RU2103706C1 - Method of radar calibration and radar - Google Patents

Abstract

FIELD: radiolocation. SUBSTANCE: calibration signal is formed in receiving section of radar. It is formed of spurious pulse soaked through antenna switch to input of radar receiving section at moment of probing pulse radiation as a result of reduction of power of soaked spurious pulse. Radar is provided additionally with controlled switch, switch, integrator, subtraction unit, control pulse former, and synchronizer. Radar variants and diagram are presented. EFFECT: higher accuracy of calibration. 5 cl, 4 dwg

Description

 The invention relates to radar and can be used during radio meteorological measurements.

 Known methods for calibrating the radar [1], based on the use of passive or active reflectors. The use of these methods is limited due to the difficulty of detecting the target against the background of local reflections.

There is a known method of calibrating a radar [3], selected as the closest analogue, which consists in counting the received echo signals from the level of the calibration signal generated from the local oscillation signal by shifting by the value of the intermediate frequency with subsequent separation of the harmonic and generating a pulse, the duration of which is equal to the duration of the emitted pulse, and the level is a certain part of the transmitter power, for which the generated signal is compared with the probe pulse weakened several times Maintaining a constant ratio _{P t τ / P k = K} , where P _t τ - cumulative transmitter power, P _k - pulsed power calibration signal, K - constant, wherein the calibration signal is fed into the receiving channel immediately before the probe signal radiation, and comparison calibration and echo signals are performed after the conversion of signals carried out in the receiving part.

 Known radar [2], containing a synchronizer, modulator, pulsed microwave generator, transmitting and receiving antenna, antenna switch, receiver, detector and terminal device. Such a construction of the circuit does not allow the calibration of the radar.

 Known radar [3], selected as the closest analogue, containing serially connected transmitter, antenna-equivalent switch, antenna switch, second directional coupler and antenna connected to the second output of the antenna switch mixer, receiving path (intermediate frequency amplifier and detector) and indicators, as well as a first directional coupler, an input whose first and second outputs are connected respectively to the second output of the antenna-equivalent switch, the antenna equivalent the second input of the second directional coupler and, in addition, short-delay line connected to the output of the mixer or to the second output of the receive path, and a display portion of the meter to be connected to the third output of the receive path.

 The disadvantages of the calibration method are as follows.

 When conducting radar measurements, the power of the received signal is usually measured from some initial level, which is taken as the noise level of the receiver or the signal level of the noise generator (the minimum detectable signal). The basis for quantitative measurements in weather radar is the multiple target equation that relates the characteristics of the target with the received power and radar parameters.

где

- средняя мощность принятого сигнала;
P_t - излученная мощность;
G - эффективный коэффициент усиления антенны;
λ - длина волны;
c - скорость распространения радиоволн;
τ - длительность зондирующего импульса;
θ - ширина диаграммы направленности;
η - суммарное поперечное сечение обратного рассеяния единичного объема;
K_o - коэффициент ослабления;
R - расстояние до выделенного объема.

Where

- average power of the received signal;
P _t is the radiated power;
G is the effective antenna gain;
λ is the wavelength;
c is the propagation velocity of radio waves;
τ is the duration of the probe pulse;
θ is the width of the radiation pattern;
η is the total backscattering cross section of a unit volume;
K _o - attenuation coefficient;
R is the distance to the allocated volume.

Здесь δ - отношение мощности принятого сигнала

к минимально обнаружимому сигналу

, П_м - характеристика метеорологического потенциала МРЛ, определяемая соотношением:

Однако за начальный уровень, от которого отсчитывается мощность принятого сигнала, в ближайшем аналоге принят не уровень шумов приемника, а реперный (калибровочный) сигнал, формируемый из части зондирующего импульса и подаваемый на вход приемника. При этом уравнение (3) приведено к виду:

где Δ - отношение мощности принятого сигнала

к величине реперного сигнала P_k.In quantitative measurements, equation (1) is represented as:

Here δ is the ratio of the received signal power

to minimally detectable signal

, P _m - the characteristic of the meteorological potential of the satellite, determined by the ratio

However, for the initial level, from which the received signal power is measured, the nearest analogue is not the receiver noise level, but the reference (calibration) signal generated from part of the probe pulse and fed to the receiver input. In this case, equation (3) is reduced to the form:

where Δ is the ratio of the received signal power

to the value of the reference signal P _k .

 From equation (4) it follows that the radar parameters that you need to know when measuring radar signals reflected from weather patterns are only antenna characteristics, wavelength and constant K. Due to the fact that the reference signal undergoes the same transformations in the receiving path, as the received signals, and its value at the input of the receiver is strictly connected with the transmitter power, the values of the output signals counted from the level of the reference signal will not depend on the parameters of the transmitter, as well as the sensitivity , gain and receiver tuning accuracy.

 In fact, there is no need to maintain a constant calibration signal, and the accuracy of measurements is determined by the stability of K.

 The disadvantage of the considered calibration method is that it does not take into account the difference in the spectral characteristics of the probe and reference signals, as well as the imperfect coincidence of their frequencies. These differences are due to the fact that the reference signal in this method is formed from the local oscillator by shifting by the value of the intermediate frequency, followed by the allocation of harmonics.

будет осуществляться со значительной погрешностью, величина которой будет непостоянной и зависящей от параметров реперного и зондирующего сигналов.To maintain the constancy of the ratio of the frequencies of the main and reference signals in the considered method, a system of automatic frequency adjustment (AFC) of the radar is used. As you know, the frequency drift of the magnetron generator is 0.1 - 0.5%, which corresponds to about 15 MHz, for example, for the MPL-2 radar [4]. At the same time, it is known [2] that the duration of transient processes in the AFC system is significantly longer than the pulse repetition period (for example, in the Groza radar station (radar), the time constant is 15–30 s [5], and the passband of the intermediate-frequency amplifier (IFA) (on average) - 0.4 - 3.2 MHz. Obviously, in the case of fast changes in the magnetron frequency, the reflected signal may not be converted in the mixer-IFA system, which will lead to the loss of information. reduces calibration accuracy because Since the local oscillator generation frequency is quite stable, the reference signal will be completely converted in the receiving path, creating a false idea that the system is functioning normally. Since the echo signals will be counted from the fully converted reference, the value

will be carried out with a significant error, the value of which will be unstable and depending on the parameters of the reference and probing signals.

 Another disadvantage is the presence of a hardly accountable error due to the difference in the spectral characteristics of the signals. The creation of a calibration signal from a heterodyne signal by modulation with the subsequent separation of the harmonic forms a spectrum that does not coincide with the spectrum of the pulse signal, and the latter may have variable spectral characteristics.

где F(f) - амплитудный спектр сигнала;
S_o(f) - нормированный энергетический спектр зондирующего импульса;
H_n - коэффициент преобразования излучаемого сигнала исследуемой целью.This statement can be illustrated using the following constructions. The echo signal is, as you know, a pulsed random process, the energy spectrum of which can be described by the relation [6]:

where F (f) is the amplitude spectrum of the signal;
S _o (f) is the normalized energy spectrum of the probe pulse;
H _n - conversion coefficient of the emitted signal of the target.

вычисляется из соотношения:

Поскольку оба сигнала проходят преобразования в приемном тракте РЛС, то при переходе от (5) к (6) необходимо ввести соответствующие коэффициенты преобразования (КП). Тогда

где μ_o - КП эхо-сигнала;
μ_k - КП репера;
S_o(f)_k - нормированный энергетический спектр калибровочного импульса.When counting the echo from the calibration error determination

calculated from the relation:

Since both signals undergo conversions in the receiving path of the radar, then, when switching from (5) to (6), it is necessary to introduce the corresponding conversion coefficients (CP). Then

where μ _o - KP of the echo signal;
μ _k - KP benchmark;
S _o (f) _k is the normalized energy spectrum of the calibration pulse.

В силу неравенства спектров S_o(f) и S_o(f)_k не совпадают и величины μ_o и μ_k . Возможные быстрые изменения спектра излучаемого сигнала, обусловленные частотной модуляцией, делают величины S_o(f) и S_o(f)_k практически несопоставимыми, в силу чего величина δ P_r становится постоянно меняющейся в ту или иную сторону. В результате КП также постоянно изменяется, что влияет на точность калибровки.Introduction to (7) μ _o and μ _k is explained by the fact that, when signals are converted in the receiving path of the radar, the spectrum is fragmented, which cannot be the same for different signals. As a result, you can write:

Due to the inequality of the spectra S _o (f) and S _o (f) _k , the values of μ _o and μ _k do not coincide either. Possible rapid changes in the spectrum of the emitted signal due to frequency modulation make the values of S _o (f) and S _o (f) _k practically incomparable, due to which the value of δ P _r becomes constantly changing in one direction or another. As a result, the KP also constantly changes, which affects the accuracy of calibration.

 Thus, it can be argued that the formation of the calibration signal in accordance with the considered method leads to the appearance of an uncontrolled and difficult to account error.

 To implement this method, significant changes must be made to the radar circuit. Moreover, in the case of the equipment of the existing serial radar, quite serious changes in the microwave path are inevitable, which are not always possible due to the very dense layout of the unit (for example, in a Thunderstorm aircraft radar [2]).

 In addition, the presence of several control elements determines the dependence of the constancy of the value of K on the stability of the parameters of these blocks, which either implies the presence of a constant error introduced by these blocks, or the use of additional stabilization measures that further complicate the block, leading to an increase in the cost of the device.

 The disadvantage of this radar scheme is that calibration is not possible during the measurement process, which reduces the accuracy of the results. In addition, the use of this device in commercially available radars leads to the need to change the microwave path, which is not always possible due to the density of the layout of the unit.

 The problem to which the invention is directed includes increasing the accuracy of calibration, simplifying the circuit and reducing the cost of radars implementing this method, as well as providing the possibility of automatic calibration.

 The solution to this problem is achieved by the fact that in the method of calibrating the radar, which consists in counting the received echoes from the level of the calibration signal, which is a known part of the transmitter power, carried out after the echo signal is converted in the receiving part of the radar, the calibration signal is formed from a spurious pulse that has leaked through antenna switch to the input of the receiving part of the radar at the time of emission of the probe pulse by attenuation (power) of the leaked spurious pulse by a known amount to a level included in the dynamic range of the receiver signals.

 The solution to this problem is achieved by the fact that in the device of the radar station, containing a series-connected transmitter, antenna switch and antenna, as well as a receiver, a controllable switch connected between the input of the receiver and the output of the antenna switch, a series-connected key, integrator and subtraction unit, is additionally introduced, the second the input of which is connected to the output of the receiver, and the output is the output of the radar, the synchronizer, the output of which is connected to the control input of the transmitter, f rmirovatel control pulses, and a synchronizer connected between the control input of controlled switch, the input and the control input of switch is connected respectively to the output of the receiver and to the synchronizer.

 According to the second variant, the radar station is characterized in that a delay line is connected between the output of the synchronizer and the output of the driver of the control pulses.

 According to the third embodiment, the radar station is characterized in that the control pulse shaper consists of series-connected delay lines and a standby multivibrator, and the input of the delay line and the output of the standby multivibrator are respectively the input and output of the shaper.

 In the fourth embodiment, the radar station is characterized in that a delay line is connected between the output of the synchronizer and the control input of the transmitter.

 In FIG. 1 shows a diagram of a radar; figure 2 - 4 - scheme options.

 The radar contains a synchronizer (Cx) 1, a control pulse shaper (FCI) 2, a transmitter 3, a controlled switch (HC) 4, an antenna switch (AP) 5, an antenna 6, a receiver 7, a key 8, an integrator 9, a subtraction unit (BV) ten.

 The device operates as follows. The clock pulse from the output of Cx 1 is fed to the input of the IGF 2 and the control (triggering) input of the transmitter 3. The latter, under the influence of the clock pulse, generates a powerful microwave pulse, transmitted through AP 5 to the antenna 6 and radiated into space. At the same time, part of the energy of the probe pulse through AP 5 “leaks” to the input of HC 4. Due to the fact that the sync pulse also arrives at the input of the IGF 2, the latter generates a control signal supplied to the control input of HC 4. As a result, HC 4 attenuates the leaked signal to level corresponding to the dynamic range of the receiver 7, i.e. to a level sufficient to count the received echo signals from it, and safe for input circuits, for example, mixing diodes. From the output of the receiver 7, the reference (calibration) signal thus formed, through the key 8 controlled by the clock pulse, is supplied to the integrator 9 and, after averaging, is fed to the second input of the BV 10.

 The reflected echo pulses are received by the antenna 6 and, passing through the AP 5, HC 4 and the receiver 7, are fed to the first input of the BV 10. After comparison with the benchmark, this signal is transmitted to subsequent processing devices.

The inclusion in the receiving path of HC 4, controlled by the IGF 2, makes it possible to introduce a constant attenuation into the signal leaking at the input of the receiving part. When applying to the control input of HC 4 a signal that performs complete "locking", a constant ratio K = P _t τ / P _{k is} maintained. \\ 2 Since the control signal at HC 4 is supplied only at the time of the radiation of the probe pulse, HC 4 does not interfere with the free passage of echo signals to the input of receiver 7.

 The control signal is generated by the IGF in such a way that the “blocking” of the HC 4 occurs somewhat earlier, and the “unlocking” is somewhat later than the probe pulse. This ensures the protection of the mixing diodes of the receiving part from the leaked pulse and ensures the stability of K.

 In the case of the implementation of the radar according to the scheme of Fig. 2, the clock coming from the output of Cx 1 is delayed by LZ 11 by the time t necessary for the IGF 2 to generate a control pulse immediately before the radiation of the probe pulse.

 The driver pulse generator can be implemented, for example, according to the scheme of figure 3. The clock coming from the output of Cx 1 is delayed by LZ 11 by the time t necessary for the waiting multivibrator (FM) 12 to generate a control pulse immediately before the radiation of the probe pulse.

 The radar according to the scheme of FIG. 4 differs in that LZ 11 is connected between the output of Cx 1 and the transmitter 3. This solution allows the use of a line with a short delay time. The operation of the main nodes is no different from other schemes.

Using the proposed method allows to eliminate errors associated with short-term frequency drift and changes in the spectrum of the emitted signal. Since the calibration signal is formed from the main one, we can talk about the coincidence of their spectral characteristics [S _o (f) = S _o (f) _k ], due to which (8) takes the form:
δP _r = 1-H _{n /} (9)
(due to the identity of the spectra μ _o = μ _k ).

 Thus, the error will be determined only by the accuracy of determining the signal conversion coefficient of the studied object and will not depend on the difference between the emitted and calibration signals.

 Using the proposed solution will improve the accuracy of calibration by eliminating errors introduced by devices for comparison and power control. The control of the switch 4 is carried out by a signal stabilized in amplitude, which is easily ensured using simple methods (for example, using logic circuits). The HC 4 itself operates in a key mode, providing either the free passage without attenuation of the useful echo signal or the passage of the "spurious" signal at a given attenuation. Moreover, any changes in the transmitter power are automatically taken into account, since the value of K is constant.

 In addition, the implementation of the calibration method is significantly simplified (see radar diagram), since serious changes in the microwave path are excluded. According to the proposed option, changes in the microwave path are reduced to replacing the arrester, usually used to protect the mixing diodes, with a controllable switch that can be implemented, for example, on the basis of p-i-n diodes and has overall dimensions comparable with the dimensions of the arrester.

 Literature.

 1. Potemkin I.G. Methods and devices for absolute and relative calibration of meteorological radar. Proceedings of the Central Administrative District, issue 126, 1971, p. 63-73.

 2. Davydov P.S., Sosnovsky V.A. and Khaimovich I.A. Aviation radar. Directory. Transport, 1984, 223 p.

 3. Latin S. M., Sharapov V. I., Ksenz S. P. and Afanasyev S.S. Theory and practice of operating radar systems. / Ed. CM. Latin. M .: Sov.radio, 1970, p. 133-141.

 4. Potemkin I.G. Automatic calibration and stabilization of the potential of weather radars. Proceedings of the 4th All-Union Conference on Radio Meteorology. M .: Gidrometeoizdat, 1978, p. 177-184.

 5. Bulkin V.V. and Kostrov V.V. On the possibility of using the Thunderstorm aircraft radar in radio meteorological measurements. Measuring equipment, 1996, N 2, p. 55-57.

 6. Yurchak B.S. On the influence of the characteristics of the meteorological radar receiver on the accuracy of measuring radar reflectivity of meteorological objects. Proceedings of the IEM. 1975, no. 9 (52), p. 137-151.

Claims (5)

 1. The method of calibrating the radar, which consists in counting the received echo signals from the level of the calibration signal, carried out after converting the signals in the receiving part of the radar, and the calibration signal is formed from a spurious pulse, leaked through the antenna switch to the input of the receiving part of the radar at the time of radiation of the probe pulse, wherein the calibration signal is a certain part of the transmitter power, characterized in that the calibration signal is generated by the weakening of the leaked spurious impulse by a known amount to a level that is included in the dynamic range of the receiver signals.  2. A radar containing a series-connected transmitter, antenna switch and antenna, as well as a receiver, characterized in that it further includes a controlled switch connected between the input of the receiver and the output of the antenna switch, a key, an integrator and a subtraction unit connected in series, the second input of which connected to the output of the receiver, and the output is the output of the radar, the synchronizer, the output of which is connected to the control input of the transmitter, the driver pulse control included between the synchronizer and the control input of the controlled switch, and the input and control input of the key are connected respectively to the output of the receiver and to the synchronizer.  3. The radar according to claim 2, characterized in that a delay line is included between the output of the synchronizer and the input of the driver of the control pulses.  4. The radar according to claim 2, characterized in that the driver pulse shaper consists of a series-connected delay line and a standby multivibrator, and the input of the delay line and the output of the standby multivibrator are respectively the input and output of the shaper.  5. The radar according to claim 2, characterized in that a delay line is connected between the output of the synchronizer and the input of the transmitter.

Images