Module 2 Principles of work key parameters of

Module 2. Principles of work, key parameters of radio location systems Topic 2. 2. Range of radar Lecture 2. 3. 2. SIDE-LOOKING 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 coef ficient 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. 2 a 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 stron ger 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 permittiv ity 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 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 math ematically 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 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 approxi mation 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 side looking 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 map like 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 electrostatic deflection 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 1950 s and 60 s, 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 long persistence 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 WW 2 radar Fu. G 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

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