A METHOD FOR THE DETERMINATION OF THE OPTIMAL ALTITUDE FOR A VEHICLE, FLYING AT A LOW ALTITUDE OVER A SEA SURFACE

Abstract: In a method and a corresponding device for determining the optimal altitude for altitude control for a vehicle (1), flying at a low altitude over a sea surface, like an anti-ship missile, the distance between the sea surface and the vehicle is measured by an altimeter (2), placed in the vehicle. The altimeter gives an altitude signal, corresponding to this distance, which is varying due to the wave structure of the sea surface. The al­ titude signal is processed in a signal processing unit (3), which gives to the steering means (4) of the vehicle (1) an altitude control signal, which is dependent on the difference between the actual altitude of the vehicle and the optimal altitude (H o ). The altitude signal is filtered in the signal processing unit (3) to eliminate any superimposed signals, the frequencies of which lie outside of a predetermined range of frequencies, comprising the range of frequencies of the wave movement. The rms-value of the filtered altitude signal is determined in the signal processing unit (3). An acceptable risk of a wave hit for the vehicle (1) is established. The optimal altitude (H o ) is calculated, considering the rms-value determined and the established risk of a wave hit for the vehicle (1).

Description: The present invention relates to a method for the determination The altitude of the optimal altitude for control of the altitude of a vehicle, flying at a low altitude over a sea surface, like an anti-ship missile. The objectmissile, which method comprises the determination of the distance between the sea surface and the vehicle by an altimeter, placed in the vehicle, which altimeter gives an altitude signal, which varies depending on the wave movements of the sea surface, which method comprises the processing of the altitude signal in a signal processing unit, which gives an altitude control signal to the steering means of the vehicle, depending on the difference between the actual altitude and the optimal altitude. The invention also relates to a device for carrying out the method. In order to avoid detection by radar or other reconnaissance equipment, and in order to, if possible, hit a marine target directly, an anti-ship missile should fly towards the target at the lowest possible altitude, but with a safe margin to the crests of the waves. For high sea-states, the missile must be commanded to a considerably higher path than for calm sea. As the sea-state often varies locally along a missile trajectory, it is desirable to control the altitude of the missile according to the local sea-state. The optimal altitude for the missile has to be determined according to the local sea-state. For this purpose it is necessary to provide the missile with an altimeter, which measures the vertical distance between the vehicle and the sea surface along the trajectory of the missile towards the target. From DE Auslegeschrift 24 36 791 a method is known for the control of the altitude of a vehicle flying towards a marine target. This known method comprises the determination of the distance between the vehicle and the sea surface by an altimeter, carried by the vehicle. Every determined minimal value of this distance, which thus corresponds to a wave crest, is stored in a sample-and-hold circuit during a certain period of time, which corresponds approximately to a wave period. The stored minimal value is compared, in a command circuit, with a reference altitude. If the stored minimal value should not coincide with the reference altitude, the command circuit will give such an altitude control signal to the steering means of the vehicle, that the flight path of the vehicle will be placed on an altitude above the wave top in question, corresponding to the reference altitude. The optimal altitude consists, according to DE Auslegeschrift 24 36 791, of the reference altitude plus the height of the wave top, that is to say the wave amplitude. As the reference altitude is constant, the optimal altitude in this case consists of a constant term and a variable term, which is equal to the wave top height. The last term thus forms the part by which the optimal altitude can be varied as a function of the sea state. This term, however, only makes possible rather slight variations, which means, that the reference altitude must be chosen unnecessarily high at low sea-states in order to maintain a low risk for a wave hit at high sea-states. The object of the present invention is to provide a method and a corresponding device for the determination of the optimal altitude, in which the information on the wave movement obtained by a measurement of the distance between the vehicle and the sea surface, combined with knowledge of the statistics of the wave movements, is utilized efficiently in order to determine a term, which is dependent on the locally prevailing sea state, and which corresponds substantially to the variable term, mentioned above, which makes it possible to change the optimal altitude considerably, when needed, and which thus admits, that a term, corresponding substantially to said constant term, is given a rather low value, without increasing the risk of a wave hit. This object is achieved, according to the present invention, by a method, which is of the art mentioned introductorily, and is characterized by the fact that the altitude signal is filtered in the signal processing unit to eliminate any superimposed signals, the frequencies of which are outside a predetermined range of frequencies, comprising the range of frequencies of the wave movement, that the rms-value, that is to say the standard deviation or a value related to this of the filtered altitude signal like the mean amplitude, is determined in the signal processing unit, that an acceptable risk P o for a wave hit of the vehicle is established, and that the optimal altitude is calculated from the rms-value determined and from the established risk for a wave hit for the vehicle. The optimal altitude is advantageously calculated considering the systematic errror of measurement of the altimeter and the variation of altitude of the vehicle around the reference altitude. Here it is preferably presumed, that the systematic error of measurement of the altimeter and the variation of altitude of the vehicle are normally distributed with the mean value zero and the standard deviations σ F and σ h , in which case the optimal altitude H o is calculated from the relationship where σ v is the standard deviation of the sea-surface deviation due to the wave movement, L o is the flight range of the vehicle and λ o is the characteristic wave length of the wave movement. If the altitude variation of the vehicle around the reference altitude is presumed to be independent of external effects said predetermined range of frequencies is preferably chosen such, that the altitude variation of the vehicle around the reference altitude is eliminated, when the altitude signal is filtered, whereby the filtered altitude signal corresponds to the wave movement and its rms-value corresponds to the local standard deviation of the wave structure, in which case the optimal altitude H o is calculated from the relationship where H oo is the optimal altitude H o (σ v = 0) at a sea surface without any wave movement. Then H o can be calculated from the approximate relationship H o = H oo + K o σ v . If one cannot look away from the impact of the external effects on the variation of the altitude of the vehicle around the reference altitude, said predetermined range of frequencies is preferably chosen such, that the filtered altitude signal corresponds to the sum of the wave movement and the variation of the altitude of the vehicle around the reference altitude, whereby its rms-value corresponds to The optimal altitude H o in this case can be calculated from the approximate relationship where H′ oo is the optimal altitude H o (σ v = 0) at a sea surface without any wave movement and is the value for σ h at a calm sea, or from the approximate relationship The invention will now be described more in detail, reference being made to the enclosed drawings. Figure 1 shows an anti-ship missile, flying at a low altitude over a sea surface. Figure 2 is a block diagram showing an altimeter, a signal processing unit and the steering means for the missile. Figure 3 shows the signal spectrum of the altitude signal. In figure 1 an anti-ship missile 1 is shown, flying at a low altitude over a rough sea surface. The wave movements of sea surfaces have been studied extensively both empirically and theoretically. The statistic properties of the waves have been given a special attention. According to "Fundamental of the behaviour of ships in waves", Int. Shipbuilding Progress - Vol. 6, No. 63, - Nov. 1959 it has been found, that the momentary value v of the wave movement (fig. 1) whereby here is meant the momentary height deviation of the sea surface from the average value at smooth sea, can be regarded as normally distributed where σ v is the standard deviation of the momentary value of the sea surface deviation. According to the same reference the wave height H v (fig. 1) whereby here is meant the vertical distance between the wave top and the wave through, can be regarded as Rayleigh-distributed The so-called wave spectrum determines how rapidly the momentary wave height varies along a flight path. Several models for the determination of the dependence of the wave spectrum on wind force, the duration of the wind etc. have been proposed. As an example the Neuman-Pierson model can be mentioned, described in G. Neuman, Pierson, "A comparison of various theoretical wave spectra, Proceedings of a symposium of the behaviour of ships in a seawave", Vol. 1 - 2, 1957, and its successor the Pierson-Moskowitz model, which was adopted in 1964 by ISSC (International Ship Structurers Congress) as a model for completely developed sea. From measurements from lighthouses along the Swedish coast Statens Skeppsprovningsanstalt has calibrated these models for Swedish conditions. In the missile 1 there is provided an altimeter 2 (fig. 2), which is for instance a radar altimeter. The altimeter 2 continuously measures the distance between the missile 1 and the sea surface and gives an altitude signal H(t) corresponding to this distance to a signal processing unit 3, which in the example shown (fig. 1) is placed in the missile, but which can also be placed in a fixed site. The signal processing unit 3 receives the altitude signal H(t) and converts it to give to the steering means 4 of the missile 1 an altitude control signal R, which is dependent on the difference between the actual altitude of the missile 1 and the optimal altitude H o (in the example shown in fig. 1 there is no difference, but the actual altitude is equal to the optimal altitude). The altitude signal H(t) contains several components and may be written H(t) = H o + F + h(t) - v(t) + n(t) , where H o as has been mentioned is the optimal altitude, F is a constant deviation from the reference altitude, dependent on systematic errors in the altimeter, h(t) is the slow variation of the altitude of the missile around the reference altitude, v(t) is the variation generated by the wave movement and n(t) is a noise component. In figure 3 the signal spectrum of the altitude signal H(t) is shown. The components of the altitude signal H(t) lie in different frequency ranges. H o and F have the frequency zero, h(t) is concentrated to the frequency range 0 < f < f₁ in fig. 3. v(t) lies in the frequency range f₁ < f < f₂ in fig. 3, whilst n(t) substantially has frequencies, which are larger than f₂. Usually f₁ is of the order of magnitude of 1 Hz or less and f₂ is about 5 - 20 Hz depending on the sea state. The signal processing unit 3 comprises a high-pass filter 5, a low pass filter 6, means 7 for the determination of rms-value (root mean square value) and a calculation means 8 for calculation and emitting of said control signal R. The altitude signal H(t) is allowed to pass the high-pass filter 5, which has the pass-band f > f₁ and thus suppresses the components H o , F and h(t) but lets the wave movement component v(t) and the noise component n(t) pass. The filtered signal H₁(t), which is the output of the high-pass filter 5, is allowed to pass the low-pass filter 6, which has the pass-band f < f₂ and thus suppresses thehigh-frequency noise component n(t) but lets the wave movement component v(t) pass. The wave movement component v(t) thus forms the only remaining component of the altitude signal H(t) in the filtered signal output H₂(t) from by the low-pass filter 6. The filtered signal H₂(t) is connected to a rectifier 9, which forms part of the means 7. The rectified output signal H₃(t), is connected to a low-pass filter 10, which also forms part of the means 7. The signal H₄(t), emitted from the low pass filter 10, is directly proportional to the rms-value of the wave movement and thus also to its standard deviation v . Here H₄(t) = k o σ v . As the momentary value of the wave movement is normally distributed, k o is = √ 2/π if the rectifier 9 is a full-wave rectifier, and k o is = 1/2 √ 2/π , if the rectifier 9 is a half-wave rectifier. The signal H₄(t) is connected to an amplifier 11, which also forms part of the means 7, and which has the gain 1/k o and thus emits a signal H₅(t), which is equal to the standard deviation v for the momentary value of the wave movement. The signal H₅(t) is received by the means 8, which calculates the optimal altitude H o in a way to be described more in detail below. The means 8 also receives the signal H(t) from the altimeter 2 and therefrom it determines the actual altitude of the missile 1. The means 8 then compares the actual altitude with the optimal altitude H o and emits an altitude control signal R, depending on the difference between them, to the steering means 4 of the missile 1. To calculate the optimal altitude H o firstly an acceptable risk P o is established for a wave hit by the missile 1. If the systematic error of measurement F of the altimeter 2 and the variation of altitude h(t) of the missile 1 around the reference altitude is presumed to be normally distributed with the mean value 0 and the standard deviation σ F and σ h , the risk or probability P o that the missile should hit the waves during a flight range L o can be given by the following approximate relationship, provided that H v is Rayleigh-distributed according to the formula, which has been given earlier. Hereλ o is the characteristic wave length of the waves, which can be determined from the altitude signal H(t), for instance by counting the number N o of zero-crossings for the filtered signal H₂(t) during a certain period of time T, whereupon λ o is calculated from where V o is the speed of flight for the missile 1. Alternatively empiric relationships between σ v and λ o can be utilized for an estimation of λ o for instance according to Pierson-Moskowitz model, which has been mentioned earlier. The relationship (1) may be written where In this expression, λ o as well as L o vary, but they have fairly little influence on K o , and thus K o can often be regarded as constant. As an example it can be mentioned, that K o = 4.8, if L o = 50 km, λ o = 50 m and P o = 0.01, and that K o = 5.4 if L o = 200 km, λ o = 10 m and P o = 0.01. If the variation of the altitude h(t) of the missile around the reference altitude is presumed to be independent of external conditions, which here means the weather, that causes the wave movement and thus has an influence on σ v , the relationship (2) can be written where H oo is optimal altitude at calm sea (σ v = 0) is according to the presumption constant, that is to say independent of the external conditions. σ F and σ h can be determined by simulations or experiments. A linear approximation of the relationship (3) gives H o = H oo + K o σ v (4) A comparison between the relationships (3) and (4) shows, that the relationship (4) gives a somewhat too high optimal altitude H o at low states of sea. In the approximation above of the relationship (2) the variation of the altitude h(t) of the missile 1 around the reference altitude was presumed to be independent of the external conditions. If this influence cannot be neglected, the limit frequency of the high-pass filter 5 is given a value, that is so much less than f₁, that the high-pass filter will, like before, suppress the components H o and F of the altitude signal H(t), but will let through the components v(t) and n(t) and also the component of altitude variation h(t). Instead of σ v the amplifier 11 will now emit a signal H₅(t), that is The relationship (2) for this case can be written where H' oo is optimal altitude at calm sea (σ v = 0) and is the value for σ h at calm sea. which is constant. σ F and can be determined by simulations or experiments. A linear approximation of the relationship (3') gives If is considerably smaller than σ h the relationship (4') can be approximated further by For certain algorithms the realization is facilitated, if the rectifier 9 is instead formed as a quadratic detector, so that H₃(t) = k o H₂(t)² . The outgoing signal from the amplifier 11 is then given by H₅(t) = σ v ², if only the component of wave movement v(t) is allowed to pass both filters 5 and 6, and by H₅(t) = σ v ² +σ h ², if the component of wave movement v(t) as well as the component of variation of the altitude h(t) are allowed to pass.

Claims: A method for the determination of the optimal altitude for altitude control of a vehicle (1), flying at a low altitude over a sea surface, like an anti-ship missile, which method comprises the determination of the distance between the sea surface and the vehicle by an altimeter (2), placed in the vehicle, which altimeter gives an altitude signal (H(t)) corresponding to this distance, which signal varies depending on the wave movements of the sea surface, which method comprises the processing of the altitude signal in a signal processing unit (3), which gives an altitude control signal (R) to the steering means (4) of the vehicle, depending on the difference between the actual altitude and the optimal altitude H o , characterized by the fact that the altitude signal (H(t)) is filtered in the signal processing unit (3) to eliminate any superimposed signals, the frequencies of which are outside of a predetermined range of frequencies, comprising the range of frequencies of the wave structure, that the rms-value, that is to say the standard deviation, or a value related to this, of the filtered altitude signal (H₂(t), like the mean amplitude is determined in the signal processing unit (3), that an acceptable risk P o for a wave hit of the vehicle (1) is established, and that the optimal altitude H o is calculated from the rms-value determined or a value related to this and from the established risk P o of a wave hit for the vehicle. A method according to claim 1, characterized in that the optimal altitude H o is calculated considering the systematic measurement error of the altimeter (2) and the variation of altitude around the reference altitude of the vehicle (1). A method according to claim 2, characterized in that it is presumed, that the systematic error of measurement of the altimeter and the variation of altitude of the vehicle are normally distributed with the mean value zero and the standard deviation σ F and σ h , and that the optimal altitude H o is calculated from the relationship where σ v is the standard deviation of the momentary value of the wave movement, L o is the flight range of the vehicle and λ o is the characteristic wave length of the wave movement. A method according to claim 3, characterized in that the altitude variation of the vehicle (1) around the commanded altitude is presumed to be independent of outer circumstances, and that said predetermined range of frequencies is chosen such, that the altitude variation of the vehicle around the reference altitude is eliminated, when the altitude signal (H(t)) is filtered, whereby the filtered altitude signal (H₂(t)) corresponds to the wave movement and its rms-value corresponds to the standard deviation σ v of the momentary value of the wave movement, and that the optimal altitude H o is calculated from the relationship where H oo is the optimal altitude H o over a calm sea surface ( σ v = 0). A method according to claim 4, characterized in that the optimal altitude H o is calculated from the approximate relationship H o = H oo + K o σ v . A method according to claim 3, characterized in that said predetermined range of frequencies is chosen such, that the filtered altitude signal (H₂(t)) corresponds to the sum of the wave movement and the variation of the altitude of the vehicle around the reference altitude, whereby its rms-value corresponds to A method according to claim 6, characterized in that the optimal altitude H o is calculated from the approximate relationship where H′ oo is the optimal altitude H o (σ v = 0) at a sea surface without any wave movement and is the value for σ h at a calm sea. A method according to claim 6, characterized in that the optimal altitude H o is calculated from the approximate relationship where H′ oo is the optimal altitude H o (σ v = 0) at a sea surface without any wave movement. A device for altitude control to optimal altitude for a flying vehicle (1) like an anti-ship missile, intended tfly over a sea surface at a relatively low altitude, comprising an altimeter, placed in the vehicle (1), which altimeter, provided to measure the distance between the sea surface and the vehicle and to give an altitude signal (H(t)), corresponding to this distance, which signal varies in dependence on the wave structure of the sea surface, further comprising a signal processing unit (3) provided to receive said altitude signal (H(t)) and to give to the steering means (4) of the vehicle (1) an altitude control signal (R) in dependence on the difference between the actual altitude of the vehicle and the optimal altitude H o , characterized in that the signal processing unit (3) is provided to: filter the altitude signal (H(t)) to eliminate any superimposed signals, the frequencies of which lie outside of a predetermined range of frequencies comprising the range of frequencies of the wave movement, determine the rms-value of the filtered altitude signal (H(t)), calculate the optimal altitude H o considering the rms-value determined and an established acceptable risk P o of a wave hit for the vehicle (1).

Inventors: AXELSSON, SUNE
Applicants: SAAB MISSILES AKTIEBOLAG
Application year: 1987
Main classification: G05D1
IPC codes: G05D1