FCM signal in radar. A method for increasing the effectiveness of radar for detecting people behind optically opaque obstacles. Intrapulse modulated signals
FCM radio pulses are characterized by an abrupt change in phase within the pulse according to a certain law, for example (Fig. 1.66):
– three-element signal code
– law of phase change
or seven-element signal (Fig. 1.67):
Thus, we can draw conclusions:
· ASF of signals with chirp is continuous.
· The ASF envelope is determined by the shape of the signal envelope.
· The maximum ASF value is determined by the signal energy, which in turn is directly proportional to the amplitude and duration of the signal.
The spectrum width is 
where the frequency deviation and does not depend on the signal duration.
· Signal base (bandwidth factor) can be n>>1. Therefore, chirp signals are called broadband.
FCM radio pulses with a duration are a set of elementary radio pulses following each other without intervals, the duration of each of them is the same and equal to . The amplitudes and frequencies of the elementary pulses are the same, but the initial phases may differ by (or some other value). The law (code) of alternation of initial phases is determined by the purpose of the signal. For FCM radio pulses used in radar, corresponding codes have been developed, for example:
1, +1, -1 - three-element codes
- two variants of four-element code
1 +1 +1, -1, -1, +1, -2 - seven-element code
The spectral density of coded pulses is determined using the additivity property of Fourier transforms, in the form of the sum of the spectral densities of elementary radio pulses.
Chapter 1 - Methods for processing digital PCM signals
1.1 Problem statement
1.2 Classification of side lobe suppression methods compressed! about us nala
1.2.1 Primary and secondary processing methods
1.2.2 Spectral and time domain processing methods
1.2.3 Iterative and reentrant processing methods
1.2.4 Adaptive methods
1.3 Description is original! o &gp of the algorithm adaptively1 o pulse compression
1.4 Chapter Conclusions
Chapter 2 - Mathematical description systems
2.1 Generalized representation of the system
2.2 Description of the probing FKM-ssh pala
2.2.1 Description of the probing signal for the PJIC odpocapalpa
2.2.2 Description of the vector1 probing signal for polarization PJIC
2.3 Modeling of radar objects
2.3.1 Impulse response of a radar object for the PJIC single capalpole
2.3.2 Description of radar object models for polarized PJIC
2.4 Factors affecting the accuracy of the estimate impulse response radar object
2.5 System noise
2.6 Criteria for assessing the level of side lobes of the signal at the output of the compression filter
2.7 Chapter Conclusions
Chapter 3 - Adaptive Filtering Algorithms
3.1 Using adaptive processing when filtering signals
3.2 Adaptive algorithm for odpocapalpoy PJIC
3.2.1 Using a matched filter as part of an adaptive filter for a single-drop PJIC
3.2.2 Description of the adaptive rhythm for the PJIC unipocapalpa
3.2.3 Description of the adaptive filter for single-channel PJIC
3.3 Adaptive Alurhythm for Polarized PJIC
3.3.1 Using a matrix-matched filter as part of an adaptive filter for polarization PJIC
3.3.2 Description of adaptive alurit for polarized PJIC
3.3.3 Description of the adaptive filter for polarizing PJIC
3.4 Conclusions on the first lava
Chapter 4 - Study of the proposed adaptive algorithms
4.111rimspspie adaptive! about the algorithm for single-channel PJIC
4.1.1 Application of aluritma for different models radar objects
4.2 11implementation of the adaptive speed rhythm for polarization PJIC 96 4.2.1 11implementation of the algorithm for different models of radar objects
4.4 Conclusions on Chapter I 4 109 Conclusion 111 C11 and literature 113 Appendix A 119 Appendix B
List of abbreviations
LCF - autocorrelation function;
ASI - adaptive pulse compression;
LF - adaptive filter;
ICF - intercorrelation function;
DD - dynamic range;
IH - impulse response;
Chirp - linear frequency modulated;
MSO - minimum mean square error;
PJI - radar;
PJIC - radar station;
MSD - root mean square deviation;
UBL - side lobe level;
FKM - phase-code-manipulated;
FN - uncertainty function;
ESR - effective scattering surface.
Recommended list of dissertations
Study of simulation algorithms for transforming complexly modulated radar signals for measuring the parameters of radar stations 2005, Candidate of Technical Sciences Nguyen Huu Thanh
Development and research of a method for increasing the noise immunity of radars with complex quasi-continuous signals 2003, candidate of technical sciences Nilov, Mikhail Alexandrovich
Synthesis of signals with a pseudo-random law of amplitude-phase keying and methods for their processing in radars with a quasi-continuous operating mode 2005, Doctor of Technical Sciences Bystrov, Nikolai Egorovich
Suppression of correlation noise when processing discrete radio signals using the method of conjugate matched filtering 2003, candidate of technical sciences Melnikov, Alexey Dmitrievich
Improving the parameters of radar target observability in air traffic control radars using digital adaptive spatial-Doppler processing of echo signals 2000, Candidate of Technical Sciences Savelyev, Timofey Grigorievich
Introduction of the dissertation (part of the abstract) on the topic “Adaptive algorithms for reducing the level of side lobes of the response at the output of the PCM compression filter of radar signals”
Since its inception, radar theory has mainly developed as a theory that considers so-called point targets. However, the real target is often a complex object consisting of a set of elements, and there is a need to determine their ranges and the intensity of reflections reflected from these elements! catch
Many modern radar stations (PJIC) use a complex Foundation signal. The use of complex signals compared to simple ones has a number of advantages, in particular, achieving a high radar potential with limited radiation power and increased noise immunity. Among the wide variety of complex US signals, fire-code-manipulated (FCM) signals have found application. The uncertainty function of such signals has a form that excludes the equivalence of the shift in time and frequency, characteristic, for example, of lipid-frequency modulated (chirp) signals.
During processing, the reflected signals are folded into short pulses in a compression filter (compressed). As a rule, a matched filter (MF) is used for this. The duration of the compressed phase-code mapped signal is taken to be the width of the main peak, but outside it, side maxima (side lobes) are observed.
The mixed PJIC receiver filter can be considered optimal if the radar object is represented by a single point reflector and the signal is received against a background of additive white noise. When observing a complex radar object consisting of a set of reflecting elements, the use of SF is not optimal.
The level of side lobes (US1) of a compressed signal can significantly exceed not only the noise level, but also the levels of useful signals. The interfering influence of side lobes manifests itself in the masking of information peaks from weak signals. In practice, it is often important not to miss a useful weak radio signal from a reflector with a small effective scattering surface (ESR) against the background of interfering reflections from objects with a large ESR. For example, a fully reflected impulse is often lost in stronger reflections from foreign objects close to the target. This phenomenon significantly limits the dynamic range of amplitudes of useful signals processed by PJIC, and the ability to expand it using one accepted implementation is especially attractive.
The problem of reducing the level of side lobes by compressed sishala is relevant for both unicapital and polarization (mpojucapal) PJIC. In polarized PJIC, the situation is aggravated by the fact that with full polarization sensing, two orthogonal signals are simultaneously emitted, and the level of the side lobes is determined by both their auto- and cross-correlation.
Considering that the location of reflectors within a radar object and the intensity of the signals reflected from them are random, the problem arises of synthesizing adjustable (adaptive) spur rhythms and corresponding filters, the parameters and structure of which change over time. In order for the system characteristics to be variable and able to adapt to changing radar surveillance conditions (to various objects), it is necessary to use adaptive filters.
Thus, research work on the synthesis of adaptive alurhythms for reducing the side lobes of the response at the output of the FCM compression filter of radar systems! catches scattered by complex radar objects can be considered relevant.
The focus of this dissertation is the development of adaptive algorithms for reducing the level of side lobes of the response at the output of the FCM signal compression filter for single-drop PJIC and polarization PJIC, which makes it possible to increase the radar observability of small-sized targets against the background of objects with a large RCS, and to study their effectiveness.
In accordance with this, the following main tasks were set and solved.
1. Definition mathematical models probing signal for single-drop and polarized PJIC.
2. Determination of models of radar objects for single-drop and polarization PJIC.
3. Expansion dynamic range amplitudes of the useful received PJIC signals according to one accepted implementation by reducing the level of the side lobes of the compressed signal.
4. Carrying out numerical modeling of the adaptive algorithm for single-drop PJIC and developing on its basis an adaptive algorithm for polarization PJIC.
5. Study of the effectiveness of the developed adaptive filtering algorithms.
Relevance of the study
Solving these problems is relevant at the present stage of development of radar, since expanding the dynamic range of useful signal amplitudes allows us to avoid complications associated with the presence of large side lobes of a compressed complex signal and is relevant [1].
These tasks are especially relevant for polarization radar, since they make it possible to reduce errors in determining the elements of target scattering matrices both during sequential and simultaneous measurements.
Research methods. The research carried out is based on methods of adaptive signal processing, the theory of complex signals, best practices radio reception and statistical theory of radar. Mathematical modeling methods were used in the research process.
When conducting mathematical modeling the application package MatLAB 7.0 was used.
The practical significance of the work is determined by its focus on increasing the efficiency of radar systems.
The results obtained in the work allowed:
1. Using professional methods, optimize the processing of radar channels according to one accepted implementation.
2. Develop an adaptive algorithm for single-loop PJIC, which allows reducing the level of side lobes of the received compressed sleeper. The dynamic range of single-channel PJICs for the radar object models described in the work was increased by 7-23 dB, and the average square error of estimates of the impulse characteristics of radar objects was reduced by 8-32 dB. A comparison was made with the signals at the output of a matched filter, all other things being equal.
3. Develop an adaptive algorithm for polarization PJIC, which allows reducing the level of side lobes of the received compressed signal. The dynamic range of polarization PJICs for the radar object models described in the work was increased by 8 - 19 dB, and the average squared error in estimates of the impulse characteristics of radar objects was reduced by 8 - 17 dB. The comparison was made with the signals at the output coi of the matrix filter, all other things being equal.
Structure and scope of the dissertation
The dissertation consists of an introduction, 4 chapters, a conclusion and a list of references, including 72 titles, and 2 appendices. The work contains 122 pages, 36 figures and 7 tables.
Similar dissertations in the specialty "Radarlocation and radio navigation", 05.12.14 code VAK
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Conclusion of the dissertation on the topic “Radar and radio navigation”, Babur, Galina Petrovna
The results of the work were used in the educational process when giving lectures and conducting practical classes with students of the radio engineering faculty at the department of radio engineering systems in the disciplines “Design of radio engineering systems” and “Theory of electrical communications”. Also, the results of the dissertation work were implemented during the research work “Solving the problem of using complex singals in the problem of correctly estimating the scattering matrix of a radar object” under the project RI-111/004/006 FTSPTN “Research and development in priority areas of development of science and technology” for 2002-2006 gg., (state registration numbers: registration No. 01200611495, inventory No. 02200606700).
CONCLUSION
The dissertation work solved the current scientific and technical problem of expanding the dynamic range of the amplitudes of useful received ultrasonic pulses in single-channel and polarization radars with PCM sounding signals. The expansion of the dynamic range occurs due to adaptive processing, which makes it possible to significantly reduce the level of side lobes of the response function at the output of the received signal compression filter.
The following main results were obtained in this work.
1. An adaptive algorithm has been developed for reducing the level of the side lobes of the response at the output of the FCM signal compression filter for a single-drop radar according to one accepted implementation by step-by-step processing of the input signal.
2. Developed functional diagram adaptive filter for single-drop radar, using a matched filter.
3. For a polarization radar, an adaptive algorithm has been developed for reducing the level of side lobes of the response at the output of the PCM signal compression filter according to one accepted implementation by step-by-step processing of the vector input signal. The algorithm makes it possible to reduce the level of side lobes of the compressed signal, caused by both non-bullet autocorrelation and non-zero cross-correlation of the signal signals used.
4. A functional diagram of an adaptive filter for a polarization radar has been developed, using a matched matrix filter. A feature of the filter is the presence of cross connections between signal processing channels.
The developed adaptive processing algorithms allow:
1. Expand the dynamic range of amplitudes of useful received radar signal signals due to adaptive processing of signal signals at the filter output, coordinated with a complex PCM signal signal. For the given PJI object models, the expansion of the dynamic range for a single-channel radar was 7-23 dB, for a polarization radar - 8-19 dB.
2. Reduce the level of side lobes of compressed signals without increasing the base of probing FCM signals.
3. Increase the accuracy of estimating the impulse response of radar objects. The reduction in the mean squared error in IR estimation of various radar objects at the output of the adaptive filter for a single-channel radar compared to a matched filter was 8-32 dB; for the case of polarization PJIC, the mean squared error in IR estimation was reduced by 8-17 dB.
4. The adaptive algorithm for polarization PJIC makes it possible to reduce the side lobes, which are determined not only by the autocorrelation of complex signals that are part of the vector sounding signal, but also by their mutual (cross-) correlation.
The theoretical results of the work have a practical orientation:
1. The developed adaptive algorithms make it possible to expand the dynamic range of amplitudes of useful received signals in single-capacity and polarization PJICs with FCM signals by reducing the level of side lobes of compressed signals. This makes it possible to reduce the masking effect of targets with a large RCS on nearby targets with a small RCS, this improves radar detectability and recognition of small targets.
2. The presented adaptive algorithms make it possible to increase the accuracy of estimating the impulse response of observed radar objects.
3. When performing radar detection of distributed objects, which can be described by a set of point reflectors, the developed algorithms make it possible to increase the contrast of the radar image.
An analysis of the algorithms developed and presented in this work shows the advantage of adaptive algorithms for processing complex radar signals compared to traditional methods, namely matched filtering.
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Please note that the scientific texts presented above are posted for informational purposes only and were obtained through original dissertation text recognition (OCR). Therefore, they may contain errors associated with imperfect recognition algorithms. IN PDF files There are no such errors in the dissertations and abstracts that we deliver.
Currently remain relevant in radar, the task is resolution, and in information transmission systems, the task is to distinguish signals.
To solve these problems, one can use FCM signals encoded by ensembles of orthogonal functions, which, as is known, have zero cross-correlation.
To resolve signals in radar, you can use a burst signal, each pulse of which is encoded by one of the rows of an orthogonal matrix, for example, the Vilenkin-Chrestenson or Walsh-Hadamard matrix. These signals have good correlation characteristics, which allows them to be used for the above-mentioned tasks. To distinguish between signals in data transmission systems, you can use the same signal with a duty cycle equal to one.
The Vilenkin-Chrestenson matrix can be used to form a polyphase ( p-phase) FCM signal, and the Walsh-Hadamard matrix, as a special case of the Vilenkin-Chrestenson matrix for the number of phases equal to two, to form a biphasic signal.
Polyphase signals are known to have high noise immunity, structural secrecy and a relatively low level of side lobes of the autocorrelation function. However, to process such signals, it is necessary to spend a greater number of algebraic addition and multiplication operations due to the presence of real and imaginary parts of the signal samples, which leads to an increase in processing time.
Discrimination and resolution challenges can be exacerbated by the a priori unknown Doppler shift of the carrier frequency due to the relative motion of the source and subscriber or radar and target, which also complicates real-time signal processing due to the presence of additional Doppler processing channels.
To process the above-mentioned signals having a Doppler frequency addition, it is proposed to use a device that consists of an input register, a discrete conversion processor, a cross-connection unit and a set of identical ACF signal generation units, which are sequentially connected shift registers.
If we take the orthogonal Vilenkin-Chrestenson matrix as a basis matrix for processing a polyphase burst signal, then discrete transform will go into the discrete Vilenkin-Chrestenson-Fourier transform.
Because the Vilenkin-Chrestenson matrix can be factorized using the Goode algorithm, then the discrete Vilenkin-Chrestenson-Fourier transform can be reduced to rapid transformation Vilenkin-Chrestenson-Fourier.
If we take the orthogonal Walsh-Hadamard matrix as a basis matrix - a special case of the Vilenkin-Chrestenson matrix for processing a biphase burst signal, then the discrete transformation will turn into a discrete Walsh-Fourier transform, which by factorization can be reduced to the fast Walsh-Fourier transform.
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UDC 621.396.96:621.391.26
A method for increasing the efficiency of radar for detecting people behind optically opaque obstacles
O. V. Sytnik I. A. Vyazmitinov, E. I. Miroshnichenko, Yu. A. Kopylov
Institute of Radiophysics and Electronics named after. A. Ya. Usikova NAS of Ukraine
The possibilities of reducing the level of side lobes of the autocorrelation function of FCM probing signals and the problems of their practical implementation in equipment are considered. An optimal phase-amplitude intrapulse modulation has been proposed, which makes it possible to reduce the side lobes and at the same time increase the repetition rate of probing messages. The factors influencing the characteristics of such signals are studied and a criterion for their feasibility in equipment is proposed.
Introduction.
Signal processing algorithms in a radar with a quasi-continuous probing signal designed to detect objects hidden behind optically opaque obstacles are usually built on the principle of optimal correlation processing or matched filtering [ – ].
Probing signals for such radars are selected based on the requirement to ensure the necessary resolution and noise immunity. In this case, they try to make the signal uncertainty function pencil-shaped in the corresponding plane with a minimum level of side lobes. For this, various complex types of modulation are used [, ,]. The most common of them are: frequency-modulated signals; a lot of frequency signals; phase-shift keyed signals; signals with code phase modulation; discrete frequency signals or signals with code frequency modulation; composite signals with code frequency modulation and a number of signals that are a combination of several types of modulation. The narrower the main peak of the signal uncertainty function and the lower the level of its side lobes, the correspondingly higher the resolution and noise immunity of the radar. The term “noise immunity” in this work means the radar’s resistance to interference caused by reflections of the probing signal from objects that are not targets and located outside the analyzed strobe (frequency, time). Such signals are called long-baseline signals or ultra-wideband (UWB) signals in the literature.
One of the varieties of UWB signals are phase-keyed signals, which represent a coded sequence of radio pulses, the initial phases of which vary according to a given law. Code sequences of maximum length or M-sequences have very important properties for radar:
· M-sequences are periodic with period , where is the number of elementary pulses in the sequence; − duration of an elementary pulse;
· The level of the side lobes of the uncertainty function for a periodic sequence is − , and for a single sequence of pulses − ;
· Pulses in one period of the sequence, differing in phases, frequencies, durations, are distributed with equal probability, which gives grounds to consider these signals to be pseudorandom;
· Formation M-sequences are carried out quite simply on shift registers, and the number of bits of the register is determined by the length of one period of the sequence - from the relation.
The purpose of this work is to study the possibilities of reducing the level of side lobes of the uncertainty function of signals modulated M-sequences.
Formulation of the problem.
Figure 1 shows a fragment of a modulating function formed by a periodic sequence (here there are two periods M-sequences with
).
Section along the time axis of the uncertainty function of a radio signal modulated by such M-sequence is shown in Fig. 2. The side lobe level, as predicted by theory, is 1/7 or minus 8.5 dB.
Let us consider the possibility of minimizing the side lobes of the uncertainty function of the FCM signal. Let us denote by the symbol M-sequence, the duration of one period is equal to . In discrete time, provided that , the algorithm for calculating the elements of the sequence can be written in the following form:
(1)
The radio signal emitted by the locator is the product of the carrier harmonic signal
, (2)
Where
− vector of parameters for the modulating function (1) -
. (3)
The signal power is distributed between the side lobes of the uncertainty function -
(4)
and the main petal -
, (5)
where the symbol *− denotes the operation of complex conjugation, and the limits of integration in the time and frequency domains are determined by the corresponding type of signal modulation.
Attitude
(6)
can be considered as the objective function of a parametric optimization problem.
Algorithm for solving the problem.
The solution to the optimization problem (6) is to estimate the parameter -
, (7)
where is the domain of definition of the vector.
The traditional way to calculate estimate (7) is to solve the system of equations -
. (8)
The analytical solution (8) turns out to be quite labor-intensive, so we will use a numerical minimization procedure based on Newton’s method
, (9)
where is the quantity that determines the step length of the procedure for searching for the extremum of the objective function.
One way to calculate stride length is to calculate:
. (10)
In the simplest case, when the vector is composed of one parameter, for example or , the probing signal is generated relatively simply. In particular, when optimizing the objective function by parameter, the signal is generated in accordance with the relation
. (11)
In Fig. Figure 3 shows a fragment of the module of the autocorrelation function of the signal (11) at , which corresponds to a PCM radio signal without intrapulse phase modulation.
Level side lobe this function corresponds to a theoretical limit equal to , where . In Fig. Figure 4 shows a fragment of the module of the autocorrelation function of the signal (11) with the parameter obtained by optimizing the function (). The side lobe level is minus 150 dB. The same result is obtained when amplitude modulation M-sequences. In Fig. Figure 5 shows the appearance of such a signal at the optimal value.
Rice. 5. Fragment of an amplitude-modulated FCM signal
The probing signal is generated in accordance with the algorithm
. (12)
Simultaneous amplitude-phase modulation leads to a decrease in the side lobe by another order of magnitude. It is not possible to reach the zero level of the side lobe due to the inevitable computational errors of the recurrent procedure for minimizing the objective function (), which do not allow one to find the true value of the parameter , but only its certain vicinity - . In Fig. Figure 6 shows the dependence of the values of the optimal phase modulation coefficients on the parameter , which determines the length of the sequence.
Rice. 6. Dependence of optimal phase shift on length M- sequences
From Fig. 6 it can be seen that as the sequence length increases, the value of the optimal phase shift asymptotically tends to zero and at we can assume that the optimal signal with intrapulse phase modulation is practically no different from a conventional PCM signal. Research shows that with increasing period length of the modulating PSP relative sensitivity to signal distortion will drop.
An analytical criterion for choosing the limit sequence length can be the following relation
, (13)
where is a number that determines the possibility of technical implementation of a signal with intrapulse modulation in equipment.
Assessing the feasibility of complicating the signal.
The inevitable complication of the signal with a decrease in the side lobes of the autocorrelation function significantly tightens the requirements for generation devices and signal transmission and reception paths. Thus, if there is an error in setting the phase multiplier to one thousandth of a radian, the side lobe level increases from minus 150 dB to minus 36 dB. With amplitude modulation, the error relative to the optimal value of the coefficient A one thousandth leads to an increase in the side lobe from minus 150 dB to minus 43 dB. If the errors in setting the parameters are 0.1 from the optimal ones, which can be implemented in the equipment, then the side lobe of the uncertainty function will increase to minus 15 dB, which is 7 - 7.5 dB better than in the absence of additional phase and amplitude modulation.
On the other hand, the side lobe of the uncertainty function can be reduced without complicating the signal by increasing . So at the side lobe level will be approximately minus 15 dB. It should be noted that ordinary (i.e., without additional AM-FM modulation) PCM signals are sensitive to errors that arise during their formation. Therefore the length M-sequences in real radar devices are also impractical to increase indefinitely.
Let us consider the influence of errors that occur in equipment during the formation, transmission, reception and processing of FCM radio signals on their properties.
Assessment of the influence of errors in the formation of a FCM signal on its properties.
The entire set of factors influencing the characteristics of the signal can be divided into two groups: fluctuation and deterministic.
Fluctuation factors include: phase-frequency instabilities of reference oscillators;
noises of various kinds;
signals leaking from the transmitter directly to the receiver input and, after correlation processing with the reference signal, forming noise-like processes, and other factors. M Deterministic factors include: insufficient broadband of the forming circuits;
, (14)
asymmetry of the modulating function; incoherence of the modulating function and the carrier oscillation; difference in the shape of the reference and probing signals, etc. More generally, the analytical expression for a signal modulated by a pseudorandom=2- sequence, represent it in the form-1; Where ; - constant amplitude; or
p
(15)
- signal phase;
N k k
-integer; -duration of the elementary pulse forming the sequence.
Its two-dimensional correlation function is written as: at, , and its normalized spectrum is shown in Fig. 7. Here, for clarity, a fragment of the frequency axis is shown, where the main components of the signal spectrum are concentrated.
Characteristic feature
Such a signal, as can be seen from Fig. 7, is a reduced level of the unmodulated carrier oscillation, which in the ideal case tends to zero.
Fig.7. Normalized signal spectrum More generally, the analytical expression for a signal modulated by a pseudorandom The wide spectrum band and the absence of periodic unmodulated oscillations makes it possible to implement algorithms for detecting and identifying objects in location systems such as
Fig.9. Dependence of the ACF side lobe level on the bandwidth
transmission of the forming path for k=4
Here, the ordinate axis shows the value that determines the maximum achievable level of the side lobe of the autocorrelation function - a signal modulated by a pseudorandom M- sequence, and on the abscissa - expressed as a percentage, the ratio of the bandwidth of the forming circuit to maximum value frequencies of the effective spectrum of the signal. The dots on the graph show the ACF side lobe level values obtained from numerical simulation of hardware effects. As can be seen from Fig. 9, in the absence of frequency distortions in the radio paths, the level of the side lobe of the ACF signal modulated by the phase of the periodic PSP with a period More generally, the analytical expression for a signal modulated by a pseudorandom, is – 1/ More generally, the analytical expression for a signal modulated by a pseudorandom. This corresponds to the known theoretical limit. When the spectrum of the modulated signal is limited, the side lobe level increases and at 50% limitation reaches the level, which corresponds to a non-periodic autocorrelation function. Further limitation of the radio signal spectrum leads to almost complete collapse of the ACF and, as a result, to the inability to use the signal for practical purposes.
Distortions of the spectrum of the signal emitted by the locator and the reference oscillations arriving at the correlator, due to the asymmetry between positive and negative levels and durations of modulating oscillations, lead to a significant increase in interference in the area of the side lobes of the ACF and deterioration of the spatial resolution and detection characteristics of the locator. The dependence of the side lobe level on the asymmetry coefficient is shown in Fig. 10
The asymmetry coefficient was determined as
, (16)
where is the duration of the undistorted elementary pulse forming M- subsequence; the indices “+” and “−” mean the duration of the positive and negative elementary pulse with asymmetric distortions.
Fig. 10. Dependence of the ACF side lobe level on the magnitude of asymmetric signal distortions for k=4.
Conclusion.
The choice of signal and the degree of complexity of its modulating function is determined primarily by the nature of the tasks for which the radar is intended. The use of a fairly complex FCM signal with intrapulse modulation requires the creation of precision equipment, which will inevitably lead to a significant increase in the price of the design, but at the same time will make it possible to create universal units that can be used both in radars for rescuers and in radars for detecting fast-flying aircraft. goals. This possibility arises because the characteristics of a complex signal with a short sequence length, i.e. high sending repetition rate, allow you to have the necessary resolution and noise immunity with the ability to measure Doppler frequencies in a wider range. In addition, the construction of radar systems with continuous radiation and pseudo-random phase modulation of the carrier wave requires a detailed analysis and consideration of all factors that cause signal distortion in both the transmitting and receiving paths of the locator. Taking into account distorting factors comes down to solving engineering problems to ensure sufficient broadband, stability of electrical parameters and stability of the characteristics of the forming paths. In this case, the radar probe signals must be coherent with the modulating and auxiliary signals. Otherwise, technical solutions are needed that would minimize the difference distortions between the radiated and reference oscillations. One of possible ways, allowing the implementation of such technical solutions is the introduction of symmetrical amplitude limitations of signals in the output stages of the transmitter and at the input of the receiver correlator. In this case, although part of the signal energy is lost, it is possible to form an ACF of the modulated signal with acceptable parameters. Such technical solutions are acceptable in portable radars, where the cost and dimensions of the system play a decisive role.
The most promising at present, from the authors' point of view, should be considered the construction of devices for generating and processing radio signals of complex structure for radar equipment, based on high-speed signal processors operating at clock frequencies of several gigahertz. Structural scheme radar with this approach becomes extremely simple. This linear amplifier power, low-noise linear receiver amplifier and processor with peripheral devices. This scheme allows not only to almost completely realize the properties of signals inherent in their fine structure, but also to create technologically easy-to-set up radar systems, the information processing of which is based on optimal algorithms.
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