AU658410B2

AU658410B2 - Frequency dependent beamforming

Info

Publication number: AU658410B2
Application number: AU15993/92A
Authority: AU
Inventors: Samuel Brengle Colegrove
Original assignee: Commonwealth of Australia
Current assignee: Commonwealth of Australia
Priority date: 1991-05-02
Filing date: 1992-05-01
Publication date: 1995-04-13
Anticipated expiration: 2012-05-01
Also published as: AU1599392A

Description

40529 HKS:MAH:LL 6 5 8 4 0 P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPL ETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

SAMUEL BRENGLE COLEGROVE Address for Service: COLLISON CO.,117 King William Street, Adelaide, S.A. 5000 Invention Title: *FREQUENCY DEPENDENT BEAMFORMING Details of Associated Provisional Applications: Australian Patent Application No. PK5954 dated 2nd May 1991 The following statement is a full description of this invention, including the best method of performing it known to us: This invention relates to a method of beam-forming in frequency agile radar systems.

In its most general sense the invention can be applied to any frequency agile system in which antenna array elements are manipulated to form muitiple simultaneous receive beams. In providing a background discussion of the beamforming, reference will be made to its application to High Frequency (HF) Band, Over-the-Horizon radar (OTHR) systems with one dimensional arrays.

An OTHR uses refraction of HF waves by the ionosphere to propagate to long ranges, eg over 1000km. The effective height of the ionosphere is about 100 1 0 to 300 km above the earth's surface. The ionosphere changes with geographic location, time of day, season of year, sunspot activity and other influences. The effect of these ionospheric changes is that the optimum operating frequency varies over the radar coverage and from time to time.

Also for a particular frequency and bearing from the transmitter, there is a finite 1 5 range depth over which adequate performance is achieved.

In practice an OTHR beams a signal from a transmitter antenna which is then refracted by the ionosphere to illuminate a geographical region. The echo from the illuminated region travels by a similar path back to a receiving array.

Because of equipment cost factors, scanning rate and resolution requirements for tracking, the receiving array has a much larger aperture than the transmitting array and it forms multiple simultaneous receive beams. Typically there might be 6 receive beams formed.

The angular extent of a processed geographical region, is determined by the total span of the simultaneous receive beams while the range extent of the 25 region is determined by the ionosphere and selected radar waveform and processing. The selected radar waveform is repeated at a preset rate herein called the waveform repetition frequency. The OTHR processing involves *Doppler processing a preset number of waveform repetitions to separate targets from clutter. Therefore the radar dwells for about 1 second or more to 3 0 collect data from a region. The beamforming for the simultaneous receive beams involves combining the steer directions and beamwidth of each beam so as to cover the region of interest. There is necessarily some overlap between each receive beam to ensure that there are no parts of the region from which no signal is detected. A number of regions together form a total coverage pattern.

The angular beamwidth of the receive beams is effected by the receiving array frequency of operation (carrier frequency) and array element amplitude and phase weights. For example the beamwidth is increased by adding extra attenuation to the signals from the ends of the antenna array. Also beamwidth decreases with increasing frequency because beamwidth is approximately proportional to the inverse of the aperture length in wavelengths.

At any point in time there is an optimum carrier frequency for a region.

1 0 Therefore to achieve optimum radar performance the carrier frequency must be changed from time to time. Because of the frequency dependence of the beamforming, the antenna steer directions and weights in amplitude and phase weights must be recalculated to maintain the coverage pattern. In practice the main oojective is to limit any changes to the location of the 1 5 regions and minimise losses in radar sensitivity.

S Typically there are three different ways U) cope with the required changes in carrier frequency. The first way is to keep the receiver beams pointing in the same steer direction with the antenna amplitude weights fixed. As the S frequency increases the beamwidth decreases and overlap between beams 20 reduces. To avoid losses in target detection, the steer directions are set for adequate overlap at the highest frequency. In this case reducing the frequency increases the overlap. The increase in overlap leads to redundant information which makes for inefficient utilisation of the receiver processing.

The second way to deal with frequency changes is to keep a fixed overlap 25 between beams. Thus with increasing frequency the beamwidth again decreases and the steer directions must adjust to maintain the constant overlap. The maximum radar sensitivity is achieved, however the region positions change which greatly complicates the display of data.

The third way is to hold the beam steer directions constant and adjust the antenna amplitude weights to broaden the beam and maintain beam overlap as frequency increases. In this case the antenna gain reduces due to a decrease in the effective aperture from the change in weights.

It is the intended object of this invention to alleviate one or more of the above mentioned problems or at least provide a useful alternative.

Therefore, in one form of this invention, though this need not be the only or indeed the broadest there is proposed an improved method of frequency dependent beamforming in frequency agile systems which maximises antenna gain and minimises computational load comprising the steps of determining for each region the optimum carrier frequencies, beam steer directions, antenna weights and waveform repetition frequencies to form multiple simultaneous beams to cover each region and subsequently altering within predetermined values the carrier frequencies and waveform repetition frequencies only, the predetermined values determined in their range by the optimum carrier frequencies, while maintaining constant beam steer directions in each region to account for ionospheric changes, interference and to decorrelate ambiguous clutter and target signals.

In preference the optimum frequencies and waveform repetition frequencies are determined by a frequency management system which determines the necessary parameters for a particular task from setting parameters derived from a plurality of sub-systems which measure the ionospheric environment, each of the sub-systems being adapted to obtain and process a set of particular setting parameters and to maintain a register of the particular setting parameters such that the management system may poll them as necessary.

o: Such a system is described in co-pending PCT Application No.

PCT/AU92/00174 entitled Modular Frequency Management System.

In preference the waveform repetition frequencies and carrier frequencies S°predetermined values are contained within and are selected from a jitter table, which is a table of frequencies provided by the Frequency Management System.

In preference the step of monitoring the clutter and noise to dynamically remove from the table of frequencies those carrier frequencies and, or waveform repetition frequencies which degrade performance; the system therefore does not use those frequencies when choosing new carrier and waveform repetition frequencies and thereby the system sensitivity is maximised.

In preference the beam steer directions and antenna weights are chosen to achieve a desired coverage pattern.

In preference the step of monitoring the performance of the frequency agile system and redetermining the optimum frequencies if the performance falls below a threshold.

In detail, two levels of frequency agility are implemented. On the first level the radar, at a given time, has a required coverage pattern which is divided into r,.%gions with a reference carrier frequency, range of waveform repetition frequencies, beam steer directions and antenna weights chosen to optimise the radar operation. The upper and lower limit in the range for the waveform repetition frequencies i. based on the range and velocity ambiguity change to expose target returns masked by clutter. On the second level, the radar carrier frequency and waveform repetition frequency for each region is altered about **the reference carrier frequency and within the range of waveform repetition frequencies from dwell to dwell to account for ionospheric changes, interference and to decorrelate ambiguous clutter and target signals. This go:o° second level is called jitter. The jitter frequencies are taken from a table of frequencies derived from the Frequency Management System and the range oooeo S• of waveform repetition frequencies. During jitter, the performance for each combination of carrier frequency and waveform repetition frequency is measured for a region and those combinations which give poor performance are removed from the jitter table list.

A number of methods of measuring performance are possible. One approach proposed here is to divide the Doppler frequencies into three areas. The first 0 o contains clutter from land and sea, the second contains clutter from the ionospheric environment and the third contains external noise. These clutter N)~h~q measurements are derived from the power in the Doppler frequency cells. In the case of land and sea clutter, its measurement is made by summing the power about the maximum clutter return nearest zero hertz. TlKh external noise is derived from the noise power over the Doppler frequencies about plus or minus one quarter the waveform repetition frequency from zero. The clutter from the ionospheric environment is estimated from the remaining Doppler frequencies.

During jitter the beam steer directions are held constant until such time that ionospheric conditions change so that the jitter is unable to maintain adequate 1 0 performance in one or more of the regions. At this time a coverage pattern with new region divisions based on new reference carrier frequencies, range of waveform repetition frequencies, beam steer directions and antenna weights are determined and jitter recommences.

This approach alleviates the previ'us situation of sacrificing radar 1 5 performance by achieving maximum antenna gain and radar sensitivity for each region's simultaneous receive beam. There is a drop in gain controlled by the overlap of receive beams and the increase in frequency from the initial reference frequency. Also there is no need to relocate regions whenever there is a change of frequencies used for the regions in a coverage pattern.

20 This simplifies the radar operation because the changes to the reference frequencies for the regions are made infrequently, say at most about once every 15 minutes.

Another advantage of this two-tiered frequency agility system is that time animated displays of data obtained from a region are available at all times except for the short time after the regions ar, relocated.

For a better understanding of this invention a preferred embodiment will now be described with reference to the attached drawing in which: FIG. 1 schematically shows the position of regions to fill a coverage pattern.

3 0 In FIG. 1 the required coverage pattern 7 of an OTHR is divided into six regions (1 to Each region will have a different carrier frequency and range of waveform repetition frequencies to achieve optimum operation for the OTHR task. The optimum carrier frequency is dependent on, amongst other factors, the required range, bearing of the coverage pattern and time of day.

The range of waveform repetition frequencies is dependent on the ambiguities in range and velocity and the location of clutter. Since the region width 8 is carrier frequency dependent each region has a different width. In addition, to ensure total coverage, there is a degree of overlap (not shown) between regions and for each simultaneous receive beam in a region.

The carrier frequency and waveform repetition frequency in each region is altered about the optimum frequency from dwell to dwell. The maximum deviation from the optimum frequency is based on propagation conditions and 1 0 beamforming losses. The choice of carrier frequencies at any instant is based on interference from noise and ionospheric clutter. The choice of waveform repetition frequencies is based on ambiguous target and clutter returns. The carrier frequency and waveform repetition frequency combinations used at any instant is based on those which give minimum noise and ambiguous 1 5 clutter. This results in variations in either the antenna weights, beam widths or both while maintaining the steer directions 9 unchanged. Maintaining steer directions of the simultaneous receive beams unchanged has the advantage of eliminating bearing jitter from time animated displays which would otherwise appear if the region steer directions were changing.

e° Ce oe 20 As the carrier frequency deviates from the optimum carrier frequency there is a change in receive beam overlap. Eventually this overlap becomes unacceptable at which time new operating parameters are calculated and the jitter process recommences.

Throughout this specification the purpose has been to illustrate the invention and not to limit this.

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Claims

1. An improved method of frequency dependent beamforming in frequency agile systems which maximises antenna gain and minimises computational load comprising the steps of determining for each region the optimum carrier frequencies, beam steer directions, antenna weights and waveform repetition frequencies to form multiple simultaneous beams to cover each region and subsequently aitering within predetermined values the carrier frequencies and waveform repetition frequencies only, the predetermined values determined in their range by the optimum carrier frequencies, while maintaining constant beam steer directions in, each region to account for ionospheric changes, interference and to decorrelate ambiguous clutter and target signals.

2. The method of claim 1 in which the optimum frequencies and waveform repetition frequencies are determined by a frequency management system which determines the necessary parameters for a particular task fiom setting parameters derived from a plurality of sub-systems which measure the ionospheric environment, each of the sub-systems being adapted to obtain S• and process a set of particular setting parameters and to maintain a register of the particular setting parameters such that the management system may poll them as necessary.

3. The method of claim 2 in which the waveform repetition frequencies and carrier frequencies predetermined values are contained within and are selected from a jitter table, which is a table of frequencies provided by the S°Frequency Management System.

4. The method of claim 3 further comprising the step of monitoring the clutter and noise to dynamically remove from the table of frequencies those carrier frequencies and, or waveform repetition frequencies which degrade performance; the system therefore does not use those frequencies when choosing new carrier and waveform repetition frequencies and thereby the system sensitivity is maximised. /s' i' k~si NB ~a- 8 The method of claim 1 in which the beam steer directions and antenna weights are chosen to achieve a desired coverage pattern.

6. The method of claim 1 further comprising the step of monitoring the performance of the frequency agile system and redetermining the optimum frequencies if the performance falls below a threshold.

7. The method as herein described with reference to the Figure. Dated this 8th day of February 1995 THE COMMONWEALTH OF AUSTRALIA By its Patent Attorneys, COLLISON CO. 0 0 0* 0 *0*0 0 00r 0 0 0 0 00 0 00000 0 0 0 0 00 0 0000 0 00 00 00 0 0 0 00 ABSTRACT An improved method of frequency dependent beamforming in frequency agile systems which maximises antenna gain and minimises computational load and disruption to data displays comprising the steps of using the optimum carrier frequencies to determine beam steer directions and antenna weights to form multiple simultaneous beams to cover each region within a radar coverage and altering the carrier frequencies and waveform repetition frequencies to account for ionospheric changes by interference and ambiguous clutter and target returns. a. *r a. I, a *4 Sr *r S a. a S a.. SS a. C S *r S S