METHOD AND MEANS FOR DETERMINING DIRECTIONAL CHARACTERISTICS OF LARGE SENSOR OR RADIATOR ARRAYS

Abstract: A method and apparatus for the measurement of the directional characteristics, both in azimuth and in elevation, of sensors or radiators or arrays of sensors or radiators in which a reference antenna is located in close vicinity to the array under test such that the reference antenna and the array substantially simultaneously transmit or receive signals provided by a signal source. The signal is modulated to facilitate analysis in both range and Doppler.

Description: METHOD AND MEANS FOR DETERMINING DIRECTIONAL CHARACTERISTICS OF LARGE SENSOR OR RADIATOR ARRAYS This invention relates to a method of measurement of the directional characteristics, both in azimuth and in elevation, of sensors or radiators, or arrays of sensors or radiators. BACKGROUND OF THE INVENTION A particular application of this method is the measurement of the polar radiation pattern of the large receiving and transmitting antenna arrays for an Over-the Horizon Radar. The method is not restricted to this application. The apparatus used to demonstrate the method of measurement, is not the only possible implementation of the method, nor is it necessarily the broadest implementation of the method. The following discussion of the method will be based on the measurement of the directional gain characteristics of a large array for electromagnetic transmissions in the frequency band 3-30 MHz, known as the High Frequency Band, or simply HF band. Signals in this frequency range can travel for large distances via the ionosphere, and the band is typically used for long range communications, and more recently for Over-the-Horizon Radar. While the discussion uses an electromagnetic array as the example, it must be understood that the technique applies equally well to other sensing or radiating arrays, and that the method is not confined to a particular band of electromagnetic radiation, but could also be applied to other frequency bands in the electromagnetic spectrum, or other wave phenomena such as acoustic waves for example. The method can be applied to the measurement of single elements, but it is especially effective, when compared to previous methods, for the measurement of the patterns of large arrays of elements. The difficulty that arises in testing large arrays is that the test probes (whether receivers of test transmissions or transmitters of test signals) must be remote from the array under test by a sufficient distance that the wavefronts of the signals can be considered to be almost planar, that is, within some acceptable tolerance on the geometric error in measurement. The large separation between the array under test and the test probe, however, ensures that uncertainties about the magnitude, phase, and polarisation of transmission in the signal propagation path are increased, thus inhibiting accurate measurement. The difficulty is compounded if the intervening medium of propagation itself imposes fluctuations in the magnitude, phase or polarisation of transmission in the signal paths, or imposes multiple propagation paths between transmitter and receiver. This behaviour is especially characteristic of the ionosphere, for example. An object of this invention is to provide an apparatus and method whereby measurement techniques for the above purpose can be improved. The present invention incorporates one or more of the following features : 1) the use of a signal which allows processing to discriminate between multiple propagation paths, including those paths arising from reflections from objects or the earth; 2) the use of a signal which allows processing to discriminate between propagation paths which have imposed a modulation (typically Doppler or velocity) on a dimension other than range; 3) the use of a modulator in a reference channel to impose a modulation in a second dimension without affecting range discrimination; 4) a single receiver (including the two dimensional processing) which receives the propagated signals from the array being tested and the reference antenna simultaneously, and which separates in range the signals that have travelled by different paths, and which separates in Doppler (or a second dimension) the test and reference signals which have travelled by a common path. According to one form of this invention, there is proposed a method of measurement of the directional characteristics of an array of sensors or radiators which comprises the steps of locating a reference antenna in close vicinity to the array, causing the array and reference antenna to substantially simultaneously transmit or receive signals provided by a signal source such that the signal associated with the reference antenna is distinguishable from the signal associated with the array and analysing and comparing said signals in a receiving system. The array under test may be a transmitting or receiving array. The method in each case is substantially the same although the arrangement to perform the method varies in detail. According to a further form of this invention, there is proposed a method of measurement of the directional characteristics of a transmitting array which comprises the steps of locating a reference antenna in close vicinity to the array, locating at least one suitably adapted antenna and receiving system at a distantly spaced location from the array, transmitting a signal substantially simultaneously from the reference antenna and array such that the remote receiving system can distinguish the signal transmitted from the reference antenna from the signal transmitted from the array and effect a comparison of the received magnitude of at least one characteristic of the respective signals. In preference, the signals are transmitted from the reference antenna and the array simultaneously. In preference, the receiving system is adapted to distinguish between the signals radiating from the array and from the reference antenna. The receiving system may consist of a receiver and signal processor adapted to the processing of the radiating signals. The receiving system effects a recording of a magnitude of that signal received from the array as compared to a magnitude of that signal received from the reference antenna. In preference, the signal transmitted through the reference antenna and array has at least two dimensions, one dimension of which contains identifying information to allow discrimination in range between signals travelling by way of different paths, and another dimension of which contains information to allow discrimination between signals travelling by way of substantially equal path lengths to the receiving system from respectively the reference antenna and the array. In preference, the method further includes the step of modifying the signal transmitted through either the one or the other of the reference antenna and array, to allow the remote receiving system to effect a discrimination between signals arriving at the receiving system by substantially equal path lengths. According to a stilf further form of this invention, there is proposed a method of measurement of the directional characteristics of a transmitting array which comprises the steps of locating a reference antenna in close vicinity to the array, locating at least one suitably adapted antenna and receiving system at a distantly spaced location from the array, transmitting a signal substantially simultaneously from the reference antenna and array such that the remote receiving system can distinguish the signal transmitted from the reference antenna from the signal transmitted from the array and effect a comparison of the received magnitude of at least one characteristic of the respective signals wherein the signals are propagated through a propagating medium, reflected towards the receiver from a substantially large reflecting surface, such as the Earth, and traverse the propagating medium a second time. The remote receiving system is adapted to receive signals primarily from the direction of the reflecting surface. In a yet further form of the invention there is proposed a method of measurement of the directional characteristics of a receiving array which comprises the steps of locating a reference antenna in close vicinity to the array, locating at least one suitably adapted transmitting system at a distantly spaced location from the array, transmitting a signal from the transmitting system to be substantially simultaneously received by the array and the reference antenna such that the signal received by the array can be distinguished from that received by the reference antenna, and effecting a comparison of the received magnitude of at least one characteristic of the signals received by the array and the reference antenna. In preference, the method further includes the step of modifying the signal received by either the one or the other of the reference antenna and array, to allow the receiver or receivers to effect a discrimination between signals received by respective array and reference antenna, and to allow the signals from these two antenna systems to be measured substantially simultaneously. The propagation medium with its uncertain losses and propagation paths cannot be removed entirely as a source of error in measurement. However, making simultaneous measurements of the signals received by the array and the reference antenna, located in close proximity, ensures that there is a high statistical correlation between the signals in the antenna and array, and consequently smaller error variance in the measurement compared to former types of measurement. The use of only one receiver is also more economical than multiple receiver methods. In another form of the invention there is proposed an arrangement for the measurement of the directional characteristics of a transmitting array including a transmitting array to be measured, a reference antenna having known directional characteristics located in close vicinity to the array, a signal source adapted to provide signals to be transmitted by the transmitting array and reference antenna, a splitter adapted to direct signals from the signal source to both the array and the reference antenna, a modulator adapted to modulate the signals directed to either the transmitting array or the reference antenna and an antenna and receiving system distantly spaced from the array location and adapted to receive and analyse signals from the array and the reference antenna. In another form of the invention there is proposed a method of measurement of the directional characteristics of a receiving array which comprises the steps of locating a reference antenna in close vicinity to the array, locating at least one suitably adapted transmitting system at a distantly spaced location from the array, transmitting a signal from the transmitting system to be substantially simultaneously received by the array and the reference antenna such that the signal received by the array can be distinguished from that received by the reference antenna, and effecting a comparison of the received magnitude of at least one characteristic of the signals received by the array and the reference antenna wherein the signals are propagated through a propagating medium, reflected towards the receiver from a substantially large reflecting surface, such as the Earth, and traverse the propagating medium a second time. The transmitting system is adapted to transmit primarily towards the reflecting surface. " In a still further form of the invention there is proposed an arrangement for the measurement of the directional characteristics of a receiving array including a receiving array to be measured, a reference antenna having known directional characteristics located in close vicinity to the array, a signal source distantly spaced from the array location and adapted to effect the transmission of signals to be received by the respective array and reference antenna, a combiner adapted to combine signals received by the receiving array and the reference antenna, a modulator adapted to modulate the signals received by either the receiving array or the reference antenna and a receiving system adapted to receive and analyse signals from the array and the reference antenna. In preference the receiving system consists of a receiver and a processor. The processor analyses the received signals in two dimensions with the first dimension preferably containing information allowing discrimination between signals travelling by way of different paths and the second dimension allowing discrimination between signals travelling by way of similar paths. In yet another form of the invention, once again stressing that it is not intended this further statement should indicate limitations or restrictions, as such, to the broader concept or basis of the invention, there is proposed apparatus for the measurement of directional characteristics of arrays comprising: (1) A signal source which allows signal analysis in at least two dimensions, (A first dimension can contain information to allow discrimination in range between signals travelling via different paths, and a second dimension can allow discrimination between signals travelling via substantially equal path lengths to and from the array and the reference antenna. Signals containing different Doppler frequency shifts are examples of signals which can be discerned as separate, even though they may have the same range information.) (2) A reference element with substantially known characteristics, (This will usually be a simple structure having a pattern that is well known with respect to an isotropic element and exhibiting close agreement between theoretical and measured characteristics.) (3) A modulator to modify the second dimensional characteristics of a signal without changing its range discrimination and without interfering with the unmodulated signal, (The signal received in either the reference antenna or array when measuring a receiving array, or transmitted through either the reference antenna or array when measuring a transmitting array, is modulated so that the signals from the reference antenna and array may be distinguished. The modulation is selected so as not to affect the range discrimination.) (4) A signal combiner or splitter and receiver or transmitter, (In the case of measuring a receiving array, this allows the unmodulated and modulated signals from the array and the reference element to be combined and received through a common receiver and signal processor. In the case of measuring a transmitting array, this allows the unmodulated and modulated signals to be passed to the array and the reference element respectively.) (5) A signal processor capable of processing a two-dimensional signal, (The processor processes the combined signal from the receiver in range, and then in the second dimension, typically a Doppler dimension. Signals travelling via different path lengths will appear separated in range, and the pairs of signals that have travelled via nearly equal path lengths will be discriminated in the second dimension.) The present proposals follow substantial deliberations concerning existing significant difficulties presently being experienced. It is possible for instance to provide a careful calibration of signal power and measure absolute received signal power, building up sufficient data to establish a statistical description of the characteristics of the array under test. The difficulty with this type of measurement is caused by the remoteness of the test probe and the consequent uncertainties in the signal powers to the array under test. The statistical distribution of received power in this type of measurement can be very broad, as for example in the case of the ionospheric propagation medium, leading to a corresponding large uncertainty, and hence a possible large error, in the estimate of the array directional gain. The key to reducing the uncertainty in these measurements is to make the signal paths to the array and reference antenna as close as possible, and also to make the measurement simultaneously. The only source of error in this case would be the uncertainties in the two signal paths, and by design, these uncertainties are reduced by the physical proximity of the array and the reference antenna. It might be considered that by measuring the signal from the reference antenna and array sequentially, and as closely in time as permitted by the signal acquisition and processing, that the difficulties mentioned above would be overcome. However, the time scale of the perturbations imposed by the propagation medium may be of the same order as the signal acquisition and processing time. For example the time scale of ionospheric perturbations is of the order of seconds and is similar to the time required to acquire and process range and doppler information. The present invention allows simultaneous measurement of the signals, thereby ensuring that the perturbations imposed by the propagation medium are substantially reduced. For a better understanding of the invention this will now be described with reference to actual experimental data, it being emphasised that any description throughout is intended to illustrate the invention and not to limit this. Reference will be made to the figures in which: FIG. 1 shows schematically the arrangement for measuring the directional characteristics of a transmitting array, FIG. 2 shows schematically the arrangement for measuring the directional characteristics of a receiving array, FIG. 3 shows three possible signal paths via the ionosphere, each with a discrete angle of arrival, FIG. 4 shows the use of FMCW waveforms to determine range as a function of frequency, FIG. 5 shows a typical receiver output, FIG. 6 is a plot of the front to back ratios of an antenna array for three different ranges, and FIG. 7 is an illustration of the azimuth/elevation pattern of an array determined by the method. Referring now in detail to the figures, FIG. 1 shows the arrangement for measuring the directional characteristics of a transmitting array. A linearly frequency modulated continuous wave (FMCW) signal source provides a signal which is split and provided to both the array and a reference antenna. A modulator provides the modulation to the signal to facilitate analysis in a second dimension. Remote from the transmitting system is the receiving antenna and receiving system consisting of a receiver and a processor. The arrangement for measuring the directional characteristics of a receiving array is shown in FIG. 2 and is analogous to the arrangement for the transmitting array. In this case the transmitting system consists of the signal source and an antenna. The modulation is provided at the receiving system. In the embodiment described the modulation is provided to the signal received by the reference antenna. In our application of measuring the directional characteristics (particularly the front to back ratio) of a medium sized HF receiving array the signal sources were located about 1200 km from the receiver array, and the ionospheric propagation allowed angle of arrival over a range from about 10 degrees to 30 degrees for the demonstration measurement. FIG. 3 illustrates the . geometry of the demonstration measurement. The signal source was a radar signal generator which provided a repetitive linearly frequency modulated signal in the HF band. This signal waveform has the property that signals arriving from different ranges have a frequency difference as shown in FIG. 4, with the frequency being proportional to the range difference. Analysis of the signals in the first dimension, namely the dimension of frequency, therefore gives range discrimination between signals travelling via the different ionospheric paths. The modulator for the signals in the output of the reference antenna was a double balanced mixer which simply functions as a signal reversing switch. The effect of the switching is to change the received radar signal into a pair of doppler shifted radar signals equi-spaced in doppler about the original radar signal, with the spacing being equal to the switching frequency. Provided that the switching frequency is properly chosen, the modulation process does not affect the range discrimination. Analysis in the second dimension, namely the doppler frequency dimension, of corresponding range-formed spectra from the first processing pass therefore separates in the doppler domain signals from the two antennas, which have travelled nearly equal path lengths. FIG. 5 shows a typical output from the receiver, when signals have travelled via five ionospheric paths to the two antennas, and where all signals from the reference antenna are modulated by a suitable doppler frequency (half of the maximum frequency that can be unambiguously computed is used here for illustration). The figure illustrates how a simultaneous comparison of the signals received by the two antennas is made, and how the comparison can be made for multiple paths, and hence how the directional pattern can be determined if both the reference directional pattern and the geometry of. the system are known. FIG. 6 shows the results of measurements made with this method on the ratio of forward gain to rearward gain of a medium sized array. The measured points are shown for propagation via three ionospheric layers. If the measurements are repeated with the test signal source at different azimuth's, the whole azimuthal and a large portion of the elevation pattern can be calculated with respect to the reference element, with a high degree of precision. This is illustrated in FIG. 7 where the plan and elevations of several test source locations and signal paths are shown. It can be seen that the signal paths from a particular test source location arrive at different elevation angles and hence describe some portion of the test array's vertical pattern. When the process is repeated at a different location in azimuth, a new portion of the vertical pattern is measured. When the process is repeated at closer or further ranges, the elevation angles change and the measurements thus describe more points on the vertica pattern. The invention ensures that the error in measurement is substantially reduced compared with previous methods of measurement, by virtue of the ability (a) to distinguish between signals which have travelled nearly equal path lengths but which emanate from different antennas (b) to make these measurements simultaneously, thus ensuring high correlation between the measurements, and a consequent reduction in the variation caused by fluctuations in the propagation medium over time. Throughout this specification the purpose has been to illustrate the invention and not to limit this.

Claims: CLAIMS: 1. A method of measurement of directional characteristics of an array of sensors or radiators which comprises the steps of : locating a reference antenna in close vicinity to the array; causing the array and reference antenna to substantially simultaneously transmit or receive signals provided by a signal source such that the signal associated with the reference antenna is distinguishable from the signal associated with the array; and analysing and comparing said signals in a receiving system. 2. A method of measurement of the directional characteristics of a transmitting array which comprises the steps of : locating a reference antenna in close vicinity to the array; locating at least one suitably adapted antenna and receiving system at a distantly spaced location from the array; transmitting a signal substantially simultaneously from the reference antenna and array such that the remote receiving system can distinguish the signal transmitted from the reference antenna from the signal transmitted from the array; and effecting a comparison of the received magnitude of at least one characteristic of the respective signals. 3. The method of claim 2 wherein the signals are propagated through a propagating medium, reflected towards the receiver from a substantially large reflecting surface, such as the Earth, and traverse the propagating medium a second time. 4. The method of claim 3 wherein the remote receiving system is adapted to receive signals primarily from the direction of the reflecting surface. 5. The method of claim 2 wherein the signal transmitted through the reference antenna and array has at least two dimensions, one dimension of which contains identifying information to allow discrimination in range between signals travelling by way of different paths, and another dimension of which contains information to allow discrimination between signals travelling by way of substantially equal path lengths to the receiving system from respectively the reference antenna and the array. 6. The method of claim 2 further including the step of modifying the signal transmitted through either the one or the other of the reference antenna and array, to allow the remote receiving system to effect a discrimination between signals arriving at the receiving system by substantially equal path lengths. 7. A method of measurement of the directional characteristics of a receiving array which comprises the steps of : locating a reference antenna in close vicinity to the array; locating at least one suitably adapted transmitting system at a distantly spaced location from the array; transmitting a signal from the transmitting system to be substantially simultaneously received by the array and the reference antenna such that the signal received by the array can be distinguished from that received by the reference antenna; and effecting a comparison of the received magnitude of at least one characteristic of the signals received by the array and the reference antenna. 8. The method of claim 7 wherein the signals are propagated through a propagating medium, reflected towards the receiver from a substantially large reflecting surface, such as the Earth, and traverse the propagating medium a second time. 9. The method of claim 8 wherein the transmitting system is adapted to transmit primarily towards the reflecting surface. 10. The method of claim 7 further including the step of modifying the signal received by either the one or the other of the reference antenna and array, to allow the receiver or receivers to effect a discrimination between signals received by respective array and reference antenna, and to allow the signals from these two antenna systems to be measured substantially simultaneously. 11. An arrangement for the measurement of the directional characteristics of a transmitting array including : a transmitting array to be measured; a reference antenna having known directional characteristics located in close vicinity to the array; a signal source adapted to provide signals to be transmitted by the transmitting array and reference antenna; a splitter adapted to direct signals from the signal source to both the array and the reference antenna; a modulator adapted to modulate the signals directed to either the transmitting array or the reference antenna; and an antenna and receiving system distantly spaced from the array location and adapted to receive and analyse signals from the array and the reference antenna. 12. An arrangement for the measurement of the directional characteristics of a receiving array including : a receiving array to be measured; a reference antenna having known directional characteristics located in close vicinity to the array; a signal source distantly spaced from the array location and adapted to effect the transmission of signals to be received by the respective array and reference antenna; a combiner adapted to combine signals received by the receiving array and the reference antenna; a modulator adapted to modulate the signals received by either the receiving array or the reference antenna; and a receiving system adapted to receive and analyse signals from the array and the reference antenna. 13. The arrangement of claim 11 or 12 wherein the receiving system consists of a receiver and a processor said processor being adapted to analyse the received signals in two dimensions with a first dimension containing information allowing discrimination between signals travelling by way of different paths and a second dimension allowing discrimination between signals travelling by way of similar paths. 14. An apparatus for the measurement of directional characteristics of arrays comprising: a signal source whiGh allows signal analysis in at least two dimensions; a reference element with substantially known characteristics; a modulator to modify a dimensional characteristic of a signal without changing its range discrimination and without interfering with the unmodulated signal; a signal combiner or splitter and receiver or transmitter; and a signal processor capable of processing a two-dimensional signal. 15. The apparatus of claim 14 wherein the first dimension is range and the second dimension is a Doppler dimension. 16. A method as herein described with reference to the attached figures. 17. An apparatus or arrangement as herein described with reference to the attached figures.

Inventors: EARL GEORGE FREDERICK, MASSIE ANGUS DAVIDSON, ROBERTS PETER MICHAEL, EARL, GEORGE, FREDERICK, MASSIE, ANGUS, DAVIDSON, ROBERTS, PETER, MICHAEL
Applicants: COMMW OF AUSTRALIA, EARL GEORGE FREDERICK, MASSIE ANGUS DAVIDSON, ROBERTS PETER MICHAEL, THE COMMONWEALTH OF AUSTRALIA
Application year: 1993
Main classification: G01R29
IPC codes: G01R29, G01S7, H01Q3