short-range surveillance radar systems

Short-ranae surveillance radar systems

by C. J. Baker and B. D. Trimmer

Small, light-weight, coherent radar systems have been successfully exploited in the military domain for many years. Recent advances in both technology and signal

processing techniques are enabling the production of more versatile systems aimed a t a wider range of applications spanning both the military and civil markets. In this

paper the fundamental design concepts underpinning this class of radar are briefly introduced to place in context current and emerging technical developments. The

relationships between these developments, an increasing range of future potential applications and the areas of most significant technical challenge are discussed.

1 Introduction

In the modern military environment it is universally recognised that accurate, timely and reliable knowledge of enemy activity is a vital ingredient in the provision of overall situational awareness, potentially malting the difference between success and failure of operations.

The detection and location of movement of, amongst others, vehicles, men, enemy fire and ‘the fall of shot’ are key situation awareness constituents leading to an understanding of enemy dispositions, actions and intent. The availability of this class of information on an all- weather, day-and-night basis in a hostile and extremely complex environment can only be provided by radar systems. This advantage together with the ability to view relatively large areas makes radar a vital component within a commander’s overall portfolio of surveillance and target acquisition assets. Indeed, soon after its initial invention radar was used for military surveillance of the battlefield and this continues to the present day.

These advantageous characteristics of radar systems have been exploited in numerous other applications. Examples which fall into the category ‘short-range surveillance’ include harbour surveillance, border control, traftic monitoring, airport and building security.

Humans are used to viewing scenes at optical frequencies and our natural abilities to interpret context and content are extraordinarily good. However, for radar the situation is somewhat more complicated for three main reasons. Firstly, radar operating frequencies are much lower (up to 100 GHz) and result in objects appearing relatively smooth with little or no textural information. Secondly, radar resolutions are typically of the order ol 10 m by a few hundred metres and consequently the detected backscatter will include contributions from both objects of interest and their background; there is therefore an effective loss of definition and of contextual information.

Thirdly, radar is inherently coherent and consequently backscatter from a distributed scene will exhibit constructive and destructive intederence, leading to the well-known ‘speckle’ phenomenon. This manifests itself as scene-induced multiplicative noise.

These characteristics result in the radar returns being presented to an operator as a single bright spot on a display. This may lead to the return froin a vehicle, €or example, being indistinguishable lrom other bright spots on the display caused by similar returns from other man- made objects or the surrounding terrain. However, the deployment pattern of a group of vehicles and other radar derived data such as their speed and direction of travel may make their identity and intent clear to a trained operator. In order to aid the interpretation task the raw radar data is processed into a form making its assimilation as simple as possible. Overall the radar designer must have a deep understanding of hardware, processing algorithms, display of processed data, the role of the radar operator and the operating environment and all o€ their interactions if applications are to be addressed successfully.

The underlying design principles which determine detection and classification performance, location accuracies and overall effectiveness ol short-range surveillance radar systems are briefly introduced in the next section. This is followed by a review of past and current battlefield radars, highlighting the proliceration of small light-weight systems. This background places current and likely future developments in a context which clearly demonstrates their importance and leads naturally to the increasingly comprehensive capability and widening range of applications to which these systems are being put.

2 Basic design principles

All radar systems work on the principle of transmitting and receiving electromagnetic radiation, which may take the

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000 181

Table 1: Typical battlefield radar requirements

Detection ranges Man walking 3 km Moving vehicle 10 km Fall of shot (artillery) 8 km

Range accuracy 20 m

Environment Bearing accuracy 0.3'

Rain up t o 4 mm/h Heavy clutter > - 10 dBm2/m2

Weight 30 kg Volume man-portable loads Power consumption 30 W

form of a train of pulses or may be a continuous transmission and reception. The presence of a target is determined from the amount of incident radiation that is reflected back in the direction of the radar receiver and this is a complex function of radar specification, target typej characteristics and environment. The range to the target is a function of the round-trip transit time from transmitter to target and back to receiver. The cross-range resolution and accuracy are functions of the radar's beamwidth.

Used together these measurements enable a target or object of interest to be detected and its position to be located in a convenient co-ordinate system. By examining the returns from moving targets over a period of time, radial velocity can be calculated. Usually this is achieved in modern radars by measurement of the phase histoiy of the radar returns as this can be related directly to radial velocity. As stated above, target location accuracy in the cross-range dimension is a function of beamwidth. For a 1.5" beam and no additional processing, this implies a location accuracy of approximately 160 m at a radar range

Table 2: Typical system parameters

of 10 km. Using techniques such as monopulse or azimuth matched filtering this accuracy can be improved by approximately an order of magnitude. There are many excellent introductory radar textbooks available that establish and describe these relationships (e.g. References 1-3). Fig. 1 shows a very simplified schematic diagram of the major components comprising a radar system. Note that a reference signal is used to retain a knowledge of phase by comparison of the outgoing transmission and subsequent incoming reception.

The overall design of a radar system to meet a particular application must take into account the complex interactions between the characteristics of the transmitter, receiver, antenna, target, environment, size, weight, power consumption, affordability and role of the operator. All of these design aspects need to be carefully considered together in order to provide the most cost-effective systems. Next, the design of an example radar system is considered against the requirements of a hypothetical man-portable battlefield surveillance radar as in the outline specification listed in Table 1. This will highlight the significance of the role of the parameters described above in system design options.

The technical specification of a radar system, at the simplest level, is expressed by the well known radar equation, which can be used to indicate the high-level system parameters and the effects of their interactions. The radar equation for noise-limited detection may be written as:

PG2 o A z (47~)~ LSkTBF

R 4 =

where R is the detection range, Pis the peak power, G is the antenna gain, o i s the radar cross-section of the target, A is the wavelength, S is the minimum detectable signal, L represents the system losses and kTBFis the noise power in bandwidth B and with noise figure F.

Clcarly, to dct(vt a target oi a Rivcn cross-section at a chosen range t l i c w are various u)nibin:itiotis of powcr, Radar system parameter Value

Average transmitted power Transmitter duty radio Receiver noise figure System losses Elevation beam loss Scan rate Antenna size Antenna gain Azimuth 3dB beam width Elevation 3dB beamwidth Wavelength a t 15 GHz Dwell time at 15 GHz

1w 0.1 5.0 dB 7.0 dB 1 .O dB 36 degls 1 x 0.5 m 38.5 dB 1.5" 3.0" 0.02 m 0,0625 s

System losses breakdown Frequency weighting and scalloping 2.0 dB Receiver range matching and scalloping 2.0 dB Detection thresholding 1 .O dB In-service degradation 2.0 dB

wavelength and antenna gain that have to be optimised against the constraints of external factors. One example, for a man-portable system that shows this in a simple way, is the contrasting needs to propagate through adverse weather whilst maintaining a small and manageable antenna size. The former will limit the upper frequency whilst the latter will tend to impose a limit on the lower frequency.

A further example is provided by the way in which a predetermined antenna gain may be realised in practice for the chosen application. This will be subject to external factors such as coverage optimisation, terrain masking, the likelihood of third party detection and jamming. For example it may be advantageous to use a fan beam that is narrower in azimuth than elevation so that the radial extent of the area of interest (as projected onto typical terrain) can be increased to provide an extended field of coverage. This will also improve cross-range location accuracy, which is a direct function of azimuth beam width, Alternative applications may demand a wider azimuth beam, which

182 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000

can scan an area more quickly, or a pencil beam, which concentrates energy onto a smaller area of intersected ground.

By optimising the principal radar design parameters against a well understood requirement in this way the high-level design goals of the radar may be outlined. This can then be used as preparation for iurther, more detailed refinement. Table 2 shows a representative set of the principal parameters able to meet the requirement for a man-portable battlefield surveillance radar system meeting the requirements in Table 1. The relative importance and significance of the parameters in design optimisation can now be seen. For example, consider the case of two classes of targets - moving vehicles and moving people - which are

transmit oscillator and power modulator amplifier

reference signal

receive

Fig. 1 Schematic diagram of radar system

assumed to have arbitrary radar cross-sections of 10 m2 and 0.5 m', respectively. The radar equation can now be used to examine performance characteristics, such as the influence o i frequency on detection range. It is relatively straightforward to calculate range and cross-range accuracies, unambiguous Doppler bands and Doppler resolutions.

Consider the calculation of the optimum operating frequency, for which some typical results are summarisecl in Table 3. The optimum frequency is shown for detection range, range and cross-range accuracy and Doppler ambiguity. The frequency range over which the performance falls by 6 dB is also shown, which in turn reduces the range by approximately 30% and doubles the measurement errors. A frequency of around 15 GHz probably offers the best compromise all-round performance but is clearly not optimal in all cases. For example, at longer ranges or in adverse weather conditions, lower frequencies would be more suitable, or, il ranges of approximately 7 kin or less are required, this could be achieved at a higher frequency, around 35 GHz, which might have size and weight advantages.

3 Systems

From its beginnings in the early 1940s the development oi short-range surveillance radar has been mainly aimed at providing battlefield targeting information from a man- portable platform for lorward artillery observers. Notable radars have included the ZB298, which entered service in the UK in the late 1960s. It operates in the I and J bands using largely solid-state technology with the exception of the magnetron power source. It is man-portable in two segments with the radar head tripod-mounted and the control system able to be located up to 20 m away. The system displays target returns on a gallium phosphide screen and additionally has an audio output used by the operator to aid detection and act as a crude means of classification. The ZB298 has been incorporated into two tracked armoured personnel carriers, the FV 103 and the FV 432 with the antenna mounted on a telescopic mast.

A similar system, the AN/PPS-15 radar, is in service in the USAand worldwide.'I'here are currently many varieties of battlefield surveillance radar. Anumber (not all) of these are listed in Table 4, which shows the enormous variety of

Table 3: Optimum operating frequencies showing a 6 dB performance spread

Detection range Optimum 15 15 1 1 8 6 dB spread 4-35 4-35 2 - 20 2-16

Range accuracy Optimum 15 15 1 1 8 6 dB spread 4-35 4-35 2 - 20 2-16

Cross-range accuracy Optimum 35 35 15 12 6 dB spread 12 - 35 1 1 -35 7-25 6 - 19

Doppler ambiguity Spread (+IO0 km/h) 0 - 32 0- 16 0 - 32 0 - 16


Table 4: Battlefield and short-range ground surveillance radars. Detection ranges are given for large shells (e.g. 155 mm). Data is taken from open literature sources.

Name Country Radiated Weight, Frequency Azimuth Azimuth Azimuth Detection range, km Minimum power kg bandt bandwidth, coverage, accuracy, detect

deg deg Men Vehicle FOS* velocity,

*FOS = fall of shot t I Band: 8-10 GHz. J Band: 10-20 GHz. B Band: 250-500 MHz. C Band: 0.5-1 GHz.

extant systems and widely ranging radar specifications. It is worth noting that it is often quite sinal1 changes in the operational requirements for these systems that have resulted in very significant diflerences in the final design. Table 4 illustrates that although there is a clear correlation between weight, power and detection range it is not necessarily the case that the heavier and the more powerful the radar the longer the detection range. A sound understanding of the radar design fundamentals and the electromagnetic backscattering properties of the target and environment can result in much more efiicient, compact designs resulting in greater all-round operational versatility and effectiveness. A note of caution should also be sounded as these specifications do not necessarily tell the whole story and other characteristics, such as usability, cost, reliability and technology vintage, also need careful consideration.

Advances in technology beyond that incorporated in the ZB298 have allowed the capability of later generation radar systems to improve further and have led to manportable surveillance radar systems such as MSTAR (Moving and Stationary Target Acquisition and Kecognition) , currently in service with the British Army and IUSIT (Radar d'Acquisition et de Surveillance dans les Intervalles Terrestres) in service with the French Army.

MSI'AK is a pulse Doppler scanning radar system optimised for the detection of moving men and vehicles

against a typical ground environment. Fig. 2 shows the MSL'AR equipment as a typical instance of the man- portable ground surveillance radar.

MSTAR uses a solid-state power amplifier and retains phase information Cor subsequent coherent processing. The radar detects men and vehicle targets with a location accuracy oi approximately 0.3", enables classification from the Doppler signature (presented as an audio signal to the operator) and is able to detect and correct artillery fire. MSTARhas alow peak power to minimise hostile detection by electronic surveillance measures and operates from a standard field battery4.

One of the most important aspects of this class of radar design is the required simplicity of the display to the operator. In general, he or she is under high stress and cannot assimilate complex information presentations. Figs 3 and 4 show typical MSTAR displays, representative of the displays available lrom this general class ol radar system. Fig. 3 shows the area surveillance mode, where some targets have been detected at the edge of the scanned sector. Fig. 4 shows the display in acquisition mode; effectively it is an enlarged section of the surveillance display using a 'B Scope' format and showing the trails from two moving targets that have been observed over a period of time. Note the absence of extraneous data that could distract the operator.

Current developments in research and technology are


Fig. 2 MSTAR deployed: (a) man-portable and (6) vehicle-mounted roles

resulting in systems with an ever-increasing radar Technology capability with ever-reducing sizes and weights. Not only can this lead to an increase in the capability of man-portable battlefield surveillance radar systems but it is also enabling systems to be hosted on a greater range of platform types and a greater range oi applications to be addressed. However, the key to the successlul realisation of any system lies in optimisation of the radar performance in the

The design of radar systems Tor short-range surveillance applications needs to strike a balance between leading- edge technology and application-driven requirements. For many applications weight, volume, power consumption, performance, ease of use and cost are the principal design

technology topic. box


drivers that severely constrain the radar designer’s options. For military systems low probability of intercept by hostile ESM (Electronic Support Measures) systems and electronic protection measures is also important. It will be seen in this section how technology advances are now increasing the freedom of the radar designer from these constraints.

Transmitters and receivers All radar systems have to generate and amplify an

electromagnetic source prior to transmission and it is in the design of power amplifiers where most recent progress has been made. There has been an evolving trend to higher power devices (>lo W peak in high J-band, with corresponding advances scaled to the other radar bands) in smaller packages with higher efficiencies. The waveform generator and power amplifier combination of tomorrow will also be capable of enhanced agility and will be able to host a wider variety of waveform types which can be tailored to a given application. For example, this may enable the duty cycle to be varied such that when coupled with power management techniques the system is able to minimise the likelihood of intercept by ESM systems at all ranges by automatically controlling the output power to suit the operating conditions. CW (continuous wave) operation may also be easily supported resulting in significant potential cost savings that accompany this simpler radar design approach.

Future envisaged amplifiers will be compatible with substantial increases in the resolution of the radar, which will be enhanced by more than an order of magnitude to less than 1 m. This offers the potential for improved target detection and clutter rejection and additionally opens up one method for the classification of targets (discussed later in this paper). The sensitivity of receivers (coupled with improvements in receiver signal processing) has also improved notably, resulting in overall system losses being reduced by several decibels, Together these improvements enable the detection range of systems to be extended by up to a factor of two without weight, volume or cost penalties. Indeed, future systems may be envisaged with overall weights of considerably less than 30 kg. These advances instantly translate into significant improvements in the capability of current systems. Coupled with corresponding improvements in signature understanding and processing they will allow future systems to increase greatly the versatility of short-range surveillance radars.

Antennas The radar antenna is a key component that directs the

Table 5: Military battery parameters

generated electromagnetic power to the areas of interest ancl is the first point ol reception of the backscattered energy. Current systems use a mechanically scanned dish antenna that is relatively simple to fabricate and is efficient and easy to operate. However, it has the disadvantage that the beam shape is fixed and therefore not always optimised for purpose. The pointing of the beam is slow and not very agile and the size and shape of the antenna make it cumbersome and physically vulnerable in many applications. Many o l these disadvantages may be overcome with phased-array antennas, which can be operated either passively or actively.

There is much research ongoing into phased arrays and they are beginning to find application in larger systems. The single biggest difficulty to be overcome in order to be able to use phased arrays in small radars is cost. Current phased arrays are prohibitively expensive. Conventional phased arrays, although reducing in cost, seem destined to be out of reach of the class of radar considered here. There is research ongoing into low-cost phased arrays that on maturity would revolutionise the capability of short-range surveillance radar systems. These will enable nonlinear scanning so that, for example, dwell times on targets of interest could be increased whilst still maintaining an all- round scanning capability. Laboratory prototypes have been fabricated, however much research is still required before they will be mature enough for production systems.

Power generation If the radar is to be self contained then a source of power

will be required, most likely a battery if the radar is to be used remotely from a mains-like power source. Battery technology is advancing, with longer lifetimes available from smaller, lighter weight packages largely developed for a wide range of applications.

The advances in battery technology as applied to this type of radar can be seen in the 24 V ‘Clansman’-type power source suitable for an MSAR-like radar. The typical Ni- Cad form has been augmented with lithium or nickel- metal-hydride based disposable and rechargeable units. Typical parameters of available military batteries are shown in Table 5. Quite clearly there has been a factor of two improvement in power available per unit weight, particularly if the user is prepared to consider a disposable battery.

This is a very active field of engineering and significant improvements are expected over the next few years as the commercial imperatives of computer and automotive systems force battery development.

It is also important to manage power with the utmost

Capacity, Ah Weight, kg Size (L x W x H), inches


care to ensure that it isn’t being used when not needed and that processing algorithms are designed in a way that makes them as power eMcient as possible. This is an excellent example of the importance of taking a holistic view of the radar system design to optimise performance in all operational conditions.

Display technology and human computer interaction Display technology is advancing at a considerable rate,

largely driven by the demands of the mass consumer market. Research prototypes of light-weight, low-power- consumption high-resolution flat screens have been developed and there is the possibility of ultra-thin liquid crystal display (LCD) screens that could be rolled or folded for ease of storage and carriage. Clearly these forms of display have to be tailored to the operating environment to ensure that they can be viewed in all likely conditions. The way in which data is displayed and the method by which the operator interacts with the radar system are also key factors in ensuring that the system meets its overall design goals. This may become simplified through the adoption of PC standards such as Windows NT, although it is not clear that this necessarily provides a route to the most efficient system performance, nor will the commercial human interface necessarily be well suited to operation in extreme conditions (for instance, operation in the pouring rain, at night, in a foxhole, and under attack may not be the best time to be finding the right submenu or fighting the mouse).

The battlefield of the future will exploit digital technology and next-generation system designs will need to consider carefully electronic forms of reporting and communicating with the commander. It will also be important to examine interoperability issues, particularly as most future conflicts are likely to be coalition based.

Implications of technology advance Overall, the above advances in technology are enabling

future radar systems to function at longer ranges in smaller, lighter more cost-efficient packages offering a much more versatile set of operating modes. For example, the man-portable radar of tomorrow will be lighter, easier to carry and able to operate at longer ranges but able to manage its radiated power so that the likelihood of hostile intercept is minimised. Further it will be able to adjust its operating parameters so that they are automatically tailored to the task in hand. An example might be using a lower resolution wide-area surveillance mode to automatically cue a high-resolution classification mode towards detected targets of interest. Naturally these radar attributes translate into advantages in vehicle mounting where real estate for sensors is at a premium but demands on performance are equally severe. The likely future performance of these radar systems also makes them prime candidates for mounting in either manned or unmanned air platforms. The advantages are that the field of regard is greatly enhanced and also that the forward motion of the platlorm can be used to add a synthetic aperture imaging mode to the current moving-target detection mode without major changes to the basic radar

hardware. These and other signal processing issues are considered next.

Signal processing

Ever increasing signal processing power from ever more efficient devices has been a major technology trend for many years and is set to continue for the foreseeable future, driven again by the requirements of the mass consumer market. The consumer market may not, however, progress the processing power per watt as fast as it increases processing power per chip. A telling example is the increasing need for on-chip fans as the power consumption of the PC processor rapidly expands.

There is some hope for improvements in this area via laptop PC developments, but the pace of development of processing per watt can be illustrated by the example of an update to an existing radar, currently using ten year old technology. In this case the processing available (for the same power consumption) by using all of the modern technological advances has only increased by a factor of five. This contrasts with the processing power per chip, which has increased over the same period by a factor between 30 and 100. Clearly, when prime power is at a premium, this issue is of fundamental importance.

Of much more importance is the move from function- specific hardware to general-purpose processing. Although this seems at first to be just another way of achieving the required process, it does offer a qualitatively different approach.

In this new scheme a pre-process operating at the signal processing rate can cue a much more complex algorithm, but at the much lower information rate. Critically, because of the availability oilarge memories accessible to the signal processor devices, these ‘cued’ complex algorithms can now work on the raw data stored while the cueing algorithms operated.

This ability to mix algorithm types on a single processing engine allows the designer to use much more sophisticated radar algorithms without greatly increasing the power consumption, size or weight.

Target detection As discussed earlier, in the principal mode of operation

of current systems targets are distinguished from their background by virtue of differences in the magnitude and radial Doppler velocity of the backscatter. When the radial velocity of the target with respect to the background is high, this is not too demanding. However the radial velocity of many targets of interest will naturally be low, for example a man walking or a vehicle moving close to a cross-radial direction. Consequently the resulting Doppler velocity may be comparable with the background, particularly if the latter consists of wind-blown vegetation.

If this is the case complex adaptive detection schemes must be employed that take account of the unpredictable and ill-behavecl nature of the background clutter whilst, simultaneously, positively exploiting the characteristics of target signatures, the overall objective being to maximise target detection whilst minimising false alarms. Fig. 5.


Fig. 5 Example of adaptive thresholding: (a) real data; (b) targets found using constant thresholds; (c) targets found using adaptive thresholds (CFAR - constant false alarm rate)

shows a comparative example of fixed and adaptive thresholding. In the fixed case the threshold removes all the clutter but conceals two targets with radial velocities close to that of the clutter. However, in the adaptive case, where prior knowledge of clutter behaviour is embodied in the algorithm, all the targets are detected with no false alarms. This leads to a less confusing radar display, and allows the operator to concentrate their attention on evaluating targets rather than being distracted by false alarms.

It is well recognised that the behaviour of the clutter is a function of both environment and radar specification. As future systems may be capable of operating at higher radar

Fig. 6 Time records of radar targets

188

resolutions the nature of the clutter returns will change. The energy backscattered will be reduced but its nature will become progressively more ill-behaved. This requires a detailed understanding so that detection algorithms can be adjusted accordingly. Various empirical and statistical models have been proposed. One of these, the compound form of the K-distribution, has been successfully used to describe the amplitude and correlation properties of clutter5, However, when man-made clutter dominates a resolution cell the K-distribution model breaks down and an alternative must be used. The fact that more than one model is needed to represent all lorms of clutter illustrates the complexity of the operating environment.

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL AUGUST 2000

Understanding this environment and target signatures is one of the major challenges for the future. Overall detection performance at high resolutions will generally improve as there will be less clutter in a resolution cell competing with the target. However there will come a point where the target will start to traverse resolution cells during the period of the processing interval and this will cause its energy to be smeared out; consequently detection will not be enhanced. The detailed relationship governing the detection performance under these conditions is also the subject of ongoing research but the performance of prototype systems is extremely encouraging.

Target classification There can be little

doubt that if detected targets could be classified into type, such as men and vehicles, the utility of the information would be greatly enhanced, enabling the battlefield

Fig. 7 Spectrograms (Doppler frequency against time): (a) of man walking; (b) of tank

commander to form a much better understanding of enemy dispositions. The utility would be improved still further if the type of vehicle, for example, could also be discerned. Having detected a target of interest one method of classification is to use the magnitude of the time-varying backscatter to modulate an audio system and provide an acoustic representation of the target Doppler signal to the operator. Experience with ZB298 and MSTAR show this to be effective in certain conditions but overall the performance is variable and the relationship between signature and classification is ill understood.

Doppler classification: Fig. 6 shows two examples of the time-varying intensity (as might be used to modulate a speaker) of a single resolution cell of a target return over a period of a few seconds. It is possible to observe gross movement of the target as evidenced by the modulation and it is this that the human listener hears and uses to make the classification decision. The experienced operator is able to make quite subtle distinctions between differing target modulations. Research work is attempting to identify the key classification criteria in order that they

may be built into automatic or semi-automatic algorithms, either to make the classification decision or to cue the operator to targets 01 greatest interest. Recent research has taken this form of classification a stage further.

To understand the machine view of the target returns we must view them with the processing available to the machine. In general the Doppler radar sees the target as a series of Doppler spectra against time. This representation of typical targets of interest is shown in Fig. 7. The two spectrograms clearly show the difference between a radar return from a man walking and a vehicle (in this case a tank).

Of particular interest for our understanding of this form of classification is the periodic nature of the returns from men (Fig. 7a) , representing the movement of arms, legs etc.; the returns from a vehicle (Fig. 7b) have much less periodicity, representing a relatively smooth motion. In addition to the longer term modulation behaviour of the backscattered radiation there is important target-related information contained in the shorter-term fluctuations. For example, reflections from wheeled vehicles may exhibit a

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL. AUGUST 2000 189

characteristic Doppler signature, enabling them to be distinguished from other targets, such as tracked vehicles.

Machine classifiers working on short-term Doppler classification have been shown to be capable of distinguishing classes of vehicle, and capable of achieving this with very short bursts of information (such as could be gathered as the scan of a radar passes a target - a few tens of milliseconds).

High range resolution classification: Another technique for target classification is to increase the spatial resolution by increasing the radar bandwidth. If the radar resolution is sufficiently high it is possible that the target can be classified spatially. The simplest approach is to form a ‘range profile’, which is in effect a one-dimensional target signature formed from a series of sequential range gates. It is one dimensional in the sense that the second dimension is bounded by the antenna beamwidth, which is relatively large and therefore provides little or no additional information. The range profile will contain backscatter from the different parts of the target under interrogation. If the nature of the profile is characteristic then it is possible to perform classification. The simplest method would be to build a library set of ‘known’ targets and compare the profile under test with the library sets. Library sets can be compiled either from measurements or from simulation.

Pattern recognition techniques for classification: Standard pattern recognition or matched-filtering techniques can be used to perform the classification for either the Doppler or high range resolution technique. For Doppler classification, the simpler techniques appear adequate to achieve an acceptable and robust performance.

For improved performance in high range resolution systems, more advanced neural network or genetic programming approaches can be employed. This has been demonstrated successfully for maritime applications but

Fig. 8 Example of SAR imagery

the smaller land-based targets provide a further challenge due lo the greater variability of target types and background environments. Early research results are encouraging but algorithms are far from being either optimal or robust and target signature understanding is still a relatively immature science with much further work required. For example, little research has yet been carried out to establish the trade-offs between algorithm performance, processing demands and power consumption. The role of the radar operator is key in overall systems operation and must be included in performance trade-offs.

Target classification is a statistical technique, and future systems are likely to employ multiple methods to classify targets in an attempt to make the results more robust. Current developments in Doppler and high range resolution techniques will complement each other in future ground surveillance radar systems.

Imaging for target detection and classification II there is relative motion (or angular change) between

the radar and the target or area of interest then an image can be formed by synthesising a virtual antenna that results typically in a cross-range resolution that matches that of the radial direction. Two of the more common forms of this technique are known as ISAR (inverse synthetic aperture radar) and SAR (synthetic aperture radar).

In ISAR relative motion is provided via the velocity or rotation of the target. However, if the motion of the target is unknown this results in defocusing and uncertainty in the scaling of the image. Techniques aimed at removing these imaging errors have been developed for ship targets but their successful application to smaller land targets is still in its infancy.

An obvious extension to the military utility offered by this class of radar is to mount it on an airborne platform (such as an unmanned aerial vehicle or a helicopter), which affords a much less inhibited view of the battlefield. In addition, the forward motion of an airborne platform can be used to synthesise a large antenna and achieve high resolution in two dimensions. This is known as SAR and typically the direction of view is perpendicular to that of flight of the air platform. The attributes of the radar systems described here lend themselves ideally to this form of imaging. The techniques for producing a correctly focused image are relatively mature and have been demonstrated to resolutions of a few metres; in principle it is possible to form an image with a resolution of the order of 1 m or less. Fig. 8 shows an example SAR image. At very high resolutions an image of a target can be formed as a basis for classification. This is the subject of intense worldwide research and is likely to lead to techniques that can be used reliably and effectively in operational systems.


Airborne short-range surveillance radar systems can be used in both moving-target and imaging modes to provide a more comprehensive surveillance capability. Indeed they can be used simultaneously so that the moving targets can be overlaid on the SAR image, thus providing a valuable context in which interpretation decisions can be made. Not surprisingly there are penalties to be paid, largely in the form o l cxtra system complexity and consequently in cost. For example, the rate at which data is produced, particularly by imaging radars, is somewhat prodigious (up to several hundred megabits per second) and a means of communicating this to the ground for exploitation must form part of the overall system. This of course also implies a ground segment and operations will require mission planning and a much higher level of all-round sophistication and support. Clearly the increased capability olYered has to be carefully balanced against the increased complexity and system costs. Nevertheless the potential for using small lightweight coherent radars to perform many functions is considerable and likely to grow in the near future. Implicit reductions in production costs should increase further the range ol applications and accelerate developments in this important category of radar. Applications o l SAR imaging include 3-D mapping, flood management, oil exploration, volume estimation, treaty verification, surveying, urban planning, ocean monitoring and coastal erosion monitoring.

5 Summary

In this paper the essential design parameters of short- range battlefield surveillance radars have been outlined and examples of operational systems given. Against this backdrop current and likely future technological and signal processing advances have been described pointing the way to future system developments. Rapid advances are impacting almost every aspect ol the system and are quickly increasing the capability ol man-portable battlefield radars whilst simultaneously finding application in vehicle and air platform based surveillance. The quality of information about target type and status is likely lo enable the operator to perform increasingly wide ranging and sophisticated tasks, so enabling the battlefield commander to gain a more comprehensive understanding of his situation. This trend is set to continue. As costs continue to be driven downwards it is likely that additional commercial applications, from harbour surveillance to the protection of buildings and other lacilities requiring security measures, will emerge. The range of applications clearly increases greatly with the ability to form SAR imagery, although airborne systems will be accompanied by higher manufacturing and operating costs.

Acknowledgments

The authors would like to thank Professor Miltc Dean and Dr. Simon Watts for their helpful comments and assistance during the writing of this paper.

Chris Baker graduated lrom the University of Hull in 1980 with a first class honours degree in Applied Physics. After completing a PhD in laser physics (also at Hull) he joined DERA (formerly RSRE) in 1984. Initially, Chris worked in maritime radar, pioneering pulse by-pulse analysis techniques aimed at characterising electromagnetic backscatter from the sea surface. In 1990 he was appointed leader of the Battlefield surveillance research group and played a key role in the development of airborne radar techniques, including ultra-high- resolution SAR, SL4P and interferometry. This culminated in the highly successful MOD procurement programme ASTOR. Chris is currently head of short-range radar researc lising in seeker sensors and surveillance and target ac systems. Chris is a Fellow of the IEE, is chairman of professional group E15 (Radar, Sonar and Navigation) and is a member of the organising committee for RADAR 2002

Address: PE 304, DERA Malvern, St. Andrews Rd., Malvern, Worcs. WR14 3PS, UK. Email: [email protected];ov.uk

Barry Trimmer graduated with a degree in Physics from Warwick University in 1978 and was awarded a Master’s degree in Astronomy by Sussex University in 1979. He joined the radiation laboratory at EM1 Electronics, Hayes, working on radar antennas for the Searchwater radar and naval ESM systems. During the 198Os, he developed RF and system modelling within EMI, leading to system design of ground surveillance and weapon locating

IEE and a chartered engin

Electronics, Manor Royal, Crawley, West Sussex, R H l O Email: [email protected]

References

1 SKOLNIK, M. I.: ‘Introduction to radar systems’ (McGraw Hill, 1980)

2 S I M S O N , G. W..: ‘An introduction to airborne radar’ (SciTech, 1998,2nd edn.)

3 NATHANSON, I;. E.: ‘Ibadar design principles’ (McGraw Hill, 1969)

4 WATTS, S., ‘I’IUMMER, E., PRIESTLEY, B., and BAKEK, C. J.: ‘Battlefield surveillance radars’ (Defence Systems Internalional, 1996)

5 WARD, K. I)., BAKER, C. J., and WATTS, S.: ‘Maritime survcillance radar part 1: Radar scattering k o m the ocean surface’, IEE Proc. F, Radar Signal Process, 1990,137, pp.51-62

OIEE:2000 Received 11th Januaiy 2000

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short-range surveillance radar systems

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