Introduction
22. It has been explained earlier how a pulse radar can be used to detect a reflecting target, and measure its range and relative direction from the transmitter. This principle is used in ground mapping radars to present the operator with an image of terrain features, which can then be used to aid navigation, locate targets, and determine weapon aiming parameters. A ground mapping radar is often one component of an integrated navigation and weapon aiming system such that radar derived data can be used directly to update the system, and conversely data from the rest of the system is used to enhance the radar facilities. Doppler or inertial velocities, for example, may be used to stabilize the radar image and superimposed electronic cursors. In many cases a ground mapping radar will have a secondary air intercept mode of operation, although often with rather limited facilities.
Similarly, many air intercept and most cloud warning radars have ground mapping as a secondary mode.
23. A radar map of an area of the surface is achieved by scanning the radar beam either by mechanically moving the aerial or by using a phased array antenna. The radar image is presented on a CRT, the time base of which sweeps in synchronization with the radar beam, and is intensity modulated in response to the signal strength of received echoes. The image brightness is thus related to the nature of the reflecting object and, in general, built-up areas appear bright, water appears dark, and land appears in an intermediate tone. The persistence of the CRT phosphor ensures that a continuous image of the ground is maintained between sweeps. Although a few installations provide a 360° PPI display, more normally the radar is mounted in the aircraft nose and scans a sector ahead of the aircraft, typically 60° either side of the aircraft heading. The radar aerial must be stabilized, within limits, to the true horizontal both in roll and pitch to avoid image distortion and this is normally achieved by using inputs from the aircraft attitude system. The degree of roll and pitch, for which compensation can be provided, varies between aircraft types. In addition to the radar image, it is possible to superimpose electronically produced symbols and cursors, and in some systems a topographical map can be projected onto the display.
Beam Characteristics
24. The beam of a ground mapping radar must be ‘narrow in azimuth’ so that the bearing of any echo can be accurately defined, and ‘broad in the vertical plane’ in order to illuminate all of the ground between a point beneath the aircraft and the horizon (or effective range).
25. Azimuth Beamwidth. It is impossible to produce a beam in which all of the radar energy is distributed and confined within a finite beam. However, with a well designed antenna most of the radiated power can be constrained to a given direction, and a typical polar diagram is shown in Fig 16-10. It is impossible to eradicate
the sidelobes completely but it is desirable to minimize them since they represent wasted power, and their presence makes the radar more vulnerable to EW interference. Two
definitions of beamwidth are recognized: Fig 16-10: Half Power Beam Width
(a) Nominal Beamwidth. The nominal beamwidth is defined as the angle subtended at the source by the lines joining the two points on the radiation diagram where the power has fallen to a certain proportion (usually a half) of its maximum value. Radiation patterns are normally plotted showing relative field strengths, and since field strength is proportional to the square root of power, the corresponding half power points A and C on the field strength diagram shown in Fig 16-10 are where the field strength has fallen to √5, i.e. 0.707, of the
maximum value OB. Conversely, the power radiated in the direction OA and OC = 0.7072
= 0.5 of the power transmitted along the centreline OB. The angle θ is the nominal beamwidth and is proportional to the wavelength (λ) of the radiation and inversely proportional to the size of the aerial:
Nominal Beamwidth α _____Kλ_____ Degrees (16.1)
Dish Diameter
Where λ is the wavelength and K is a constant which varies with the side lobe level, but for a simple parabolic aerial is typically 70.
(b) Effective Beamwidth. From an operator’s perspective, the effective or apparent beamwidth is of more concern than the nominal beam width since it is one factor which influences the accuracy with which radar returns are displayed. The effective beamwidth is the angle through which the beam rotates whilst continuing to give a discernible image from a point response. Fig 16-11 illustrates the effect of a radar beam with an effective
beamwidth of 4°, scanning clockwise, through a point target. The target will be displayed on the CRT once the leading edge of the beam intercepts it, however it will not be portrayed on its correct bearing of 090°, but in the direction in which the aerial is pointing, i.e. along 088°.
The target continues to be displayed until the trailing edge of the beam passes through it.
The effect is to spread the image of the point response across the effective beam width, thus, for example, a point target at a range of 60 nm would appear to be 4 nm wide (1 in 60 rule), i.e. 2 nm either side of the correct bearing. The effective beamwidth is largely a function of receiver gain. Both transmitted power and receiver sensitivity are maximum along the beam centre line, decreasing towards the beam margins. Receiver gain determines the overall amplification of the received signal and may be reduced to a level which is only sufficient to ensure that signals near to the centre line of the beam are displayed, generating a narrow effective beamwidth, or may be increased so that signals at the edge of the beam are amplified sufficiently to exceed the video threshold.
Fig 16-11: Effective Beamwidth
26. Vertical Beamwidth. A pencil beam is not ideal for mapping purposes since it does not illuminate a sufficient area of land, instead, a diffuse beam known as a spoiled or cosecant2 beam is used. The main characteristic of such a beam is that greater power is transmitted to greater ranges so as to compensate for range attenuation and in this way similar targets will give similar returns
regardless of range (Fig 16-12). The cosecant2 beam dilutes the power per unit area of ground coverage and is therefore normally restricted to the shorter range scales and at longer ranges reversion to a pencil beam is necessary.
Radar Parameters
27. The radar parameters have been described in detail in the chapter
on Basics of Radar. Theses are, even at the cost of repetition, described again in order to refresh the readers’ memory. These parameters are:
Fig 16-12: Cosecant² Beam
(a) Operating Frequency. High frequencies allow narrow beamwidths to be achieved with relatively small aerials, and pulselengths can be relatively short, both attributes leading to improved resolution. Additionally, high frequency equipment tends to have size and weight advantages. Conversely, high frequency radar is restricted in the power that can be employed, and therefore in the effective range. Furthermore, the higher the frequency, the more the radar will be susceptible to interference and atmospheric attenuation. In practice, the majority of airborne mapping radars operate in the I or J band with frequencies around 10 GHz (wavelengths around 3 cm).
(b) Pulse Length. The energy content of a pulse is directly proportional to its length and thus the shorter the pulse, the weaker any echo will be. However, shorter pulses are desirable for good discrimination in that if two targets are separated in range by less than half the distance occupied by a pulse, they will be seen as a single echo. Pulse length also determines the minimum range that the radar can measure. In practice the minimum range will be greater than this since some finite time will be necessary for the aerial to switch from transmission to reception. A pulsed radar signal contains a spectrum of frequencies which broadens with decreasing pulse length, and a receiver for short pulses therefore requires a wide bandwidth, making it more susceptible to noise. Pulse lengths for mapping radars are usually between 0.5 and 5 µs.
(c) Pulse Recurrence (or Repetition) Frequency (PRF). The selection of pulse recurrence frequency is closely associated with maximum unambiguous range and with scanning rate.
(i) PRF and Range. In order to avoid ambiguity in range measurement it is essential that the echo of any one pulse is received before the next pulse is
transmitted. The PRF will therefore determine the maximum range at which the radar can be used, or conversely the PRF must be selected with regard to the desired maximum range of the radar.
Runamb = ___c___ where c = Speed of light
2 X PRF
In practice, the maximum range of a ground mapping radar is more likely to be limited by attenuation effects than by considerations of range ambiguity.
(ii) PRF and Scanning Speed. The PRF must be sufficiently high to ensure that at least one pulse of energy strikes a target while the scanner is pointing in its direction. A very narrow radar beam with a high rate of rotation therefore needs a high PRF. In practice the relationship between scanner rate, beamwidth, and PRF is adjusted so that any target will receive between 5 and 25 pulses each time it is swept by the beam.
The PRF of ground mapping radars is typically between 200 and 800 pulses per second and in some systems it is variable.
(d) The Transmitter Duty Cycle. Because a pulse radar transmits short pulses with relatively long interpulse periods, the transmitter is only functioning for a small fraction of the time (typically 0.001). The magnetron, which is the normal power source, must therefore be capable of handling high peak powers (typically around 200 kW) in order to achieve the necessary mean power of a few tens of watts. The product of pulse length and PRF is known as the duty cycle, and magnetrons are limited in the length of the allowable duty cycle in order to avoid overheating.
Image Distortion
28. A radar reflective target will not be portrayed accurately in size or shape on the CRT due to a combination of beamwidth, pulse length, and spot size distortions.
29. Beamwidth Distortion. The cause of beamwidth distortion has already been examined in para 25. The effect is to add one half of the effective beamwidth to each side of the target as shown in Fig 16-13. Beamwidth distortion increases with increasing range.
Fig 16-13: Beamwidth Distortion
30. Pulse Length Distortion. The range to the near edge of a target is correctly determined by half the time taken for the leading edge of the
pulse to reach the target and return multiplied by the propagation speed. However, although the range to the far side of target is similarly determined by the leading edge of the pulse, the CRT continues to paint until the whole pulse is completely received. The effect is to extend the far edge of the target by an amount equivalent to half the pulse length. The distortion is added to the effect of beamwidth distortion as shown in Fig 16-14.
31. Spot Size Distortion. The electron beam which produces the image on the CRT has a finite size, and although the centre of the spot draws the correct outline, the image is blurred by the addition of a margin with a thickness equal to the radius of the spot as illustrated in Fig 16-15. Adjustments to focus, brilliance, and gain affect the spot size, but it is independent of selected range scale and therefore represents a greater distortion on
smaller scales (i.e. on greater ranges). Spot size distortion is added to the effects of beamwidth and pulse length distortions as shown in Fig 16-16.
Fig 16-14: Pulse Length Distortion
Fig 16-15: Spot Size Distortion
32. Resolution Rectangle. Echos separated in azimuth by less than the effective beamwidth plus spot diameter, and echos separated in range by less than half the pulse length plus the spot diameter will merge together on the CRT. The combined effect of beamwidth, pulse length and spot size distortion is to limit the size of the smallest image which can appear on the CRT to the approximate rectangle, known as the resolution rectangle, of Fig 16-16.
Reflecting objects will be resolved as separate images only if the distance between them is greater than the appropriate dimension of this rectangle.
Fig 16-16: Resolution Rectangle
33. Height Distortion. The range measured by a mapping radar is slant range, whereas for a completely accurate display plan range is needed. At larger distances the difference between slant range and ground range is small and will not be conspicuous on the picture. At closer ranges, however, the picture will be distorted and the distortion would increase with increase in height. Fig 16-17 shows the picture of a straight coastline six nm ahead. If such accuracy is necessary, the CRT time base can be made non-linear such that the electron beam producing the time base is made to move faster at the start of its movement from the centre of the display than towards the edge. Even so, it is not possible to remove all of the
distortion at the centre of the display. In the majority of cases height corrections are only worthwhile if the aircraft is operating at very high altitudes.
Fig 16-17: Straight Coastline Six Miles Ahead
Radar Reflection Characteristics
34. The creation of a map-like image on a radar display relies on the differing reflectivities of the various terrain features. Radar energy is reflected in the same way as other electromagnetic waves, such as light, and two types of reflection situations may be recognized, specular and diffuse.
35. Specular Reflection. If the radar energy impinges on a smooth surface the reflection is known as specular and is the same as light being reflected from a mirror, i.e. with the angle of reflection equal to the angle of incidence. A surface may be considered smooth if it is approximately planar and contains no irregularities comparable in size with, or larger than, the wavelength of the radar. From a horizontal surface, specular reflection
causes the energy to be directed away from the receiver and so such a surface would appear dark on the display. Such specular reflection is typical of smooth water and fine sand.
Fig 16-18: Reflection from Two Plane Surfaces
36. Corner Reflectors. In order for specular reflections to arrive back at the receiver from a single plane surface, that surface must be very close to normal to the radar beam. A pair of plane surfaces, one vertical and one horizontal, will return a signal at the same angle of elevation as the incident energy, but not necessarily in the same angle of azimuth (Fig 16-18). However, if there are
three mutually perpendicular surfaces the geometry is such that energy is reflected back to the source regardless of the angle of incidence. This arrangement is known as a corner reflector.
Although rare in nature, corner reflectors frequently occur in built-up areas, as in Fig 16-19, and are largely responsible for the bright display of such areas. They are also widely manufactured to enhance the radar reflectivity of, for example, bombing targets, runway thresholds, and small boats.
37. Diffuse Reflection. When a reflecting surface is rough, i.e. when its irregularities are comparable in size with, or larger than, the radar wavelength, the reflected energy is diffused in all directions, the rough surface acting as a mosaic of randomly orientated specular reflecting surfaces.
The amount of energy reflected in any direction is less than would occur in a single specular reflection.
Diffuse reflection is uncommon in man-made structures, but is typical of normal undeveloped land and accounts for the intermediate tone of such terrain on the display. The proportion of the energy which is reflected back to the receiver depends largely on the angle of incidence, as illustrated in Fig 16-20.
Fig 16-19: Corner Reflector in Built-Up Area
Fig 16-20: Diffuse Reflection
38. Influence of Materials. In addition to the geometrical considerations, the amount of energy arriving back at the receiver from an object depends on the material from which the object is made.
All objects transmit, absorb, or reflect different proportions of any incident electromagnetic energy depending on their material. Broadly, metals reflect strongly, and naturally occurring materials such as earth and wood reflect weakly. Brick, concrete, and stone have intermediate reflecting characteristics.
Display Interpretation
39. Map reading from a radar display requires skill, experience, and care. The normal technique requires the identification of pre-selected fix points from which present position can be determined or targets can be located. In most modern systems it is possible to place cursors over the fix point and
allow the system to calculate present position from knowledge of the fix point co-ordinates and its relative position. Alternatively, the cursor position can be transferred to a target or waypoint by the insertion of offset values.
40. The ideal radar fix point should be sufficiently large, a good reflector, unique, and exhibit good contrast against its background. Since water in general reflects little or no energy back to the receiver (specular reflector), the best contrast is usually afforded by a cultural return against a water background. Such features are normally of sufficient size, are man made and therefore good reflectors, and tend to be unique, they are however rather uncommon. More commonly, fix points will be cultural features against a terrain background like sharp bends and crossings of power cables might be suitable, as might some of the smaller towns. Power stations and large factories are often suitable. In these cases, however, the features are unlikely to be truly unique, and confident identification must be achieved by relating the radar returns one to another. Coastal features are often easily identified and suitable, however they must be used with some caution since their appearance can vary with tide changes, especially in shallow and estuarine waters. Precipitous and rocky coastlines (particularly small islands) are more reliable than sandy or muddy ones. However, man-made coastal features such as piers, harbours, and lighthouses, usually show significantly regardless of tide state.
Radar Interpretation at Low Level
41. Most of the factors discussed above are equally applicable to operation at low level. However, in addition, consideration must be given to the effect of terrain shadowing on the display. The short wavelength radar energy travels in straight lines and so any obstruction, such as a hill, will cast a shadow on the far side, and any feature in the shadow will not be apparent. Fig 16-21 gives an impression of the sort of display that might result from a low-level viewpoint over hilly terrain. Fig 16-22 shows a terrain cross-section together with a simplified diagram of the
the radar display that might be generated, although it should be appreciated that variations in tilt and gain settings, and in the aircraft height will make significant differences.
Fig 16-21: Impression of Typical Low Level Radar Display over Hilly Terrain
42. If the terrain elevation is equal to, or greater than, the aircraft altitude the radar can no longer
42. If the terrain elevation is equal to, or greater than, the aircraft altitude the radar can no longer