1 INTRODUCTION TO PASSIVE RADAR

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INTRODUCTION TO PASSIVE RADAR

1.1. Introduction

Passive bistatic radar (PBR), or Passive Coherent Location (PLC), is the name given to a bistatic

radar system that makes use of emissions from broadcast, communications or radio navigation transmitters rather than a dedicated, co-operative radar transmitter. The interest in this subject is fast grown up since the hardware required for an experimental system is simple and low cost [2], and there are no licensing issues because the transmitter sources already exist. As well as this, such sources tend to be high power and sited to give wide coverage [8]. PBR may also allow VHF and UHF frequencies to be used which are not normally available for radar, and where in a defense context there may be an advantage against stealthy targets compared to conventional microwave radar frequencies. It is also difficult to deploy countermeasures against PBR receivers, since if the location of the receiver is not know any jamming has to be spread over a range of angles, diluting its effectiveness.

PBR can be thought of as fitting into the overall subject of Waveform Diversity, since the waveforms of broadcast, communications and radio navigation sources are not explicitly designed for radar use and so may be far from optimum for radar purposes. It is therefore necessary to understand the effect of the waveform on the performance of the passive bistatic radar, so as to be able to choose the most appropriate illuminator, and to process the waveform in the optimal way.

Furthermore, the electromagnetic spectrum represents a finite resource with many different applications, of which radar is only one, competing for frequency allocation. So there is great motivation and scope for techniques which use the spectrum in a more intelligent and adaptive manner, and PBR represents one such technique. In that sense PBR has been described as “green radar”, since it does not generate any electromagnetic pollution.

4 1.2. Bistatic Radar

Bistatic radar is defined as a radar in which the transmitter and receiver are at separate locations, sufficiently separated that the properties are significantly different to those of a monostatic radar. The distance L between the transmitter and the receiver is called bistatic baseline. The angle at the target subtended by the transmitter and receiver is the bistatic angle β. The range from the transmitter to the target is 𝑅_𝑇 and the range from the target to the receiver is 𝑅_𝑅. The target direction at the transmitter and receiver, measured w.r.t. North, are given by 𝜃_𝑇 and 𝜃_𝑅 respectively. The target velocity is ν, in a direction which makes an angle δ w.r.t. the bisector of the bistatic angle (Fig. 1.1):

Fig. 1.1: Bistatic radar geometry [1].

Usually the locations of the transmitter and receiver, and hence the baseline L, are known and the quantities that can be measured for a given target at the receiver are:

 the difference in delay between receipt of the direct signal from the transmitter and the target echo, which provides the bistatic range sum (𝑅_𝑇+ 𝑅_𝑅).

 the direction of arrival of the target echo 𝜃_𝑅.  the Doppler shift 𝑓_𝐷 of the target echo.

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The range of the target from the receiver can be deduced from [1]:

𝑅_𝑅 = (𝑅𝑇+ 𝑅𝑅)

2_{− 𝐿}2

2(𝑅_𝑇+ 𝑅_𝑅+ 𝐿 sin 𝜃_𝑅) (1.1)

The Doppler shift 𝑓_𝐷 is given by the rate of change of the bistatic range sum

𝑓_𝐷 =1 𝜆

𝑑

𝑑𝑡(𝑅𝑇+ 𝑅𝑅) (1.2)

In the simplest case where the transmitter and receiver are both stationary and only the target is moving

𝑑𝑅_𝑇 𝑑𝑡 = 𝜐 cos (𝛿 − 𝛽 2) (1.3) 𝑑𝑅_𝑅 𝑑𝑡 = 𝜐 cos (𝛿 + 𝛽 2) (1.4) so that 𝑓_𝐷 =2𝜐 𝜆 (cos 𝛿 cos 𝛽 2) (1.5)

It can be seen that at the point at which the target is crossing the baseline, 𝛽 = 180° and so 𝑓_𝐷 = 0. This can be appreciated since for such a target the transmitter-to-target range is changing in an equal and opposite way to the target-to-receiver range. Furthermore, the echo arrives at the receiver at the same instant as the direct signal from the transmitter, irrespective of the location of the target on the baseline. This means that for a target on the baseline the bistatic radar provides neither range nor Doppler information. Nevertheless, this configuration, which is known as forward scatter, has some attractive properties in respect of target RCS enhancement.

6 1.3. Bistatic radar equation

In the case of bistatic radars the radar equation is relative simply to derive [4] but its application is much more complicated. The bistatic radar equation holds:

𝑆𝑁𝑅_𝑅 = 𝑃_𝑇𝐺_𝑇𝐺_𝑅 𝑓𝑇(Φ, Θ)𝑓𝑅(Φ, Θ)𝜆 2_𝜎 𝐵 (4𝜋)3_𝑅 𝑇2𝑅𝑅2𝐿𝑇𝑅𝑘𝑇0𝐵𝐹 (1.6)

where 𝑃𝑇 and 𝑃𝑅 are the transmitted and received powers; 𝐺𝑇 and 𝐺𝑅 are the transmitter and receiver

antennas gains; 𝑓_𝑇 and 𝑓_𝑅 are the normalized transmitter and receiver antenna patterns; λ is the wavelength of the transmitted signal; 𝜎_𝐵 is the target effective bistatic RCS; 𝐿_𝑇𝑅 is the loss on the transmitter – receiver path, k is the Boltzmann’s constant; 𝑇₀ is the noise reference temperature (290 K at environment temperature); B is the receiver effective bandwidth; F is the receiver effective noise figure. Obviously, the product 𝑃_𝑇𝐺_𝑇 is the transmitter’s EIRP.

There are several competitive signals reducing the maximum range and complicating the signal processing, for example they are: the direct signal; correlated reflections of terrain objects (clutter) and non-correlated signals transmitted by other sources at the same frequency (electromagnetic noise).

In the case of omnidirectional transmit and receive antenna patterns (𝑓_𝑇(Φ, Θ) = 𝑓_𝑅(Φ, Θ) = 1), contours of constant signal-to-noise ratio are defined by 𝑅𝑇𝑅𝑅 = 𝑐𝑜𝑛𝑠𝑡, which are Ovals of Cassini (Fig.

1.2), but when directional antenna patterns are used the contours are weighted accordingly and no longer have the same shape.

The 1 (𝑅⁄ _𝑇2𝑅_𝑅2) factor also means that the signal-to-noise ratio has a minimum value when 𝑅𝑇 = 𝑅𝑅 and is maximum when the target is either close to the transmitter or close to the receiver.

7 1.4. Target signatures

One of the advantages sometimes ascribed to bistatic radar is that it may offer a counter-stealth capability, since targets that are shaped and/or treated to minimize their monostatic RCS may nevertheless have higher bistatic RCS.

Early theoretical work on bistatic electromagnetic scattering from targets led to the bistatic

equivalence theorem [1], which states that the bistatic RCS of a given target at a bistatic angle β will be

the same as the monostatic RCS measured at the bisector of the bistatic angle, and scaled in frequency by the factor cos(𝛽 2⁄ ). This depends on a number of assumptions:

 the target is sufficiently smooth.

 there is no shadowing of one part of the target by another.  retroreflectors persist as a function of angle.

In practice these conditions are never met, so the theorem is usually useless, particularly for complex targets and at large values of β.

Enhancement of bistatic target RCS at frequencies where the dimensions of target features are comparable to the radar wavelength (typically at VHF or HF frequencies for aircraft targets) will occur, just as with monostatic signatures. This resonance effect occurs when contributions from different scatterers comprising the target add in phase at a particular radar frequency and geometry.

1.5. Forward scatter

One mechanism by which the bistatic RCS of a target may be enhanced substantially is the

forward scatter geometry introduced by the paragraph 1.2. The enhancement in RCS can be understood

by making reference to Babinet’s Principle from physical optics [1]. Suppose that an infinite screen is placed between the transmitter and receiver, so that the signal received is zero. Now suppose that a target-shaped hole is cut in the screen, between the transmitter and receiver. Babinet’s Principle states that the signal that would be diffracted through the target-shaped hole must be equal and opposite to the signal diffracted around the target, since the two contributions must add to zero.

Determination of the signal diffracted through an aperture of a given size and shape is a standard electromagnetics problem, and for simple shapes the results are well known. For example, for a

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rectangular aperture of sides a and b, the pattern in each plane has a sinc shape with main lobe whose angular widths are

𝜆 𝑎

⁄ and 𝜆 𝑏⁄ (radians) (1.7)

and the peak scattered signal corresponds to a forward scatter RCS of

𝜎_𝐹𝑆 = 4𝜋𝑎

2_𝑏2

𝜆2 (1.8)

Fig. 1.3 plots these quantities as a function of frequency for an ideal small target for which 𝑎 = 10 𝑚 and 𝑏 = 1 𝑚 and hence 𝐴 = 10 𝑚2. In comparing this with the monostatic RCS (which for small target such as small aircraft might be of the order of 0 𝑑𝐵𝑚2_{), it is seen that the forward scatter RCS can be}

several tens of dB higher, particularly at microwave frequencies, although the scatter is concentrated in a fairly narrow beam.

Fig. 1.3: Forward scatter RCS 𝜎𝐹𝑆 and angular width of scatter 𝜃𝐵 for an idealized small aircraft target with 𝐴 = 10 𝑚2 [1].

At lower frequencies the forward scatter RCS is lower, but the scattered energy is spread over a wider range of angles, which might be preferable for the purpose of detection. This suggests that the optimum frequency range to exploit this phenomenon will be at VHF or UHF, which are precisely the bands in

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which many high power PBR Illuminations sources are to be found. In our case, using satellite communications, we will work in the Ku band, i.e. the 10.7 – 14.5 GHz portion of the electromagnetic spectrum in the microwave range of frequencies, so we will have a big target’s RCS, but a really narrow beam.

1.6. PBR waveforms

A key feature of PBR is that waveforms of the signals are not explicitly designed for radar purposes, and therefore that their performance when used for PBR may be far from optimal. It is important to understand the nature of the wide range of waveforms that may be used for PBR, so as to be able to choose the most suitable ones and to process them in the most appropriate way. The classical tool for analyzing and displaying waveform properties is the Ambiguity Function:

|𝜓(𝑇𝑅, 𝑓𝐷)|2 = |∫ 𝑠𝑡(𝑡)𝑠𝑡∗(𝑡 + 𝑇𝑅)𝑒𝑗2𝜋𝑓𝐷𝑡 +∞ −∞ 𝑑𝑡| 2 (1.9)

The ambiguity function is the square magnitude of the output from a filter matched to the signal 𝑠_𝑡(𝑡), and represents the point target response of the radar as a function of delay 𝑇_𝑅 and Doppler shift 𝑓_𝐷. A plot of this function shows the resolution, sidelobe pattern, and presence of ambiguities in range and in Doppler in an elegant and easily-visualized manner.

The ambiguity function for a bistatic radar not only depends on the waveform properties, but also, when exploiting an analogically modulated format, varies significantly with the instantaneous program content [1].

In contrast, digital modulation formats such as DAB and DVB give a signal spectrum that is flatter and more noise-like, and which does not depend on instantaneous program content, so the ambiguity performance does not vary with time and does not depend on the program content.

1.7. Applications

1.7.1. Low – cost scientific measurements

The principal applications in which PRB has been deployed thus far have been for low – cost scientific remote sensing measurements. As a general comment, radar remote sensing relies on a model

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to relate the desired remotely – sensed quantity to some parameter of the radar echo. The model may be the result of rigorous electromagnetic scattering calculations, or empirical measurements, or a mixture of the two.

1.7.2. Border or perimeter surveillance

Forward scatter fences based on PBR illuminators may be used for border or perimeter surveillance, to detect intruders or smugglers, or for maritime surveillance, for example for harbor protection, fisheries protection or counter-piracy [2]. Success here will depend on the availability of illuminating sources with appropriate coverage, and suitable locations for receivers (whether on land or at sea), as well as understanding the forward scatter signature of maritime targets and of sea clutter.

It may also be desirable to attempt to exploit the micro – Doppler signature of targets, so as to be able to distinguish between different types of target (e.g. human that is walking and one that is running) [1].

1.7.3. Air surveillance

PBR can be used for air surveillance as well, to detect a variety of air targets including military and civil aircraft, an anti-tank missile [3] [5].

1.7.4. Airborne PBR

PBR receiver may be also located in an aircraft. This might be allowed (for example) airborne early warning (AEW) or detection of ground vehicle targets [1] [3].

1.7.5. PBR imaging

PBR may also used as the basis for bistatic radar imaging [1] [9]. The synthetic aperture may be formed by a moving transmitter or a moving receiver, or if both transmitter and receiver are fixed, by the motion of the target itself (bistatic ISAR, as in our case). Bistatic images and High Resolution Range Profiles (HRRPs) may be used in target classification and Non – Cooperative Target Recognition (NCTR) processing, just as with monostatic imaging.

The azimuth resolution of such a system may be derived in the same way as for a monostatic SAR, recognizing that the phase history of the sequence of echoes is determined by the change in transmitter – to – target path (if the receiver is stationary) or target – to – receiver path (if the transmitter is stationary) rather than the two – way path in a monostatic SAR. Thus in general the azimuth resolution is a factor 2 coarser than the equivalent monostatic SAR.