronger echoes. Bottom: East half of a map display of a WSR-88D radar near the middle of the night. Just before the beam passes completely over the biological layer, the lower periphery of the beam encounters only the highest tail of the distribution of birds, causing the radar to register relatively weak echoes (yellow).

5. RADAR 101 This discussion assumes a radar uses the same antenna to transmit the radar signal and receive the returning echo and the radar’s operational frequencies are microwaves. Microwaves, can be described in terms of wavelength (), or distance between successive troughs (or crests) in a traveling wave. Radar wavelengths include X-band(about 3 cm), C-band (about 5 cm) and S-band (about 10 cm). L-band, used in aircraft surveillance, is longer than S-band. K-bandis shorter than X-band and is being used in applications such as automotive radar. Most radar used with wildlife operates at a single wavelength.

6. A piece of rectangular waveguide. Microwave energy travels on the inside surface of the tube, which is machined to tight specifications and must be smooth, clean, and dry. Usually 0.9 wavelengths >X>0.6 wavelengths, and Y = 0.5X.

7. The Radar Equation (Pt)(G2)( λ2)(σ) Pr = (4π2)(R4) where Pr (W) = received power from the echo, Pt (W) = radar transmitted power, G = antenna gain, or amplification, λ (m) = wave length of the radar, σ (m2) = radar cross section (RCS), and R (m) = range (straight-line distance) to the target. Units are: m = meters, W = Watts.

8. Maximum Range Equation Knowledge of the Radar Equation is useful to understand radar. To help understand the Radar Equation, consider a person shouting across and canyon and listening for an echo. How much radar echo a bird or bat produces is a ratio in the Radar Equation. The maximum range at which an animal of a certain size can be detected is Rmax. Rmax = (Pt)(G2)(λ2)(σ) (4π)3(Pr, min)

9. Producing and Interpreting an A-scope Display An A-scope display shows time variation of echoes vs. range. The radar antenna should be either stationary, with birds and bats flying through the beam, or tracking a bird or bat. On the vertical axis the radar receiver signal produces a positive logarithmic display. The horizontal axis is the range in km (or delay, corresponding to 150 m·s-1). One can construct an A-scope using a suitable oscilloscope (less costly on the used electronics market) and 2 high-frequency cables. One cable feeds the radar “video out” or “rectified video” signal into a vertical “signal in” or “voltage in” connection on the oscilloscope, the other feeds the radar “transmitter trigger” or “pulse out” into the oscilloscope’s “trigger in” connection.

10. The nonlinear relationship between apparent size of a target on radar (vertical axis) and actual mass (or volume) of the target, at 10 cm wavelength (S-band) (redrawn from Vaughn 1985).

11. Antennas and Scanning A radar antenna’s main lobe, or beam points at targets and its direction, along with range, gives their location. Direction is expressed in polar coordinates. Returning echoes from birds, bats, and other objects take the reverse path and the antenna concentrates received echoes the same way it concentrates transmitted microwave energy. Large antennas may have beams as narrow as 1˚. Side lobes are weaker concentrations of energy that are, to some extent, symmetrical about the main lobe. All directional antennas have side lobes. If the antenna is pointing at a bird, another bird of similar size illuminated by a side lobe in a different (deceptive) direction will appear equally prominent on the radar display if it is as close as 0.3 R.

12. The 3 principal concentrations of energy in a reflector antenna. Microwave energy emerges from the feed, which directs it toward a solid or mesh reflector, in this case a parabolic reflector. Bottom: A parabolic reflector antenna with a radar fence constructed of 3.04-m (10 ft.) lengths of corrugated sheet metal.

13. The polar coordinate system used by radars consists of azimuth (angle from north), elevation (angle up from horizontal), and slant range.

14. Antennas and Scanning Spillover radiation includes any energy that escapes past the edge of the antenna. Like side lobes, spillover radiation produces spurious echoes from the ground, structures, and vegetation (ground clutter) in directions different from the main lobe.Only a few radar beam shapes are commonly used in wildlife biology. Many radars used in meteorology have a narrow, conical pencil beam to provide height information (e.g., on storms). A large radar of this type can operate at great range, but its resolution is coarse even though its beam might be narrow. For instance, a 1˚ beam is 1 km across at a range of ~60 km. This type of radar must rotate several times with its antenna at different elevations (a volume scan) to achieve coverage of different heights. Smaller, specialized ornithological research radars use spatially precise conical beams to obtain information on animals at specific heights, often even single flocks and individual animals.

15. Surveillance Radar Antenna Surveillance radar is any radar designed to scan a wide geographical area repeatedly, usually in the horizontal plane. Some surveillance radars use a beam that is narrow in azimuth, but broad in elevation; marine radar beams are shaped this way to detect objects on the surface even while the radar platform is pitching and rolling. These radars provide almost no information on height of flying animals. Animals near the edges of the beam are partially illuminated and the size of the echo they produce depends on their exact position in the beam. Paths of observed targets flying at high elevation on these radars are distorted. The antennas are usually swiveled (rotated in azimuth) through 360˚, but, in wildlife biology, also can be held stationary to monitor bird or bat traffic passing through the beam.

16. Conical Scanning Radar Antenna They may not have a conical beam and they rotate in azimuth while elevation is held constant. A nearly horizontal conical scan generates a Plan Position Indicator (PPI) display that is projected onto the earth as a map. This is the display shown in weather forecasts with stronger targets coded as more intense spots or brighter colors. A PPI scan performed like a windshield wiper (<360°) is called a sector scan. At times, a conical beam is held stationary and animals are counted as they fly through the beam. Specially constructed radar that looks vertically and spins rapidly in azimuth provided useful data on insects with simple wing beat patterns. Biologists may use conical-beam radars in conjunction with surveillance radars

17. Top: A narrow conical beam (“pencil beam”) produced by a small feed evenly illuminating the surface of a paraboloid. The antenna swivels in azimuth and tilts in elevation. A short cylindrical cuff partly shields the paraboloid from ground clutter. Middle: A view of beams narrow in azimuth, but wide in elevation. The hatched pattern is produced by a slotted waveguide antenna typical of marine radars (photo shown) and the additional shaded region at high elevation is typical of airport surveillance radars (not shown). Bottom: A marine radar modified to perform a vertical scan (arrow).

18. Echo strength from a single flying animal passing through the beam of a stationary, vertically-pointing pencil-beam radar. Overall echo strength increases as the target comes into the beam, peaks as it flies through the beam’s center, and decreases again as it exits (adapted from Atlas 1965).

19. Other Radar Antennas Some radars have a stacked-beam arrangement in which several stationary narrow beams are arrayed vertically to provide height information as the array is swiveled in azimuth. Vertically scanning radars intercept animals crossing the plane of elevation through which they scan. Radar antennas are designed to scan sufficiently slowly to receive multiple echoes from each target, reducing the effects of many kinds of noise and clutter, yet sufficiently fast to provide information that is timely. Similarly, long radar pulses give stronger echoes whereas short pulses give greater detail in range. In wildlife biology, advantages of rapid data updates and fine spatial detail may outweigh need for detecting weak echoes.

20. A radar is sited in a gravel pit behind an earth barrier to reduce ground clutter during observations of wildlife flying at low height over a ridge. The operators positioned the trailer-mounted antenna so that the radar’s pencil-beam can point low over the ridge in the background while the earth bank 30 m away shields the radar from side lobe reflections off the ridge itself. The aluminum cuff around the antenna further reduces return from side lobes.

21. Types of Radar Marine radars: Used on boats and ocean vessels to track other vessels, detect weather, aid in navigating land hazards, and, in the fishing industry, spot birds feeding on large schools of fish. Marine radar can be used to record the horizontal tracks of birds as they move through an area, including their size, speed, track, and position. These radars can precisely register the shape of large flocks of birds and can be mounted to point upward to measure height, size, and numbers of flying animals passing overhead. Doppler radar: Refers to large weather radars, but also are excellent wildlife radars. Doppler weather radars have lower spatial resolution, significantly higher power, longer range, highly sensitive receivers, are expensive and generally not portable, and via networking, usually send data rapidly to a central location for display and archiving. Fortunately data from existing radars can be obtained easily and cheaply.

22. A target moving tangential to the radar has a Doppler shift and radial velocity of zero. In this typical fall nighttime Doppler image of migrants from the Corpus Christi, Texas WSR-88D, echoes with negative velocities (blue) approach the radar, positive velocities (red) recede, and a zero velocity line (white) is perpendicular to the direction of travel of migrants at each range on either side of the radar. Birds at farther ranges are higher and flying more toward the northeast.

23. WSR-88D radars at Green Bay and west of Milwaukee, Wisconsin, USA present clear images of perched dunes on the east shore of Lake Michigan 200 km away. These have zero Doppler velocity, confirming they are on the ground rather than in the air. In a “normal” atmosphere without ducting, the height of the bottom of the radar beam would exceed 3 km AGL at such ranges.

24. A Doppler PPI display at low elevation angle (0.5 deg) from a WSR-88D radar near Tucson, Arizona, USA shows birds (and probably some insects and bats) during spring migration except where relief in terrain interferes with propagation of the radar beam. Height of terrain above sea level is shown in gray-scale and increases from black to white. White areas signify partial or complete blockage of the radar beam.

25. Insects (mayflies, Plecoptera) emerging from the Mississippi River, military chaff over Utah, and a ray from the setting sun over Pennsylvania, seen on radar images.

26. Top: A-scope data from a pencil-beam radar pointed across the path of migrating animals. Botom: Behavior of the animals over time revealed in successive traces that descend in 50-ms increments from the top. The outgoing pulse and short-range ground clutter are omitted. Echo strength of the prominent target at slightly over 200-m range varies regularly at 19.4 s-1. Field workers noted, “Bird-like target, range a little less than 2 s”. At greater ranges smaller biological targets wax, then wane over time as they fly through the beam.

27. Types of Radar Airport Surveillance Radar (ASR): Used to detect aircraft with beams that are broad in elevation. High-power ASRs can detect flying animals, especially larger birds. Newer, lower-power ASRs may lack the sensitivity to be of use for studying wildlife beyond the immediate airport environs. Lack of height information also is a problem with these radars. Military Tracking Radar: They can track a single bird or flock and map its trajectory.A powerful tracking radar can followed single birds at a range of more than 80 km. These radars are useful to understand flight dynamics and flocking behavior of birds and have been fitted with telephoto cameras to aid in target identification.

28. Types of Radar Police Radar: Provide no information on target range because they send and receive a fixed-frequency signal continuously rather than in pulsed form, but can be used measurement of speeds of birds. Harmonic Radar: is a directional transmitter and receiver at different frequencies combined with miniature tags to convert the transmitted frequency to the received frequency. It has the potential for following tagged subjects, even terrestrial wildlife, at useful ranges when line-of-sight visibility is possible.

29. Acquiring Radar Data Wildlife researchers and managers work either with existing radar equipment operated by another agency or with dedicated wildlife radar. Whatever the source of data, obtaining information useful for statistical analysis, model development, data visualization, and management is the key to radar study of wildlife as a science rather than a curiosity. One can use existing equipment by borrowing a radar, visiting a radar facility, hiring a consulting company to gather radar data on wildlife, or by acquiring already archived data on computer media or from the Internet. Purchase of a new or used radar system or assembly of a radar dedicated to biological studies can be productive. The purchase cost of a radar that is truly suitable for acquiring the needed data can be too expensive and months or years can be wasted trying to make cheaper, unsuitable purchased equipment do a job that is beyond its capability.

30. Recording Radar Information Recording radar information by videography or time-lapse photography has immediate intuitive appeal and can be handy for obtaining images to accompany oral presentations or proposals. However, for monitoring animals one should avoid photography, preferring direct recording on a computer medium. Signals in the radar exist as voltages and information is lost when the signals are converted into an optical display and subsequently converted back to voltages in a camcorder or camera. More importantly, the deferred labor of quantifying radar data from photographs will quickly become the most expensive part of a radar project and the most tedious. Radars can quickly generate large amounts of data. Infrastructure to clearly label, efficiently quantify, and readily summarize those data is critical and should receive equal importance as the data.

31. Interpreting Radar Data Useful data require timely information on antenna position and/or scan pattern. Animal flight takes place in 4 dimensions (3 spatial and time) or frequently reported as direction, speed, height, and time. Enumeration of flying animals can be accomplished in several ways depending on the questions asked. One can convert a radar signal to meaningful measures such as numbers of animals or biomass. Questions of social behavior or probability of encounter with an animal will require volumetric densities (animals per length³). Habitat-related questions will require areal densities (animals per length²) summed over height. Rate-of-passage questions will require rates of crossing a line on the earth summed over height (animals per length per time) or rates of passing through a vertical plane (animals per length² per time).

32. Ground Truth One should acquire sufficient radar data, but equal weight should be given to concomitant field observations that establish the identity and numbers of targets. Ground truth includes visual observations, infrared, sound, and use of separate small radars. Ground truth should be simultaneous with radar operations, because daily monitoring of migrating birds on the ground is usually a poor indication of numbers of birds actually migrating overhead. Although the difficulty of discriminating small birds from insects with radar has been appreciated for a long time, failing to do so remains one of the most common mistakes in designing radar studies of wildlife. Software algorithms using wing beat patterns and other signal characteristics can distinguish between bird and insect echoes in many cases. Such algorithms require that targets dwell in the radar’s beam for a period of time of at least several wing beats, which is not possible when radars scan at high rates.

33. Ground truth as supplied by continuous scanning with binoculars and telescopes. Radar observations have shown that some raptors and storks fly higher than visual observers on the ground can detect.

34. Wildlife as Unwanted Radar Echo Flying wildlife can be important sources of unwanted echoes for those using radar to observe aircraft and weather. Engineers regard birds as clutter when echoes persist despite use of anti-clutter techniques.

35. Applications of Radar in Wildlife • Aviation Safety: Estimated annual losses from collisions of aircraft with wildlife (including “bird strikes”) total at least $500 million. • Human Impacts on Wildlife: Because radar can monitor flying vertebrates at night and beyond human vision, it can provide data helpful for assessing and/or reducing wildlife collisions with electrical power distribution lines, wind turbines, and tall structures. • Animal Control and Insect Pests in Agriculture: Birds that flock in large numbers can damage crops, impact farmers economically, and even contribute to food shortages. Radar offers an opportunity for long- and short-term monitoring of flocking species that cause economic hardship. Data collected with radar can be used to assess effectiveness of management techniques and long-term impacts of the management on the species. • Population Monitoring: Radar has potential as a conservation tool in population monitoring.

36. Doppler velocity image of a dawn exodus from a roost of 1.5 x 108 European starlings (Sturnus vulgaris) and a few brown-headed cowbirds (Molothrusater). Data were taken with a large research radar then operated by the Illinois State Water Survey.

37. A spatially extended flock of 6 x 104 snow geese departing from Coffeen Lake in west-central Illinois. The birds were counted during an aerial census by the Illinois Natural History Survey and followed on KLSX (Weldon Springs, Missouri WSR-88D) until after they crossed the Mississippi River (black) into Missouri. The flock (circled) extends 53 km E-W at 1050 CST at ~100-km range. Other echoes around the radar are caused by ground clutter and probably other biological targets.

38. The Future • We should see better understanding of the relationship of animal sizes, taxa, flight patterns, speeds, numbers, and density to radar measurements of the properties of echoes as well as Doppler speed and its variation. This will permit new inferences about wildlife and application of the new knowledge to management. Inferences about types of biological targets aloft will primarily be limited by the amount and quality of ground-truth data available from visual observations, small local radars, and acoustic and infrared sensors. • Networks of Doppler weather radars are revolutionizing atmospheric sciences and will revolutionize knowledge of organisms from pollen and insects to the largest birds. • As data from these powerful instruments are fused with data from other sensors, particularly within GIS, opportunities will emerge to compile data useful for land-use and other decisions in wildlife management and to put displays of this technology to use directly in the classroom and in nature centers.

39. SUMMARY Radar has been used for over half a century to observe flying animals. Its application to wildlife research and management continues to blossom, primarily because of the availability of capable and reliable small radars and copious data from large networks of Doppler radars designed for monitoring weather. Properly placed radar can observe flying animals at different spatial scales without affecting their behavior. However, radar offers little if any information on species identity. Radar is useful for informing aircraft of flying birds and bats, locating roosts, following birds that depredate crops, and monitoring populations including threatened and overabundant species and waterfowl. Locating areas critical for stopover of migratory birds is an especially useful application. Acquiring radar data necessitates care and accumulation of meaningful numbers. Challenges in using data from radars lie in establishing the identity of radar echoes (ground truth), recognizing different kinds of artifacts, and especially in coping with large amounts of automatically-generated data.