BISTATIC WITH THINNED RECEIVED

M. Cherniakov University of Birmingham Edgbaston, B15 2TT, UK [email protected]

Abstract The presented concept of bistatic radar can have significant benefit for airport surveillance. It may be described as a bistatic semi-active radar which utilizes one of the available airport area surveillance as the and a receiving only, slave radar. The passive radar contains an electronically scanned phased array which is essentially thinned. The grating lobes are suppressed in this system by the transmitting radar acting as a space filter. At the conceptual level of the paper the advantages and limitations of the proposed radar architecture are considered.

1 Introduction

The primary goal of this conceptual paper is to introduce the topology and basic performance analysis of bistatic radar (BR) with essentially different transmitting and receiving antennas. This BR can be described as bistatic semi-active radar that utilize as the illuminator (transmitter) one of the available surveillance radars (master radar - MR) and a passive, receiving only, radar (slave radar - SR). Presumably MR and SR are operating independently, but if expedient or necessary the obtained data could be collected at one position for further data or information fusion. In the SR radar, a phased array is proposed which provides essentially better angle resolution (order or more) in comparison with the master radar within a given sector.

360 0 observation

Mechanical scanning MR

Synchronization channel

:0 SR observation

Phased array antenna Electronic scanning

Figure 1: System topology

An example of possible system topology is shown in Figure 1. In the system we assume presumably mechanical scanning in the azimuth MR antenna, with circular or sector coverage and a relatively broad beam which results in a low angle resolution. In contrast, SR has electronically scanning phased array antenna with sector coverage and an essentially narrower beam in comparison with the MR antenna. During one MR antenna scan, the sector of SR coverage is illuminated over a significant time period TD, and MR and SR could act in a bistatic radar mode. We assume at this stage that BR has a quasi-monostatic configuration, i.e. the bistatic angle is small enough to avoid a pulse-chasing problem. It will be shown that for the considered configuration the SR array can be essentially thinned. Instead of the traditional 0.5-0.7  (wavelength) spacing between the array radiators, it can be enlarged up to 0.5-0.7 D, where D is the MR antenna effective aperture length in azimuth. The penalty for this technical advantage is a reduction in the power budget which will be analyzed later in the paper. The key factor for the possible reduction in the number of antenna elements is the interaction of the relatively broadbeam transmitting antenna and the narrowbeam receiving phased array. The transmitting antenna pattern acts as the space filter for the grating lobes rejection (Figure 2).

MR antenna SR antenna Main beams

SR antenna grating lobes

Figure 2: Combination of the transmitting and receiving antennas patterns

Potentially the passive radar has the millirad resolution. The system can be used for many applications where the maximal operational range of the slave radar can be less than the operational range of the active radar, but where the enhanced angle resolution is the key factor, i.e. airfield monitoring (cars, pedestrians, left luggage, etc), landing aircrafts, etc; where the array is positioned on the airport roof (Figure 3).

MR

SR

Figure 3: Airport layout example [6]

The proposed system has a passive nature and does not require an appropriate spectrum allocation that makes its‘ application specifically attractive in areas sensitive to an introduction of new active sensors (airports as a typical example). For instance in an airfield monitoring scenario, high-resolution monitoring information can be obtained by the data fusion from all kind of sensors: optical, infrared, laser, MM wave, etc. In spite of the range of systems used, there are still grey spots in the monitoring area. Potentially this could lead to a disaster similar to the Concorde crash in , due to a small metal object present on the runway; or the collision between Singapore Airline B-747 and a civil engineering machine at an airport in .

2 Antenna elements spacing

A comprehensive introduction to radar with phased arrays is presented in [1]. The traditional radar contains Transmitting/Receiving antenna(s). If two antennas are used and they are significantly separated spatially, the system is referred to as bistatic radar [2]. In general in a bistatic system, transmitting and receiving antennas can be different in terms of their patterns and steering approaches. One of the antennas could be, for example, mechanically scanning when another could be an electronically steering phased array. Assume a scenario where a transmitting antenna is mechanically rotated over 360 0 with an antenna beamwidth TR in the azimuth plane. Only this plane is considered here, as it is of specific interest in this paper. The approximate relationship between an antenna aperture size œ DTR ,  and TR in radians is: TR ≈ /D TR . Commonly the size of mechanically rotated radar antennas is less than ~ 8-10 m [4], which limits the minimum antenna beamwidth. It should also be mentioned here that the antennas‘ size critically influences the system cost. Figure 4 contains an approximate graph of antennas beamwidth MIN in degrees, for two specific wavelengths 0.03 and 0.03 m vs antenna size in meters œ DTR .

Mechanically Phased scanning antennas arrays

 N deg number of elements λ=0.3

3  5.0 10 λ=0.03

0.5 10 2

0.05 10 1 λ=0.03 N λ=0.3

0.3 3.0 30 D [m]

Figure 4: Beamwidth -  and radiating elements number œ N vs antenna size - D

Thus for the frequency bands traditionally occupied by surveillance radars, the best practically achievable beamwidth (8 m antenna) is between 0.2 (X-band) and 2 degrees (L-band). For example, at a distance of 5 km, this provides a cross range linear resolution between ~15 and ~150 m respectively. If a narrower beam is needed for scanning antennas, phased array technology could be used. In this case the antenna itself is not mechanically rotated, but the beam is controlled electronically. Potential advantages of phased array technology are known. Here we will highlight the major drawbacks: high cost and a restriction to approximately ±60 0 scanning angle. Actually, more than one flat array could cover 360 0, but this further affects the system cost. In the first approximation the phased array cost is proportional to the number of the antenna elements - N and the number of appropriate channels in an array. For an array scanning in one plane over ±60 0: N ≈ 1.4D/ . This number could be reduced by not using all of the elements for large arrays ( N>100). In practice, a thinned array could have 2-4 + times element reduction in 2-D arrays [4]. Nevertheless, at this stage we consider 1.4D/ as the estimate. The number of elements in a phased array N vs D is also shown in Figure 4. For example at X band it would be N~200 for D~3 m (beamwidth M~0.5 0) and N ~2000 for D~30m (beamwidth M~0.05 0). An antenna with 0 M~0.05 at a distance of 5 km provides an excellent linear resolution, )l ≈ 5m. Not considering here specific technical problems of antennas with a so narrow beam design, we will focus attention on the high cost of such arrays. At L band, to achieve 0.05 0 resolutions, the antenna should have a length of about 300 m. Even if the cost of the antenna module at this frequency is less than for X band, a maintenance construction cost could be higher. Another peculiarity of such large size antennas array is the aperture - range resolution relationship. When the aperture becomes of an order or bigger than the range resolution the antenna control technique and its structure requires some specific modification. First of all the phase shifter or their baseband equivalent, in the case of digital array, should be replaced by appropriate delay lines [3]. In point of fact, radars with an extremely large array and high range resolution have already been considered in the literature and are known as —radio-camera“ [5]. We should also take into account one further technical problem, specifically if analogue phased array is used, that is the accuracy of the elements phasing. Narrowbeam array is traditionally associated with a large number of antenna elements and all non-ideality in a circuits could be averaged. In out case this statistical approach cannot be directly applied and an individual components precision could be taken into account. For the systems discussed so far, it is difficult to develop cost affordable scanning antennas with a very high angle resolution and specifically at relatively low frequency bands. For mechanically scanning antennas, this is first of all due to the limitations of the mechanical system; and for the phased array it is due to the quickly increasing cost/angular resolution dependence. On the other hand, low frequency bands are preferable in many cases due to a better power budget and weather condition robustness, higher target detectability, less shadow area, etc.

3 Thinned array for the BR

The spacing s between antenna elements specifies the array pattern‘s grating lobes. To avoid ambiguity θ the distance between these lobes should be more than a scanning angle 0 and for the broad scanning array it is s ~ 0.5 Q - 0.7 Q. The angle distance M 1 between gratings is specified by a simplified equation [8]: n sin(θ ) ≈ sin(θ ) ± (1) 1 0 s / λ where n is the lobes‘ number. For the considered case, the transmitting antenna pattern θTR is acting as a space θ θ filter (see Figure 2) and the distance between grating lobes could be potentially reduced from 0 to TR . For relatively narrow beam transmitting and receiving antennas the conditions (1) for n =1 will be:

θ ≈ λ s but θ ≈ θ ≈ D λ and s ≈ D 2 1 / TR 1 TR / TR 2/ (2)

The total transmitting-receiving antenna pattern will be a product of these two patterns: transmitting and receiving. Thus the array has a narrow beam specified mainly by the receiving antenna size with antenna element spacing DTR /2. For example, if the transmitting antenna size is DTR = 8 m and the receiving array has the length DRE = 30 m, the minimal number of antenna elements in the thinned array will be only N~8, when for the full array the number of elements is N=160 . The gain . in angle resolution will be about four times in this example. Following the discussion above, a simplified equation could be derived which merges the required number of elements in the phased array and the resolution gain in the BR relevant to the MR in the bistatic radar under discussion: D N ≈ 2 = 2 θTR = 2 TR (3) η D θRE RE

Thus, if we want to increase the angle resolution by 10 times, the minimum required number of the array elements is 20. The proposed approach could transform a system that is unaffordable for many applications due to large elements number phased array, into a practically applicable and cost effective system. 4 Power budget

Of course, this BR has some drawbacks in comparison with equivalent angular resolution radar utilising a full-scale (non-thinned) array. We have to pay the penalty for this simplification. The thinned receiving phased array antenna itself has a lower gain in comparison with the full array. According to [4] the gain of the array can be evaluated as:

G ≈ G RE ∑ i (4) N

Where N is the number of the array elements and Gi is the individual element gain. Reduction in the number of elements leads to appropriate changes of the antenna gain and the system power budget reduction. This is true for any method of array thinning. So, the upper boundary of power loss 0 due to the thinned array utilization is the ration of the elements number in the full and the thinned array:

N F DTR 2 DTR ≈ = η = (5) ξ N Th λ2η λ

This dependence as a function of the wavelength for DTR equal 2, 4 and 8 m is shown in Figure 5. As one can see, when the number of elements in the thinned array depends only on the angular resolution improvement factor ., the power loss factor 0 depends on the operational frequency and the transmitting antennas effective size. Another parameter which influences the target‘s detectability in the BR being discussed is the time of targets illumination in the BR which will be less than in the MR. This is the direct consequence of improving the angle resolution and does not depend on the number of elements in the array. This dwell time TD reduction is equal to . and should be taken into account when the system power budget is considered.

0 dB

8m 25 4m

15

5.0 2m

0.3 0.1 0.03 Q[m]

Figure 5: Power losses in the BR

Fundamentally in the BR, TD could be increased to the dwell duration in the master radar. We can exploit the fact that the transmitting antenna beamwidth is . times broader than the receiving one. So, using . parallel channels in the phased array, for example Battler Matrix, the time TD could be expanded. In this case we should consider one extra power loss factor 01 that is between 3 and 6 dB . When we are using only one beam phased array and it tracks the direction of the maximum of the transmitting antenna pattern. In case of a parallel surveillance within the MR antenna pattern, the loss factor 01 follows the transmitting pattern shape and at the edges could be about 6 dB, or some average loss ~ 3 dB can be considered . This is illustrated in Figure 6. The power budget reduction could be calculated for any particular scenario and the appropriate decision made by the designer. Nevertheless, for a number of cases the penalty discussed above could be acceptable. For example, the range of airport monitoring systems may be approximated as ~5 km where the air traffic control radar is actually designed to operate over a range of more than one hundred km (that is 50 dB spare power). This means the proposed system has a sufficient power budget, even operating over the master radar antenna sidelobes ( -30 dB order). The same could be applied for other cases.

Power loss

Receiving antenna Transmitting antenna patterns œ pattern maltibeam antenna and multichannel receiver

Figure 6: Transmitting antenna pattern as an envelope for the multibeam phased array patterns

5 Pulse chasing

In bistatic radars, and specifically when at least the receiver has a narrow beam antenna, there could be essential loss in the power budget due to the different antenna coverage at the transmitting and receiving sides [2]. As it follows from Figure 7, the receiving beam should be pointed in the direction of where the transmitting pulse is at in a particular moment in time, ti. This is the general problem of bistatic radar operating with narrow beam of the receiving antenna. In our case, when the receiving antenna is fundamentally electronically steered, the problem of flexible scanning has a solution.

Transmitting pulse position at the time instant t i Grating lobes of the Phased Array

Main lobe of the Phased Array

Transmitting beam Tr Re DTR

Figure 7: Pulse chasing via the thinned phase array The technique that helps to resolve this problem is referred to as ”pulse chasing‘, in which the receiving beam should follow or track the position of the transmitting pulse; potentially performed by a modern antenna phased array, specifically a digital one. An example is considered below where the distance between the transmitter and the receiver D TR =3 km and the observation range is 1-6km. The maximum angle tracking 0 speed corresponds to a 90 bistatic angle β and the shortest range relevant to the receiver position. As the 8 speed of light is 3 •10 m/s, the angle tracking speed could be estimated in the first approximation [2] as 0.3 deg/microsecond. This requirement fits well into both the analogue phased array concept with GaAs phase shifter and digital beamforming. The difference between the full and the thinned array considered in this paper is the presence of the grating lobes (see Figure 7). When the transmitting radar has a non-ambiguous pulse repetition frequency, and this is true for most airport surveillance and related radars, the grating lobes of the receiving antenna never illuminate the position of the transmitting pulse. In spite of the pulse chasing bistatic radar, its development is a rather complex problem. The proposed system at least fundamentally presents an electronically scanning receiving antenna which is the key component of this technique‘s implementation.

6 Airfield monitoring example

Let us illustrate the subject with an example. At any airport, highly powerful traffic control radar is present. A typical airport layout is shown in Figure 3 and we will consider how it is possible to use such radar for airfield monitoring. For the sake of solidity, assume that NRL‘s Senrad air traffic control radar system [7] is used. It operates around 1000 MHz, with an average transmitting power of 10 kW and a signal bandwidth up to 200 MHz, which provides ~1-2 m range resolution. Let the detection performance be 1m 2 radar cross section (RCS) target at a distance of 100 km. Antenna aperture size in the azimuth plane is ~8 m, which corresponds to the angle resolution ~2 0. The semi-active bistatic configuration can be used where the MR is Senrad and the SR antenna is deposited along the air terminal. In the scenario it is assumed that the array with 8 times bigger effective aperture i.e. .=8 is used, and that the angle resolution will be about 0.26 deg or ~0.005 rad. At a distance of 5 km from the radar the linear azimuth resolution will be about 25 m. The array length would be about 80m, possible to accommodate on a typical terminal roof. We also consider an 8 channel Battler matrix (or similar) at the reception side which provides a parallel surveillance within the transmitting antenna beam. As discussed, an average 3 dB power losses in dwell time should be taken into consideration. Using the graph from Figure 5, the power loss factor 0= 14 dB . It is assumed that the object of interest is a human at the far end, relevant to the radar, of the airfield, i.e. 5 km range with RCS of 0.5 m 2. So the expected power budget loss relevant to the MR is 20 dB. We could also take into account that only a sidelobe of MR illuminates the airfield which introduces ~ 30 further power reduction. On the other hand, due to the distance reduction from 100 to 5 km, the expected power gain is ~ 50 dB . Thus, the power balance is achieved. Of course the performance analysis introduced above does not pretend to be precise, but rather accurate enough to illustrate the practical applicability of the proposed bistatic system.

References

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