RU2038612C1 - Range finder for locating low-altitude and low-speed targets in earth return background - Google Patents

Abstract

FIELD: radar for locating low-altitude and low-speed targets. SUBSTANCE: range finder has transmitter 1, transmit-receive switch 2, antenna array of two radiating elements 3 and 4, subtracter 5, receiver 6, controlled phase shifter 7, indicator 8, and synchronizer 9. EFFECT: improved design. 4 dwg

Description

 The invention relates to radar, in particular, to the fundamentals of the construction and construction of low-altitude radars, and can be used in radars to suppress reverse interference reflections of sounding signals from the earth's surface.

 A known method and device for suppressing passive interference in a Doppler radar containing a transmitter and a receiver of echo signals [1] From the received signals are generated signals including Doppler frequency components, which are supplied to a signal processing device containing a Doppler processor. This processor performs selective frequency filtering of signals. At least one filtered signal is generated in the Doppler processor, which is compared with the received signal in the amplitude comparison device. As a result of the comparison, the last device generates output signals of the first or second level. The signal of the first level is generated in those cases when the ratio of the intensities of the compared signals lies within a given range of values. The second level signal is generated when the specified ratio is outside the specified range. The output signal with the amplitude comparison device is supplied to a gating device through which the radar output signal with suppressed reflections from passive interference is selectively passed. The signal passes through the gating device only if the ratio of the signal passed through the filter to the pleasant signal exceeds a predetermined level.

 However, such a device cannot detect low-altitude low-speed and motionless air targets against the background of reverse interference reflections, probe signals of the transmitter from the earth's surface. The echo signals of such targets have almost no Doppler frequency shift and will not be selected by the Doppler processor of this device. As a result, the target echo will not pass through the gating device and the target will not be detected.

 Also known is the coherent-pulse system for moving target selection (SDC) [2]. In this system, the oscillations of a special so-called coherent local oscillator are used as reference oscillations with which echo signals are compared. These oscillations are tightly phase locked to the oscillations of the transmitter. Phasing of a coherent local oscillator is carried out at an intermediate frequency.

 The pulses of the high-frequency energy of the transmitter are emitted by the antenna into space. Oscillations of the transmitter, converted by means of the local oscillator voltage into pulses of intermediate frequency (phasing pulses), are also transmitted to the coherent local oscillator for its phasing. During phasing, the frequency and phase of oscillations of a coherent local oscillator is equal to the frequency and phase of oscillations of the phasing pulse. In this way, coherence of the oscillations of the transmitter and the coherent local oscillator is ensured.

 The phased voltage of the coherent local oscillator is supplied to a phase detector, which also receives the received echo signal, converted by the same local oscillator into an intermediate frequency signal.

 For echo signals from stationary objects, the video signals at the output of the phase detector have a constant amplitude from period to period of pulses, and for echo signals from a moving target at the output of the phase detector, video pulses appear, the amplitude of which varies from period to period in a cosine law with a Doppler frequency.

 After the phase detector, the signals are supplied to the compensating device. In the latter, the periodical subtraction of the video signals from the output of the phase detector is carried out, moreover, video signals with a constant amplitude are suppressed, and pulses whose amplitude varies from period to period are emitted. Thus, for signals from stationary objects (earth's surface, cloud, etc.), the signal at the output of the compensating device is zero, and for signals from a moving target it is non-zero.

 The disadvantage of such a coherent-pulsed radar is that it cannot detect low-speed and stationary air targets against the background of reverse interference reflections of the probe signals of the transmitter from the earth's surface. The echo signals of such targets have almost no Doppler frequency shift and will be suppressed by the SDC system described above in the same way as passive interference.

 Closest to the invention is a radar system for detecting low-flying targets [3] In this system, low-altitude targets are detected using a radar transceiver, the antenna of which emits centimeter-wave energy during the transmission transmission interval. The received high-frequency energy during the reception interval is divided into two separately distributed components. This separation is carried out using two vertically spaced emitters forming part of the antenna of the transceiver. Between the antenna and the receiver, high-frequency energy is distributed along two parallel branches, which are then combined in the total and difference channels, forming the total and difference components of the received signal. To detect low-altitude targets, the channel in which there is a more intense signal is selected. In this case, the difference channel is formed using a high-frequency subtraction device, the input of which receives signals received by the lower and upper radiating elements of the antenna system, and the total channel using a high-frequency adder. An antenna switch was used to switch the antenna from transmission to reception, and a computing device and a channel switch with its control device were used to select the best receiving channel.

 However, this radar system cannot detect targets against the background of reverse interference reflections of probing signals from the earth's surface. When a target is found in the field of such interfering reflections, the level of passive interference from the ground is often significantly higher than the target echo level. In this case, the interference level in the sum channel will usually be higher than in the difference channel. In this case, the computing device of the prototype will often choose the total receiving channel, focusing on a higher level of interference. As a result, the echo of the target will be lost in passive interference, it becomes impossible to detect the target and measure its distance in the field of interference reflections from the ground.

 The objective of the invention is the suppression of reverse interference reflections of the probe signals of the transmitter from the earth's surface and thereby improving the ability to detect low-altitude low-speed and stationary air targets in the field of these reflections.

Z_o+h

+Z

Z

-2Z_o-2h

+

1+

×
×

1+

1-

1-

1-

-

(Z_o+h_в)²+Z

Z

-2Z_o-2h

+

1+

×
×

1+

1-

1-

1-

(1) на трассе со сложным рельефом местности, где Δφ(t) сдвиг фаз, на который фазовращатель изменяет фазу принимаемого сигнала;
t время запаздывания;
λ- длина волны;
с скорость света;
h_н, h_в высоты над землей нижнего и верхнего элементов антенны;
а_э эквивалентный радиус Земли;
Z_о- высота над уровнем моря точки расположения дальномера;
Z( Z

) -высотный профиль рельефа местности на трассе между дальномером и целью.To do this, in the range finder for detecting low-altitude low-speed targets, containing a pulse transmitter, a receiver, an antenna switch, an antenna system of two identical emitting elements spaced in height and a high-speed subtraction device that subtracts the echo signals received by the lower and upper elements of the antenna system between the lower element the antenna system and the subtraction device included controlled phase shifter, periodically changing the phase of the received signals synchronously with the probing pulses of Occupancy is inversely proportional to the delay time of echoes of a flat underlying surface or legally
Δφ (t)

Z _o + h

+ Z

Z

-2Z _o -2h

+

1+

×
×

1+

1-

1-

1-

-

(Z _o + h _in ) ² + Z

Z

-2Z _o -2h

+

1+

×
×

1+

1-

1-

1-

(1) on a track with a complex terrain, where Δφ (t) is the phase shift by which the phase shifter changes the phase of the received signal;
t lag time;
λ is the wavelength;
with the speed of light;
h _n , h _in height above ground of the lower and upper antenna elements;
and _{e is the} equivalent radius of the Earth;
Z _about - the height above sea level of the location of the rangefinder;
Z (Z

) is the elevation profile of the terrain on the highway between the range finder and the target.

 In this case, only the upper radiating element of the antenna system is used as the transmitting antenna, which is switched from transmission to reception using the antenna switch, and the total channel with the adder, the switch of the total and difference channels with its control device and a computing device are selected from the prototype device best channel.

 The technical nature and principle of action proposed.

 In FIG. 1, 2 presents a simplified structural diagram of the proposed range finder for detecting low-altitude low-speed targets against a background of interfering reflections from the ground and the principle of its operation, respectively; in FIG. 3 dependence of the phase shifter phase on the delay time; in FIG. 4 normalized visibility zones in the vertical plane for the prototype over a flat underlying surface (dashed curve), for the proposed device over complex mountainous terrain (dashed curve) and for the proposed device over flat ground.

 The range finder consists of a pulse transmitter 1 connected through an antenna switch 2 "transmit-receive" with the upper transceiving radiating element 3 of the antenna system, the lower element 4 of which is used only for receiving signals. The signals received by the upper and lower elements of the antenna system are subtracted in the high-frequency subtraction device 5 and fed to the receiver 6 and then to the indicator 8 for measuring the target range. Between the lower element 4 of the antenna system and the subtraction device 5, a phase shifter 7 controlled by a synchronizer 9 is included, periodically changing (delaying) the phase of the signals received by this element according to the above law.

 The principle of operation of the proposed device is illustrated as follows. Echoes of a low-altitude target come to the elements of the receiving antenna system of the rangefinder in two ways: a direct wave and a wave reflected from the earth's surface. Passive interference, which is the result of backscattering of the energy of the probe pulses of the transmitter on the elements of the earth's surface, comes to the rangefinder antenna mainly from negative elevation angles only by a direct wave. This is schematically shown in FIG. 2, which shows the lower and upper elements of the antenna system of the rangefinder, a low-altitude target, the elevation profile of the terrain Z (D) on the path between the antenna and the target, and the rays of the radio waves of the target echoes and reverse interference reflections from the current resolution cell with τ / 2 (s the speed of light, τ is the duration of the probe pulse) on the earth's surface. When the target is in the area of interfering reflections from the ground, the target echoes and interfering reflections from the corresponding resolution cell arrive at the range finder antenna at the same time, this makes it difficult to detect the target whose echo is lost in these interfering reflections.

(r_в-r_н) (2) с помощью управляемого фазовращателя и вычесть помеховые отражения с верхнего и нижнего элементов антенны с помощью высокочастотного устройства вычитания, то на выходе этого устройства помеховые отражения от земли будут почти полностью подавлены. При этом полагается, что верхний и нижний элементы антенны одинаковы, направлены горизонтально и имеют симметричные диаграммы направленности вертикальной плоскости в свободном пространстве. Это обеспечивает примерное равенство амплитуд помеховых отражений от земли, принимаемых этими элементами.The phase of interfering reflections in the upper element of the receiving antenna system usually lags behind the phase of these reflections in the lower antenna element, since the distance r _in from the current resolution cell on the earth's surface to the upper antenna element is greater than the corresponding distance r _n to the lower antenna element (Fig. 2). If you delay the phase of interference reflections from the lower element of the antenna by
Δφ

(r _to -r _n ) (2) using a controlled phase shifter and subtract interference reflections from the upper and lower antenna elements using a high-frequency subtraction device, then at the output of this device, interference reflections from the ground will be almost completely suppressed. It is assumed that the upper and lower elements of the antenna are the same, directed horizontally and have symmetrical radiation patterns of the vertical plane in free space. This provides an approximate equality of the amplitudes of the interfering reflections from the earth received by these elements.

λ

F_э(θ)e

e

+

e

, (3) где λ- длина волны;
G_m коэффициент усиления элемента антенны;
R_вх входное сопротивление фидерного тракта;
S плотность мощности прямой радиоволны от цели вблизи антенны;
F_э(θ) нормированная диаграмма направленности элемента антенны в вертикальной плоскости в свободном пространстве;
θ- угол места цели;
φ- фаза радиоволны, приходящей от цели в точку расположения антенны на поверхности земли;
К 2 π/λ- волновое число;
h_а высота элемента антенны над землей (h_a h_н для нижнего элемента и h_a h_в для верхнего элемента);

комплексный коэффициент отражения радиоволн от земной поверхности при вертикальной или горизонтальной поляризации.The echo signals of a low-speed low-speed target at the output of the subtraction device 5, in contrast to interfering reflections, will not be completely suppressed and often their level will be higher than in any one of the antenna elements. This is explained by the following. Echoes of a low-altitude target come to the lower or upper element of the antenna in two ways: a direct wave and a wave reflected from the ground. In this case, the complex amplitude of the target echo voltage at the output of any receiving element of the antenna (upper or lower) is determined by the well-known formula

λ

F _e (θ) e

e

+

e

, (3) where λ is the wavelength;
G _m the gain of the antenna element;
R _I input resistance of the feeder path;
S is the power density of the direct radio wave from the target near the antenna;
F _e (θ) normalized pattern of the antenna element in the vertical plane in free space;
θ is the elevation angle of the target;
φ is the phase of the radio wave coming from the target to the antenna location point on the earth's surface;
K 2 π / λ is the wave number;
h _{a is the} height of the antenna element above the ground (h _a h _n for the lower element and h _a h _in for the upper element);

complex coefficient of reflection of radio waves from the earth's surface with vertical or horizontal polarization.

-1 при любой поляризации радиоволн. Учитывая это, можно представить формулу (3) для комплексной амплитуды напряжения на выходе приемного элемента антенны в следующем виде

2λ

F_э(θ)e

sin(kh_asinθ) (4)
Из этой формулы видно, что напряжения эхо-сигналов маловысотной цели на выходах нижнего и верхнего элементов антенны находятся либо в фазе, либо в противофазе и не могут иметь какого-либо иного сдвига фаз. Кроме того, эти напряжения (в отличие от соответствующих напряжений помеховых отражений) обычно отличаются друг от друга по амплитуде. Из этого следует, что сдвиг фазы напряжения эхо-сигнала цели с нижнего элемента антенны в управляемом фазовращателе отличается на Δφ(формула (2)) и последующее вычитание напряжений эхо-сигнала с верхнего элемента антенны и с выхода фазовращателя обычно не приводит к полному подавлению эхо-сигнала цели на выходе устройства вычитания.It is known that at small slip angles the complex reflection coefficient from the ground

-1 for any polarization of radio waves. Given this, we can present formula (3) for the complex amplitude of the voltage at the output of the receiving element of the antenna in the following form

2λ

F _e (θ) e

sin (kh _a sinθ) (4)
This formula shows that the voltage of the echo signals of the low-altitude target at the outputs of the lower and upper elements of the antenna are either in phase or out of phase and cannot have any other phase shift. In addition, these voltages (in contrast to the corresponding voltages of interfering reflections) usually differ in amplitude. It follows that the phase shift of the voltage of the target echo signal from the lower antenna element in the controlled phase shifter differs by Δφ (formula (2)) and the subsequent subtraction of the echo voltage from the upper antenna element and from the output of the phase shifter usually does not completely echo - the signal of the target at the output of the subtraction device.

 Thus, at the output of the subtraction device, the echo signal of the low-altitude target will be highlighted, and the back interference reflections from the earth's surface will be almost completely suppressed. In this case, the target echo may not have a noticeable Doppler frequency shift, i.e. the proposed device will be able to effectively detect low-speed low-altitude air targets against the background of intense interference reflections from the earth's surface.

, (5) где D горизонтальная дальность от дальномера до текущей ячейки разрешения по поверхности земли, изменяющаяся с течением времени запаздывания t по формуле
D

, (6) где с скорость света;
λ- длина волны;
h_н, h_в высоты над землей нижнего и верхнего элементов антенной системы;
а_э эквивалентный радиус земли;
Z_о -высота над уровнем моря точки расположения дальномера;
Z(D) высотный рельеф местности на трассе между дальномером и целью.The phase shift Δφ, which must be controlled by a phase shifter 7 to ensure subsequent suppression of interference reflections from the ground, is determined by formula (2) (with a minus sign) and depends on the delay time t, since over time the radio-frequency reflection cell cτ / 2 on the surface the earth moves along the route and accordingly the distances r _in , r _n from this cell to the upper and lower elements of the receiving antenna of the range finder change. Using the geometry of the figure (Fig. 2) and the cosine theorem, we can imagine the dependence of the phase shift Δφ (t), phase shifter 7 on the delay time t, i.e. formula (2), in the following form
Δφ (t)

, (5) where D is the horizontal distance from the range finder to the current resolution cell over the earth’s surface, changing over time lag t according to the formula
D

, (6) where c is the speed of light;
λ is the wavelength;
h _n , h _in height above ground of the lower and upper elements of the antenna system;
and _{e is the} equivalent radius of the earth;
Z _{about the} height above sea level of the location of the rangefinder;
Z (D) high-altitude terrain on the highway between the range finder and the target.

степенным рядом и используя элементарные математические преобразования, можно представить формулу (5) в более удобном для расчетов виде (1).Formula (5) is not convenient for calculations, since in it under the radicals there are small differences of very large quantities. Representing the cosine of the small argument cos

power series and using elementary mathematical transformations, it is possible to represent formula (5) in a more convenient form for calculations (1).

-1

≈

h $\genfrac{}{}{0ex}{}{\text{2}}{\text{н}}$ -h

, (7) т.е. в этом случае фаза фазовращателя 7 должна изменяться в течение периода следования импульсов обратно пропорционально времени запаздывания.Over a flat underlying surface or over the sea (Z (D) 0), formula (1) is significantly simplified:
Δφ (t)

-1

≈

h $\genfrac{}{}{0ex}{}{\text{2}}{\text{n}}$ -h

, (7) i.e. in this case, the phase of the phase shifter 7 should change during the pulse repetition period in inverse proportion to the delay time.

Examples of calculated dependences of the phase Δφ (t) of the phase shifter 7 on the delay time t are presented in FIG. 3. These graphs were calculated for a particular embodiment of the apparatus (wavelength λ = 0,35 m, the height above ground of the antenna radiators h _n and h 15 m ₃₀ m) for a flat underlying surface (dashed line) and for a particular example of a complex mountainous terrain terrain on the highway between the range finder and the target. These examples show that the phase characteristics of the phase shifter necessary for the operation of the device can be realized using known constructions of controlled phase shifters.

The above elements of the structural diagram of the proposed device (Fig. 1) are made as follows. The lower 4 and upper 3 radiating elements of the antenna system are the same, directed horizontally and have symmetrical radiation patterns in a vertical plane in free space. As these elements, for example, conventional horn or mirror antennas can be used. The elevation heights of these elements above the ground must necessarily be different. A large (in comparison with the wavelength) height spacing of these elements is not advisable, since this increases the necessary interval of the phase change of the controlled phase shifter 7. In addition, with a large height spacing of these elements in the region of interfering reflections from the earth, areas of considerable size appear shaded for the bottom element and not shaded for the top. Interference reflections from such areas, received only by the upper element, will not be suppressed by the subtraction device 5. A small height difference of radiating elements 3 and 4 is also not advisable, since in this case the device will greatly weaken the echo signals of low-altitude targets, therefore, for each specific position of the range finder in difficult terrain, you can find the optimal lifting heights of the lower h _n and upper h _{in the} elements above land in which the use of the proposed device in a given responsible sector will be most effective.

 The controlled phase shifter 7 can be implemented, for example, in the form of a ferrite phase shifter, for the magnetization of which DC pulses with a steep leading and slowly falling trailing edge should be used. The repetition rate of these pulses should be equal to the repetition rate of the probe pulses of the transmitter.

 Another implementation of the controlled phase shifter 7 is possible, for example, in the form of a set of short segments of strip lines switched by p i n diodes. To connect such lines, control rectangular pulses of constant voltage of appropriate duration should be used. In this case, the repetition rate of the control pulses for p i n diodes should also be equal to the repetition rate of the probe pulses of the transmitter.

 Other elements of the apparatus shown in FIG. 1, are no different from the corresponding elements of the prototype.

 The range finder works as follows.

 The transmitter 1 generates, and the upper element 3 of the antenna system emits a sounding radio pulse during the transmission interval. During the reception interval, elements 3 and 4 of the antenna system receive target echoes and interference reflections from the earth's surface. Interference reflections from the ground, received by element 4 of the antenna system, are compared in phase with the corresponding interference reflections from the upper element 3 using a controlled phase shifter 7 and fed to the high-frequency subtraction device 5. Interference voltages (and target echoes) and received by the upper element 3 of the antenna system also come to the same device through the antenna switch 2. At the output of the subtraction device 5, interfering reflections from the ground are almost completely suppressed, and the echo signals of low-altitude targets are allocated and fed further to the receiving and measuring range.

For a comparative assessment of the capabilities of the prototype and the proposed device for the detection of low-altitude targets, the visibility ranges of rangefinders in the vertical plane were calculated. The calculation results are presented in FIG. 4, where normalized visibility zones in the vertical plane are constructed (i.e., the dependence of the detection range r _m on the elevation angle θ) for the prototype over a flat underlying surface (dashed curve), for the proposed device over a complex mountainous terrain (dashed curve), and for the proposed devices above flat ground. The visibility zones were normalized with respect to the maximum detection range r _mo in free space for rangefinders with a transceiver antenna from one radiating element of the same type as in the proposed device.

 As seen in FIG. 4, the proposed device is fully capable of detecting low-altitude targets in the sector of small elevation angles. Outside the field of interference reflections from the ground, the capabilities of the proposed device are not much inferior to the prototype. In the field of interfering reflections, the proposed device significantly exceeds the prototype and can detect low-altitude, low-speed targets against the background of intense interfering reflections from the ground, and the prototype in this area can hardly detect such targets.

Claims (1)

RANGE FOR DETECTION OF LOW-SPEED SHORT-SPEED TARGETS ON THE BACKGROUND OF INTERFERENCE REFLECTIONS FROM THE EARTH, containing an antenna system of two identical emitting elements spaced apart in height, a pulse transmitter connected to the upper radiating element of the antenna system through an antenna switch, a receiver whose output is connected to the indicator with a transmitter, receiver and indicator, a high-frequency subtraction device, the output of which is connected to the input of the receiver, and the input to the upper radiating element ohm of the antenna system through the antenna switch, characterized in that between the lower element of the antenna system and the other input of the high-frequency subtraction device, a controlled phase shifter connected to the synchronizer at the control input is connected, periodically changing the phase of the received signals synchronously with the probe pulses of the transmitter in inverse proportion to the delay time of the echo signals above flat ground or on a track with a difficult terrain according to the law

where Δφ (t) is the phase shift of the controlled phase shifter;
t lag time;
λ wavelength;
c is the speed of light;
h _n , h _in height above the Earth of the lower and upper elements of the antenna system;
a _{e is the} equivalent radius of the Earth;
z _{0 the} height above sea level of the location of the rangefinder;

elevation profile of the terrain on the highway between the range finder and the target.

Images