RADAR TECHNIQUES FOR
1
WILDLIFE
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
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 lowflying animals because the earth, and anything bound to it, curves
from under the radar beam or topography prevents a clear view of
the animals.
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.
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).
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-band is shorter than Xband and is being used in applications such as automotive radar.
Most radar used with wildlife operates at a single wavelength.
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.
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.
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)
Producing and Interpreting an Ascope 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.
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).
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.
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.
The polar
coordinate system
used by radars
consists of
azimuth (angle
from north),
elevation (angle
up from
horizontal), and
slant range.
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.
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.
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
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).
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).
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.
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.
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.
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.
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.
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.
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.
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.
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, lowerpower 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.
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.
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.
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.
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).
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.
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.
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 anticlutter techniques.
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.
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 (Molothrus ater). Data were taken with a large research
radar then operated by the Illinois State Water Survey.
A spatially extended flock of 6 x 104 snow geese departing from Coffeen Lake in westcentral 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.
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.
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.