EP0254649A1 - Process and device for compensating the frequency dispersion in an electronically scanned antenna and its use in a landing system of the MLS type - Google Patents

Abstract

Method and device for correcting as a function of the frequency of the aiming error in an electronically scanned antenna, in the context of a MLS-type landing-aid system. The method consists in compensating the aiming error by modifying the beam-scanning rate as a function of the transmission frequency and, in a variant, also as a function of the angle of aim. <IMAGE>

Description

The subject of the present invention is a method and a device for compensating the frequency dispersion of an electronic scanning antenna, more particularly intended for an MLS type landing aid system.

It is recalled that the MLS system (initials of the Anglo-Saxon expression "Microwave Landing System") is a system making it possible to assist an airplane in landing by providing it with its position, in coordinates spherical in a coordinate system linked to the track:
- its azimuth angle;
- its elevation angle;
- possibly other additional information, such as rear azimuth;
- a certain number of data;
- distance information, provided by an autonomous system called DME (for "Distance Measuring Equipment").

This different information, called "functions", is permanently transmitted from the ground in time multiplexing on the same frequency, close to 5 GHz, according to characteristics standardized by the ICAO (International Civil Aviation Organization), appendix 10 , paragraph 3.11. This information is decoded by each aircraft concerned.

Each of the preceding "functions" is broken down into two parts, issued successively:
- a preamble, the role of which is to provide the aircraft with a identification of the function which will follow; this preamble is transmitted by a so-called "sectoral" antenna, that is to say a fixed antenna transmitting throughout the area, or sector, which the MLS system must cover;
- the function itself:
. in the case where this function is a datum, it is transmitted by the sectoral antenna;
. in the case where this function is angular information, it is transmitted using an antenna with electronic scanning according to the principle known as the beating beam with time reference, described below in FIGS. 1 and 2.

In Figure 1 is illustrated the principle of scanning intended to code, for example, the azimuth angle.

From an azimuth station, according to the above, two different radiations are emitted by two separate antennas which, for simplicity, have been represented at the same point A _Z.

Starting from point A _Z, there is therefore, on the one hand, the emission diagram of the preamble, denoted P _Z , emitted by the sector antenna throughout the coverage area of the MLS system.

From this point A _Z we have, on the other hand, the diagram of a beam B, flat and vertical, said beating beam, emitted by an antenna with electronic scanning. The beam B carries out at a constant angular speed a scanning then, after a stopping time (T _o ), a scanning back and, this, in a scanning zone making an angle 2ϑ _M (+ ϑ _M and - ϑ _M ) on the face. There has also been illustrated, respectively by an arrow A _L and an arrow R, the outward and backward scanning paths of the beam B. We have finally illustrated an airplane A _V , by way of example not correctly aligned with the axis PPʹ of the track, that is to say making an angle ϑ _A with the latter.

In the diagram in FIG. 2a, the successive transmissions from the antennas of the azimuth station are illustrated, as a function of time.

From an instant zero to an instant t _P is therefore transmitted the preamble by the sectoral antenna. Then, from an instant t₁ to an instant t₂, the beam B emitted by the scanning antenna described at an angle 2ϑ _M (beam go A _L ). The value of the angle (ϑ) thus scanned, which goes from -ϑ _M to + ϑ _M during the duration t₁ to t₂, is shown in diagram 2b, still as a function of time.

After a duration T₀, the beam scans the same angle in the other direction, from an instant t₃ to an instant t₄ (diagram 2a), that is to say from the angle + ϑ _M to the angle - ϑ _M (diagram 2b).

. (Δt - T_o)      (1)The signals received on board the aircraft A _V are shown in diagram 2c, again as a function of time. The airplane first receives the preamble, between instants 0 and t _P. It then receives two pulses, marked 1 and 2, at times t₅ and t₆ which correspond to the moments when the beam passes over the airplane A _V , in one direction then in the other. The duration separating the instants t₅ and t₆ (Δt) is characteristic of the angle ϑ _A of azimuth of the airplane A _V :

ϑ _A =

. (Δt - T _o ) (1)

It has been said above that the transmission of the MLS functions takes place on a frequency close to 5 GHz. More specifically, each MLS station has a frequency chosen from a band ranging from 5.03 to 5.09 GHz.

As is known, the electronic scanning antenna is made up of a large number of radiating sources and it is generally calculated by the manufacturer at the central frequency of the band, namely 5.06 GHz. When the emission frequency is different from this center frequency, the real aiming angle of the beam is different from the angle calculated for the center frequency, and it turns out that the error thus made is greater than the errors tolerated by ICAO standards. This error must therefore be corrected.

One possible correction solution consists in recalculating the phase shifts to be made to each of the radiating sources for each emission frequency. This solution is generally used in electronically scanned radars, since these usually include calculation means which can also recalculate the phase shifts. In the case of the MLS system, we does not have such means of calculation; the values of the phase shifts are usually stored in read only memories and changing the values of the phase shifts involves replacing the memory. This is obviously a drawback, particularly in terms of flexibility and cost.

The subject of the present invention is a method making it possible to compensate for this error by modifying the scanning speed (v) of the beam; in this way, the measured time Δt is modified (see expression (1) above) and, consequently, the corresponding value of the angle ϑ is corrected.

According to the various embodiments of the invention, the scanning speed can be modified by taking into account only the transmission frequency or by taking into account both the transmission frequency and the angle scanned.

• - la figure 1, déjà décrite, illustre le principe du codage de l'angle azimut par un faisceau battant ;
• - les figures 2, a à c, déjà décrites, sont des diagrammes explicatifs relatifs à la figure précédente ;
• - les figures 3, a et b, illustrent schématiquement le principe de correction selon l'invention ;
• - la figure 4 est une courbe explicative ;
• - la figure 5 est un schéma synoptique d'une station MLS ;
• - la figure 6 représente un mode de réalisation du dispositif selon l'invention ;
• - la figure 7 est un schéma explicatif ;
• - la figure 8 représente un autre mode de réalisation du dispositif selon l'invention.

Other objects, features and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended drawings in which:

• - Figure 1, already described, illustrates the principle of coding the azimuth angle by a beating beam;
• - Figures 2, a to c, already described, are explanatory diagrams relating to the previous figure;
• - Figures 3, a and b, schematically illustrate the principle of correction according to the invention;
• - Figure 4 is an explanatory curve;
• - Figure 5 is a block diagram of an MLS station;
• - Figure 6 shows an embodiment of the device according to the invention;
• - Figure 7 is an explanatory diagram;
• - Figure 8 shows another embodiment of the device according to the invention.

In these different figures, the same references relate to the same elements.

It was indicated above (expression 1) that the angle measurement (ϑ _A ) carried out by the aircraft is reduced to a measurement of time: the angle is a linear function of time Δt which separates the two receptions ( pulses 1 and 2, FIG. 2c) of the beam B. According to the invention, the frequency dispersion of the antenna is compensated for by the modification of the scanning speed (v) of the beam, as illustrated in FIGS. 3.

In FIG. 3a, the preceding diagram 2b is reproduced in dotted lines, illustrating the angle values scanned as a function of time. Similarly, in FIG. 3b, the signals received on board the aircraft (pulses 1 and 2) are reproduced in dotted lines as illustrated in FIG. 2c.

In diagram 3a, the effects of a change in the scanning speed of the beam B (modified speed but constant over the entire coverage) are also shown in solid lines: the outgoing beam covers the angle 2ϑ _M of a time t _1c at time t _2c ; in the same way, the return beam covers the same angle for the same duration, going from an instant t _3c to an instant t _4c . In diagram 3b, there is also shown in solid line a pulse 1 _c corresponding to the passage of the outgoing beam and a pulse 2 _c corresponding to the passage of the return beam, at times respectively t _5c and t _6c . The duration which separates these two pulses 1c and 1b is noted Δt _c . It is less than the preceding duration Δt, for the same angle ϑ _A. It therefore appears that the same duration Δt then corresponds to an angle different from ϑ _A (lower in the case of FIGS. 3).

According to one embodiment of the invention, to vary the scanning speed of the beam, the clock frequency of the logic circuit controlling the scanning of the beam is varied.

avec :
    - dϑ' exprimé en radians ;
    - f : fréquence de l'onde émise ;
    - f_o : fréquence centrale de la bande de fréquence des émissions MLS (5,06 GHz), avec : f = f_o + df.
Démonstration :
On sait en effet qu'à la phase (Δφ) du n^ème déphaseur correspond un angle de pointage ϑ de la manière suivante :
    Δφ = 2π.

. sin ϑ      (3)
avec :
    - L : distance du n^ème déphaseur au centre de l'antenne ;
    - λ : longueur d'onde correspondant à la fréquence d'émis­sion (f).
A phases constantes, on peut calculer l'erreur dϑʹ commise sur l'angle ϑ lorsque la fréquence d'émission varie de f_o à f = f_o + df en opérant la dérivée logarithmique de l'expression (3) :

d'où :

d'où il vient l'expression (2) mentionnée plus haut.First, we show that the error (noted dée) on the angle ϑ that the beam B makes with the axis PP l'axe of the track, or pointing angle, is written:

with:
- dϑ 'expressed in radians;
- f: frequency of the transmitted wave;
- f _o : center frequency of the frequency band of MLS transmissions (5.06 GHz), with: f = f _o + df.
Demonstration:
We indeed know that to the phase (Δφ) of the n ^th phase shifter corresponds a pointing angle ϑ in the following manner:
Δφ = 2π.

. sin ϑ (3)
with:
- L: distance from the n ^th phase shifter to the center of the antenna;
- λ: wavelength corresponding to the emission frequency (f).
At constant phases, we can calculate the error dϑʹ made on the angle ϑ when the emission frequency varies from f _o to f = f _o + df by operating the logarithmic derivative of expression (3):

from where :

whence comes the expression (2) mentioned above.

Démonstration :
A la fréquence centrale (f_o), on a :
    ϑ_o = v_o.t
avec :
    - ϑ_o : angle variable dans le temps, correspondant à f = f_o ;
    - v_o : vitesse angulaire déterminée pour f = f_o.
A la vitesse v_o est liée une valeur F_o de la fréquence F par la relation :
    v_o = K. F_o      (5)
A une fréquence d'émission f quelconque, on a :
    ϑ = v_o.t + dϑʹ
Les expressions (2) et (5) ci-dessus permettent d'écrire :

Si on fait varier la fréquence d'horloge, qui s'écrit alors F, avec F = F_o + dF, on peut écrire :

ou encore, en remplaçant K (expression 5):

A partir de cette dernière expression, on peut exprimer l'erreur dϑ commise sur ϑ par rapport à la loi idéale ϑ_o = v_o.t ci-dessus (avec dϑ = ϑ - ϑ_o), en fonction des fréquences F et f :

ce qui peut encore s'écrire :

En négligeant dF/F_o devant 1, on obtient l'expression (4) mentionnée plus haut.If we now express the angle ϑ as a function of the angular scanning speed (v), or also as a function of the frequency (F) of the clock signal of the logic circuit, which corresponds to it with a proportional constant near (K ), we obtain :

Demonstration:
At the center frequency (f _o ), we have:
ϑ _o = v _o .t
with:
- ϑ _o : variable angle over time, corresponding to f = f _o ;
- v _o : angular speed determined for f = f _o .
At the speed v _o is linked a value F _o of the frequency F by the relation:
v _o = K. F _o (5)
At any emission frequency f , we have:
ϑ = v _o .t + dϑʹ
The expressions (2) and (5) above allow to write:

If we vary the clock frequency, which is then written F, with F = F _o + dF, we can write:

or again, by replacing K (expression 5):

From this last expression, one can express the error dϑ committed on ϑ compared to the ideal law ϑ _o = v _o .t above (with dϑ = ϑ - ϑ _o ), according to the frequencies F and f :

which can still be written:

By neglecting dF / F _o before 1, we obtain the expression (4) mentioned above.

Il ressort de cette dernière formule que la correction de la fréquence de l'horloge de balayage, pour annuler l'erreur dϑ, est fonction de la fréquence d'émission f mais également de l'angle ϑ de pointage du faisceau.To cancel the error dϑ, it is therefore necessary that the variation dF of the frequency of the scanning clock obeys the following relation:

It appears from this latter formula that the correction of the frequency of the scanning clock, to cancel the error dϑ, is a function of the transmission frequency f but also of the beam pointing angle l'angle.

According to the invention, this correction is made, in a first, simplified embodiment, solely as a function of the transmission frequency (f). In a second, more elaborate embodiment, the correction is made both as a function of the frequency f and as a function of the angle ϑ.

In the first embodiment, a correction is therefore made only as a function of the transmission frequency f .

On peut alors écrire l'expression de l'erreur δ de pointage en fonction de l'angle ϑ, pour la valeur de dF₁ ci-dessus :

For a given angle ϑ = ϑ₁, according to formula (6) above, we obtain a corresponding value dF₁ of dF which is written:

We can then write the expression of the pointing error δ as a function of the angle ϑ, for the value of dF₁ above:

FIG. 4 shows the variation of the angle ϑ over time:
- in dotted lines, the theoretical curve: linear variation;
- in solid line, the real variation: the error δ is zero for ϑ equal to zero; it then goes through a maximum, noted δ _{1 MAX} ; it is canceled for the value ϑ₁ then increases again for the higher values of the angle ϑ, up to δ _{2 MAX} for ϑ _M. In one embodiment, the value ϑ₁ of the angle ϑ is chosen so that the values δ _{1 MAX} and δ _2MAX are substantially the same. In other words the value ϑ₁ is a mean value which minimizes the error δ.

In another embodiment, since the ICAO standards admit a larger error for large angles ϑ than for small angles, a value is chosen for ϑ₁, which minimizes the error δ for small angles, while retaining the value δ _2MAX below the ICAO limits.

Before describing the implementation of this first correction mode, FIG. 5 recalls the general organization of an angle station of an MLS system, azimuth or site for example.

This station essentially comprises a transmitter 10, two antennas: a sectoral antenna 30 and an antenna with electronic scanning 40, and control circuits (20 and 50).

The transmitter 10 conventionally comprises, in cascade:
- a frequency generator providing a carrier frequency of approximately 5 GHz according to the ICAO standard;
a phase modulator, performing a DPSK phase modulation with two states (0, π) making it possible to transmit the preamble and the data on command from a logic control device 50, such as a microprocessor;
- an on / off control device, also controlled by the microprocessor 50;
- a power transmitter.

The transmitter 10 supplies a signal, via the switch 20, either to the sector antenna 30 for the transmission of the preamble and the data, or to the scanning antenna 40.

The latter is typically broken down into:
a power divider 41, dividing the power received from the switch 20 into N;
- N digital phase shifters 42, supplied by the divider 41;
- N radiating elements 43, supplied by the preceding phase shifters;
a scanning logic circuit 44, supplying the phase shifters 42 with the values of the phase shifts to be introduced, in order to carry out an electronic scanning from static radiating elements.

FIG. 6 shows an embodiment of the scanning logic circuit (44) carrying out the compensation according to the invention.

It will be recalled that the scanning logic circuit controls the phase shifters 42 so as to produce a succession of pointing of the lobe, adjacent to each other, thus simulating continuous scanning.

The phase shifters 42 are for example 4-bit digital phase shifters; in this case, they each allow a phase shift between 0 and 360 °, in steps of 22.5 °. The positions of the N phase shifters for successive pointing (not usual: 0.1 to 0.2 °) are calculated beforehand and generally stored in a PROM type read-only memory (446 in FIG. 6), in the form of words of 4 bits. Each of the N phase shifters is identified by an address and the function of the scanning logic 44 is to successively supply the values of the phase shifts (data bus 447) with their respective addresses (bus addresses 448) to the phase shifters 42.

To this end, the block 44 further comprises:
- a clock generator 441, which has the function of supplying the frequency F;
a sequencing logic circuit 442, controlling the triggering and the operating mode of a set of counters 443, controlled by the device 50 for controlling the station;
- the assembly 443 comprising two counters:
. an up-down counter 445 for pointing the lobe, receiving the frequency F and which provides at all times the value of the pointing of the lobe; the latter is used by the memory 446 as the address (partial, see below) of a datum which it supplies (bus 447) to the phase shifters 42;
. an up-down counter 444 for the addresses of the phase shifters: for a given pointing of the lobe (information provided by the counter 445), it allows the successive addressing of the N phase shifters; its frequency is therefore N times greater than that of counter 445; for this purpose, it also receives the frequency F.

According to the invention, the clock signal generator 441 supplies a clock signal whose frequency F depends on the transmission frequency f . In an alternative embodiment, shown in FIG. 6, the various values of the possible transmission frequency f are grouped together to simplify, so as to minimize the number of possible clock frequencies; for example, we can choose twenty groups of ten frequencies f each. The clock generator 441 is conventional, except that it is capable of supplying different frequencies F on the control of the frequency f or of the selector 440; it can be constituted for example by a quartz clock, where the single quartz is replaced by a set of switchable quartz under the previous order; it is also possible to use a logic synthesizer: the frequency F can be selected for example by coding wheels.

As said above, expression (6) shows that the variation of the frequency F which cancels the error dϑ is also a function of the angle ϑ. In a second embodiment of the invention, dF is therefore varied not only as a function of the transmission frequency f but also as a function of the angle ϑ, as illustrated in FIG. 7.

qui correspond à l'expression (7) ci-dessus pour l'intervalle I_i et l'angle ϑ_Ii, avec i variant de 1 à n.FIG. 7 represents, in a similar manner to that of FIG. 4, the variation of the pointing angle ϑ as a function of time. The theoretical curve is, as before, shown in dotted lines and the actual curve, in solid lines. The latter is in the form of a succession of segments I₁, I₂ ... I _n . Indeed, it appears in this figure 7 that we no longer choose an average angle ϑ₁ as in the first embodiment but intervals I₁, I₂ ... I _n . For each interval I thus defined, an angle ϑ _I is chosen for which the frequency correction dF to be applied for the entire interval is calculated:

which corresponds to expression (7) above for the interval I _i and the angle ϑ _Ii , with i varying from 1 to n .

Pour ϑ = ϑ_Ii , on a :

pour f = f_i, on a :

avec δ = δ₁ + δ₂.The length of the interval I as well as the angle ϑ _I must be chosen so as to minimize the residual pointing error. The calculation of dF is made for a given angular interval (of index i ) and for a given emission frequency interval (denoted f _i ). The residual pointing error, as a function of the angle ϑ and the frequency f is then, for the values ϑ _Ii and f _i :

For ϑ = ϑ _Ii , we have:

for f = f _i , we have:

with δ = δ₁ + δ₂.

To minimize δ, one thus seeks to minimize separately δ₁ and δ₂, experimentally or by computation.

For example, if we limit ourselves to an error in the entire frequency band and the entire system coverage of ± 0.01 °, we can choose δ _{1 MAX} = ± 0.005 ° = δ _{2 MAX} . As a result, one is led to choose frequency groups f of approximately ± 0.5 MHz and to carry out the additional correction as a function of the angle ϑ for angular intervals of ± 3.2 °.

FIG. 8 shows an embodiment of this correction as a function of the transmission frequency and the pointing angle at the level of the scanning logic circuit 44.

In this figure, we find the sequencer 442, the set of counters 443 and the memory 446.

There is also a third counter, marked 451, controlled by the sequencer 442, a clock 449 at fixed frequency, the pulses of which are counted by the three counters, and a correction memory 450.

In operation, the transmission frequency f and the angle ϑ select an address of the memory 450, of the PROM type for example, which contains the loading values (N) of the up-down counter 451, which counts from N to zero at rhythm of the clock 449. When the counter 451 goes to zero, it sends a signal (INHIB) which inhibits the counting of counters 444 and 445 on my clock period. In this way, the scanning speed is varied, the lower the number N, the lower the number.

The angular information (ϑ) is taken at the output of the counter 445 destined for the memory 450.

Furthermore, as before, the different possible frequencies f can be grouped into groups, upstream of the memory 450.

Claims (11)

1. Method for compensating the frequency dispersion of an electronic scanning antenna, characterized in that it consists in modifying the scanning speed (v). 2. Method according to claim 1, characterized in that the scanning being controlled by logic means comprising a clock signal, the scanning speed (v) is modified by modification of the frequency (F) of the clock signal . 3. Method according to one of the preceding claims, characterized in that said modification is carried out as a function of the frequency (f) of the signal emitted by the antenna. 4. Method according to claim 3, characterized in that the transmission frequencies (f) are predefined and grouped into a limited number of groups, said modification being identical for the same group. 5. Method according to one of claims 3 or 4, characterized in that said modification is carried out further depending on the pointing angle (ϑ) of the antenna. 6. Compensation device for the frequency dispersion of an electronic scanning antenna, characterized in that it ensures the implementation of the method according to one of the preceding claims and that it comprises logic means (44) controlling the sweep. 7. Device according to claim 6, characterized in that the logic means (44) comprise means (441) of generation of a clock signal, controlled as a function of the frequency (f) of the signal emitted by the antenna. 8. Device according to Claim 7, characterized in that the generation means (441) comprise a set of switchable quartz as a function of the frequency (f) of the signal emitted by the antenna. 9. Device according to claim 7, characterized in that the generation means (441) comprise a logic synthesizer controlled as a function of the frequency (f) of the signal emitted by the antenna. 10. Device according to claim 6, characterized in that the logic means (44) comprise a clock (449) at fixed frequency, counting means (443) addressing the phase shifters of the antenna, means (451) of correction count and a correction memory (450), the latter receiving the transmission frequency (f) and the pointing angle (ϑ) and supplying the correction counting means (451) with a corresponding value (N) of counting, the correction counting means inhibiting, at the end of counting, the means (443) for counting the phase shifters. 11. Application of the method according to one of claims 1 to 5 to at least one antenna with electronic scanning of an MLS type landing aid system.

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