The radar and its ground environment
1.6 RADARS WITH SEPARATELY LOCATED TRANSMITTERS AND RECEIVERS
There are principally two reasons for placing a radar transmitter with its transmitting antenna and the receiver with its antenna in two separate p laces or bu ildings:
• Lack of suitable transmit-receive switch devices with sufficient isolation. Early radars, the British GL Mark II, were split into transmitting a nd receiving cabins which were place d 100 ya rds apart. Later, microwave radars, for example, the British Radar AA No. 3 Mk II, used separate transmitting antennas on the same mount, and currently, higher power continuou s wave radars.
• Radar transmitters are expensive to develop, purchase, and operate. It has long been an aim to use echoes from another transmitter and antenna to b uild up a radar picture of the surro undings.
The use of echoes from a distant transmitter was employed by the G ermans (Parasit) using the Chain Home radar stations as illuminators during the Second World War. A conventio nal radar is like a high-pow er searchlight a t night, and antiradiation missiles (ARM) have been developed to seek and destroy such radiating sources. There is a current military interest in passive radar systems using illuminators “from the other side” which are not likely to be jammed.
Further examples are the use of frequency modulated radio transmitters in VHF band II as illuminators.
Civil uses are to use a loca l receiving-on ly radar to give better coverage in relatively small but important areas where the covera ge of a prim ary radar is no t adequate .
When the transmitting site and the receiving site or sites are widely separated, the radar system is called bistatic and a typical arrangement for a bistatic radar system is shown in Figure 1.16. Notice that the bearing angles are referenced to the base line, the line between the transmitting and rece iving sites.
Part of the p ulse from the tra nsmitting site follows the dire ct path to the receiving site. The signals giving echoes will have traveled from the transmitting site to the scatterer and then to the receiving site , which takes lon ger. Bistatic radars measure range by measuring the time that the echoes arrive after the direct pulse, so that lines of constant range are ellipses. No range measurements are possible along the base line.
The first process is to change the range sum , , into radar radial range, normally the slant range to the receiver,
(1.13)
Figure 1.16 The geo metry of a bistatic radar.
(1.14) RR. For this the azimuth of the echoes at the receiving antenna must be available. The slant range is [4, p. 25.9]
When the elevation angle of arrival is available, then the height and the ground range may be calculated.
1.6.1 Elliptical coordinates
Ellipses with the major axis along the x-axis are commonly defined by two of the following
• Major a xis, 2a;
• Minor ax is, 2b;
• Distance be tween the foc i, 2c, or the base line length, LB.
The ellipticity is the ratio of the distance between the foci to the length of the major axis, or c/a.
Classical elliptical coordinates [5, p. 529] are defined in terms o f u and v. The transformation into Cartesian coordinates is given by
where 2c is the distance between the foci or the length of the base line;
1/cosh u is the ellipticity;
tan v is the slope o f the line for large va lues of u.
The range sum is the major axis of the ellipse, or 2c cosh u. Elliptical coordinates for a base line length of unity (foci
Figure 1.17 A grid of elliptical coordinates.
(1.15)
(1.16) at ±½) and a number of values of u and v are shown in Figure 1.1 7. The ellipses are lines of constant range sum, and the hyperbo lae are the co urses for scatter ers that give a co nstant Dop pler freque ncy.
The radar measures tim e delay or ra nge in terms o f the range sum , RT + RR, which is related to the coor dinate u by
1.6.2 Bistatic radar maximum range
Chapter 6 deals with the scattering of energy back to a monopulse radar only. With a bistatic system, the energy is not scattered back direc tly but at an angle, $, that changes the scattering characteristics. Military aircraft that see themselves as targets for anti-aircraft defenses have reduced scattering characteristics for monopulse radars and this extends over a wide range of the bistatic ang le, $, except where $ approaches 180 degrees. The rang e, R, in the monostatic radar equation, (1 4.1), is replac ed by the ge ometric me an, in the bistatic radar equation:
where RTis the distance from the transmitter to the scatterer;
RR is the distance from the scatterer to the receiver;
GT is the gain of the transmitting antenna;
GR is the gain of the receiving antenna;
FB is the bistatic radar cross-section;
Figure 1.18 Curves showing lines of equ al bistatic signal-to-no ise ratio in terms o f
/(Rt Rr)/L.
FT is the propagation factor for transmission;
FR is the propagation factor for reception;
c is the velocity of light;
k is Boltzma nn’s constant;
T is the system noise temperature, K;
DS is the signal-to-noise ratio required for detection;
f is the frequency of the radar;
L is the product of the losses.
The radar cross-section for scattering depend s on the angle to the transmitting a nd receiving sites. For military airc raft, the cross-section is minimized towards the direction o f the transmitter site, so the bistatic cross-section may be greater.
The low-altitude coverage of monostatic radars is limited by the shadow formed by the Ea rth’s curvature. W ith bistatic radar, there are shadows in the coverages of both transmitting and receiving sites. Echoes occur only in the volume of comm on cover age, and airb orne elem ents may help in reducing these shadows. The clutter echoes come from cells defined not only by the pulse length and the antenna beam widths but also from the angle between the two antenna beams.
Cassini’s ovals. Figure 1.1 8 shows the curves for a c onstant value of the geom etric mean divided by the base line length, , for a base line distance of unity. For values of 1.5 and greater, the quasi-mo nostatic rada r case is
approached with similar range s and signal-to-n oise ratios in directions along and across the base line. Below this value, the coverage elongates and finally splits into two, giving only local coverage arou nd the two sites. Notice that this also applies to the elevation coverage. This has the effect of changing the traditional coordinates as shown in Table 1.7.
Table 1 .7
Comparison of monostatic and bistatic radar characteristics
Monostatic radar Bistatic radar
Measured Coverage
Range Slant range, R Slant range, RT + RR
Elliptical coordinates Cassinian coordinates Ranges to transmitter or receiver must be calculated Radial velo city Doppler frequency Hyperbolic Doppler frequency contours Azimuth and elevation Radar antenna Usually measured by the receiving antenna