EP0664574A1 - Error compensating device for an antenna with electronic scanning - Google Patents

Abstract

The device includes distribution of power (3) to a set of phase shifters (2) and antennae (1) which form a phased array. A monitor (21) measures the state (24) of the phase shifters to detect faulty phase shifters. The array is arranged so that each array phase shifter has a symmetrical phase shifter. In the case of a detected fault, the symmetrical phase shifter is positioned (23) with a 180 degrees phase shift offset from the faulty phase shifter. The technique allows phase array errors due to faulty phase shifters to be nulled.

Description

The present invention relates to a device for compensating for pointing errors caused by breakdowns of phase shifters of electronically scanned antennas or of coefficients of beam-forming antennas by calculation.

It applies in particular to antennas with electronic scanning when one or more phase shifters with electronic control used in the antenna to deflect its beam are broken down, these breakdowns causing a degradation of the pointing accuracy in free space of the beam.

In an antenna with electronic scanning, the pointing of the beam at a given time towards a given direction of space is made by acting on the radiation phase of the radiating sources called elementary sources constituting the antenna. So that the direction of pointing changes are rapid, the modification of the phase of the elementary sources is obtained by the insertion of electronically controlled phase shifters connected in series between a microwave power distributor and the elementary sources. A phase shifter can serve several elementary sources, but the most generally adopted solution is to provide a phase shifter for each elementary source.

The electronic control of the phase shifters is made so that the radiated energy is focused over a long distance in a desired direction. This is done by positioning the different phase shifters in a certain phase state determined in a manner known to those skilled in the art. Chapter 7 of the second edition of Merril I. Skolnick's "Radar Handbook" published by Mac Grawhill editions gives a broad description of the techniques used and their applications to radars.

It is shown and verified in practice that, provided that there is a sufficient number of elementary sources, it is not necessary, to ensure good performance, to have a large number of phase states on the phase shifters. . In practice, the phase shifters are therefore controlled by digital data in the form of messages giving the phase to be displayed on N bits, which corresponds to 2 ^N phase positions theoretically spaced every 360 ° / 2 ^N. The spacing thus corresponds to 45 ° in an example where N = 3 or 22.5 ° in an example where N = 4. Depending on the technology used for the phase shifter, it is economically desirable, especially in the case of diode phase shifters, or indifferent, in particular in the case of ferrite phase shifters, reduce the number N of bits as much as possible. In practice, it is possible to limit oneself to N = 1 to 4.

The failures affecting the phase shifters and the resulting drop in performance constitute a phenomenon which is more suffered than combated. Additions of additional safety margins compared to what would be necessary to maintain performance with all phase shifters in working order allow this to be accommodated. These safety margins are dimensioned such that, with a given number of phase shifters broken down, whatever their distribution in the antenna, the required performance is always maintained.

The known methods for compensating for the effect of phase-shifter failures therefore notably consist in providing performance for the antenna in the absence of failures on the phase-shifters more severe than necessary so that, in the presence of failures on the phase-shifters, the required performance are always obtained. The phase shifters are monitored to determine either permanently or at close intervals whether they are in working order. The number of suspect phase shifters is kept up to date and constantly monitored so as to alert the operator or the services responsible for maintenance when their number approaches or reaches the maximum number that the system can support, without falling below the performance level. required. A maintenance action is necessary to replace the suspect phase shifters.

In the particular case of the MLS system so called from the Anglo-Saxon expression "Microwave Landing System", relating to terminal guidance for landing at airports, using antennae with electronic scanning and for which the pointing accuracy is a priority feature, a patent of the United States of America US-4,041,501 describes a particular embodiment of an MLS antenna with electronic scanning, then another patent US-4,359,740 describes a means for canceling the pointing error caused on such an antenna by the failure of a phase shifter. The invention described in this last patent applies to diode phase shifters using as a 0-180 ° cell a 3 dB coupler connected to two switching diodes controlled independently of one another. As soon as a phase shift fault is detected, the two diodes of the 0-180 ° cell are switched to two separate states. One is then passing, therefore in a low impedance state, possibly capacitive. The result is that the microwave signal which crosses the phase shifter is then cut or rather strongly attenuated. Therefore, it no longer participates in the radiation of the antenna and the pointing error in free space of the beam resulting from the failure of the phase shifter in question is canceled. This is a direct result of the fact that by creating an amplitude hole at the level of the corresponding radiating element, which inhibits the effect of phase errors, the antenna pattern is deformed symmetrically and the direction of maximum radiation remains unchanged.

These methods have several drawbacks. Methods requiring a margin on the required performance require an expensive design. The associated additional cost is directly linked to the level of performance sought and the number of failures that the system can accept. This additional cost can be considerable if the level of performance required is high or if the number of failures to be tolerated is significant.

The method described in US Pat. No. 4,359,740 provides a solution only in the case where the phase shifter is with diodes and uses a 0-180 ° cell comprising two diodes operating by reflection of the high frequency signal. On the other hand, it is ineffective when the device for cutting the high-frequency signal is itself broken down.

The object of the invention is to overcome the aforementioned drawbacks, in particular by making it possible to cancel the pointing error in free space due to one or more breakdowns of phase shifters without it being necessary to complicate the latter.

To this end, the subject of the invention is a device for compensating for pointing errors caused by failures of phase shifters of a plane electronic scanning antenna having a symmetrical amplitude power distribution, characterized in that it comprises means positioning a faulty phase shifter and its symmetrical phase shifter in additional phase states.

The main advantages of the invention are that it frees the antenna design from the constraints of the effect of breakdowns on pointing accuracy, that it applies to all types of electronic phase shifters, that it does not require incorporate in the phase shifter means to prevent it from radiating when it breaks down, that it allows flexibility of use, that it is simple to implement and that it is economical.

• la figure 1, la constitution d'une antenne à balayage électronique plane ;
• la figure 2, un synoptique de l'implantation de moyens constitutifs du dispositif selon l'invention ;
• la figure 3, un exemple d'antenne active susceptible d'utiliser un dispositif selon l'invention.

Other characteristics and advantages of the invention will become apparent from the following description given with reference to the accompanying drawings which represent:

• Figure 1, the constitution of a planar electronic scanning antenna;
• Figure 2, a block diagram of the layout of the means of the device according to the invention;
• Figure 3, an example of active antenna capable of using a device according to the invention.

Figure 1 shows the constitution of a planar electronic scanning antenna. It includes radiating elementary sources 1. In order for changes in pointing direction to be rapid, the phase modification of the elementary sources 1 is obtained by means of electronically controlled phase shifters 2, mounted in series between a microwave power distributor 3 and the elementary sources 1. A phase shifter 2 is for example associated with each elementary source 1. The input of the power distributor 3 is for example connected to the output of a power transmitter 4.

The principle of the invention exploits a particular formulation of the pointing error which shows that there are other means for canceling the pointing error caused by the failure of a phase shifter than preventing it from radiating in particular.

où n est le numéro d'une source élémentaire, A_n l'amplitude du signal qu'elle rayonne, λ la longueur d'onde dans l'air et x_n son abscisse et où φ_n représente l'erreur de phase sur le déphaseur associé à la source élémentaire numéro n.On a plane electronic scanning antenna, comprising N aligned sources, the amplitude of the electric field resulting at great distance in the direction of direction cosine u₀ is, when the antenna is positioned to point in the direction of direction cosine u, given in free space by the following relation:

where n is the number of an elementary source, A _n the amplitude of the signal it radiates, λ the wavelength in air and x _n its abscissa and where φ _n represents the phase error on the phase shifter associated with the elementary source number n.

annulation réalisée pour u₀=u+δ _u où δ _u représente l'erreur de pointage.If the phase shifter in question works correctly, then φ _n = 0 with errors near completion and quantification. The pointing direction obtained is obtained by searching for the maximum radiation as a function of the director cosine u₀, which is obtained by canceling the derivative of the function | E _u ( u ₀) | ², that is: $$\frac{\mathit{\text{d}}\text{|}{\mathit{\text{E}}}_{\mathit{\text{u}}}\text{((}\mathit{\text{u}}\text{₀) | ²}}{\mathit{\text{of}}\text{₀}}\text{= 0}$$

cancellation made for u ₀ = u + δ _u where δ _u represents the pointing error.

• le nombre de déphaseurs en panne est petit devant le nombre de déphaseurs, inférieur à 10% par exemple,
• le nombre total de déphaseurs mis en oeuvre est suffisamment grand pour que l'hypothèse précédente ait un sens, ce nombre total est par exemple supérieur ou égal à 20.

The calculation of the pointing error δ _u is relatively complicated in the general case but is considerably simplified under the following assumptions, which are carried out in most cases of practical application:

• the number of phase shifters broken down is small compared to the number of phase shifters, less than 10% for example,
• the total number of phase shifters used is large enough for the preceding hypothesis to make sense, this total number is for example greater than or equal to 20.

où P représente l'ensemble des numéros de déphaseurs en panne.Once these assumptions have been taken into account, the result of the calculation is a pointing error δ _u given by the following relation:

where P represents the set of faulty phase shifter numbers.

It appears clearly in this relation that the phase shifters which are not faulty do not participate in the error δ _u and that this error is canceled when no phase shifter is faulty.

étant entendu que l'antenne a une distribution symétrique, c'est-à-dire dans laquelle les sources élémentaires situées symétriquement, aux tolérances mécaniques près, par rapport au centre rayonnent avec des amplitudes identiques et des phases opposées, aux erreurs de réalisation près.The principle of the invention consists in canceling the pointing error δ _u not by canceling the amplitudes A _p but by canceling the term at numerator of relation (3), i.e. the term

it being understood that the antenna has a symmetrical distribution, that is to say in which the elementary sources located symmetrically, except for the mechanical tolerances, with respect to the center radiate with identical amplitudes and opposite phases, with the errors of realization near .

et

For this antenna, it is then possible to reformulate the relation (1) giving the field by the following relations:

and

et sa source symétrique, d'abscisse x _-n =-x _n rayonne un signal d'amplitude A _-n =A _n avec une phase opposée :2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{-n}}}}{\text{λ}}$$

u=-2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{n}}}}{\text{λ}}$$

u.If in addition, the origin of the abscissa is located at the center of symmetry, then each abscissa source x _n radiates a signal of amplitude A _n with, to point in the direction of the director cosine u, a phase equal to 2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{not}}}}{\text{λ}}\mathit{\text{u}}$$

and its symmetrical source, of abscissa x _-n = -x _n radiates a signal of amplitude A _-n = A _n with an opposite phase: 2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{-not}}}}{\text{λ}}$$

u = -2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{not}}}}{\text{λ}}$$

u .

The device according to the invention comprises means enabling it, as soon as a phase shifter has failed, to put the symmetrical phase shifter in a state at any additional time 2π near the state in which the phase shifter is found to be faulty. Thus, if the faulty phase shifter is in the phase state φ, its symmetrical phase shifter is positioned in the phase state π-φ or 3π-φ, or more generally (2 k +1) π-φ, k being an integer relative number.

In a first possible embodiment, the faulty phase shifter is for example blocked in a fixed phase state φ₀ and kept in this state permanently. Its symmetrical phase shifter is then blocked in the additional phase state, π-φ₀ or 3π-φ₀ for example, and permanently maintained in this phase state.

In a second possible embodiment, the phase states remaining available on the faulty phase shifter continue, for example, to be used and the symmetrical phase shifter is positioned at all times in the additional phase state.

The phase states of the two phase shifters therefore vary with the antenna beam, while remaining additional to within 2π.

avec $${\text{φ}}_{\mathit{\text{p}}}\text{=φ₀-2π}\frac{{\mathit{\text{x}}}_{\mathit{\text{p}}}}{\text{λ}}\mathit{\text{u}}$$

The pointing errors resulting from the fault are thus canceled and this without resorting to the cancellation of the signal radiated by the faulty phase shifter. In fact, the pointing error resulting from the failure of a phase shifter supplying a source p is given by the following relation:

with $${\text{φ}}_{\mathit{\text{p}}}\text{= φ₀-2π}\frac{{\mathit{\text{x}}}_{\mathit{\text{p}}}}{\text{λ}}\mathit{\text{u}}$$

avec $${\text{φ}}_{\mathit{\text{-p}}}\text{=(π-φ₀)-2π}\frac{{\mathit{\text{x}}}_{\mathit{\text{-p}}}}{\text{λ}}\mathit{\text{u}}$$

ou $${\text{φ}}_{\mathit{\text{-p}}}\text{=(3π-φ₀)-2π}\frac{{\mathit{\text{x}}}_{\mathit{\text{-p}}}}{\text{λ}}\mathit{\text{u}}$$

De x _-p =-x _p il résulte que : φ _-p =π-φ₀+2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{p}}}}{\text{λ}}$$

u=π-φ _p ou φ _-p =3π-φ₀+2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{p}}}}{\text{λ}}$$

u=3π-φ _p , ce qui donne dans les dans les deux cas sin φ _-p =sin φ _p .
Comme A _-p =A _p et x _-p =-x _p , il s'ensuit que : $${\mathit{\text{A}}}_{\mathit{\text{-p}}}{\mathit{\text{x}}}_{\mathit{\text{-psin}}\text{φ}\mathit{\text{-p}}}\text{=-}{\mathit{\text{A}}}_{\mathit{\text{p}}}{\mathit{\text{x}}}_{\mathit{\text{p}}}\mathit{\text{sin}}{\text{φ}}_{\mathit{\text{p}}}$$

et donc que δu _-p =-δu _p Once the faulty phase shifter is blocked in a phase state φ₀ and its symmetrical phase shifter in an additional phase state, π-φ₀ or 3π-φ₀ for example, it appears that the pointing error resulting from the blocking in the additional fixed state, φ₀-π or 3π-φ₀ for example, of the symmetrical phase shifter is equal and opposite to the error caused by the fixed state φ₀ of the phase shifter inoperative, regardless of the pointing direction and the radiated frequency . Indeed, the phase-shifter in failure, once blocked in the chosen state φ₀ produces an error given by the relation (5) and the symmetrical phase-shifter, once blocked in the state π-φ₀ or 3π-φ₀ for example, produces an error δ _{u -p} given by the following relation:

with $${\text{φ}}_{\mathit{\text{-p}}}\text{= (π-φ₀) -2π}\frac{{\mathit{\text{x}}}_{\mathit{\text{-p}}}}{\text{λ}}\mathit{\text{u}}$$

or $${\text{φ}}_{\mathit{\text{-p}}}\text{= (3π-φ₀) -2π}\frac{{\mathit{\text{x}}}_{\mathit{\text{-p}}}}{\text{λ}}\mathit{\text{u}}$$

From x _-p = - x _p it follows that: φ _-p = π-φ₀ + 2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{p}}}}{\text{λ}}$$

u = π-φ _p or φ _-p = 3π-φ₀ + 2π $$\frac{{\mathit{\text{x}}}_{\mathit{\text{p}}}}{\text{λ}}$$

u = 3π-φ _p , which in both cases gives sin φ _-p = sin φ _p .
As A _-p = A _p and x _-p = - x _p , it follows that: $${\mathit{\text{AT}}}_{\mathit{\text{-p}}}{\mathit{\text{x}}}_{\mathit{\text{-psin}}\text{φ}\mathit{\text{-p}}}\text{= -}{\mathit{\text{AT}}}_{\mathit{\text{p}}}{\mathit{\text{x}}}_{\mathit{\text{p}}}\mathit{\text{sin}}{\text{φ}}_{\mathit{\text{p}}}$$

and therefore that δ u _-p = -δ u _p

Consequently, the addition of errors δu _-p and δu _p produces a zero global error, whatever the direction cosine u, therefore the pointing of the beam, and whatever the wavelength λ of the radiated signal.

In the case of the second possible embodiment mentioned above, where the phase shifter is not locked in a fixed phase state but where its available phase states continue to be used, the command displayed at a given instant may be the same as in the absence of failure or then take into account the type of failure detected to, for example, best approximate the desired state by means of the phase states remaining available. In both cases, the phase φ _A actually obtained is a function of the command displayed and of the fault affecting the phase shifter. The symmetrical phase shifter is then not locked in a fixed state but receives at all times a command which puts it in an additional phase φ _B of phase φ _A carried out on the phase shifter inoperative, that is to say that φ _B = π-φ _A or 3π-φ _A for example.

The device according to the invention can for example be used for beating beam antennas of an MLS landing system. In this case, each of the two antennas of the system, the azimuth antenna and the antenna elevation, radiates a fan-like beam, that is to say angularly narrow in one dimension and wide in the other. The spatial scanning of these beams is carried out by electronic scanning, at the speed of 50 microseconds per degree for example. The device according to the invention makes it possible to obtain an improved MLS system in terms of pointing accuracy, and therefore of guiding precision of aircraft, by the fact that pointing errors of the beating beam antennas caused by breakdowns on the electronically controlled phase shifters are compensated for by the device according to the invention.

An azimuth electronic scanning antenna of an MLS system is for example constituted by a planar network of regularly spaced radiating guides, supplied through a power distribution and connected to the output of a transmitter. The antenna comprises for example N radiating guides, N being even. These guides are for example numbered from 1 to N / 2 for the right part and from -1 to -N / 2 for the left part. The guides numbered n and -n are symmetrical with respect to the plane of symmetry of the antenna and the power distribution is also symmetrical.

Each radiating guide is associated with a 4-bit phase shifter, for example. There are therefore N phase shifters each capable of taking 2⁴ = 16 phase states from 0 ° to 337.5 ° in steps of 22.5 °. These phase shifters include, for example, a succession of four phase shift cells, 180 °, 90 °, 45 ° and 22.5 °, respectively. Each cell uses for example two diodes which are controlled either in the on state or in the blocked state. Two diodes of the same cell receive identical control signals at all times, except in the event of a fault, that is to say that they are both positioned at all times, ie in the on state, which corresponds to a first phase state, ie in the blocked state, which corresponds to the second phase state of the cell.

The device according to the invention comprises for example means for permanently monitoring the state of the diodes. The state of the diodes is for example monitored via the control circuit by the value of the current in the control line and by the value of the voltage on the control line, each diode having a control line. Indeed, a normally functioning diode consumes in the passing state a significant current under a low voltage and in the blocked state a quasi-zero current under a voltage of several volts. The failure of a diode puts it either in open circuit, which is an infrequent case, or in short circuit, which is the general case. The control circuit self-monitors, for example, itself and transmits to the control system of the phase shifters, called a pointer, the overall state of the control circuit and of the phase shifters. The device according to the invention is for example located in this pointer. It shares, for example, hardware circuits of the latter and includes, for example, software installed in addition to the pointer software.

FIG. 2 illustrates, by way of example and by means of a block diagram, the installation of a device according to the invention in a pointer 21. A bus 22 comprising all the control lines for the diodes of the phase shifters 2 connects by these lines the phase shifters 2 to the pointer 21. The latter includes for example the device according to the invention which is at least made up of means 23 for putting a phase shifter out of order and its symmetrical phase shifter in additional phase states. It further comprises, for example, means 24 for reading the state of the phase shifters. These states are obtained for example in the aforementioned manner by comparing the currents and voltages of the phase-shifting diodes with the positioning orders conveyed by the control lines of the bus 22. The other functions of the pointer 21, known to those skilled in the art trade are not represented. The phase shifters 2 are for example equipped with means 25 for establishing their state, these means acting for example by comparing the diode voltages with control voltages conveyed by the bus 22.

To treat the case where a detected fault concerns a single diode of a phase cell comprising two diodes used in a phase shifter, a processing of the other diode of the cell can be carried out, so that the failure of only one diode only causes blocking in one of the two normal states of the phase cell of which the diode is a part. The second cell diode can for example be placed in the same microwave impedance state as the faulty diode by means for controlling the state of the diodes of the device according to the invention. So for example, if the faulty diode is in open circuit, the second diode is put in the blocked state. The two diodes then have the same impedance state, open circuit, for a high frequency signal as if they were both in working condition and blocked. In this way, any failure of a single diode results in the blocking of the phase cell of which it is part in one of its two normal states.

The device according to the invention can be applied not only to a passive antenna supplied through a power distribution and diode phase shifters from a centralized transmitter, but it can also be applied to an active antenna incorporating the transmission function such as illustrated in FIG. 3. In this case, a plurality of amplifiers 31 connected to the outputs of a low level distribution 32 feeds the different radiant elementary sources. The phase shifters 2 are this time for example placed upstream of the amplifiers 31 and possibly integrated therein, the whole being able for example to be produced in the form of an integrated circuit.

By canceling the pointing error linked to the failure of one or more phase shifters or their control, the device according to the invention frees the design of the antennas from the constraints resulting from the effect of the failures on the pointing accuracy. It therefore allows, by eliminating a major cause of imprecision, a simpler and therefore less costly design. It can also apply to all types of phase shifters with electronic control whatever the technology used, it applies in particular to phase shifters with ferrites, phase shifters with diodes, or phase shifters in MMIC technology, these initials coming from the Anglo-Saxon expression "Microwave Monolithic Integrated Circuits". In addition, it is not necessary to incorporate means into a phase shifter to prevent the latter from radiating when it is out of order. This advantage is important because the phase shifter is an element used in numerous copies in an antenna with electronic scanning and represents, consequently, a significant part of the cost of the antenna. Any increase in the complexity of the phase shifter therefore has a negative impact on the cost of the antenna. The device according to the invention also has flexibility of use due to the fact that there are a large number of solutions for choosing the phases φ₀ and π-φ₀, or 3π-φ₀ for example, which is equivalent, in which the faulty phase shifter and its symmetrical phase shifter are respectively blocked. It follows that even in the case where symmetrical phase shifters each have a bit blocked in one or the other of their two phase states, as a result of a failure, there remain enough degrees of freedom to be able, in the almost all cases, put the two phase shifters in phase states fulfilling the condition sought by the single action on the remaining bits.

The device according to the invention can also be transposed to the case of an antenna with beam formation by calculation provided that this antenna is planar and symmetrical and provided that the laws of calculation of the beams are symmetrical in amplitude and in phase. In this case, the invention consists when one of the phase values φ₀ to be used in the calculation of a given radiation pattern is designated as erroneous or even only suspect, to set, for this radiation pattern, the value of phase concerning the symmetrical radiating source in the additional phase state. In this way, the pointing error linked to the erroneous phase value φ₀ is canceled.

Claims (10)

Device for compensating for pointing errors caused by breakdowns of phase shifters (2) of a plane electronic scanning antenna having a symmetrical power distribution in amplitude and in phase, characterized in that it comprises means (23) positioning a faulty phase shifter and its symmetrical phase shifter in additional phase states. Device according to claim 1, characterized in that the additional phase states in which the faulty phase shifter and its symmetrical phase shifter are fixed states which do not change as a function of the pointing of the antenna beam. Device according to claim 1, characterized in that the phase states remaining available on the faulty phase shifter continue to be exploited and the symmetrical phase shifter is positioned at all times in the phase state additional to that of the faulty phase shifter. Device according to any one of the preceding claims, characterized in that it comprises means (24) for monitoring the state of the diodes of the phase shifters (2). Device according to claim 4, characterized in that the phase shifters (2) being made up of phase cells each containing two diodes, it comprises means for controlling the state of the diodes so that a diode of a cell being faulty, the second diode is put in the same microwave impedance state as the faulty diode. Device according to any one of the preceding claims, characterized in that the electronic scanning antenna is active, microwave amplifiers (31) being connected in series between the phase shifters (2) and the elementary radiating sources (1) of the antenna. Device according to claim 6, characterized in that a phase shifter (2) and a microwave amplifier (31) are integrated in the same circuit. Device according to any one of the preceding claims, characterized in that the antenna belongs to an MLS type landing system. Device according to any one of the preceding claims, characterized in that it is integrated in the antenna pointer. Device for compensating for pointing errors caused by values which have become erroneous of the calculation coefficients of the phases of a beam-forming antenna by calculation with a plane and symmetrical structure and using symmetrical laws of calculation in amplitude and in phase, characterized in what it includes means positioning the erroneous phase coefficient and the phase coefficient relating to the symmetrical radiating source in additional phase states.

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