Ranging. Range measurement methods. Pulse ranging method. Frequency method of range measurement. Phase ranging methods

METHODS RANGE MEASUREMENTS

To measure the range to reflecting objects, three methods are used: pulse, frequency and phase. All three methods are based on phenomena caused by the finite speed of propagation of radio waves in space. Due to a number of advantages, the pulse method of range measurement has become most widespread in radar.

PULSE RANGING METHOD

Determining the range in pulse rangefinders comes down to measuring the delay time of the pulse reflected from the target relative to the probing one, since

This is the basic relationship for radio ranging. The delay t 3 = 1 μsec corresponds to a range of R = 150 m. To obtain high accuracy in determining R, you need to accurately measure t. The structure of the pulse rangefinder is shown in Fig. 3.1, and the corresponding time diagrams are in Fig. 3.2.

Reflected signals in the simplest case, when the target is a single small object, are high-frequency pulses of very low intensity of approximately the same duration as the emitted pulses.

A pulse radar consists of a synchronizer, transmitter, receiver and terminal device.

The terminal device is a very important element of the radar. The terminal device, in general, also includes a system for measuring angular coordinates associated with the antenna for synchronous transmission of the antenna rotation angle. The terminal device must solve two problems: firstly, to isolate the signal from the background noise while preserving all useful parameters and, secondly, to measure the coordinates of the target and the characteristics of its movement using the parameters of the isolated signal. Depending on the recipient of radar information, they can be used Various types terminal devices.

If the recipient of radar information is the operator, then the terminal device modern radar, as a rule, serves as a cathode ray indicator or liquid crystal display. Here the radar signals are converted into an image visible on the screen.

If the recipient of radar information is a computing device or directly a continuous actuator, then the terminal device of the radar must be an automatic target tracking system. The latter provides data on the target range, usually in the form of voltage, and data on angular coordinates - in the form of rotation angles of the antenna axes.

If information from the radar is supplied to the digital computer, it must be submitted in the form of binary code numbers. The terminal device that converts radar information into binary code is called an instrumental data acquisition device.

Let's consider the operation of a pulse range finder, the terminal device of which is an electron beam indicator. The synchronizer generates control pulses that follow at a certain repetition rate, with the help of which the operation of all radar elements is coordinated in time. The synchronizer pulses trigger the transmitter, which consists of a modulator and a high-frequency generator. At the output of the transmitter, pulses of high-frequency oscillations are generated, which through the antenna switch AP enter the antenna and are emitted.

Most pulse radars use the same antenna to transmit and receive signals, which is connected to the transmitter and disconnected from the receiver during transmission, and connected to the receiver and disconnected from the transmitter during signal reception. The antenna is switched by an automatic control system, the operation of which is based on the use of sections of quarter-wave lines and gas arresters.

The signals reflected from the target through the antenna and AP enter the receiver, where they are amplified to the required value and converted into video pulses. Since the AP cannot ideally turn off the receiver, significantly weakened probe pulses from the transmitter leak to its input. In amplitude they significantly exceed reflected pulses. Two types of indicators are used in radars: indicators with an amplitude mark and indicators with a brightness mark.

Let's consider an indicator with an amplitude mark. To create a range sweep, voltage is supplied to the horizontal deflection plates of the CRT from a sweep generator, which is triggered by a synchronizer pulse simultaneously with the transmitter. If the scanning voltage is linear, then the luminous spot on the indicator moves from one edge of the tube screen to the other at a constant speed v p. Pulses from the receiver output arriving at the vertical deflection plates of the indicator cause the spot to deflect vertically. The first surge on the screen is created by the transmitter pulse leaking through the AP, all other surges are created by signals reflected from objects (Fig. 3.3). Since the amount of movement of a spot across the screen is a measure of time, the distance l between the leading edge of the transmitter pulse and the leading edge of the reflected signal characterizes the distance to the object. Really,

l = v p t з = v p 2R/c =MR, (1)

where v p is the speed of movement of the spot along the tube screen (sweep speed);

t z = 2R/c - delay time of the reflected signal relative to the probing signal;

M= 2v p R/c - linear scan scale.

In the simplest case, the range is measured either on a mechanical scale superimposed on the tube screen, or on an electronic scale created in the time base plane. The main advantage of the mechanical scale method is its simplicity. Its disadvantage is the low reading accuracy. It is used in long-range detection stations and other devices where high accuracy is not required.

The invention relates to radar and is used to measure range. The method of measuring range is by radiation pulse signals with repetition frequency F 1, receiving reflected signals and measuring their Doppler frequency f 1, subsequent emission of a signal with linear frequency modulation (chirp) of the carrier frequency with slope S, receiving the reflected chirp signal, measuring the frequency difference f 2 between the emitted and received chirp signals , measuring the delay of reflected pulse signals t 1 at the repetition frequency, emitting pulse signals with a repetition frequency F 2 equal to, receiving reflected signals and measuring their delay t 2, and the range is determined by the formula The technical result is to increase the accuracy of range measurement. 1 ill.

The invention relates to radar technology, namely to ranging. This invention is intended for measuring the range to a target using a pulse-Doppler radar operating with a high repetition rate of emitted pulses in the "pass" target tracking mode (TST).

There is a known method for measuring the range to a target, which involves linear frequency modulation of the carrier frequency (chirp).

This method is described in the literature (see Handbook on Radar, vol. 3, edited by M. Skolnik, p. 383, M. "Soviet Radio", 1979).

The method includes sequential emission of a signal with a chirp of a carrier frequency with a certain slope and without a chirp, measuring the Doppler frequency shifts of the signal from the target and calculating the range to the target by dividing the difference in the Doppler shifts by the double slope of the change in the carrier frequency. The accuracy of range measurement using this method is low due to the fact that the difference in Doppler shifts is determined with low accuracy.

Airborne radar stations use various methods of alternating the emission of signals with and without linear frequency modulation of the carrier. In some stations, before detecting a target, signals without a chirp are used, and after detecting a target, the Doppler frequency shift is calculated by sequentially sending a signal with a chirp with slopes S 1 and S 2 . Two slopes are used to calculate ranges to multiple targets.

In other stations, the review is carried out by sequentially using signals without chirp and with chirp with slopes S 1 and S 2 and calculating the range in a “floating window”, where the sequence of signals used S = 0, S = S 1, S = S 2 can be arbitrary.

There are also several known methods for measuring range in pulse-Doppler systems, which make it possible to obtain high measurement accuracy, in which, during the observation period, the target produces a stepwise change within certain limits of the repetition frequency of the probing pulses.

The known method involves sequentially emitting a signal with two operating pulse repetition frequencies, determining the temporal position of the pulses reflected from the target at each repetition frequency, and determining the true range. This method provides range measurement in survey mode in a radar with an average repetition rate.

Another well-known dual-frequency method for measuring range, which is a prototype of the proposed method, is described in the literature (see "On-board radar systems"edited by D. Poveysil., R. Roven, P. Waterman, Military Publishing House of the USSR Ministry of Defense, Moscow, 1964, pp. 317-320). Pulses are emitted with two repetition frequencies F 1 and F 2. In connection with Due to the ambiguity of the measurement, a mark from the target, the true range of which corresponds to t c, will appear at each repetition period at a distance corresponding to the time delays t 1 and t 2 relative to the nearest mark from the probing pulse. Thus where n 1 and n 2 are the number of unambiguous range measurement intervals for each pulse repetition frequency; t 1, t 2 - ambiguous ranges at repetition frequencies F 1 and F 2, respectively, expressed in time units; t c is the range to the target, expressed in time units.

There are several possible relationships between n 1 and n 2: n 1 = n 2, n 1 - 1 = n 2, n 1 + 1 = n 2, etc.

Substituting these relations into (1), we obtain formulas for determining the time delay corresponding to the true target range.

Choosing the relation n 1 = n 2, we get From (2) it follows that the use of two repetition frequencies allows for unambiguous range measurement up to the required maximum distance corresponding to a time delay equal to
where is determined by the value of the required maximum range.

When operating a radar with a high repetition rate of emitted pulses, it is necessary to measure long ranges to the target with an ambiguity (the ratio of the range to the target to the range corresponding to the viewing period) equal to ~ 100, while the described dual-frequency method provides range measurement with an ambiguity of 5-6.

To further increase the distance of unambiguous range measurement, a multi-frequency method can be used using three or more pulse repetition frequencies, but when sequentially moving from one repetition frequency to another with a constant receiving channel bandwidth, the target observation time must be increased in proportion to the number of pulse repetition frequencies used. With a range of about 100 km, to estimate it with high accuracy using the above method, 10-12 repetition frequencies are practically used, which cannot be implemented in the mode of tracking targets on the pass due to the short time of radar contact with the target.

Increasing the repetition period to achieve the required unambiguous measurement distance leads to the fact that detection and tracking of targets “on the pass” due to a decrease medium power emitted signal and due to the fact that detection and tracking of targets will occur against the background of reflections from earth's surface, leads to a decrease in the radar range and to significant false measurements caused by “shiny” points against the background of the underlying surface.

The purpose of this invention is to improve the accuracy of range measurement under conditions of limited time of radar contact with the target.

This goal is achieved by combining the operations of two methods - linear-frequency modulation and dual-frequency modulation in a certain, previously unknown sequence and choosing the repetition frequency F 2 according to the formula

No other technical solutions have been found that have features similar to the distinctive ones; therefore, the proposed invention meets the “significant differences” criterion.

The proposed method for measuring range is as follows. The time of irradiation of the target with frequency F 1 is divided into two intervals (cycles). At the first step, frequency modulation of the carrier is not carried out and the Doppler shift of the signal from the target is measured. During the second clock cycle, the frequencies of the transmitter and the local local oscillator change linearly with a slope S. During the propagation of the signal to the target and back, the frequency of the local local oscillator changes so that each signal from the target after heterodyning, in addition to the Doppler shift, receives a shift proportional to the range. The frequency difference F of two signals (reflected signal and local oscillator signal) is determined and the range is calculated using the formula

Measuring the range using the carrier chirp method does not impose practical restrictions on the maximum measured range, but gives a large (6 km) error D.

To increase the accuracy of range determination, the uncertainty D in the calculated chirp D value is revealed using two pulse repetition rates. To do this, select an interval of possible range values ​​equal to 2 D:
[D lchm -D; D lchm +D].
Within this interval, the use of two repetition frequencies makes it possible to determine an unambiguous range value with high accuracy. Since the range measurement using the chirp method was carried out at the repetition frequency F 1, the second frequency F 2 is selected from the condition of unambiguously determining the range at an interval of 2 D:

in other words, on an interval of 2 D, the number of repetition periods corresponding to F 2 is 1 less than the number of periods corresponding to F 1 .

But since the required maximum range when the radar operates in the SNP mode significantly exceeds the value of the 2 D interval, the measured range value using two repetition frequencies will in fact be ambiguous. The true range can be represented as
D and = D not one + n T deviation,
where D neodn is the ambiguous range at the deviation frequency F 1 - F 2, the period of which T deviats = equal to the accuracy interval 2 D;
n is the largest number of deviation periods at which the range, a multiple of the specified number of deviation periods n T deviats, does not exceed D chirp.

Measuring the range using a chirp allows you to determine the value of n T deviats, and the use of two repetition frequencies allows you to determine the value of D not one with high accuracy.

Moving on to the notation adopted in the description of the prototype, we note that the true range to the target corresponds to a delay at time t c, which can be calculated using the formula

where the integer part, the integer part is the number of repetition periods in the interval from 0 to D chirp for each frequency F 1 and F 2
K 1 ; K 2 - the number of repetition periods corresponding to the displacement of the range measured by the chirp method relative to the true range to the target (for F 1 and F 2, respectively).

There are two relationships between K 1 and K 2:
K 1 = K 2 , K 1 + 1 = K 2
Substituting these relations into (5), we obtain formulas for determining the time delay

(7)
The true range is taken to be the value t c, which is within a limited accuracy range D.

In expressions (6) and (7), the first terms represent the greatest range, not exceeding D chirp, multiple of the period of frequency deviation (F 1 - F 2), and the second terms are the ambiguous range at the period of frequency deviation F 1 and F 2.

The accuracy of determining the range t c is determined by the accuracy of determining D in more than one two-frequency way, since the quantities D chirp, F 1, F 2 included in the first term are precisely known.

Thus, from formulas (6), (7) it is clear that the combination of operations of two methods - chirp and two-frequency, and the choice of the second repetition frequency of emitted pulses from the relation F 2 = F 1 - allows, over the entire range of possible range values, to obtain range estimation with range measurement accuracy using a dual-frequency method, which is comparable to the measurement accuracy characteristic of a pulse radar station using pulses of the same duration (see “Airborne radar systems.” / edited by Poveysil et al., Voenizdat, M., 1964 ., p. 321).

The drawing shows a simplified block diagram of a device that implements the proposed method, and the area outside the dotted rectangle corresponds to the implementation of the prototype device, and inside it corresponds to newly introduced blocks; a certain relationship between all blocks allows the proposed method to be implemented.

1 - transmitter,
2 - synchronizer,
3 - controlled local oscillator,
4 - antenna with antenna switch,
5 - receiver,
6-1, 6-2, ..., 6-N - gated amplifiers,
7-1, 7-2, ..., 7-N - sets of parallel Doppler filters, each of which contains a detector and a storage device,
8-1, 8-2, ..., 8-N - switchable threshold devices,
9 - range finder,
10 - protractor,
11 - indicator,
12 - the first logical circuit "OR" for N-inputs,
13 - second logical circuit "OR" for N-inputs,
14 - counter with decoder,
15 - “time interval - code” converter,
16 - “code - time interval” converter,
17 - block for measuring target position,
18 - memory device,
19 - ambiguous range calculator,
20 - chirp range calculator,
21 - calculator of the integer part of the number of periods,
22 - second repetition frequency calculator,
23 - true range calculator.

The device given as an example implementation works as follows.

The operation of the radar is divided into cycles, the duration of which is determined by the time of coherent accumulation of the signal.

Within a cycle, the parameters of the emitted pulse and its repetition frequency (RPF) remain unchanged. The start pulse of transmitter 1, set by synchronizer 2, enters the transmitter, the second input of which receives the output voltage of the controlled local oscillator 3.

Until the target is detected, the output frequency of the local oscillator 3 is constant and the output pulse of the transmitter emitted by the antenna 4 does not have a chirp carrier frequency.

If there is a target in the 1st cycle, the signal reflected from it, which has a Doppler frequency shift proportional to the radial component of the speed of approach of the radar carrier and the target through antenna 4, enters receiver 5, the second input of which receives local oscillator voltage 3. After heterodyning, the signal from the output The receiver is supplied to the inputs of N gated channels, each of which consists of series-connected gated amplifiers 6-1, 6-2, ..., 6-N, a set of parallel Doppler filters containing detectors and accumulators 7-1, 7-2, ..., 7-N, the outputs of which are sequentially polled by a switchable threshold device 8-1, 8-2, ..., 8-N.

The channels are gated by strobes coming from synchronizer 2 to the second inputs of gated amplifiers 6-1, 6-2, ..., 6-N. The threshold device is switched using a comb of polling pulses coming from output “b” of synchronizer 2.

In each channel, when the threshold is exceeded at the output b 1, b 2, ..., b N of the switchable threshold device, the number of the polling pulse is recorded, corresponding to the filter number N f in which the target was detected. The signal of the detected target from the output a 1 , a 2 , ..., a N of the threshold device is supplied to the indicator 11 and the target position measurement unit 17. The output voltage at the output of unit 17 is proportional to the delay of the target from the probe pulse closest to the left.

At the output C 1 , C 2 , ..., C N of the threshold device, if the threshold is exceeded, a signal appears, which is supplied through the “OR” circuit 13 to the input of the counter with a decoder 14, at the 3 outputs of which the 1st, 2nd are generated separately th and 3rd control pulses corresponding to successive clock cycles of the radar.

When the 1st control pulse occurs in the memory device 18, the following parameters are stored: the number of the “ringing” filter (N f1) through the “OR” logic circuit 18, the time delay of the echo signal t 1 from the output of block 17, frequency repetitions F 1, coming from the output "a" of synchronizer 2 through the "time interval - code" converter 15. In addition, the computer 22 is started, containing a divider and a subtractor that calculates the repetition frequency F 2 = F 1 - , and in synchronizer 2 in the next clock interval a chirp strobe coming from the output “d” of the synchronizer to the input of the controlled local oscillator 3. From the output “c” of the synchronizer, the trigger pulse of the transmitter with PRF F 1 arrives at the input of the transmitter 1.

In the 2nd cycle, the radar operates with PRF F 1 and chirp of the carrier frequency. If there is a 2nd control pulse in the memory device 18, the second number of the “ringing” filter (N f2) is stored and the computer 20, containing a multiplier, a calculator and a divider, calculates the range D chirp = , where S and K are constants. When the 2nd control pulse occurs, in the next clock interval the synchronizer input receives the frequency value F 2 from the output of the computer 22 through the code-time interval converter 16, and a chirp strobe is not formed at the output “d” of the synchronizer, due to which local oscillator 3 generates a constant carrier frequency. From the output "C" of the synchronizer, a trigger pulse with PRF F 2 ​​is supplied to the input of transmitter 1.

In the 3rd cycle, the radar operates with PRF F 2 ​​without a chirp of the carrier frequency. The passage of signals through the radar blocks is similar to the 1st cycle. When the 3rd control pulse occurs, the time delay of the reflected signal t 2 from the output of block 17 is stored and calculators 19, 21 and 23 are sequentially launched.

Block 21 calculates the integer part n 1 and n 2 of the quotient - from the division of D chirp by F 1 and F 2, respectively. Block 19, containing 2 multipliers, 2 subtractors and a divider, calculates the ambiguous range

At the output of block 23, containing 2 subtracting devices, a divider and an adder, a true range voltage is generated in accordance with the formula:

The device is described above as a combined device, although it can also be made digital.

The proposed method and the device given as an example have a fundamental difference from the known methods of measuring range in survey mode, consisting in high accuracy of range measurement in conditions of limited time of radar contact with the target when application of radar with high pulse repetition rate.

The effectiveness of the proposed method for measuring unambiguous range was tested using mathematical modeling. The model included a block for generating an input signal, a block for generating a signal/noise mixture at the output of a linear receiver in each of the time gates of the receiving channel, and a range calculation block operating according to the proposed method. The signal generation unit produced random signal realizations at a given repetition rate with an initial range uniformly distributed over a distance segment of 12 km. In the receiver block, this signal was added to a random realization of noise distributed according to a normal law, and the signal-to-noise ratio varied within specified limits from one series of statistical tests to another. In the unambiguous range measurement unit, a single-threshold signal detector was used, and the threshold level corresponded to a specified false alarm probability value. The accuracy of the single-digit range measurement was assessed by statistically processing a series of random results obtained from mathematical modeling of the signal processing process.

The results obtained showed that with four strobes in the reception area, an initial accuracy of 6 km (obtained at the first stage of measuring an unambiguous range using a chirp), a signal-to-noise ratio at the meter input equal to 17 dB (7 times), a detection threshold equal to 4.5 w, probability of measurement with error //< 300 м составляет P = 0,6, с ошибкой / / = 300 - 600 м P = 0,33, с ошибкой / / = 600 - 900 м P = 0,043.

Thus, it is shown that the proposed method achieves the set goal: increasing the accuracy of range measurement under conditions of limited time of radar contact with the target.

A method for measuring range, including emission of pulse signals with a repetition frequency F 1 , reception of reflected pulse signals and measurement of their Doppler frequency f 1 , subsequent emission of a signal with linear frequency modulation /chirp/ of a carrier frequency with slope S, reception of the reflected chirp signal, measurement of the difference frequencies f 2 between the emitted and received chirp signals and determining the range D using the formula

The invention relates to radar technology and can be used for the selection of homing anti-radar missiles (ARMs) with pulsed sensing of different polarization in the quasi-optical region of radio wave reflection

Based on the properties of the propagation of electromagnetic waves in the microwave range (straightness and constancy of speed equal to the speed of light), it is possible to determine a parameter that allows one to estimate the range. This parameter is the delay time tz of the reflected signal relative to the emitted one. The range in this case is determined as follows:

where C is the speed of light propagation (C = 3·10 8 m/s).

The number 2 in the denominator takes into account that the electromagnetic wave travels from the radar to the object and from the object to the radar, i.e. twice.

When measuring range non-automatically, the all-round visibility indicator is used (Fig. 3.11). The coordinate value is measured relative to the range scale marks.

Around view indicators are indicators with a brightness mark. The detected signal is displayed as a luminous mark (with a long afterglow). The coordinates of the air object are determined by the position of the mark relative to the scale marks of range and azimuth. The range determination process is accompanied by measurement errors, the main of which are:

a) errors due to signal delay in processing circuits due to inaccurate synchronization of scan generators;

b) errors due to distortion of the shape of unfolding stresses;

c) errors due to instabilities of supply voltages;

d) errors due to counting.

The errors indicated in paragraphs a) and b) are classified as systematic and can be taken into account. The other two errors are random. The most significant reading errors arise: due to inaccurate determination of the true position of the leading edge of the reflected signal on the scan line; due to parallax and interpolation at finite sizes of the aperture (scanning spot); due to noise. The minimum range measurement error in PPI is

where d is the diameter of the spot;

m – range sweep scale.

To reduce errors, it is necessary to achieve the best focusing of the beam and choose a larger scale.

The range can be measured using a counting pulse generator that has a high repetition frequency stability (Fig. 3.12).

The radar trigger pulse puts the trigger into a state in which the coincidence cascade opens, through which the counting pulses enter the counter. The detection pulse moves the flip-flop to another state, which closes the matching cascade. A code corresponding to the number of pulses at its input is fixed at the counter output

where D is range;

с – speed of light;

F p – repetition rate of clock pulses.

The range reading will only change when the N number changes by at least one. In this case, there is a discreteness of reading equal to

The discreteness of the reading determines the range measurement error, which is equal to

.

The number of counter bits n is determined by the maximum range and permissible measurement error

. (3.9)

For example, with D max = 200 km and Δ D = 20 m, 2 n = 10 4, whence n = 14.

The device shown in Fig. 3.12 allows you to measure the range to only one object in one period of trigger pulses.

The more preferable method is automatic ranging, the algorithm of which is as follows. The radar detection zone by range is divided into separate discretes (Fig. 3.13), the value of which is determined by the pulse duration (minimum size) and the maximum range measurement error (maximum size).

The repetition period of launch pulses (IP) determines the maximum detection range of airborne objects. The range measuring device must be multi-channel, because During one probing period, it is necessary to provide range measurements to several objects that have the same azimuthal coordinate.

During automatic measurement, the range is determined by the discrete number N D in which the mark is observed

where h D is the duration of one range discrete.

The sample number can be set by counting the clock pulses that sample the range during the repetition period T n of the trigger pulses. The ranging device can be configured according to the structure shown in Fig. 3.14.

The device works as follows. The radar trigger pulse “resets” the counter. With the arrival of TI clock pulses, a current range code is generated at the counter output, which is issued to one of the inputs of the “AND” coincidence circuits. In the response signal of the transponder, a coordinate code is formed, which represents time - a pulse code consisting of two pulses. Decoding of the coordinate code is realized by a delay line and an “AND” matching circuit. As a result of decoding, a pulse appears at the output of the matching circuit, allowing the range code to be issued to the outputs of the “AND” matching circuit. The next cycle of work begins with “resetting” the counter. The number of bits of the digital range code depends on the total number of range bins. When measuring range in primary radars, permission to issue the current range code to the output of the meter is given by a pulse from the read pulse generator (GIS), generated with the arrival of the detection pulse (ID).

Azimuth measurement

The maximum method is used when measuring angular coordinates.

The maximum method refers to amplitude methods for measuring angular coordinates, which are based on the use of the directional properties of antennas. Direction finding by the maximum method (Fig. 3.15) is carried out by combining the direction of the maximum direction finding characteristic (or antenna radiation pattern) β with the direction to the direction finding object β 0 as a result of smooth rotation of the antenna. The bearing (azimuth) is counted at the moment when the voltage at the receiver output becomes maximum.

The practical implementation of the maximum method can be carried out as follows.

In the case of non-automatic determination of azimuth, the coordinate is measured in the middle of the detected mark (counting in the azimuthal plane) relative to the scale azimuth marks. The main advantages of the maximum method are: the simplicity of determining angular coordinates, and also the fact that at the moment of precise bearing the highest signal-to-noise ratio occurs, since the counting is made at the maximum signal.

One of the main disadvantages of the method is low accuracy due to the low sharpness of the signal apex. In radars with fairly narrow antenna patterns, the azimuth measurement error is

where is the width of the radiation pattern at half power level (Fig. 3.16).

To reduce azimuth measurement errors, it is necessary to take measures to make the beam narrower (for example, increase the linear size of the antenna).

With automatic methods of measuring coordinates, the area within which signals are received and detected is divided into elements by range and azimuth. The discrete value in range and azimuth is selected from the permissible errors in coordinate measurement.

The number of discrete azimuths in radar systems is 4096, which provides an error in azimuth measurement of 5 arc minutes. This satisfies the accuracy requirements.

The antenna radiation pattern in the horizontal plane is symmetrical, so the azimuth of the object can be determined as follows:

, (3.10)

where β n is the azimuth of the beginning of the burst;

β k – azimuth of the end of the pack

Δβ is a systematic error caused by the shift of β n and β k when checking the criteria for detecting the beginning and end of a burst.

The structure of the azimuth meter is shown in Fig. 3.17.

The operation of the circuit is as follows. With the arrival of the “North” impulse, the counter is “reset to zero”. When large-scale azimuthal pulses (MAP) arrive, a digital code is generated at the counter output, which is the current azimuth code. This code is supplied to one of the inputs of the “AND” matching circuits, the second inputs of which receive read pulses from the read pulse generator (GPS). In the presence of read pulses, digital codes β n and β k are issued at the outputs of the coincidence circuits, which are sent to a special computer, where the azimuth of the aircraft is determined.

The formation of pulses at the beginning and end of the reflected signal packet is carried out as follows. To weaken the influence of false pulses and signal omissions on the measurement accuracy, the beginning and end of the burst are determined by a special criterion (logic). The following criterion can be chosen. If one pulse is detected during three consecutive repetition periods, it is considered false (Fig. 3.18), if two, they are considered the beginning of a burst. The end of the burst is marked if, in three consecutive periods after the start, the omission of two pulses is detected for the first time (the omission of only one pulse is considered false).

In general, “k from t” logic can be used. Logics can be integer (k = t) and fractional (k< m). Для определения начала и конца пачки могут использоваться одинаковые логики либо различные. При использовании различных логик менее жесткая определяет конец пачки, чтобы исключить ее дробление вследствие флюктуационного выпадения отдельных импульсов. Например, если начало пачки определяется по логике «3 из 3» (3/3), тогда конец пачки будет определяться по логике «2 из 3» (2/3). В некоторых случаях конец пачки определяется тогда, когда в смежных периодах повторения импульсы отсутствуют l раз (l нулей подряд). Такую логику обозначим как «k/m – l».

Structural scheme The pulse generator for the beginning and end of the burst is shown in Fig. 3.19.

The standard pulse generator is triggered when the pulses from the receiver output exceed a certain threshold. Subsequently, the criteria for detecting pulses at the beginning and end of a burst and the formation of these pulses are checked. In Fig. 3.20, 3.21. Detectors of pulses of the beginning and end of a burst are shown.

The above detectors perform analysis according to the “2 out of 3” criterion. Pulses arriving at the input are delayed for one and two repetition periods, summed up and fed to a threshold device, which is triggered when at least two pulses are summed. The formation of pulses at the beginning and end of a burst is implemented by a circuit consisting of a delay line and a mismatch cascade that passes signals if they are simultaneously absent at two inputs.

In Fig. Figure 3.22 shows voltage diagrams that explain the process of forming pulses at the beginning and end of a burst.

It should be emphasized that in the process of checking the detection criteria for pulses at the beginning and end of a burst, these pulses shift relative to the actual position of the burst pulses. This is a systematic error that must be taken into account in the azimuth computer.

where Δβ is an amendment that takes into account the time of checking the detection criteria.

When using the “k/m – l” criterion, the beginning of the pack β n turns out to be shifted to the right by (t – 1) positions, and the end of the pack β k – by l positions. In this case, the systematic correction Δβ is equal to

where is the angular discrete (the angle between adjacent azimuthal positions).

The considered processing schemes are single-threshold.

To determine the distance to a detected object (target), it is necessary to know the delay time of the reflected radio pulse relative to the emitted one.

Delay time Δ t depends on distance D to reflecting object and speed With radio wave propagation. Since the reflected pulse travels a double path (from the radar station to the object and back), this time doubles

If you measure the delay time of the reflected pulse, you can determine the distance to the reflecting object. This distance from the above formula is

When the reflected pulse is delayed relative to the emitted one by one microsecond, the distance to the reflected object

To measure the delay time of the reflected pulse, and, consequently, the distance to the detected target, cathode ray tubes are used. In the simplest case, a cathode ray tube with electrostatic control and linear beam sweep is used for this purpose.

To measure range in radar, the pulse method is most widely used. Range measurement is based on the constancy of the speed and straightness of the propagation of radio waves, which are maintained in real conditions with fairly high accuracy. Range measurement comes down to recording the moments of emission of the probing signal and reception of the reflected signal and measuring the time interval between these moments.

To ensure this method, pulse modulation of the probing signal is used.

Rice. 3. Pulse modulation principle

A– modulating pulses; b− probing pulses

Let's consider the operation of the simplest pulse rangefinding radar.

Rice. 4. Block diagram of the simplest pulse radar (range finder)

A radar uses a single antenna for both transmission and reception. A pulse transmitter produces a radio pulse with a duration τ and, which, through an antenna switch (receive-transmit switch), enters the antenna and is emitted. At this moment, the receiver is disconnected from the antenna for a time τ and only part of the pulse energy (direct signal) “leaks” to the receiver input.

The reflected pulses perceived by the antenna are sent to the receiver through the same antenna switch in the pauses between probing pulses.

Lag time t The value of the reflected pulse relative to the probing pulse (characterizing the initial timing) is measured using a terminal device, for example a visual indicator.

Rice. 5. Temporary position of the reflected pulse in the absence of noise

Reflected pulse delay time

,

Where D- distance between the radar and the target;

- speed of propagation of radio waves.

Thus, the range to the target is:

,

To do this, it is enough to measure the delay time τ z.

Determining the range is most simply carried out using an electron beam indicator with a target amplitude mark. To do this, using a horizontal deflection voltage of a sawtooth shape, the beam of a cathode ray tube (CRT) is periodically scanned at a constant speed, i.e., a linear time scale is created that can be calibrated in range units. This sweep is called a time sweep or range sweep.

Rice. 6. Measurement of delay time using a CRT