Download Radar A- Scope

Module 2. Principles of work, key
parameters of radio location systems
Topic 2.2. Range of radar
Lecture 2.3.2.
SIDE-LOOKING RADAR.
RADAR SCOPES.
Ground Echo
Radar ground return is described by σ°, the
differential scattering cross section, or scattering
coefficient (scattering cross section per unit
area), rather than by the total scattering cross
section σ used for discrete targets. Since the
total cross section σ of a patch of ground varies
with the illuminated area that is determined by
the geometric radar parameters (pulse width,
beamwidth, etc.), σ° was introduced to obtain a
coefficient independent of these parameters.
Use of a differential scattering cross section implies that the
return from the ground is contributed by a large number of
scattering elements whose phases are independent. This is
primarily because of differences in distance that, although small
fractions of total distance, are many wavelengths. Superposition
of power is possible for the computation of average returns. If
this condition is not applicable to a particular ground target, the
differential-scattering cross-section concept has no meaning for
that target. For example, a very fine-resolution radar might be
able to resolve a part of a car; the smooth surfaces on the car
would not be properly represented by σ°. On the other hand, a
coarser radar might look at many cars in a large parking lot, and
a valid σ° for the parking lot could be determined.
Some authors use a scattering cross section per
unit projected area rather than per unit ground
area. Figure 16.2 illustrates by using a side view
the difference between ground area and
projected area.
PARAMETERS AFFECTING GROUND RETURN
Radar return depends upon a combination of system parameters
and ground parameters:
Radar systemparameters (Eqs. 16.1 and 16.2a andb):
Wavelength
Power
Illuminated area
Direction of illumination (both azimuth and elevation)
Polarization (including the full polarization matrix when available)
Ground parameters:
Complex permittivity (conductivity and permittivity)
Roughness of surface
Inhomogeneity of subsurface or cover to depth where attenuation
reduces waves to negligible amplitude
Different wavelengths are sensitive to different
elements on the surface. One of the earliest known and
most striking directional effects is the cardinal-point
effect in returns from cities: Radars looking in
directions aligned with primary street grids observe
stronger regular returns than radars at other angles.
Horizontally polarized waves are reflected better by
horizontal wires, rails, etc., than are vertically polarized
waves. Vertically polarized waves are reflected better
from vertical structures such as tree trunks, at least
when the wavelength is comparable or larger than the
trunk diameter.
If the geometry of two radar targets were the same, the returns
would be stronger from the target with higher complex
permittivity because larger currents (displacement or
conduction) would be induced in it. Because identical geometries
with differing permittivities do not occur in nature, this
distinction is not easy to measure. Effective permittivity for
ground targets is very strongly influenced by moisture content
since the relative permittivity of liquid water is from about 60 at
X band to about 80 at S band and longer wavelengths, whereas
most dry solids have permittivities less than 8. Attenuation is
also strongly influenced by moisture since wet materials usually
have higher conductivity than the same materials dry.
The roughness of surfaces (especially natural
ones) is difficult to describe mathematically but
easy to understand qualitatively. Thus it is easy
to see that a freshly plowed field is rougher than
the same field after rain and wind have been at
work on it. A forest is inherently rougher than
either a field or a city. It is harder to see the
difference between the roughness of natural
areas and the roughness of a city that has flat
walls interspersed with window sills and with
curbs, cars, and sidewalks.
Surfaces that are relatively smooth tend to
reflect radio waves in accordance with the
Fresnel-reflection direction, so they give strong
backscatter only when the look angle is nearly
normal to the surfaces. Rough surfaces, on the
other hand, tend to reradiate nearly uniformly in
all directions, and so they give relatively strong
radar returns in any direction.
The problem of radar scatter is complicated because waves
penetrate significant distances into many surfaces and
vegetation canopies, and internal reflection and scatter
contribute to the return.
Measurements of attenuation for field crops and grasses show
that most of the return is from the upper layers, with some
contribution by the soil and lower layers if the vegetation is not
very dense.
At С band and higher frequencies, most of the signal returned
from trees is usually from the upper and middle branches when
the trees are in leaf, although in winter the surface is a major
contributor to the signal.
At L band, and especially at VHF, the signal penetrates farther, so
trunks and the ground can be major contributors even when the
trees are leafed out.
Additional problems occur near grazing incidence. Because of
the low angle with the surface, shadowing frequently occurs—
some parts of the target are obscured by intervening projections
such as hills and buildings. Parts of the area that are somewhat
elevated have the signal modified by the effect of multipath
interference between the direct ray and one reflected off the
ground. Since the scattering from relatively level surfaces is very
small, any projection may give a return much stronger than the
background, thereby skewing the statistics so a Rayleigh
distribution no longer applies to the average signal. Objects such
as trees, buildings, fence posts, and power lines give localized
echoes strong relative to their surroundings.
Moreover, the signal from surfaces without
projections falls off very rapidly for depression
angles within a few degrees of grazing. This
means that the effect of small local slopes can
be very significant in modulating the return
signal, not just in shadowing.
Physical Optics Models. Theories based on applications
of the Kirchhoff-Huygens principle have been
thoroughly developed.
The Kirchhoff approximation is that the current flowing
at each point in a locally curved (or rough) surface is
the same as would flow in the same surface if it were
flat and oriented tangent to the actual surface. This
assumption permits construction of scattered fields by
assuming that the current over a rough plane surface
has the same magnitude as if the surface were smooth,
but with phase perturbations set by the differing
distances of individual points from the mean plane.
Side Looking Airborne Radar (SLAR)
Side-looking viewing geometry of imaging radar system.
The platform (aircraft or satellite) of an sidelooking airborne radar (SLAR) travels forward in
the flight direction with the nadir directly
beneath the platform. The microwave beam is
transmitted obliquely at right angles to the
direction of flight illuminating a swath. Range
refers to the across-track dimension
perpendicular to the flight direction, while
azimuth refers to the along-track dimension
parallel to the flight direction.
Swath width refers to the strip of the Earth’s surface
from which data are collected by a side-looking
airborne radar. It is the width of the imaged scene in
the range dimension. The longitudinal extent of the
swath is defined by the motion of the aircraft with
respect to the surface, whereas the swath width is
measured perpendicularly to the longitudinal extent of
the swath.
The SLAR is a real aperture radar primarily. This
requires a reasonable large antenna for adequately
angular resolution. The azimuth resolution, Ra, is
defined as
Resolution cell variation.
The equation shows, that with increasing
altitude decreases the azimuthal resolution
of SLAR. A very long antenna (i.e., large L)
would be required to achieve a good
resolution from a satellite.
Synthetic Aperture Radar (SAR) is used to
acquire higher resolution.
The size of the ground resolution cell
increases on the side of the nadir as the
distance between radar platform and the
ground resolution cell increases. This
means that the ground resolution cells are
larger towards the edge of the image than
near the middle. This causes a scale
distortion, which must be accounted for.
At all ranges the radar antenna measures the
radial line of sight distance between the radar
and each target on the surface. This is the slant
range distance. The ground range distance is the
true horizontal distance along the ground
corresponding to each point measured in slant
range. The cross-track resolution, Rr, is defined
as
For an SLAR with the following characteristics:
λ = 1 cm,
L = 3 m,
H = 6000 m,
θ = 60°, and
tp = 100 ns,
has got a resolution of
Ra = 40 m and
Rr = 17.3 m
Note: The same SLAR on a platform in a height of
600 km would achieve an azimuth-resolution of Ra =
4000 m.
RADAR INDICATORS
This lecture gives an overview, how a
target is indicated in radar. In the past
several different display types were
developed. Today modern radar
systems typically use some kinds of
raster scan displays to produce a maplike image.
Learning objectives:
The learning objectives serve as a preview of the information
you are expected to learn in the chapter. This chapter provides
the basis for understanding the specific radar indicators. Upon
completion of this chapter, the student will be able to:
- describe the purpose of the A scope, the range-height indicator
(rhi), and the plan position indicator (ppi);
- state the relationship between range and sweep speed and
length on a radar indicator;
- explain the purpose of timing triggers, video, and antenna
position inputs to a radar indicator;
- list the major units of a ppi and describe their functions;
- describe the basic operation of sweep deflection and sweep
rotation in a ppi.
Radar A- Scope
Figure 1: View of an A-scope
The A-scope display, shown in the figure, presents only
the range to the target and the relative strength of the
echo. Such a display is normally used in weapons
control radar systems. The bearing and elevation angles
are presented as dial or digital readouts that
correspond to the actual physical position of the
antenna. The A-scope normally uses an electrostaticdeflection crt. The sweep is produced by applying a
sawtooth voltage to the horizontal deflection plates.
The electrical length (time duration) of the sawtooth
voltage determines the total amount of range displayed
on the crt face.
Figure 2: the attempt to see digital signals with an A-scope
The A- scope display is using in older radar sets only as
monitoring oscilloscope. In modern digital radar sets
don't exist a similar video signal of the backscatter. The
target messages are transmitted to the displays as a
digital word. There isn't any possibility to get a
synchronizing signal for these asynchronous serial
digital signals. Well, the oscilloscope can get an internal
trigger only. Therefore it is impossible to analyze the bit
sequence with a simple oscilloscope. The one and only
statement is possible seeing this picture: a digital word
exists on this line, which means, obviously the driver
module for this line works.
Figure 3: A control-pulse shown at an A-scope of the russian VHF-radar
„Spoon Rest”
Radar B-Scope
B-Scope
The B-Scope shows a picture like a
Cartesian diagram. It provides a 2-D
“top down” representation of space.
The horizontal axis (abscissa) typically
represents the measurement of the
azimuth (bearing), and the vertical axis
(ordinate) represents the
measurement of the range. Signals
appear as bright spots.
B-scope displays were common in airborne and
fire-control radars in the 1950s and 60s, which
were mechanically or electronically scanned
from side to side, and sometimes up and down
as well. The center of the bearing usually is
movable through hand wheels in fire-control
radars. The antenna turntable then is turned
into the new direction. The screens middle is
defined as the main reception direction of the
antenna normally. The bearing area is covered
through an electro-mechanical or electronic
beam steering.
Radar PPI-Scope
The PPI-scope shown in this figure, is by far the most
used radar display. It is a polar coordinate display of the
area surrounding the radar platform. Own position is
represented as the origin of the sweep, which is
normally located in the center of the scope, but may be
offset from the center on some sets. The ppi uses a
radial sweep pivoting about the center of the
presentation. The sweep rotates on the display just as
fast as the radar antenna. This results in a map-like
picture of the area covered by the radar beam. A longpersistence screen is used so that the targets remain
visible until the sweep passes again.
Airtraffic- Controllers with PPI-scopes
Radar J- Scope
J-scope used in the German airborne WW2- radar FuG 212
The J-Scope (also called circular sweep
scope) uses deflection modulation like the
A-scope. The difference between the two
kinds of scopes is that the A-scope uses a
linear deflection, and the J-scope uses a
circular deflection. This provides a better
resolution on the screen by using the same
screen-size.
The deflection begins top at the screen and
shows a pulse caused by the transmitter,
the so called “Senderzacken”. Then the
deflection drifts clockwise and the next
pulse is the echo-signal. A scale is engraved
at the screen for measuring the range.
J-scope: View of Groundclutter on a screen of Giant Wurzburg Radar
END.