WO2007023371A1

WO2007023371A1 - Grating lobe assisted search radar system utilising antenna with multiple elements

Info

Publication number: WO2007023371A1
Application number: PCT/IB2006/002305
Authority: WO
Inventors: Petrus Johannes Wolfaardt
Original assignee: Reutech Radar Systems (Proprietary) Limited
Priority date: 2005-08-24
Filing date: 2006-08-24
Publication date: 2007-03-01

Abstract

A surveillance radar system comprises a transmit antenna, and a transmitter arranged to feed the transmit antenna with transmit signals at several different frequencies or frequency ranges. A receive antenna comprises a plurality of widely spaced radiators, each radiator having a plurality of radiating elements and defining a lobed radiation pattern. A receiver circuit receives outputs of each of the plurality of radiators and extracts position information from them. The different transmit frequencies or frequency ranges are selected to correspond to the variation in the spacing of the lobes in the radiation pattern of the receive antenna radiators, thereby to generate an overall effective receive radiation pattern with reduced ambiguity or substantially no ambiguity.

Description

SEARCH RADAR SYSTEM UTILISING ANTENNA WITH MULTIPLE

ELEMENTS

BACKGROUND OF THE INVENTION

The present invention relates to radar system architecture and to a method of operating a radar system.

More particularly, the invention relates to the use of array antennas with widely separated elements, that produce grating lobes, in a surveillance radar.

Surveillance radar systems are used to scan large volumes of space and detect targets of various kinds in the scanned volume. The volume of space to be scanned is illuminated with electromagnetic waves by a transmit antenna. Targets are detected by receiving reflections off the target from electromagnetic waves emitted by the radar system and intercepted by a receive antenna. The reflections are referred to as echoes in radar terminology. The transmit and receive functions often utilise the same antenna, but can also utilise separate antennas for the two functions. In the following, attention is focused on the receive antenna. The determination of target position may be performed:

(1) In two dimensions (2D), comprising a bearing in azimuth and the range to the target; or

(2) In three dimensions (3D), comprising bearings in azimuth and elevation as well as the range to the target; or

(3) In the case of a radar with Doppler processing in four dimensions, comprising bearings in azimuth and elevation, together with a range to the target as well as the radial velocity of the target with respect to the radar system.

The determination of the range to the target is performed by electronic means, by measuring the round trip time delay of the reflected waves, from the moment of emission from the transmit antenna to the moment of arrival back at the receive antenna. The radial velocity of the target is determined by measuring the Doppler shift in the frequency of the waves reflected by the target by electronic means.

The receive antenna is the key component in determining the angular bearings in azimuth and elevation, as these rely on the directional properties of the antenna and associated beam-forming systems.

An antenna may have fixed directional radiation properties. In the case of a 2D surveillance radar system, the antenna is rotated physically around a vertical axis to point its beam in different directions in order to determine the azimuthal bearing of a target.

Alternatively, the directional radiation properties of an antenna, typically an antenna array consisting of a multitude of radiating elements, may be controlled by controlling the electric phase of the signals to or from individual radiating elements. The phases can be controlled by electronic or mechanical means. This is usually referred to as phase steering. If an electronic means of controlling the phase is used, the antenna may be called a phased array antenna.

In the case of a 2D radar system, the antenna may remain fixed in space while the radiation pattern is pointed electronically to determine the azimuthal bearing of the target.

In the case of 3D radar systems, the antenna is often scanned mechanically to determine the azimuthal bearing to a target, while several beam patterns may be formed simultaneously on receive by combining the signals from individual antenna elements with different phase weightings in passive networks, or by electronic means through a number of parallel channels. The most versatile electronic beam-forming method is to perform beam-forming in the digital domain, after down-conversion from radio frequency (RF) to intermediate frequency (IF), followed by sampling and conversion to digital format of the received signal from each plank or radiator of the antenna. Each plank of the antenna must be equipped with a receiver to accomplish this. The associated system is called a multi-lobe system.

The multiple beams formed on receive may be fixed for a given system, but may also be varied dynamically from one instant to another when electronic beam-forming techniques are used.

The elevation bearing to the target is determined by comparing the strengths of the received echoes in the different beams. Precise determinations can, for example, be made through application of the monopulse technique. -A-

The precision with which bearings in azimuth and elevation can be determined depends on the size of the antenna in relation to the wavelength of the electromagnetic waves emitted by the radar system. At a given wavelength, a large antenna can produce narrower beams than a small antenna.

A multitude of radiating elements may be combined to build a large array antenna. In one direction, say the horizontal direction, individual radiating elements may be combined in the form of a plank combining the signals from a multitude of dipoles, as shown in Figure 1 , or a waveguide radiator with a multitude of radiating slots, as shown in Figure 2, these being only two examples of numerous possibilities. The radiator of Figure 2 is just one example of a waveguide radiator, in this case with the undesirable property that the beam produced by it squints with frequency.

In the other orthogonal direction, say the vertical direction, the signals from a number of planks or waveguides may be combined, as illustrated in Figure 3.

A multi-lobe receive antenna with lobes in the vertical plane requires that the signals from the planks are combined with different phase delays to form the individual beams. The typical radiation pattern of a multi-lobe receive antenna with multiple lobes in the vertical plane is shown in Figure 4.

The spacing of the individual radiating elements and planks or waveguides in an array antenna merits careful consideration. If the spacing is too large in relation to the wavelength, grating lobes additional to the main beam may appear in the radiation pattern, as shown in Figure 5 for the case of large horizontal spacing between radiating elements. In the present state of the art, radar antennas are carefully designed to avoid grating lobes. The primary reason for this is that grating lobes can give rise to ambiguity in the determination of the bearing to a target. To suppress grating lobes, the spacing between radiating elements and planks in an array antenna must be substantially less than one wavelength of the electromagnetic waves emitted by the radar system. Therefore an array antenna consisting of a number of planks carrying the individual radiating elements that must produce high angular accuracy, must consist of a large number of radiating elements per plank and a large number of planks.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of implementing and operating a radar system, the method comprising:

providing a receive antenna comprising a plurality of widely spaced radiators, defining a lobed radiation pattern;

driving a transmit antenna at at least two different transmit frequencies or frequency ranges to transmit an illuminating beam; and

receiving echo signals at the receive antenna, the different transmit frequencies or frequency ranges being selected to control the spacing of the lobes in the radiation pattern of the receive antenna radiators, thereby to generate an overall effective receive radiation pattern with reduced ambiguity or substantially no ambiguity.

The receive antenna may be an array antenna comprising a plurality of widely spaced radiators, each having a plurality of radiating elements. The method may include driving the transmit antenna at a plurality of different frequencies or frequency ranges simultaneously, with echo signals received at the different frequencies and at different radiators of the receive antenna array being processed simultaneously.

Further according to the invention there is provided a radar system including:

a transmit antenna;

a transmitter arranged to feed the transmit antenna with transmit signals at a plurality of different frequencies or frequency ranges;

a receive antenna comprising a plurality of widely spaced radiators, each radiator having a plurality of radiating elements and defining a lobed radiation pattern; and

a receiver circuit for receiving outputs of each of the plurality of radiators and extracting position information therefrom,

wherein the different transmit frequencies or frequency ranges are selected to correspond to the variation in the spacing of the lobes in the radiation pattern of the receive antenna radiators, thereby to generate an overall effective receive radiation pattern with reduced ambiguity or substantially no ambiguity.

Each radiator may have a plurality of radiating elements.

The radar system may include a driver circuit arranged to generate drive signals at said plurality of different frequencies or frequency ranges sequentially, to be fed via the transmitter to the transmit antenna sequentially. Alternatively, the system may include a driver circuit arranged to generate the drive signals at said plurality of different frequencies or frequency ranges simultaneously, to be fed via the transmitter to the transmit antenna simultaneously.

In one embodiment of the system, the driver circuit is operable to generate a plurality of contiguous frequency modulated pluses at three different frequencies.

For example, the three different frequencies may be in the ranges 8.5 to 9.5 GHz, 9.1 to 9.9 GHz, and 9.7 to 10.5 GHz, separated by a predetermined frequency difference.

Preferably, the receive antenna comprises an array in which the radiators are spaced apart to produce a main beam perpendicular to the plane of the array and grating lobes at angles θ according to the relationship

cos(#) = — a

The radiators of the receive antenna are preferably spaced more than two wavelengths apart at the highest transmit frequency.

For example, in one embodiment of the system, the radiators of the receive antenna are spaced approximately three wavelengths apart at the highest transmit frequency.

The receive antenna may comprise a plurality of waveguide radiators defining grating lobes in the vertical plane.

The receiver circuit may comprise a plurality of receivers, one for each radiator of the receive antenna, each receiver having a number of channels corresponding to the number of different frequencies or frequency ranges of the transmit signals.

Each receiver preferably has at least one analogue to digital converter for generating a digital signal corresponding to a received signal.

The receiver circuit may further comprise digital signal processing circuits arranged to extract position information from a digital signal from the analogue to digital converters, and to resolve ambiguities in the position information according to predetermined criteria.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a typical antenna radiator or plank consisting of a plurality of dipole elements with an accompanying combining network;

Figure 2 shows a typical waveguide antenna with a plurality of radiating slots;

Figure 3 shows an array antenna, consisting of a number of planks, each housing a plurality of dipoles;

Figure 4 shows typical radiation patterns in (a) the vertical and (b) the horizontal planes of a multi-lobe receive antenna array;

Figure 5 shows grating lobes that are formed when the elements of an array antenna are spaced far apart;

Figure 6 shows the radiation pattern of an array antenna with intentionally formed grating lobes at angles of ±22° from the main lobe at a first operating frequency; Figure 7 shows how the angular displacement of the grating lobes change to ±25° from the main beam when the frequency of operation is changed to a second frequency which is 90% of the first frequency;

Figure 8 shows two simultaneously formed beams with grating lobes at a first operating frequency, with possible angular positions x, y and z for a target that produces echoes of equal strength in the two beams, as well as possible positions a, b and c for a target that produces echoes that are 7 dB larger in beam 2 than in beam 1 ;

Figure 9 shows the two beams with grating lobes at a second operating frequency, showing the target positions x, y and z, and also positions a¹, b' and c' where the target echo in beam 1 is 5 dB larger than that in beam 2;

Figure 10 shows how full coverage of the scanned volume is provided by six simultaneous beams in the specific example chosen; and

Figure 11 is a simplified block schematic diagram showing an embodiment of the system architecture of a 3D search radar that uses an antenna with grating lobes according to the invention.

DESCRIPTION OF EMBODIMENTS

The invention provides a surveillance radar system architecture that makes use of a multi-lobe antenna that produces grating lobes to obtain high angular accuracy while at the same time drastically reducing the hardware complexity and cost. The grating lobes are typically formed in either the horizontal or vertical planes, but may in principle be formed in both the horizontal and the vertical planes, or in one or two inclined planes.

The system architecture provides for the resolution of the directional ambiguities due to the grating lobes by means of multiple transmit frequencies. The resolution of angular ambiguity is based on the principle that the angular separations between the grating lobes and the main lobe are frequency dependent.

Figures 6 and 7 show grating lobes for a given antenna at two frequencies, where the second frequency is 90% of the first. It can be noted that the grating lobes move further apart as the frequency is lowered, while the position of the main lobe remains unchanged. The amplitudes of the grating lobes relative to that of the main lobe are determined by the radiation ^■ pattern of the radiating elements. A symmetrical pattern was chosen for purposes of illustration. The preferred pattern will depend on the application. For example, it could be a cosec² shape for a search radar with grating lobes in the vertical plane.

The principle of ambiguity resolution now becomes clear if a second beam is formed on receive at the IF frequency of the receiver by an analogue or digital beam-former. Consider the situations shown in Figure 8, where two beams with main and grating lobes are formed on receive. An echo has been received at frequency 1 with equal power in the two beams. Possible angular positions for the target are marked as x, y and z in the figure. (Note that equal power is assumed for purposes of illustration only, and does not imply a limitation of the principle).

The frequency is now changed to frequency 2. The positions of the main lobes remain unchanged, but those of the grating lobes change. Figure 9 shows that a target at position x would now produce a larger signal in beam 1 than in beam 2. A target at position y will again produce equal echoes in the two beams, while a target at position z would produce a larger echo in beam 2 than in beam 1. By comparing the relative amplitudes of received echoes in the two beams, a decision can be made whether the target is at position x, y or z. The bearing ambiguity of the target can therefore be resolved. In practice, comparisons will be done at three or more frequencies to increase the robustness of the ambiguity resolution process in the presence of noise.

Apart from the resolution of the bearing ambiguity, the target bearing can be precisely determined by comparing the echo powers received in each of the beams. Since the beam shapes are known, a specific power ratio relates to a specific angular position for pairs of grating and/or main lobes. This is illustrated by the thick lines in Figures 8 and 9. Suppose it is found that a target echo at frequency 1 is 7 dB larger in beam 2 than in beam 1. This places the target at angular positions a, b or c in Figure 8. At frequency 2, the echo return is 5 dB higher in beam 1 than in beam 2. This ^■■ places the target at positions a¹, b' or c' in Figure 9. Clearly, the target must be located at angular position a. In practice, in the presence of noise, the target will be placed at the average of the angular positions corresponding to a and a'.

For the chosen example, six simultaneous (or multiplexed) beams, as illustrated for frequency 1 in Figure 10, will provide complete effective coverage of the area of interest and enable the resolution of angular ambiguities of all targets returning echoes to the grating lobes of the antenna. The resolution of ambiguity and target bearing in the six beam system is simply an extension of the principle described above. At each frequency the target echo amplitudes in the pairs of beams with the largest echo returns from a given target are compared.

An embodiment of the invention is shown in Figure 11 for a 3D pulsed Doppler search radar operating in the X band. The embodiment applies to an antenna with six non-squinting waveguide radiators with grating lobes in the vertical plane. The same basic embodiment would also be suitable for an antenna with grating lobes in the horizontal plane, or for an antenna with a different number of planks or waveguide radiators. With reference to Figure 11 , the transmit antenna (1), which is separate from the receive antenna, is driven by a radar transmitter (2). The radiation pattern of the transmit antenna covers the entire search volume in any given pointing direction. In other words, its radiation pattern will cover the complete span of the main and significant grating lobes in one plane, while it will be similar to that of the receive antenna in the orthogonal plane that does not contain grating lobes.

The transmitter is excited by an exciter (3), which generates three contiguous frequency modulated (chirped) pulses at three different frequencies separated by approximately 800 MHz in the frequency band 8.5 to 10.5 GHz. The three frequencies are in the ranges [8.5 - 9.3], [9.1 - 9.9] and [9.7 - 10.5] GHz.

The exciter generates the pulses at a frequency of 70 MHz by means of a Direct Digital Synthesizer (DDS) and converts the frequency of these pulses to the three transmit frequencies in the X-band. The exciter in turn receives reference and local oscillator signals from a frequency synthesizer (4). The frequency synthesizer simultaneously generates three agile local oscillator signals in the frequency ranges [6.43 - 7.23], [7.03 - 7.83] and [7.63 - 8.43] GHz for the second up-conversion stage of the exciter, as well as a fixed local oscillator signal at 2.0 GHz for the first up-conversion. A 1 GHz clock is also supplied for the DDS.

The receive antenna consists of six antenna elements in the form of non- squinting waveguide radiators (5) that are spaced slightly less than three wavelengths apart at the highest transmit frequency, to produce a main beam perpendicular to the plane of the array and grating lobes at angles θ where the relationship cos(#) = — - a holds. In the equation, k is an integer, λ is the operating wavelength and d is the spacing between planks. The main receive lobe corresponds to k = 0, the first pair of grating lobes to k=±1 and the second pair to k=±2. It is clear that the angular position of the main lobe is independent of wavelength (or, therefore, frequency), while the positions of the grating lobes are dependent on wavelength (or frequency). Only the first pair of grating lobes are utilised in the system. The higher order grating lobes are suppressed by the radiation patterns of the waveguide radiators.

Each antenna element or radiator (5) feeds a three channel receiver (6), of which there are six in total. The receiver splits the input signal into three channels and converts signals received at the three transmit frequencies to three IF stages at a frequency of 70 MHz in three dual-conversion receivers. The IF signal is sampled at a frequency of 40 MHz and converted to digital format by an analogue-to-digital converter. The required local oscillator and clock signals are provided by the frequency synthesizer (4).

The eighteen bit-streams from the six receivers are fed to a digital beam- former (8) via a high speed optical fibre digital link (7). The digital beam- former applies appropriate phase shifts to each of the bit-streams and combines them to form six beams at each of the transmit frequencies, for a total of eighteen beams in the digital domain.

The outputs of the digital beam-former are routed to six three-channel signal processors via a second high-speed digital bus. Each channel of the signal processor performs a pulse compression operation by means of a Fast Fourier Transform and sets up the echo returns in range bins. This is followed by a further Fast Fourier Transform across 16 adjacent range bins in the scan direction to obtain Doppler information.

The eighteen output bit streams from the signal processors are routed via digital bus (11) to the collapsing and height extractor processor (12) where the following decisions are made: 1. The presence or absence of a target echo in each bin of the eighteen sets of Doppler range bins and correlation between the eighteen sets of range bins.

2. The assignation of the strongest returns from a given target to two of the six beams at each of the three frequencies.

3. The resolution of grating lobe ambiguities and determination of target angular position according to the principles discussed above.

4. The determination of Doppler velocity and resolution of velocity ambiguities according to established practice in the radar art.

The output data from the collapsing and height extractor is fed to the data processor and track-while-scan processor (14) via a digital bus (13). The data processor provides information to a display output or a radio link to a remote display system.

The hardware savings can now be quantified. A radar antenna with the same angular resolution but without grating lobes would have required an antenna with 36 radiating elements. A radar transmitting at three frequencies, required for the same detection performance, would therefore require 36 three channel digital receivers feeding a digital beam-former with 108 input channels to form at least 12 beams without grating lobes at each frequency, i.e. 36 output bit-streams. This would require 12 three-channel signal processors performing identical functions to those described above. The collapsing function would have to be applied these 36 input bit- streams. While ambiguity resolution would not be necessary, elevation extraction would be a more intensive process. Overall, hardware savings of the order of a factor of six have been realised in the receive and detection system feeding the Data Processor.

In conclusion, a radar architecture is provided that intentionally uses antenna grating lobes to effect a substantial hardware savings for a 3D pulse Doppler search radar with high angular resolution. The invention makes it possible to build a 3D or 2D surveillance radar with high angular resolution performance, at a fraction of the cost of a comparable conventional surveillance radar using an array antenna that does not produce grating lobes.

Claims

1. A method of implementing and operating a radar system, the method comprising:

driving a transmit antenna at least two different transmit frequencies or frequency ranges to transmit an illuminating beam; and

2. A method according to claim 1 wherein the receive antenna is an array antenna comprising a plurality of widely spaced radiators, each having a plurality of radiating elements.

3. A method according to claim 1 or claim 2 including driving the transmit antenna at a plurality of different frequencies or frequency ranges simultaneously, with echo signals received at the different frequencies and at different radiators of the receive antenna array being processed simultaneously.

4. A radar system including:

a transmit antenna; a transmitter arranged to feed the transmit antenna with transmit signals at a plurality of different frequencies or frequency ranges;

a receive antenna comprising a plurality of widely spaced radiators each radiator has a plurality of radiating elements defining a lobed radiation pattern; and

5. A radar system according to claim 4 wherein each radiator has a plurality of radiating elements.

6. A radar system according to claim 4 or claim 5 including a driver circuit arranged to generate drive signals at said plurality of different frequencies or frequency ranges sequentially, to be fed via the transmitter to the transmit antenna sequentially.

7. A radar system according to claim 4 or claim 5 including a driver circuit arranged to generate the drive signals at said plurality of different frequencies or frequency ranges simultaneously, to be fed via the transmitter to the transmit antenna simultaneously.

8. A radar system according to claim 6 or claim 7 wherein the driver circuit is operable to generate a plurality of contiguous frequency modulated pluses at three different frequencies.

9. A radar system according to claim 8 wherein the three different frequencies are in the ranges 8.5 to 9.5 GHz, 9.1 to 9.9 GHz, and 9.7 to 10.5 GHz, separated by a predetermined frequency difference.

10. A radar system according to any one of claims 4 to 9 wherein the receive antenna comprises an array in which the radiators are spaced apart to produce a main beam perpendicular to the plane of the array and grating lobes at angles θ according to the relationship

cos(<9) = —

11. A radar system according to any one of claims 4 to 10 wherein the radiators of the receive antenna are spaced more than two wavelengths apart at the highest transmit frequency.

12. A radar system according to claim 11 wherein the radiators of the receive antenna are spaced approximately three wavelengths apart at the highest transmit frequency.

13. A radar system according to any one of claims 4 to 12 wherein the receive antenna comprises a plurality of waveguide radiators defining grating lobes in the vertical plane.

14. A radar system according to any one of claims 4 to 13 wherein the receiver circuit comprises a plurality of receivers, one for each radiator of the receive antenna, each receiver having a number of channels corresponding to the number of different frequencies or frequency ranges of the transmit signals.

15. A radar system according to claim 14 wherein each receiver has at least one analogue to digital converter for generating a digital signal corresponding to a received signal.

16. A radar system according to claim 15 wherein the receiver circuit further comprises digital signal processing circuits arranged to extract position information from a digital signal from the analogue to digital converters, and to resolve ambiguities in the position information according to predetermined criteria.