EP0452799A1 - Method and apparatus for the automatic calibration of a "phased array" antenna - Google Patents

Abstract

Landing aids with phase-controlled group antennas must be calibrated very carefully. Previous methods use measuring probes for this purpose, which are inserted into each individual antenna of the array antenna. This method is not sufficiently precise for phase shifters with 6 bit resolution. A method and a device are therefore proposed in which the aperture assignment of the array antenna (43) is determined from the output signal of an integral waveguide (40) and is compared with a target aperture assignment. The difference between the actual (54) and setpoint (52) is compensated iteratively with the aid of an adaptive control device (51). <IMAGE> <IMAGE>

Description

The invention relates to a method and a device for the automatic calibration of a phase-controlled group antenna, in particular group antennas for microwave landing systems.

Very high demands are placed on the accuracy of landing aids in aviation, in particular on the accuracy of microwave landing systems. In order to meet these requirements, the antennas used for the landing systems must be very well calibrated. This applies both to azimuth antennas (AZ antennas) and to the elevation antennas (EL antennas). A method for calibrating a phase-controlled AZ antenna with 4-bit phase resolution is known from US Pat. No. 4,520,361, in which test probes are introduced into each individual waveguide. It has however, it has been shown that the reproducibility of the measurements with the aid of test probes in the case of phased array antennas with 6-bit resolution does not give satisfactory results. Such an antenna could be calibrated better if one knew its aperture assignment according to amount and phase. An integral monitor waveguide is used to obtain the aperture assignment of a phase-controlled group antenna. In an integral monitor waveguide, signal components from each radiator element are coupled in via coupling holes, either shortly before the radiation or immediately after the radiation. The output signal of the integral monitor waveguide corresponds in a first approximation to the course of the far field of the antenna. The course of the far field and the aperture assignment of the antenna are linked to one another by Fourier transformation. The complex aperture assignment of the antenna can therefore be determined from the output signal of the integral monitor waveguide. Known methods use the quadrature method for this. (I / Q converter). In this method, the signal from a local oscillator is mixed with the output signal of the integral monitor waveguide once at an angle of 0 ° and a second time with a phase shift of 90 °. The mixture with 0 ° phase shift provides the real part of the output signal, the mixture with 90 ° phase shift provides the imaginary part of the output signal of the integral monitor waveguide. Subsequent Fourier transformation of the real and imaginary part of the output signal provides the aperture assignment of the antenna. The disadvantage of this process is the use of two mixers.

The object of the invention is to provide a method and a device for reproducibly calibrating phase-controlled group antennas and with an accuracy required for safety. This object is achieved by a method and a device with the combinations of features of the independent claims. The dependent claims contain further developments and refinements of the invention.

The advantages of the method and the device according to the invention are that the antenna can also be calibrated during operation. Another advantage can be seen in the fact that by choosing the Hilbert transformation to obtain the aperture assignment, only one mixer has to be used. This improves the signal / noise ratio of the useful signal.

Fig. 1

Prinzip einer Gruppenantenne mit Integralmonitorhohlleiter,

Fig. 2

einen I/Q-Konverter,

Fig. 3

den prinzipiellen Aufbau eines homodynen Meßsystemes,

Fig. 4

eine Überwachungseinrichtung für eine phasengesteuerte Gruppenantenne.

Fig. 5

eine Regeleinrichtung zur Kalibrierung einer phasengesteuerten Gruppenantenne.

An embodiment of the invention is explained in more detail with reference to Figures 1 to 5. Show it:

Fig. 1

Principle of a group antenna with integral monitor waveguide,

Fig. 2

an I / Q converter,

Fig. 3

the basic structure of a homodyne measuring system,

Fig. 4

a monitoring device for a phased array antenna.

Fig. 5

a control device for the calibration of a phase-controlled group antenna.

1 shows part of a phase-controlled array antenna. With 11 radiators of the antenna are designated. 10 denotes an integral monitor waveguide, into which signal components from each radiator element are coupled via coupling holes. The signal components overlap in the integral monitor waveguide to form a complex, time-dependent signal. The signal components coupled into the integral monitor waveguide are either signal components shortly before the radiation (with azimuth antennas) or immediately after the radiation (with elevation antennas). The signal present at the output 12 of the integral monitor waveguide 10 corresponds in a first approximation to the course of the far field diagram of the antenna. Because of the relationship between the aperture assignment of an antenna and the far field diagram of the same antenna given by the Fourier transformation, the complex aperture assignment can be calculated from the output signal of the integral monitor waveguide.

In known devices, the output signal of the integral monitor waveguide is processed in a manner shown in FIG. 2. In Fig. 2 are designated 20 and 21 mixers, to which signals from hybrids 22 and 23 are supplied. The Hybrid 22 is, for example, a 3 dB-0 ^o hybrid, the Hybrid 23 is a 3 dB-90 ^o hybrid. A signal from a local oscillator is fed to the hybrid 23 via an input denoted by 24. The hybrid 22 is connected via an input labeled 25 Output signal of the integral monitor waveguide supplied. With 26 and 27 RF terminations are referred to, which are also called RF sump. They are used to complete components for high frequency without reflection. The real part is then present at the output of the mixer 20, and the imaginary part of the signal at the input 25 is present at the output of the mixer 21. The device described is called an I / Q converter, the output signals of the two mixers are called quadrature components. The aperture assignment of the antenna is then determined in a further step by Fourier transformation. The device just described needs two mixers to display the complex output signal of the integral monitor waveguide.

3 shows the basic structure of a homodyne measuring system. 30 denotes a mixer, to which signals are fed via lines 35 and 36. The output of the mixer 30 is fed to a low-pass filter 31, at the output 37 of which the desired signal is present. 32 with a transmission element is designated, the complex transfer function is to be determined with the arrangement shown. 33 designates a high-frequency generator, the output signal of which is fed to mixer 30 via line 36. At the same time, the output signal of the generator 33 is coupled into the transmission element 32 via a coupler 34. The aim of the entire arrangement is to obtain the real part of the complex transfer function of the transfer element 32 at the output 37. Assuming that the amount of the signal at input 35 is significantly smaller than the amount of the signal at input 36, that is, the mixer 30 works in linear mode, the following results:

Phase des Monitorsignales
${\text{ψ}}_{\text{R}}{\text{= ω₀ t + α}}_{\text{R}}\text{:}$

Phase des Referenzsignales
φ(t): allgemeine Phasenfunktion des Systems 32
${\text{Δα = α}}_{\text{M}}{\text{- α}}_{\text{R}}$

gilt für eine Spannung U am Ausgang 37 die Beziehung:

A signal A _M reaches the mixer 30 via the line 35. A signal A _R also reaches the mixer 30 via the line 36

${\text{ψ}}_{\text{M}}{\text{= ω₀ t + α}}_{\text{M}}\text{+ φ (t):}$

Phase of the monitor signal
${\text{ψ}}_{\text{R}}{\text{= ω₀ t + α}}_{\text{R}}\text{:}$

Phase of the reference signal
φ (t): general phase function of the system 32
${\text{Δα = α}}_{\text{M}}{\text{- α}}_{\text{R}}$

the following applies to a voltage U at output 37:

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

The real and imaginary parts of the spectrum of complex, causal time functions are connected by an integral transformation, the so-called Hilbert transformation. In other words, it is sufficient to measure the real part of such functions, since the imaginary part can be calculated using the Hilbert transformation.

FIG. 4 shows an antenna of a microwave landing system (MLS system) in which the homodyne measuring method according to FIG. 3 is used to obtain the aperture assignment of the antenna. In the following, the same reference numerals designate the same elements as in the other figures. In FIG. 4, the elements mixer 30, low-pass 31, high-frequency signal source 33 and coupling element 34, which are already known from FIG. 3, are designated. A monitor is designated by 40, for example designed as an integral monitor waveguide, as number 10 in FIG. 1. A network is designated by 41, which distributes the electrical energy originating from the radio-frequency source 33 via phase shifters designated by 42 to antenna elements of the array antenna designated by 43. 43 'denotes the entirety of the radiators and the phase shifters. Signals are coupled from the antenna elements into the integral monitor waveguide 40. The output signal of the integral monitor waveguide is fed to the mixer 30, into which the high-frequency signal coupled in with the aid of the coupler 34 also arrives. The voltage U described in connection with FIG. 3 is available behind the low-pass filter 31. This voltage U is the real part of the output signal of the integral monitor waveguide 40. The voltage U present at the output of the low pass 31 is digitized by means of a sample-and-hold element 44 and an analog / digital converter 45. A time and value discrete signal is thus available at the output of the analog / digital converter 45. This discrete-time and value-discrete signal is converted with the aid of a signal processor 46 by means of the discrete Hilbert transformation to the still missing imaginary part of the output signal of the integral monitor waveguide 40 calculated. 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 back transformation for the aperture assignment of the antenna.

To carry out the discrete Hilbert transform or the discrete Fourier transform and the fast Fourier transform, the person skilled in the field of signal processing is referred to a wealth of specialist literature on this topic, such as on the article "Quadrature Sampling with High Dynamic Range", published in the IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-18. No. 4, November 1982, pages 736 to 739.

FIG. 5 now explains in more detail how the phase-controlled group antenna according to FIG. 4 is calibrated. The same reference numerals designate the same elements as in FIG. 4. The phase-controlled group antenna with its radiators 43 is provided here as a block with the reference numeral 43. The phase shifters also appear as a block with the reference numeral 42. A signal at 50 at the output of the integral waveguide 40 is indicated, which corresponds to the far field of the antenna. This signal 50, which corresponds to the far field of the antenna, is subjected to an integral transformation in a computing unit denoted by 46 'in order to obtain the aperture assignment of the antenna. With 51 a control device is designated, the Output signal of the computing device 46 'is supplied. The setpoint for the phase adjustment of the phase shifter, designated 42, is input via a line labeled 52, to a summation point, designated 53. The output signal of the control device 51 is subtracted from this target value via a line labeled 54. The phase shifter thus receives the difference signal between the setpoint on line 52 and the output signal of the control device 51 via line 54. The computing device 46 ′, which is designated separately in FIG. 4, the control device 51, the summation point 53 and the line with the setpoints 52 can each be implemented as a program after execution in a signal processor. All the steps required to carry out the method can be carried out, for example, in the signal processor 46 in FIG. 4. It is clear from FIG. 5 that a control circuit according to FIG. 5 is assigned to each individual radiator 43 of the phase-controlled group antenna. To adjust the antenna, a comparison is made in a first step between the setpoint and actual value of the aperture assignment. At the same time, correction values are generated by the control device. If a complete agreement between target and actual values cannot be achieved with these correction values, the control parameters are changed (adaptive control circuit) and the process just described is repeated. The process is repeated as a whole until the setpoint and actual value of the aperture assignment only differ from one another within the prescribed tolerance ranges. When carrying out the method, the sampling rate of the monitor signal must be so high that immediate aliasing effects in the reconstructed occupancy function become negligibly small, i.e. significantly above the Nyquist rate.

The aperture assignment is obtained by Hilbert transformation of the output signal of an integral monitor waveguide.

Claims (14)

Iterative method for calibrating a group antenna controlled by phase shifters, in particular for microwave landing systems (MLS), in which first signals corresponding to the far field of the group antenna are derived from an integral waveguide and corresponding second signals are transformed by integral transformation in the aperture assignment of the antenna,
characterized in that the second signals are compared with third signals stored in storage means and a difference signal corresponding to the deviation of the second signals from the third signals is generated, which is fed to a control device whose output signal acts on phase shifters connected to the array antenna. A method according to claim 1, characterized in that the first, second and third signals are discrete-time signals. Method according to claims 1 and 2, characterized in that the fast Fourier transformation (Fast Fourier Transformation (FFT)) is used to generate the aperture assignment. Method according to claim 2, characterized in that a microprocessor is used as the control device. Method according to claim 2, characterized in that a personal computer (PC) is used as the control device. Apparatus for calibrating a phase-controlled group antenna consisting of a plurality of radiators supplied with electronically controlled phase shifters with high-frequency energy, in particular for microwave landing systems (MLS), with an integral waveguide and first means which convert the output signals of the integral waveguide by Fourier transformation into an aperture assignment of the group antenna through storage means for storing a target aperture assignment and comparison means which compare the target aperture assignment with the aperture assignment of the group antenna, and through control means which influence each individual electronic phase shifter depending on the deviation between the target aperture assignment and the aperture assignment of the antenna. Apparatus according to claim 6, characterized by a microprocessor as a control and comparison means. Apparatus according to claim 6, characterized by a PC as a control and comparison means. Method for obtaining a complex aperture assignment of phase-controlled group antennas by Fourier transformation of a time-dependent complex signal originating from an integral monitor waveguide (10),
characterized by the following process steps: a) homodyne detection of the real part of the signal of the integral monitor waveguide b) Formation of the imaginary part of the signal using Hilbert transformation. A method according to claim 9, characterized by the use of the discrete Hilbert transformation. Method according to claims 6 and 10, characterized by the application of the discrete Fourier transform. Device for obtaining a complex aperture assignment of a phase-controlled group antenna, with an integral monitor waveguide (10), at the output of which a complex first signal corresponding to the radiation pattern of the antenna is taken, furthermore with a high-frequency source (33) which drives the group antenna with a carrier frequency f₀ and one with high-frequency energy network (41) distributing the antenna elements, further comprising means for multiplying (30) the first signal by a second signal and a low-pass filter (31) following the means for multiplying,
characterized in that the second signal has the frequency f₀. Apparatus according to claim 12, characterized in that an analog / digital converter (45) following the low pass digitizes the output signal of the low pass. Device according to claims 12 and 13, characterized in that a signal processor (46) following the analog / digital converter subjects the output signal of the analog / digital converter to a Hilbert transformation.

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