Method of and Apparatus for Automatically Calibrating a Phased-Array Antenna

The present invention relates to a method of and an apparatus for automatically calibrating a phase-array antenna, particularly array antennas for microwave landing systems.

This application is based on and claims priority form German Patent Applications No. 40 12 101.1 dated Apr. 14, 1990 and No. 40 14 320.1 dated May 4, 1990. To the extent such prior applications may contain any additional information that might be of any assistance in the use and understanding of the invention claimed herein, they are hereby incorporated by reference.

Aircraft landing aids, particularly microwave landing systems, must meet very stringent accuracy requirements. To be able to satisfy these requirements, the antennas used must be very well calibrated. This applies to both azimuth antennas (AZ antennas) and elevation antennas (EL antennas). U.S. Pat. No. 4,502,361 discloses a method of calibrating a phased-array AZ antenna with 4-bit phase resolution wherein probes are inserted into each individual waveguide radiator. It has been found, however, that in phased-array antennas with 6-bit resolution, the reproducibility of the measurements with the aid of probes does not yield satisfactory results. Such an antenna could be better calibrated if its aperture amplitude and phase illumination were known. To derive the aperture illumination of a phased-array antenna, use is made of integral monitor waveguides. Signal components from each radiating element are coupled through coupling holes into an integral monitor waveguide either shortly before or immediately after transmission. The output of the integral monitor waveguide corresponds, to a first degree of approximation, to the far-field pattern of the antenna. The far-field pattern and the antenna aperture illumination are related by a Fourier transform. Therefore, the complex aperture illumination of the antenna can be determined from the output of the integral monitor waveguide. A conventional method of doing this is the quadrature method (I/Q converter). In this method, the signal from a local oscillator is mixed with the output signal from the integral monitor waveguide twice, once at an angle of 0.degree. and a second time with a 90.degree. phase shift. The mixing with a 0.degree. phase shift provides the real part of the output signal of the integral monitor waveguide, and the mixing with a 90.degree. phase shift provides the imaginary part. A subsequent Fourier transformation of the real and imaginary parts of the output signal yields the aperture illumination of the antenna. A disadvantage of this method is the use of two mixers.

It is the object of the invention to provide a method of and an apparatus for calibrating phased-array antennas in a reproducible manner and with an accuracy required to meet safety standards.

In accordance with the invention, the aperture illumination of the array antenna is determined from the output of an integral waveguide and compared with a desired aperture illumination. The difference between actual value and desired value is compensated for iteratively with the aid of an adaptive control system. Preferably, the real part of the actual signal is obtained by homodyne detection of the signal from the integral monitor waveguide, and the imaginary part is computed from the real part using a Hilbert transform, whereupon the far-field signal may be calculated using a Fourier transform.

One advantage of the method and apparatus according to the invention is that the antenna can also be calibrated during operation. Another advantage is that because of the choice of the Hilbert transform to obtain the aperture illumination, only one mixer is needed. This results in an improvement in the signal-to-noise ratio of the usable signal.

An embodiment of the invention will now be explained in greater detail with reference to the accompanying drawings, in which:

FIG. 1 shows the principle of an array antenna with an integral monitor waveguide;

FIG. 2 shows and I/Q converter;

FIG. 3 shows the basic design of a homodyne measuring system;

FIG. 4 shows a monitoring facility for a phased-array antenna, and

FIG. 5 shows an automatic control system for calibrating a phased-array antenna.

FIG. 1 shows part of a phased-array antenna. The radiating elements of the antenna are designated 11. 10 is an integral monitor waveguide into which signal components from each radiating element are coupled through coupling holes. In the integral monitor waveguide, the signal components combine into a complex, time-varying signal. The signal components coupled into the integral monitor waveguide are components either shortly before transmission (in the case of azimuth antennas) or immediately after transmission (in the case of elevation antennas). The signal appearing at the output 12 of the integral monitor waveguide 10 corresponds, to a first degree of approximation, to the far-field pattern of the antenna. Because of the Fourier-transform relationship between the antenna aperture illimination and the far-field pattern, the complex aperture illumination can be calculated from the output signal of the integral monitor waveguide.

To this end, in prior art apparatus, the output of the integral monitor waveguide is conditioned in the manner shown in FIG. 2. Mixers 20 and 21 are supplied with signals from hybrids 22 and 23. The hybrid 22 is, for example, a 3-dB 0.degree. hybrid, and the hybrid 23 a 3-dB 90.degree. hybrid. Via an input 24, the hybrid 23 is supplied with a signal from a local oscillator. Via an input 25, the hybrid 22 is supplied with the output signal from the integral monitor waveguide. 26 and 27 denote RF terminations, also called "RF absorbers". They serve to terminate components for radio frequencies in a non-reflecting manner. The output of the mixer 20 then provides the real part of the signal applied at the input 25, and the output of the mixer 21 provides the imaginary part. The arrangement described is referred to as an "I/Q converter", and the outputs of the two mixers are called "quadrature components". In a further step, the aperture illumination of the antenna is determined via a Fourier transform. The arrangement just described needs two mixers to represent the complex output signal of the integral monitor waveguide.

FIG. 3 shows the basic configuration of a homodyne measuring system. A mixer 30 is applied with signals via lines 35 and 36. The output of the mixer 30 is applied to a low-pass filter 31, whose output 37 provides the desired signal. The reference numeral 32 denotes a transmission element whose complex transfer function is to be determined with the arrangement shown. A radio-frequency generator 33 has its output coupled to the mixer 30 via the line 36. The output of the generator 33 is also coupled via a coupler 34 into the transmission element 32. The purpose of the arrangement is to obtain the real part of the complex transfer function of the transmission element 32 at the output 37. Assuming that the amplitude of the signal at the input 35 is substantially smaller than the amplitude of the signal at the input 36, i.e., that the mixer 30 is operating in the linear region, the following results:

A signal A.sub.M and a signal A.sub.R are applied to the mixer 30 over the lines 35 and 36, respectively. The voltage U at the output 37 is ##EQU1## where

.psi..sub.M =.omega..sub.O t+.alpha..sub.M +.phi.(t)=phase of the monitor signal

.psi..sub.R =.omega..sub.O t+.alpha..sub.R =phase of the reference signal

.phi.(t)=general phase function of system 32

.DELTA..alpha.=.alpha..sub.M -.alpha..sub.R.

As mentioned above, the real part of the complex transfer function of the transmission element 32 is available at the output 37.

The real and imaginary parts of the spectrum of complex, causal time functions are related by an integral transform, the so-called Hilbert transform. Consequently, it suffices to measure the real part of such functions, since the imaginary part can be computed by way of the Hilbert transform.

FIG. 4 shows an antenna of a microwave landing system (MLS) which uses the homodyne measuring method of FIG. 3 to obtain the antenna aperture illumination. Like reference characters have been used to designate like elements. As in FIG. 3, a mixer 30, a low-pass filter 31, a radio-frequency-signal source 33, and a coupler 34 are provided. The element 40 is a monitor implemented, for example, as an integral monitor waveguide, like element 10 in FIG. 1. A network 41 distributes the electric energy from the radio-frequency source 33 via phase shifters 42 to radiating elements 43 of the array antenna. 43' denotes the entirety of the radiating elements and phase shifters. From the radiating elements, signals are coupled to the integral monitor waveguide 40. The output of the integral monitor waveguide is fed to the mixer 30, which is also supplied with the radio-frequency signal via the coupler 34. At the output of the low-pass filter 31 the voltage U described in connection with FIG. 3 is available. This voltage U is the real part of the output signal of the integral monitor waveguide 40. The voltage U developed at the output of the low-pass filter 31 is digitized by means of a sample-and-hold circuit 44 and an analog-to-digital converter 45. A time- and value-discrete signal is thus available at the output of the analog-to-digital converter 45. From this time- and value-discrete signal, the imaginary part of the output signal of the integral monitor wave-guide 40 is computed via the discrete Hilbert transform with the aid of a signal processor 46. After this operation, the complete complex far-field signal of the phased-array antenna is available. Use of the discrete Fourier transform (DFT) or the fast Fourier transform (FFT) then provides the inverse transform of the antenna aperture illumination.

Regarding the implementation of the discrete Hilbert transform of the discrete Fourier transform and the fast Fourier transform, the person skilled in the signal-processing art is referred to a wealth of literature on this subject, such as an article entitled "Quadrature Sampling with High Dynamic Range", IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-18, No. 4, November 1982, pages 736 to 739.

FIG. 5 shows in more detail how the phase-array antenna of FIG. 4 is calibrated. Like reference characters are used to designate like elements. The phase-array antenna with its radiating elements 43 is shown in FIG. 5 as a block 43. The phase shifters appear as a block 42. A signal 50 appearing at the output of the integral monitor waveguide 40 corresponds to the far field of the antenna. In a computing unit 46', this signal 50 is subjected to an integral transformation to obtain the aperture illumination of the antenna. The output of the computing device 46' is fed to a controller 51. Via a line 52 from storage means 56, the desired value for the phase setting of the phase shifter 42 is fed to a summing point 53. The output signal from the controller 51, which is fed to the summing point 53 via a line 54, is subtracted from this desired value. The phase shifter is thus supplied with the difference between the desired value on line 52 and the output signal from the controller 51 on line 54. The computing device 46', the controller 51, the summing point 53, and the line carrying the desired values 52 may be implemented in software in a signal processor. All the steps necessary to carry out the method may be performed, for example, in the signal processor 46 of FIG. 4. From FIG. 5 it is apparent that an automatic control system as shown in FIG. 5 is associated with each radiating element 43 of the phased-array antenna. To calibrate the antenna, in a first step, a comparison between the desired value and the actual value of the aperture illumination is performed. At the same time, correction values are generated by the controller. If complete agreement between desired and actual values should not be attainable with these correction values, the control parameters are changed (adaptive control system) and the process just described is repeated. The process is repeated until the desired and actual values of the aperture illumination differ only within prescribed tolerance bands. During the process, the sampling rate of the monitor signal must be so high that immediate aliasing effects in the reconstructed illumination function become negligibly small, i.e., clearly above the Nyquist rate.

The aperture illumination is determined using a Hilbert transform of the output of an integral monitor waveguide.