RU2483324C1 - Method for aircraft navigation on radar images of earth's surface - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: result is achieved by forming, when the aircraft is in flight, radar images of the observed terrain in a normal earth coordinate system of the corresponding area taking into account adjustments to estimates of values of flight parameters, generated by the on-board aircraft control system and subsequent cross-correlation processing of the formed radar image with a reference radar image prepared beforehand.

EFFECT: high probability of correct determination of the position of an aircraft based on radar images of the earth's surface and broader conditions for possible application of on-board radar equipment of the aircraft, which enable navigation of the aircraft on radar images of the earth's surface.

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Description

The invention relates to radar, in particular to airborne radar navigation aerial vehicles (LA), providing in flight LA radar images (RLI) of the earth's surface and the comparison of these images with pre-prepared reference images.

The obtained radar images are compared with the reference ones by calculating the maximum of a certain two-dimensional function that reflects the mutual correlation of the reference and working radar images, according to the position of which is determined when the relative position of the received and reference radar images is changed, estimates of the coordinates of the position of a known area relative to the aircraft are formed, by which determines the navigation errors of the aircraft control system.

There is a method described in [1], according to which an airborne radar system (radar), which is part of the airborne control system of an unmanned aerial vehicle (UAV), using an antenna scan, looks through the space in the horizontal plane:

in front of the UAV (in the mode of bringing the UAV to a radio-contrast object);

from the zero position of the antenna, which coincides with the firing plane, to the extreme angular position of the measurement sector (in the mode of bringing the UAV to a non-radio-contrast object).

During the review process, an array A (i, j) of measurements of radar signals is formed, with assignment to each array element of the corresponding value of the angle of rotation Ψ _i of the antenna and the range D _j corresponding to the measured reflected signal.

Further, this array is subjected to correlation processing by comparing the measured array with sequentially sorted binary sequences B (i, j) of fragments of a previously prepared reference map.

The results of this processing determine the location of the measured array on the reference map, the longitudinal axis of which is oriented along the firing plane. The offset of the near left portion of the surface where the reflected signal is measured relative to the lower left point of the reference map determines the errors in bringing the UAV to a given point at a distance of measurement.

The disadvantage of this method is that the azimuthal resolution of the generated radar image is determined by the width of the radiation pattern of the antenna of the radar coordinator, which does not allow for a sufficiently accurate "binding" of the resulting radar image to the standard. In addition, the conversion of polar coordinates (Ψ _i , D _j ) into rectangular (XOZ) is carried out by simply reassigning the numbers (i, j) of the array A (i, j) to the numbers of linear coordinates X and Z, which also significantly reduces the accuracy of navigation calculations mistakes.

This method has a significant difference from the proposed, consisting in the fact that the proposed method uses radar with the synthesis of antenna aperture (CAP).

The closest in technical essence analogue (prototype) of the proposed method is the method described in [2].

In accordance with this method, the estimation of navigational errors of the inertial control system of an aircraft (LA) is obtained using the parameters of the maximum two-dimensional cross-correlation function (CCF) of the reference and working radar images (RLI) of a given (reference) area.

The reference radar of a terrain is calculated on board the aircraft for one or more correction points using a digital terrain model (DTM), previously prepared on the basis of digital vector maps of the earth's surface.

The DTM is set in the Earth's coordinate system with the origin at the priority point, which is set in the flight task, the X axis is directed along the meridian to the north, the Y axis is vertically up, the Z axis is along the parallel to the east.

The reference radar image is calculated for each correction point in the polar coordinate system inherent in the radar sensor.

A polar range-azimuth coordinate grid is formed around a certain priority point (PT) for the given range strobe boundaries Rmin and Rmax and the boundaries of the viewing sector in the azimuth of Bbeg and Bend. The priority point is defined in the flight task.

The reference radar image is an Aet matrix of the power of the signals “reflected” from elementary areas and located at the nodes of the polar grid.

At the point of correction of the aircraft trajectory, the following procedures are performed:

- estimates of the vector of the aircraft’s own speed according to the algorithm described in [3];

- estimates of the elevation angle of the target based on the elevation angles of the bright points of the radar image;

- estimates of the height of the target relative to the aircraft based on estimates of the elevation angle.

At the correction point using FFT, a coordinated digital processing of the emitted and received bursts of pulses is carried out, the result of which is a set of frames of the working radar of the terrain, the elements of which are located in the polar coordinate grid. Using incoherent processing of the frames of the working radar in the polar coordinate system, the working radar is formed in a given area.

The inter-correlation function of the reference and working radar data obtained using CAP is calculated by the coordinates of the points of the standard relative to each point of the working radar data (relative positions).

Given the possible discrepancy between the two radar data according to the angle of rotation, when calculating the cross-correlation, the most probable angle of rotation of the standard radar image is searched. The procedure for calculating and searching for the maximum VKF is repeated for a given (test) set of angles of mutual rotation of the radar image. In this case, the selection of the largest maximum VKF is made.

Then, with the specified correlation comparison parameters, the mutual linear shift of two radar images along the axes of the normal earth coordinate system is calculated, as well as the mutual angular shift of these radar data.

However, this method has the following disadvantages.

1. The azimuthal angular size of the radar images of the observation object, formed in the coordinate system "range - azimuth", changes when changing the range to this object. For example, the observed correction section with a transverse length of 1 km when observing from a distance of 5 km has an angular width of 11.42 °, and from a distance of 6 km - 9.53 °.

Thus, in the presence of unknown planned errors of aircraft withdrawal to the starting point of the survey area of a predetermined (reference) area (radar formation point [2]), it is possible to obtain radar images of this area of significantly different azimuthal sizes, which complicates their subsequent inter-correlation processing with the prepared radar standards.

2. The shift of the real position of the point of formation of the radar image relative to the calculated one used in the formation of the standard leads to a reversal of the generated working radar image relative to the reference one, which also complicates the subsequent inter-correlation processing of the working and reference radar image. For example, with a lateral displacement of the real position of the point of formation of the radar image relative to the calculated one by 500 m, a distance to the object of observation of 5 km and an azimuthal deviation of the direction of sighting of a given area in the SAR mode from the direction of the aircraft velocity vector 15 °, the turn of the working radar station relative to the calculated one will be 9.5 ° .

To eliminate this effect, the prototype provides an assessment of the maximum VKF using brute force reference radar, rotated by predetermined angles.

3. The possibility of changing the flight parameters of the aircraft during the formation of the working radar frames, including the horizontal and vertical components of the flight speed of the aircraft, flight altitude, is not taken into account. These components are closely related to the magnitude of the Doppler frequencies of the signals reflected by the elements of the observed area, based on the estimates of which a working radar is formed.

Distinctive features of the proposed method from the prototype is that the formation of both the reference and the working radar, as well as their inter-correlation processing are carried out in the normal earth coordinate system (NSC) associated with a given (reference - OUMA) area at the correction point, and To ensure the formation of working radar data at the NZSK OUM at each correction point,

determination of the values of the amendments to the estimates of the values of the flight parameters formed by the aircraft onboard control system;

the formation of the frames of the primary working radar images of the OUMA region in the coordinate system "Doppler frequency - range";

the conversion of the frames of the primary operational radar data of the OAM area from the Doppler frequency - range coordinate system to the NSC OUM taking into account certain values of corrections to the current values of the flight parameters generated by the aircraft onboard control system;

the formation of a working radar radar system based on incoherent addition of personnel of radar radar workers at the NZSC OUM;

cross-correlation processing of the reference and working ORL RMI in the NSC OUM and determination of corrections of the aircraft’s planned coordinates with respect to a given point on the OUM (aiming point, target, priority point).

The position of the axes NZSK OUM shown in Fig.1. The OY axis is directed upward along the local vertical passing through the aiming point (TP) at the OUMA. The OX axis is perpendicular to the OY axis. The OZ axis complements the coordinate system to the right.

The direction of the OX axis is determined by the conditions of the problem in which the aircraft is navigated. In particular, the OX axis can be directed, for example, to the north or belong to the azimuthal plane, coinciding with the direction from the launch point of the aircraft to the final point of its reduction, or coincide with the planned azimuthal direction to the aiming point of the reference area, along which the trajectory should be corrected flight of the aircraft, from the correction point.

The direction of the OX axis to the OUM determines the orientation of the reference and generated working radar images.

Under the conditions under consideration, the coordinates in the NZSC OUM of each element of the terrain (elementary reflector of radar signals) corresponding to the ith element of the primary operational radar, formed in polar coordinates with the center at the point corresponding to the projection of the position of the antenna phase center (FCA) of the aircraft’s onboard radar on the horizontal plane of the NZSC OUM, are determined by the following relationships:

Where:

x _a , z _a - estimates of the current planned coordinates of the aircraft’s FAA radar in the NSC OUM obtained on the basis of data from the onboard control system of the aircraft (for example, an inertial navigation system) at the time of formation of the working radar image;

- оценка горизонтальной дальности от ФЦА БРЛС ЛА до элемента ОУМ, соответствующего i-му элементу формируемого первичного рабочего РЛИ;

- assessment of the horizontal distance from the FCA radar LA to the element OUMA corresponding to the i-th element of the formed primary working radar;

Ri is the estimate of the slant range from the FCA radar to the element of the OUMA corresponding to the i-th element of the formed primary working radar;

β _i - assessment of the angle of sight of the element OUMA corresponding to the i-th element of the formed primary working radar in the horizontal plane relative to the direction of the ground speed of the aircraft;

ε _i - assessment of the angle of sight of the element OUMA corresponding to the i-th element of the formed primary working radar, relative to the horizontal plane.

Ψ - estimate of the angle of the path of the aircraft;

φ _i = β _i + Ψ is an estimate of the angle between the OX axis and the direction of sight of the element of the object of observation corresponding to the i-th element of the formed primary working radar image in the horizontal plane.

Moreover, for the quantities β _i , ε _i , Ψ it is true:

,

,

Where:

y _a = y _la -y _c - assessment of the flight altitude of the aircraft relative to the aiming point (target) on the OUMA;

y _la - an estimate of the altitude of the aircraft;

y _c - the absolute height of the aiming point on the OUMA;

- оценка скорости сближения ФЦА БРЛС ЛА с элементом ОУМ, соответствующим i-му элементу формируемого первичного рабочего РЛИ;

- assessment of the convergence rate of the FCA radar LA with the element OUMA corresponding to the i-th element of the formed primary working radar;

- доплеровская частота, соответствующая i-му элементу формируемого рабочего РЛИ;

- Doppler frequency corresponding to the i-th element of the generated working radar;

λ is the wavelength of the probing signals of the airborne radar;

V _y = Vsin (ν a ) is an estimate of the rate of change in aircraft altitude;

V is the aircraft flight speed estimate;

ν _a is the estimate of the angle of inclination of the aircraft velocity vector to the horizontal plane of the NSCS OH;

- оценка путевой (горизонтальной составляющей) скорости полета ЛА;

- assessment of the track (horizontal component) of the flight speed of the aircraft;

V _x = V _g cos (Ψ) is an estimate of the component of the flight speed of the aircraft along the X axis;

V _z = -V _g sin (Ψ) is an estimate of the component of the flight speed of the aircraft along the Z axis.

Based on the given ratios, the coordinates in the NSC of the OUM of each element of the terrain (elementary reflector of radar signals) corresponding to the i-th element of the primary operational radar of the OUM, formed by the airborne radar in the coordinate system "Doppler frequency - range", are calculated when the frames of the working RIR in the NSC are formed OUMA on board the aircraft according to the following formulas:

The above expressions reflect the dependence of the estimates of the planned coordinates of the i-th OUM reflector in the NSC OUM on the values of the parameters characterizing the flight conditions of the aircraft.

Upon receipt of accurate estimates of parameter values characterizing:

- the relative flight altitude (y _a ) of the aircraft, the speed components (V _y , V _g , V _x , V _z and, accordingly, the angle Ψ) of the aircraft flight;

сигналов, отраженных i-м отражателем ОУМ,- oblique range R _i to the i-th reflector and Doppler frequency

signals reflected by the i-th reflector OUMA,

using the full differential formula we get:

;

;

Otherwise, if there are sufficiently accurate estimates of the values of the listed flight parameters of the aircraft, the displacement of the aircraft along the X and Z coordinates leads to the same shift of the working radar in the NSC OUM without distortion and rotation relative to the reference radar. This circumstance significantly simplifies the inter-correlation processing of the reference and working radar, increases the likelihood of their correct mutual reference.

Figure 2 shows the image in NZSK test OUM, which includes 9 radar reflectors, spaced from each other at a distance of 450 meters. Center NZSK OUM combined with a central reflector.

Figure 3 shows the marks corresponding to the actual position of the indicated radar reflectors, and marks (cruciform) obtained as a result of the formation in the NSC of the corresponding working radar image, with the following flight parameters of the aircraft:

range to the aiming point (OUM center) - 5000 m; flight altitude - 3000 m; aircraft flight speed - 400 m / s; the direction of the aircraft velocity vector in the vertical plane - minus 30 °; azimuth bearing on the aiming point - 15 °; azimuthal angle of the aircraft velocity vector - minus 3 °; error in calculating the coordinates of the aircraft along the X axis in the NZSC OUM - 700 m.

Figure 4 shows the marks corresponding to the actual position of the indicated radar reflectors, and the marks obtained as a result of the formation of the corresponding working radar image at the NSC OUM for the same flight parameters of the aircraft, but in the absence of an error in calculating the coordinates of the aircraft along the X axis and the numbering error aircraft coordinates along the Z axis, equal to 700 m

Figure 5 shows the marks corresponding to the actual position of the indicated radar reflectors, and the marks obtained as a result of the formation of the corresponding working radar image in the NSC OUM at the same flight parameters of the aircraft, but in the presence of errors in calculating the coordinates of the aircraft both on the X axis and along the Z axis, equal to 700 m.

The given radar images of test positions illustrate the absence of planned distortions and the reversal of their radar images formed in the NSC OUM on board the aircraft.

Accurate estimates of the slant ranges and Doppler frequencies of the reflected signals are provided by the radar.

Obtaining accurate estimates of the flight parameters of the aircraft can be carried out using appropriate measurements carried out before the formation of the working radar on board the aircraft.

An estimate of the aircraft flight speed vector (and, accordingly, the angle Ψ) can be obtained, as in the prototype, by determining the Doppler frequency shift according to the algorithm described in [3]. An estimate of the relative flight altitude of an aircraft can be obtained, as in the prototype, by estimating the elevation angle of the bright points of the working radar, or using a monopulse [4].

However, in the general case, for example, during the formation of operational radar data under the conditions of maneuvering aircraft, significant changes in the values of the flight parameters of the aircraft are possible, which may entail a defocusing, mismatch and distortion of the frames of the generated working radar data.

The elimination of this drawback is provided taking into account the fact that the errors of measurement, including inertial systems [5] of the aircraft onboard control systems (BCC), are slowly changing processes and, within the time frame of the formation of radar data, which can reach several seconds, these errors can be considered permanent.

In the proposed method, based on the results of measurements of the flight parameters of the aircraft, which are carried out before the formation of the operational radar sensors of the OUM, the corrections ΔV _y , ΔV _x , ΔV _z , _and Δy _а are determined to the values of the estimates of the corresponding flight parameters of the aircraft formed by the aircraft’s BCA meters.

These amendments, then, when forming the frames of the working radar, are summed up with the current values of the flight parameters of the aircraft coming from the BKU LA measuring instruments, while the formation of updated estimates of the current values of the flight parameters of the aircraft is ensured.

In view of the foregoing, the proposed method for navigating an aircraft through radar images of the earth's surface is implemented according to the following algorithm.

1. In preparing the flight mission of the aircraft, the reference areas of the terrain are determined, the radar images of which should be used to correct the flight path of the aircraft. For each OUMA, an aiming point is assigned and the directions of the axes of the NSC OUMA are set based on the conditions of the navigation task. The conditions for the beginning of the formation of a working ORL RLM are determined, for example, the achievement of a given range to the TP specified on the OUM.

2. For each OUM, before flight or in the process of performing a flight, reference radar data are generated in the NSC OUM.

3. In the flight of the aircraft along a predetermined trajectory, at each correction point, the values of the amendments to the estimates of the current values of the flight parameters of the aircraft formed by the aircraft’s aircraft are determined.

4. The frames of the primary operational RLM OAM are obtained using CAP in the “Doppler frequency - range” coordinate system taking into account amendments to the estimates of the current values of the flight parameters of the aircraft, formed by the aircraft’s spacecraft.

5. The primary working radar frames are converted using expressions (8) and (9) from the Doppler frequency - range coordinate system to the NSC OUM taking into account amendments to the estimates of the current values of the flight parameters of the aircraft, formed by the aircraft’s spacecraft.

6. The formation of a working radar radar system is being carried out on the basis of incoherent staffing of radar radar workers.

7. Mutual correlation processing of the reference and working radar data is made and the position of the maximum of the VKF is determined.

8. The planned corrections to the coordinates of the aircraft in the NSC CSA are determined by the coordinates of the maximum in the VKF matrix and the corresponding corrections to the coordinates of the aircraft in the coordinate system in which the aircraft is navigated.

Figure 6 presents a simplified structural diagram of a possible variant of the radar system that implements the proposed method for navigating an aircraft on radar images of the earth's surface, where:

1 - monopulse radar antenna;

2 - radar transmit-receive switch;

3 - radar transmitter;

4 - radar synchronizer;

5 - digital radar control device;

6 - shaper amendments to the estimates of the current values of the flight parameters of the aircraft, formed by the BCU;

7 - block storing amendments to the estimates of the current values of the flight parameters of the aircraft, formed by the BCU;

8 - calculator updated estimates of the current values of the flight parameters of the aircraft;

9 - coherent radar receiver;

10 - multi-channel analog-to-digital Converter;

11 - multi-channel multiplier of the received signal and reference function;

12 - block Fourier transform;

13 - radar distribution block;

14 - shaper working radar in NZSK;

15 - block calculating the maximum VKF and determining amendments to the planned coordinates of the aircraft;

16 - airborne control system of the aircraft.

Presented in Fig.6 version of the radar system that implements the proposed method for navigation of aircraft on radar images of the earth's surface, operates as follows.

When the point of correction of the flight path of the aircraft is reached, the monopulse antenna 1 under control coming from the digital radar control device 5 is installed in the direction of sight of a given area. Monopulse antenna emits sounding radar signals received through the receive-transmit switch 2 from the transmitter 3, controlled by the radar synchronizer 4, as well as receiving reflected signals, ensuring their spatial selection. Through the receive-transmit switch 2, the received signals are received at the input of the radar receiver 9, from the quadrature outputs of which are fed to the input of the multi-channel analog-to-digital converter 10. The converted signals are multiplied with the reference function coming from the digital radar control device 5 in the multiplication unit 11, s the purpose of focusing the synthesized radar image of the observed portion of the earth's surface formed by the Fourier transform unit 12 in the coordinates "Doppler frequency - range ". This radar image through the distribution block 13 enters the shaper amendments 6 to the estimates of the current values of the velocity vector and the flight altitude of the aircraft, formed by the onboard control system 16 of the aircraft. Corrections to estimates of the components of the velocity vector can be generated using the algorithm described in [3]. An estimate of the relative flight altitude of an aircraft can be obtained using the algorithm described in [4]. The generated correction values are stored in block 7 for storing amendments to the estimates of the current values of the aircraft flight parameters generated by the BKU 16. Using these corrections and the values of the estimates of the current flight parameters of the aircraft coming from the BKU 16, the calculator 8 refines these estimates and their output to the shaper 14 working radar data to the NSC, which also receives frames from the distribution block 13 of the primary working radar data of the observed terrain in the coordinate system "Doppler frequency-range". From the shaper 14, the operational radar images presented to the NSC are sent to block 15, in which the maximum VKF of the working radar and the standard radar image received from the digital control device 5 are calculated, and corrections of the aircraft’s planned coordinates are generated by the position of the maximum RCF.

The proposed method for navigating an aircraft along radar images of the earth's surface has undergone mathematical modeling and experimental testing.

The research results confirmed the efficiency of the proposed method for navigating aircraft on radar images of the earth's surface.

The proposed method for navigating aircraft on radar images of the earth's surface allows for high accuracy of navigation of aircraft using airborne radar tools.

Using the proposed method does not impose any additional restrictions on the element base and does not impose any significant requirements on the speed and memory size of radar computers.

Information sources

1. The control system of an unmanned aerial vehicle. RF patent No. 2189625 dated 09/20/2002, IPC G05D 1/12, F41G 7/36.

2. A method of navigating an aircraft through radar images of the earth's surface using digital terrain models. RF patent №2364887 from 09.25.2007, IPC G01S 13/90 - prototype.

3. Kozayev A.A., Koltyshev U. U., Frolov F.Yu., Yankovsky V.T. Algorithm for Doppler velocity measurement in a radar with a synthesized aperture // Radio Engineering, 2005, No. 6, p.13.

4. US patent US No. 5430445, 12.31.1992, G01S 13/90.

5. Babich O.A., Dobrolensky Yu.P., Kozlov M.S. and others. Aviation devices and navigation systems. / Ed. O.A. Babicha. - M.: VVIA named after prof. N.E. Zhukovsky, 1981.

Claims (1)

A method of navigating an aircraft through radar images of the earth’s surface, which consists in determining the navigation errors of the aircraft’s onboard control system by assessing the maximum position of a two-dimensional function that reflects the cross-correlation of the reference and working radar images of the reference areas, and the working radar images of the reference areas form during the movement of the aircraft using the airborne radar with the synthesis of the antenna aperture, characterized in that the formation of both reference and working radar images, as well as their inter-correlation processing, is carried out in a normal earth coordinate system, the origin of which is combined with the aiming point on the reference area, and the coordinate axes are oriented in accordance with the conditions of the navigation task, while at each point of correction, the values of the corrections to the values of the flight parameters measured by the onboard control system are determined aircraft, form the frames of the primary working radar images of the reference terrain in the coordinate system "Doppler frequency - range" taking into account certain values of the amendments to the values of the flight parameters formed by the onboard control system of the aircraft, convert the frames of the primary working radar images of the reference terrain from coordinate system "Doppler frequency - range" into the normal earth coordinate system of the reference area month In fact, they perform the formation of a working radar image of the reference terrain on the basis of incoherent summation of the frames of the primary working radar images, at each correction point when changing the relative relative positions of the working and reference radar images, they perform the inter-correlation processing of the reference and working radar images of the reference terrain in a normal earth coordinate system reference area, according to which based on the coordinates of the maximum of the two-dimensional cross-correlation function of the working and reference radar images, the corrections of the planned coordinates of the aircraft relative to the reference area are determined.

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