NL1024532C2 - RF multipath reduction for guided projectiles. - Google Patents

Abstract

This invention relates to a method and the corresponding device for the reduction of the influence of the multipath effect on the own position measurement of a beam guided object and the RF beam guided object control means using it. In guided ammunition control, the use of RF beam guidance induces that the own position measurements of the guided ammunition with respect to the guidance beam are affected by multipath effects in the case of low flying targets. An object of this invention is a method for the reduction of the multipath effect on the own position measurement of a beam guided object comprising the limitation of the variation of the measured position into a predetermined interval [F2, F1].

Description

RF MULTIPATH REDUCTION FOR GUIDED PROJECTILES

The present invention relates to a method and the corresponding apparatus for reducing the multipath (multi-way) effect on the self-position measurement of an object guided by an RF (Radio Frequency, high-frequency) beam, and the control system 5 of the RF beam guided object that uses this.

In RF beam guidance, the beam guided object must follow an RF beam set in the desired direction (beam riding). During the flight, the guided object measures its own position with respect to the RF 10 bundle and translates the measurements into appropriate commands for its own control means. The use of RF beam guidance means that the self-position measurements are influenced by multipath effects when the object moves low above the sea surface.

15 EM (ElectroMagnetic, electromagnetic) energy propagates like waves in the atmosphere and when they hit a surface, reflection occurs. In the case of an object located low above the sea surface, a relatively large part of the EM energy received by the object comes from reflections of the sea surface. The flatter the sea, compared to the RF wavelength, the more energy will be reflected. In the calm sea, the reflected energy has a large average value (reflective reflection); in a less calm sea, a random effect (scattered reflection) will prevail.

Due to differences in path length of the direct (R 1) and reflected (R 2) waves, these will arrive at the object with a (very small) time difference. The result is that this time difference translates into a phase difference. If the phase difference is small, the two together will have a greater value than that from the direct path alone; a phase difference of about 180 degrees can almost lead to extinction. While the difference in distance will vary according to the distance to the target, this interference or mutipath effect will also show a distance-dependent fluctuation. The shorter the wavelength (higher RF frequency), the higher will be the frequency at which these fluctuations occur: more reflective reflection peaks and troughs within a certain range of distances.

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I 2 I

I To enable the guided object to determine its own position in I

I relation to the main direction (axis) of the RF bundle, a number can be prefixed

I certain bundle position deviations are inserted, either simultaneously or in I

I a monopulse system either sequentially. By the bundle pattern I

When modulated, the different bundles will have different bar punches

I which are used by the guided object to determine the own position

I estimate. I

I The multipath effect influences the accuracy of the I

I 10 estimation process. In particular, this is due to the fact that the indirect I

I roads will have different angles (and gain factors) over I

I relative to the axis at each bundle position. In addition, it introduces multipath-I

I effect additional errors in the self-position measurement, even if the object is on I

I the axis. I

I 15 I

I In the case that the object is a guided projectile, the RF becomes I

I conductive beam directed by means of a conductive beam antenna in the I

I indication of the target to be intercepted. Then multipath effects I

I occur when on low-flying targets, such as Seasklmmers, I

I focused. I

I The multipath effects will immediately flee the guided missile I

I influence and reduce the likelihood that the guided projectile I

I will hit the bye. Even if the projectile is equipped with an I

I proximity tube, which detects the passing target and the projectile head I

I explodes, the effect of the explosion and of the fragmentation I

I on the target are smaller, with a greater miss distance (lower I

Chance of destruction) in relation to the goal. I

The multipath effects therefore impair the usefulness I

I operational effect of the RF guided projectiles in terms of I

I destruction rate and retention distance. I

A method for reducing the multipath effects is the I

I apply RF mobility, i.e. use different I

I 1024532 3 frequencies in successive measurements. Because the positions of the multipath peaks vary in distance according to the RF, some frequencies will be affected less than others at a certain distance. This can be used when processing the guided projectile data. 5, for example, by selecting only measurements that have a sufficiently high received SNR (Signal-to-Noise Ratio, signal-to-noise ratio). Multipath peaks are associated with low SNR values. The greater the bandwidth (bandwidths> 10% of the main operating frequency is preferred in this connection), the more effective the application of RF mobility will be.

Current state of the art transmitters that use TWT (Traveling Wave Tubes) are designed to provide optimum performance in a relatively narrow band around the main operating frequency. Higher or lower RFs are possible (within a bandwidth of approximately 10% of the main operating frequency), but at the expense of a reduced power output. To a certain extent, the effects can be compensated, with consequences for the costs.

20

In another possible method to reduce multipath effects on the control of RF guided projectiles, measurement data is filtered. RF beam guide essentially comprises the closed loop control of the guided projectile. Closed loop in this case means that the guide 25 influences the position of the guided projectile relative to the guide beam, which of course directly influences the input to the guide, i.e. the measured own position. Essentially, filtering slows down the influence of the measurements taken into account in the guidance. For example, consider the following case:

As a result of, for example, a turn of the target, which can be followed almost immediately by the guide beam antenna, the guided projectile builds up a deviation in position with respect to the axis of the guide beam antenna. Due to the filter delay, this deviation is not initially noticed.

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I When the guided projectile has finally noticed the deviation and I

I starts a correction change, it also only becomes after the delay I

I noted. When the guidance finally corrects for the deviation I

I has already executed, an error is still detected and the I

I 5 correction order for the guided projectile is maintained, which will I

I result in an overshoot (overshoot) of the desired position, etc. I

I The effect of filtering is a less muted movement of the I

I guided projectile (stronger and longer lasting oscillations), which will eventually I

I 10 result in larger miss distances in relation to the goal (= smaller I

I destruction performance). Note here that the guided projectile, like any I

I dynamic system, itself is already a kind of filter. If too strong on the I

I measurement data is filtered, the entire system can in fact be unstable I

I and the guided missile will never reach the target. I

I 15 I

I Therefore requires projectile control that uses RF guidance

I with a large bandwidth a more expensive transmitter and reduces measurement filters by the I

I stability of the guided projectile. I

The present invention eliminates the aforementioned disadvantages I

I by limiting the conduction beam error measurements to legal errors, i.e. I

I errors that are the result of changes in the goal or the guidance I

I projectile. I

An object of this invention is a method for the reduction

I of the multipath effect on the self-position measurement of a bundle guide I

I object, comprising limiting the variation of the measured position to I

I a predetermined interval [F2> F ^. I

Another object of this invention is a multipath-I

I effect reducing device for the position measurement of a bundle guide

I object that uses such a method for the reduction I

I of the multipath effect including variation-limiting means (100) for I

Limiting the variation in the measured position to a predetermined I

I interval [F2, Ft]. I

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5

A further object of the present invention is a control system for an RF beam guided object, which comprises the following: - A guide beam antenna which makes the guide beam in the direction of a target, in order to guide the object to the target; - Means for measuring the position of the guided object, which estimates the position of the guided object relative to the guide beam, which yields a measured position mk; - The said multipath effect reducing device, which receives the measured position mk and outputs the processed position Pk to the means 10 for controlling the guided object.

Further features and advantageous features of the invention will become apparent from the following description of examples of embodiments of the invention, with reference to illustrations showing essential details for the invention, and from the claims. The individual properties can be realized separately, all or in any desired combination in one of the possible embodiments of the invention.

- Figure 1, a representation of muitipath geometry.

20 - Figure 2, a representation of deviating guide beam positions and multipath geometry.

- Figure 3, some examples of positional accuracy of a guided object for the measured position mk, - Figure 4, some examples of positional accuracy of a guided object for the processed position Pk according to the invention.

- Figure 5, a block diagram of an example of a realization mode of the multipath effect reducing device using the method according to the invention.

Figure 1 shows the multipath geometry and its effect on the bundle.

An antenna A transmits a bundle, the pattern of which is shown in Figure 1. This bundle follows different paths: direct path R1, reflected path R2 (also called mirroring path).

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Due to differences in path length between the direct Ri and I

reflected R2 waves, the beam that follows the direct path and the I

bundle that follows the specular path with a (very small) time difference I

arrive at the O object. This time difference translates into a phase difference. I

5 If the phase difference is small, the two together will have a larger value I

then have those from the direct way alone; a phase difference of around 180 l

degrees can almost lead to extinction. While the difference in distance will be I

vary according to the distance to the target, this will cause interference

or multipath effect also show a distance-dependent fluctuation. How I

10 the shorter the wavelength (higher frequency), the more peaks and troughs I

will occur within a certain distance range. I

To enable the object O to determine its own position at I

with respect to the main antenna direction, a number can advance I

15 certain bundle position deviations are inserted either simultaneously or in I

a monopulse system either sequentially. By the bundle pattern I

modulated, the different bundles will have different response rates

which are used by the guided object to determine the own position I

estimate, in Figure 2, for example, the direct roads Ri for I

Bundle C (broken line) and bundle D (dotted line) the same I

amplification factor and consequently received the same amount of I

energy. The result is the estimate that object O is on the antenna axis I

is located. I

The multipath effect, however, influences the accuracy of the I

estimation process. In particular, because the indirect roads I also have different I

angles (and gain factors) to the axis of I

each of the bundle positions. As a result, the multipath effect introduces extra I

errors in position measurement. Compare the broken and I as an example

30 dotted indirect lines indicated by an arrowhead in Figure 2, which I

have a different length to indicate gain factors that I

differ from the relevant broken and dotted bundle patterns. I

Although the object is on the antenna axis, the contribution of the I

indirectly introduce an estimation error. I

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7

The aim is to separate the legal (as a result of turns of target and / or guided object) and illegal (as a result of multipath and / or measurement noise) measured positive errors as much as possible. The invention, taking into account the mobility of the target as well as the turning possibilities of the guided object, comprises limiting the influence of variations in the measured position error. The basic idea behind this limitation is that major changes in the measured position error can then only be attributed to multipath and / or measurement noise, not to legal twists. the distance covered by the object. In this regard, the parameters of the limiting function depend on the maximum expected target mobility (dominant at greater distance) and the turning capabilities of the guided object (dominant at short distance). Different functions are applied to the vertical and horizontal direction in the reference frame of the guided object (note that due to the rotation of this reference frame relative to the local vertical, multipath effects may also be present in the horizontal direction).

20

The result achieved is a significant reduction of both the systematic error (average) and the random error (standard deviation), virtually without affecting the legal movements of the guided object and the guide beam.

25

Figures 3 and 4 show examples of characteristic elevation accuracy as a function of the distance of the guided object for different bandwidths. Figure 3 shows these accuracies in terms of measured position mk, while Figure 4 shows these accuracies in terms of processed position Pk. By "processed position" is meant the position obtained after the restriction operation according to the invention.

The X axis represents the distance of the guided object in meters 35 and the Y axis the elevation accuracy in KT3 radians. In reality, the 1024532

8 I

measured positions of the guided object beam usually angle data I

expressed in radians. Note that, as with any radar system, this data is in I

reality cannot be measured as such, but are calculated in the I

processor of the guided object from measured voltage levels. An ADC I

5 (Analog-to-Digital Converter, analog-to-digital converter) delivers the relevant I

digital equivalent to the processor. I

The unbroken lines Ci represent the evolution of the I

accuracy in terms of the average value, respectively for I

10 the measured position and the processed position with a bandwidth of 10%. I

The dotted lines (¾ represent the evolution of the I

accuracy in terms of the average value, respectively for I

the measured position and the processed position with a bandwidth of 3%. The I

broken lines ca represent the evolution of the accuracies in I

15 terms of the standard deviation, respectively for the measured position and I

the processed position with a bandwidth of 10%. The dotted line lines c4 I

represent the evolution of the accuracy in terms of the I

standard deviation, respectively for the measured position and the processed I

position with a bandwidth of 3%. I

20 I

Both these Images show that the processed position I

more accurate regardless of the bandwidth used. I

The method for reducing the multipath effect on the I

The position measurement of a beam guided object according to the invention limits the I

influence of the variation from the measured position mk to the predetermined I

interval [F & Fi]. As noted earlier, the predetermined interval I

[F2, Fi] depend on the maximum expected target mobility and the I

maneuverability of the guided object, which are distance dependent, I

30 wherein both the distance to the target (Rtarg) and the object I guided through

Distance traveled (Ramm). I

In a possible implementation of this reduction method I

of the multipath effect, the limitation includes an estimate of the legal I

Error measurement Am) im from at least the current measured position mk and the I

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9 previous processed position Pu. The position Pk is then processed by adding the estimated legal error Am ^ to a reference position P ^: Pk = Pref + Amiim.

Figure 5 shows a possible embodiment of the method for reducing the multipath effect according to the invention as variation-limiting means (100). These variation limiting means (100) include a legal error estimator (110) which receives at least the current measured position mk and the previous processed position Pu and outputs Arniim 10. These variation limiting means (100) also include a position processor (120) which adds the estimated legal error Amum to a reference position Pref: Pk = Pref + Amnm.

The legal error measurement estimate Amk can be realized by, firstly, processing a variation by means of a subtraction operation between the current measured position mk and the previously processed position Pk-i: Amk = mk-Pk-i. Secondly, the variation Amk is limited within the predetermined interval [F2, Fi], making the estimated legal error Amum equal to: Amk, if Amk is within the predetermined interval [F2, Fi]; • F1, if Amk is greater than Fi; • F2, if Amk is less than F2.

This first step can be implemented in a variation processor 25 (111) and the second step in a processed variation limiting means (112). In this embodiment of the invention, the reference position Pref is equal to the legal error estimate Pk-i, where initially, when k = 0, Pk-i is made equal to mk.

The delay T indicates that the difference Amk at time k is calculated from the measurement sample and the processed position Pk.i at times k and k-1, i.e. separated into time by a delay determined by the measurement sampling frequency.

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The two thresholds Fx (x = 1.2) of the predetermined interval can be I

I are implemented by the following limiting function, of which is I

I demonstrated that it works well (see Figure 3): I

I ^ X ^ X I

I Fx - maximum (-—, -—) with x - [12] and kx> kx I

Rfarg Ramm 1 * I

The coefficients kxi and kx2 of this function must be I

I attuned to the expected behavior of the threat (for example very I

I mobile Anti-Ship Missiles) and the behavior of the guided object. I

Moreover, these coefficients are of course also dependent on the I

I measurement sampling frequency. I

I 10 I

As can be concluded from the comparison for Fx (see I

I above), the limitation In the first part of the flight of the guided I

I object determined by kxa / Ramm, because at that time Ramm «Rtarg- At that stage I

Of the flight, when the guided object joins the guidance beam, I

Target movements are of less importance because of the longer distance Rtaig: I

I a move of the target by M meter will only result in an I

I shift of the guide beam with respect to the position of the I

I guided object of M x Rum / Rtarg meter. I

I 20 With a longer distance, when the guided object is on the I

I has fixed the guidance beam, the target mobility becomes more important. I

I The turning point is kxi / Rtarg = IWRamm. Note that this is only significant I

I when kxi> kx2. I

So initially the function value will decrease with time, but I

I after the tipping point it will increase until the interception. I

I The function described is not necessarily the only possible I

I implementation. Other functions can work just as well or even better than I

The function described. The basic idea of the invention is that "legal 11 I

I error measurements can only be caused by twists of the I

I target or the guided object and that such movements will be less large I

I are then a certain maximum. Naturally, this maximum depends on the I

I possibilities of both the threatening goal and the I

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11 dynamics / kinematics of the guided object, both of which are distance dependent.

The introduction of distance-dependent restrictions on the increase and decrease of the beam rider position measurements reduces the influence of multipath errors to an acceptable low level. This allows the use of a conventional type of transmitter with a bandwidth of less than 3% and eliminates the need for additional filtering, thereby jeopardizing the stability of the guided object. The invention is therefore an inexpensive solution that provides good stability of a guided object.

The multipath effect reducing method can also be implemented as additional software in the existing on-board computer processor of the control system for RF guided objects.

More generally, such a multipath effect-reducing method can be applied to any system for controlling beam-tracking objects such as, for example, guided projectiles and missiles.

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Claims (12)

Method for reducing the multipath effect on the eigen II position measurement of a beam-guided object, characterized in that it comprises: II limiting the influence of the variation in the measured position to a II predetermined interval [F2 , F,]. I I 5 I Method for reducing the multipath effect according to the preceding claim II, characterized in that the predetermined interval II [F2, F ^ is dependent on the maximum expected target mobility, and that II the turning possibilities of the guided object are distance dependent , where II io is taken into account both the distance to the target (Rtarg) and the II distance (Ramm) traveled by the guided object. I 3. Method for reducing the multipath effect according to one of the preceding claims, characterized in that the predetermined interval II [Fa, Fi] is distance dependent, wherein both the distance to the target (R ^ g) and II the distance traveled by the guided object (Ramm) is taken into account II. I Method for reducing the multipath effect according to one of the preceding claims, characterized in that the value zero is included in the predetermined interval [F2, Fi]. I 5. Method for reducing the multipath effect according to one of the preceding claims, characterized in that the two thresholds Fx (x = 1.2) II of the predetermined interval are determined as the maximum between II fc k II ———, where kXi and correspond to the expected behavior II Rtarg Ramm II of, respectively, the target and the guided object. I 6. Method for reducing the multipath effect according to one of the preceding claims, characterized in that it comprises limiting: II - an estimate of the legal error measurement Δτη »η from at least the II current measured position after the previous processed position Pu; I I and I 1024532 I - processing the position by adding the estimated legal error Amih to a reference position Pref: Pk = Leek + Am ^. Method for reducing the multipath effect according to the preceding claim, characterized in that the estimation of the legal error measurement Amk comprises: - determining the variation by a subtraction operation between the currently measured position mk and the previous processed position Pu: Amk = mk-Pk.i; 10. limiting the Amk variation within the predetermined interval [F2, Fi], making the estimated Arnum legal error equal to: • Amk, if Amk is within the predetermined interval [F2, F1]; • F1, if Amk is greater than F1; · F2 if Amk is less than F2. 8. Method for reducing the multipath effect according to the preceding claim, characterized in that the reference position Prei is equal to the legal error estimate Pk.i, where initially, when k = 0, Pk-i is made equal to mk. 9. Multipath effect-reducing device for position measurement of a beam-guided object, which uses the method for reducing the multipath effect according to one of the preceding claims, characterized in that it comprises variation-limiting means (100) for limiting from the influence of the variation in the measured position to a predetermined interval [F2, F1]. Multipath effect-reducing device according to the preceding claim, characterized in that the variation-limiting means (100) comprise: - a legal error estimator (110) which receives at least the currently measured position mk and the previously processed position Pk.i and releases Amibn; and 1024532 I 14 I I - a position processor (120), which adds the estimated legal error Amih, to I I a reference position Pret: Pk - Pref + Δΐ7ΐκπ ,. I Multipath effect reducing device according to the preceding claim, characterized in that the legal error estimator (110) comprises: II - a variation processor (111) which subtracts the currently measured position mk and the II previously processed position Pm from each other ; and II - a processed variation limiting means (112) that adds the estimated legal ILI error Arniim at a reference position P ^: Pk = Ρ * * * + Antifa ,. I I io I A control system for an RF beam guided object, comprising: I - A guide beam antenna that makes the guide beam in the direction of a target, to guide the object to the target; Means for measuring the position of the guided object, which allow the guided object to determine its own position relative to the guide beam, which yields a measured position mk I I; A multipath effect-reducing device according to claim 10 or 11, which receives the measured position mk and outputs the processed position Pk to the means for controlling the jellized object. 1024532 I