CN108020824B - SAL signal coherence maintaining method based on local oscillator digital delay - Google Patents

SAL signal coherence maintaining method based on local oscillator digital delay Download PDF

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CN108020824B
CN108020824B CN201711213470.6A CN201711213470A CN108020824B CN 108020824 B CN108020824 B CN 108020824B CN 201711213470 A CN201711213470 A CN 201711213470A CN 108020824 B CN108020824 B CN 108020824B
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李道京
胡烜
赵绪锋
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Institute of Electronics of CAS
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/483—Details of pulse systems
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    • G01S7/4865—Time delay measurement, e.g. time-of-flight measurement, time of arrival measurement or determining the exact position of a peak
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/88—Lidar systems specially adapted for specific applications
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    • G01S17/90—Lidar systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques

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Abstract

The invention provides a method for maintaining the signal coherence of a synthetic aperture laser radar based on local oscillator digital delay, which comprises the following steps: setting a reference channel to perform local oscillator signal self-heterodyne detection and extracting a differential phase of a local oscillator signal; estimating the instantaneous frequency of the local oscillation signal according to the differential phase; performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal; and performing phase compensation on the target echo signal after delaying the phase estimation result in a digital domain. The SAL signal coherence maintenance is realized based on local oscillator digital delay, and the azimuth/transverse resolution of SAL imaging is greatly improved.

Description

SAL signal coherence maintaining method based on local oscillator digital delay
Technical Field
The disclosure belongs to the field of Synthetic Aperture laser radar (SAL) system design and signal processing, and particularly relates to SAL signal coherence maintenance, in particular to an SAL signal coherence maintenance method based on local oscillator digital delay.
Background
Synthetic Aperture Lidar (SAL) is an application form of Synthetic Aperture Radar (SAR) in a laser frequency band, and obtains high resolution in distance dimension by pulse compression of broadband laser signals and high resolution in azimuth dimension by Synthetic Aperture technology (ludajing, zhangqingjuan, liu wave. airborne Synthetic Aperture lidar key technology and implementation scheme analysis [ J ] Radar science, 2013, 2 (2): 143-151.).
As a coherent radar, after calibration and correction are carried out on a transmitting signal, the azimuth/transverse resolution which can be realized by SAL mainly depends on the coherence of a local oscillator signal (Duxibo synthetic aperture laser radar broadband signal generation and imaging processing technology research [ D ]. Beijing: university of Chinese academy of sciences, 2017.), the coherence of the local oscillator signal mainly comprises parameters such as frequency stability and phase noise, and the frequency stability of the laser signal is mainly represented by a line width at present. The local oscillator signal of the SAL is in a laser wave band, the frequency of the local oscillator signal is more than three orders of magnitude higher than the frequency of the microwave signal, and the frequency stability and coherence of the SAL are poorer in principle compared with the microwave signal (great waves, smelling and spending, microwave photonics principle and application [ M ]. beijing: electronics industry press, 2015.), which limits the azimuth/transverse high-resolution imaging capability of the SAL to a great extent.
Under the condition that the observation geometric relationship is not changed, the SAL can adopt a method of delaying the local oscillator signal optical fiber to keep the coherence of the signal (Gschwendtner A B, Keicher W E.development of coherent laser radar at a linear resonator [ J ]. Lincoln Laboratory Journal, 2000, 12 (2): 383-. When the method is used, the length of the delay optical fiber is difficult to change time, so the method is only suitable for the conditions that the target distance is short and the variation of the target distance in the synthetic aperture time is small, and is difficult to apply in SAL aiming at the imaging of a long-distance moving target.
Taking Inverse Synthetic Aperture laser Radar (ISAR) for imaging of a GEO-orbit space target as an example, the Synthetic Aperture time is in the order of 10s, the echo delay is in the order of 0.2s, which has a high requirement on the coherence of local oscillator signals, and since the distance between the target and the Radar is about 36000km, the local oscillator signals are delayed by optical fibers and then mixed with the target echo signals, which is difficult to implement, and a new SAL signal coherence maintenance method needs to be researched.
Disclosure of Invention
Technical problem to be solved
The present disclosure provides a method for maintaining the coherence of a synthetic aperture laser radar signal based on local oscillator digital delay to at least partially solve the above-mentioned technical problems.
(II) technical scheme
According to an aspect of the present disclosure, a method for maintaining coherence of a synthetic aperture laser radar signal based on local oscillator digital delay is provided, including: setting a reference channel to perform local oscillator signal self-heterodyne detection and extracting a differential phase of a local oscillator signal; estimating the instantaneous frequency of the local oscillation signal according to the differential phase; performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal; performing phase compensation on the target echo signal after delaying the phase estimation result in a digital domain; when the local oscillation signal is detected by self-heterodyne, one path of local oscillation signal in a reference channel is subjected to frequency shift; when local oscillation signal self-heterodyne detection is carried out, a frequency source of an electronic system is used as a reference frequency signal input by a frequency shifter and is strictly synchronous with the phase of an AD clock, so that the synchronization of the phase of an electronic signal and the phase of a laser signal is realized.
In some embodiments of the present disclosure, when performing local oscillator signal self-heterodyne detection, the optical fiber delay length is set within a predetermined interval according to a local oscillator signal parameter, which satisfies the following equation:
Figure GDA0003103807360000021
wherein R is the optical fiber delay length; c is the speed of light;
Figure GDA0003103807360000022
the standard deviation of random phase noise in the local oscillation signal is obtained; phi is a0An upper limit of the standard deviation of the phase estimation error; t issThe time length of the local oscillation signal is; fsIs the AD sampling rate; a. theFThe amplitude of the frequency sinusoidal variation in the local oscillator signal; deltafrIs the standard deviation of the random variation of the frequency in the local oscillation signal.
In some embodiments of the present disclosure, when the instantaneous frequency is subjected to the integration processing, the integration interval is an AD sampling interval.
In some embodiments of the present disclosure, when the phase estimation result is delayed in the digital domain and the target echo signal is compensated, the delay length should be close to the distance from the target to the radar.
In some embodiments of the present disclosure, when the phase estimation result is delayed in the digital domain and the target echo signal is compensated, the delay length is adjusted in the digital domain according to the distance from the target to the radar.
In some embodiments of the present disclosure, the step of setting the reference channel to perform local oscillator signal self-heterodyne detection includes: dividing signals of a laser signal source (1) into a transmitting local oscillator, a receiving local oscillator and a reference channel local oscillator; the local oscillation signal of the reference channel is divided into two paths of signals by a beam splitter (2), one path of signal passes through a delay optical fiber (3), and the other path of signal passes through a frequency shifter (4); the two paths of signals are mixed on a photoelectric detector (6) through a coupler (5), and the two paths of optical signals are mixed and form an electric signal through photoelectric conversion; an AD sum signal recorder (7) performs AD sampling and recording on the electric signal, and sends the electric signal to a signal processor (8).
In some embodiments of the present disclosure, the laser model of the laser signal source (1) is as follows:
s(t)＝exp{j2πfct}·exp{jφsin(t)}·exp{jφf(t)}·exp{jφr(t)} (2)
wherein f iscIs the nominal center frequency of the laser signal;
Figure GDA0003103807360000031
the phase introduced for the sinusoidal variation of the laser signal frequency;
Figure GDA0003103807360000032
phase introduced for random variation of laser signal frequency, fr(t)～N(0，σfr 2) Random frequencies that are gaussian distributions; phi is ar(t)～N(0，σφr 2) Random phase noise that is gaussian distributed; a. theFAmplitude of sinusoidal variation of the signal frequency, fFIs a letterFrequency, σ, at which the sign frequency varies sinusoidallyfrIs the standard deviation of the random frequency and,
Figure GDA0003103807360000033
is the standard deviation of random phase noise;
the signal processor (8) is configured to perform the following operations:
extracting local oscillator signal phase difference information contained in the phase of the electric signal obtained by sampling:
Figure GDA0003103807360000041
wherein the content of the first and second substances,
Figure GDA0003103807360000042
is the phase difference of the local oscillator signal, T is the time delay of the delay optical fiber, phi (T) is the phase introduced by the unstable frequency of the local oscillator signal, phir(t) is the random phase noise of the local oscillator signal, fmIs the reference frequency of the frequency shifter;
cancellation in the digital domain
Figure GDA0003103807360000043
Phase 2 pi f introduced by medium nominal frequencycT and phase 2 pi f introduced by frequency shifter (4)mAnd T, assuming that the instantaneous frequency of the signal corresponding to phi (T) is unchanged in the time delay T of the time delay optical fiber (3), the estimated value is as follows:
Figure GDA0003103807360000044
performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal:
Figure GDA0003103807360000045
wherein, FsFor AD sampling rate, M ═ Fs·TsIs the number of sampling points, TsThe time length of the local oscillation signal is;
will be provided with
Figure GDA0003103807360000046
And performing phase compensation on the target echo signal in the time length corresponding to the distance between the digital domain delay target and the radar.
(III) advantageous effects
According to the technical scheme, the method for maintaining the signal coherence of the synthetic aperture laser radar based on the local oscillator digital delay has at least one of the following beneficial effects:
(1) the SAL signal coherence is maintained by adopting the digital delay based on the local oscillator, so that the SAL signal coherence is effectively maintained, and the azimuth/transverse resolution of SAL imaging is greatly improved;
(2) the method adopts the self-heterodyne detection of the local oscillator signal, effectively estimates the phase introduced by the unstable frequency of the local oscillator signal, carries out phase compensation on the target echo signal after delaying the phase estimation result in a digital domain, and can randomly adjust the delay length, so that the SAL can realize azimuth/transverse high-resolution imaging of a remote target and is also suitable for SAL imaging of a remote moving target.
Drawings
Fig. 1 is a flowchart of a method for maintaining coherence of a synthetic aperture laser radar signal based on local oscillator digital delay according to an embodiment of the present disclosure.
Fig. 2 is a schematic block diagram of a method for estimating a local oscillation phase by local oscillation self-heterodyne detection according to an embodiment of the present disclosure.
Fig. 3 is a schematic diagram of the root mean square error of the phase estimation result under different fiber delay distances according to the phase estimation method of the embodiment of the disclosure.
Fig. 4(a) is a schematic diagram of a phase estimation result and a phase estimation error of the phase estimation method in the case of the fiber delay distance 700m according to the embodiment of the disclosure.
Fig. 4(b) is a schematic diagram of a phase estimation result and a phase estimation error of the phase estimation method according to the embodiment of the disclosure under the condition of the fiber delay distance of 5000 m.
Fig. 4(c) is a schematic diagram of a phase estimation result and a phase estimation error of the phase estimation method in the embodiment of the disclosure under the condition of the fiber delay distance 7500 m.
Fig. 5(a) is a slow-time spectrum diagram of a target echo signal according to an embodiment of the present disclosure.
Fig. 5(b) is a slow-time spectrum diagram of the target echo signal after phase compensation according to the embodiment of the disclosure.
[ description of main reference numerals in the drawings ] of the embodiments of the present disclosure
1. A laser signal source; 2. beam splitter
3. A delay fiber; 4. frequency shifter
5. A coupler; 6. photoelectric detector
7. AD sum signal recorder 8, signal processor
Detailed Description
The disclosure provides a synthetic aperture laser radar signal coherence maintaining method based on local oscillator digital delay. Here, the signal coherence mainly refers to the coherence of local oscillator signals, and the coherence difference of local oscillator signals mainly refers to the difference of frequency stability and large phase noise, which can introduce large phase error in the SAL signals, thereby causing the azimuth/lateral resolution of SAL imaging to be reduced. The present disclosure aims to address the problem of SAL signal coherence preservation, especially in the case of targets at a large distance from the radar.
For the purpose of promoting a better understanding of the objects, aspects and advantages of the present disclosure, reference is made to the following detailed description taken in conjunction with the accompanying drawings.
Certain embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the disclosure are shown. Indeed, various embodiments of the disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements.
In a first exemplary embodiment of the present disclosure, a method for maintaining coherence of a synthetic aperture laser radar signal based on local oscillator digital delay is provided. Fig. 1 is a flowchart of a method for maintaining coherence of a synthetic aperture laser radar signal based on local oscillator digital delay according to an embodiment of the present disclosure. The invention discloses an SAL signal coherence keeping method based on local oscillator digital delay, which comprises the following steps:
step S1: setting a reference channel to perform local oscillator signal self-heterodyne detection and extracting a differential phase of a local oscillator signal;
step S2: estimating the instantaneous frequency of the local oscillation signal according to the differential phase;
step S3: performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal;
step S4: and performing phase compensation on the target echo signal after delaying the phase estimation result in a digital domain.
The following describes each step of the SAL signal coherence maintaining method based on local oscillator digital delay in this embodiment in detail.
In this embodiment, the following laser signal model is established:
s(t)＝exp{j2πfct}·exp{jφsin(t)}·exp{jφf(t)}·exp{jφr(t)}
wherein f iscIs the nominal center frequency of the laser signal;
Figure GDA0003103807360000061
the phase introduced for the sinusoidal variation of the laser signal frequency;
Figure GDA0003103807360000062
phase introduced for random variation of laser signal frequency, fr(t)～N(0，σfr 2) Random frequencies that are gaussian distributions; phi is ar(t)～N(0，σφr 2) Is gaussian distributed random phase noise. The parameters of the signal model are as follows: amplitude A of sinusoidal variation of signal frequencyFFrequency f of sinusoidal variation of the signal frequencyFStandard deviation of random frequency σfrStandard deviation of random phase noise
Figure GDA0003103807360000071
The invention discloses a method for maintaining the signal coherence of a synthetic aperture laser radar based on local oscillator digital delay, which mainly comprises the following steps:
step S1: setting a reference channel to perform local oscillation signal self-heterodyne detection, and extracting a differential phase of a local oscillation signal;
fig. 2 is a schematic block diagram of a method for estimating a phase of a local oscillation signal by local oscillation signal self-heterodyne detection according to an embodiment of the present disclosure. As shown in fig. 2, the signal of the laser signal source 1 is divided into a transmitting local oscillator, a receiving local oscillator and a reference channel local oscillator. The local oscillation signal of the reference channel is divided into two paths of signals by a beam splitter 2, one path of signal passes through a delay optical fiber 3, the other path of signal passes through a frequency shifter 4, the two paths of signals are mixed on a photoelectric detector 6 through a coupler 5, the two paths of optical signals are subjected to photoelectric conversion to form an electric signal after being mixed, AD sampling and recording are carried out through an AD and signal recorder 7, and then the electric signal is processed by a signal processor 8. The phase of the electric signal obtained by sampling contains the difference information of the local oscillator signal phase:
Figure GDA0003103807360000072
wherein T is the time delay of the time delay optical fiber, phi (T) is the phase introduced by the unstable frequency of the local oscillator signal, phir(t) is the random phase noise of the local oscillator signal, fmIs the reference frequency of the frequency shifter.
When local oscillator signal self-heterodyne detection is carried out, frequency shift needs to be carried out on a local oscillator signal of a reference channel, and the purpose is to prevent low-frequency components in a differential phase of the local oscillator signal from being filtered out in the processing process. In step S1, the reference frequency signal input by the frequency shifter 4 must be from the frequency source of the electronic system and strictly synchronized with the phase of the AD clock, so as to achieve synchronization between the phase of the electronic signal and the phase of the laser signal.
When local oscillator signal self-heterodyne detection is performed, the optical fiber delay length must be optimized and set in a certain interval according to local oscillator signal parameters, which aims to ensure that the differential phase of the local oscillator signal is not twisted and the phase estimation error is small, and is specifically embodied as follows:
Figure GDA0003103807360000081
wherein R is the optical fiber delay length; c is the speed of light;
Figure GDA0003103807360000082
the standard deviation of random phase noise in the local oscillation signal is obtained; phi is a0An upper limit of the standard deviation of the phase estimation error; t issThe time length of the local oscillation signal is; fsIs the AD sampling rate; a. theFThe amplitude of the frequency sinusoidal variation in the local oscillator signal; deltafrIs the standard deviation of the random variation of the frequency in the local oscillation signal.
Step S2: estimating the instantaneous frequency of the local oscillation signal according to the differential phase;
specifically, the step S2 includes: cancellation in the digital domain
Figure GDA0003103807360000085
Phase 2 pi f introduced by medium nominal frequencycT and phase 2 pi f introduced by frequency shifter 4mT, assuming that the instantaneous frequency of the signal corresponding to phi (T) is unchanged in the delay T of the delay fiber 3, the estimated value is:
Figure GDA0003103807360000083
step S3: performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal;
specifically, the step S3 includes: performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal:
Figure GDA0003103807360000084
wherein, FsFor AD sampling rate, M ═ Fs·TsIs the number of sampling points, TsIs the duration of the local oscillator signal.
When the instantaneous frequency is subjected to integration processing, the integration interval can be selected as an AD sampling interval, and the AD sampling rate is improved, so that the phase estimation error is reduced.
Step S4: and performing phase compensation on the target echo signal after delaying the phase estimation result in a digital domain.
Specifically, the step S4 includes: will be provided with
Figure GDA0003103807360000091
And (3) performing phase compensation on a target echo signal at the time length corresponding to the distance between the digital domain delay target and the radar:
when the phase estimation result is delayed in a digital domain and the target echo signal is compensated, the delay length is about the distance from the target to the radar, namely the delay length should be as close to the distance from the target to the radar as possible.
Preferably, when the phase estimation result is delayed in the digital domain and the target echo signal is compensated, the delay length can be adjusted arbitrarily in the digital domain according to the distance from the target to the radar. This adjustment makes the method of the present disclosure applicable to SAL for imaging of distant targets as well as SAL for imaging of distant moving targets.
The implementation effect of the method of the present invention is shown below by combining with the simulation example, and the simulation parameters are shown in table 1.
Table 1 simulation example parameters
Figure GDA0003103807360000092
Fig. 3 shows the Root Mean Square Error (RMSE) of the phase estimation result for different fiber delay lengths, where the delay length corresponding to the middle part of the asterisk is the range of fiber delay lengths calculated according to equation (1) in the claims, and it is apparent that the RMSE of the phase estimation result is smaller within the fiber delay length corresponding to equation (1). Fig. 4(a), 4(b), and 4(c) show the phase estimation results and the phase estimation errors at the fiber delay lengths of 700m, 5000m, and 7500m, respectively.
Fig. 5(a) shows the slow-time spectrum of the echo signal of the GEO-orbit space target with the time duration of 0.25s, fig. 5(b) shows the slow-time spectrum of the target echo signal after phase compensation by using the method provided by the present invention, and it is obvious that the slow-time spectrum width shown in fig. 5(b) is much smaller than the slow-time spectrum width shown in fig. 5(a), thereby illustrating that the method provided by the present invention can effectively maintain the coherence of the SAL signal and improve the azimuth/lateral resolution of SAL imaging.
The invention provides an SAL signal coherence maintaining method based on local oscillator digital delay, which can effectively maintain the coherence of an SAL signal and greatly improve the azimuth/transverse resolution of SAL imaging. Based on the local oscillator signal self-heterodyne detection, the method provided by the invention can effectively estimate the phase introduced by the unstable frequency of the local oscillator signal, and performs phase compensation on the target echo signal after delaying the phase estimation result in a digital domain, wherein the delay length can be adjusted at will, so that the SAL can realize azimuth/transverse high-resolution imaging of a remote target. The method is also applicable to SAL for imaging of a remotely moving object.
So far, the embodiments of the present disclosure have been described in detail with reference to the accompanying drawings. It is to be noted that, in the attached drawings or in the description, the implementation modes not shown or described are all the modes known by the ordinary skilled person in the field of technology, and are not described in detail. Further, the above definitions of the various elements and methods are not limited to the various specific structures, shapes or arrangements of parts mentioned in the examples, which may be easily modified or substituted by those of ordinary skill in the art.
Unless otherwise indicated, the numerical parameters set forth in the specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the present disclosure. In particular, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". Generally, the expression is meant to encompass variations of ± 10% in some embodiments, 5% in some embodiments, 1% in some embodiments, 0.5% in some embodiments by the specified amount.
In addition, unless steps are specifically described or must occur in sequence, the order of the steps is not limited to that listed above and may be changed or rearranged as desired by the desired design. The embodiments described above may be mixed and matched with each other or with other embodiments based on design and reliability considerations, i.e., technical features in different embodiments may be freely combined to form further embodiments.
The algorithms and displays presented herein are not inherently related to any particular computer, virtual machine, or other apparatus. Various general purpose systems may also be used with the teachings herein. The required structure for constructing such a system will be apparent from the description above. Moreover, this disclosure is not directed to any particular programming language. It is appreciated that a variety of programming languages may be used to implement the present disclosure as described herein, and any descriptions above of specific languages are provided for disclosure of enablement and best mode of the present disclosure.
The disclosure may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. Various component embodiments of the disclosure may be implemented in hardware, or in software modules running on one or more processors, or in a combination thereof. Those skilled in the art will appreciate that a microprocessor or Digital Signal Processor (DSP) may be used in practice to implement some or all of the functionality of some or all of the components in the relevant apparatus according to embodiments of the present disclosure. The present disclosure may also be embodied as apparatus or device programs (e.g., computer programs and computer program products) for performing a portion or all of the methods described herein. Such programs implementing the present disclosure may be stored on a computer-readable medium or may be in the form of one or more signals. Such a signal may be downloaded from an internet website or provided on a carrier signal or in any other form.
Those skilled in the art will appreciate that the modules in the device in an embodiment may be adaptively changed and disposed in one or more devices different from the embodiment. The modules or units or components of the embodiments may be combined into one module or unit or component, and furthermore they may be divided into a plurality of sub-modules or sub-units or sub-components. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and all of the processes or elements of any method or apparatus so disclosed, may be combined in any combination, except combinations where at least some of such features and/or processes or elements are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Also in the unit claims enumerating several means, several of these means may be embodied by one and the same item of hardware.
Similarly, it should be appreciated that in the foregoing description of exemplary embodiments of the disclosure, various features of the disclosure are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various disclosed aspects. However, the disclosed method should not be interpreted as reflecting an intention that: that is, the claimed disclosure requires more features than are expressly recited in each claim. Rather, as the following claims reflect, disclosed aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this disclosure.
The above-mentioned embodiments are intended to illustrate the objects, aspects and advantages of the present disclosure in further detail, and it should be understood that the above-mentioned embodiments are only illustrative of the present disclosure and are not intended to limit the present disclosure, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present disclosure should be included in the scope of the present disclosure.

Claims (8)

1. A method for maintaining the signal coherence of a synthetic aperture laser radar based on local oscillator digital delay comprises the following steps:
setting a reference channel to perform local oscillator signal self-heterodyne detection and extracting a differential phase of a local oscillator signal;
estimating the instantaneous frequency of the local oscillation signal according to the differential phase;
performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal;
performing phase compensation on the target echo signal after delaying the phase estimation result in a digital domain;
when the local oscillation signal is detected by self-heterodyne, one path of local oscillation signal in a reference channel is subjected to frequency shift;
when local oscillation signal self-heterodyne detection is carried out, a frequency source of an electronic system is used as a reference frequency signal input by a frequency shifter and is strictly synchronous with the phase of an AD clock, so that the synchronization of the phase of an electronic signal and the phase of a laser signal is realized.
2. The method for maintaining the signal coherence of the synthetic aperture laser radar according to claim 1, wherein when the local oscillator signal is detected by self-heterodyne, the optical fiber delay length is set within a predetermined interval according to the local oscillator signal parameter, and the following formula is satisfied:
Figure FDA0003103807350000011
wherein R is the optical fiber delay length; c is the speed of light;
Figure FDA0003103807350000012
the standard deviation of random phase noise in the local oscillation signal is obtained; phi is a0An upper limit of the standard deviation of the phase estimation error; t issThe time length of the local oscillation signal is; fsIs the AD sampling rate; a. theFThe amplitude of the frequency sinusoidal variation in the local oscillator signal; deltafrIs the standard deviation of the random variation of the frequency in the local oscillation signal.
3. The method of claim 1, wherein the integration interval is an AD sampling interval when the instantaneous frequency is subjected to integration processing.
4. The method of claim 1, wherein the delay length is close to the distance from the target to the radar when the phase estimation result is delayed in the digital domain and the target echo signal is compensated.
5. The method of claim 1, wherein the delay length is adjusted in the digital domain according to the distance from the target to the radar when compensating the target echo signal after delaying the phase estimation result in the digital domain.
6. The method for maintaining the coherence of the signals of the synthetic aperture laser radar according to claim 1, wherein the step of setting the reference channel to perform local oscillator signal self-heterodyne detection includes:
dividing signals of a laser signal source (1) into a transmitting local oscillator, a receiving local oscillator and a reference channel local oscillator;
the local oscillation signal of the reference channel is divided into two paths of signals by a beam splitter (2), one path of signal passes through a delay optical fiber (3), and the other path of signal passes through a frequency shifter (4);
the two paths of signals are mixed on a photoelectric detector (6) through a coupler (5), and the two paths of optical signals are mixed and form an electric signal through photoelectric conversion;
an AD sum signal recorder (7) performs AD sampling and recording on the electric signal, and sends the electric signal to a signal processor (8).
7. The method for synthetic aperture lidar signal coherence maintenance according to claim 6, the laser model of the laser signal source (1) is as follows:
s(t)＝exp{j2πfct}·exp{jφsin(t)}·exp{jφf(t)}·exp{jφr(t)} (2)
wherein f iscIs the nominal center frequency of the laser signal;
Figure FDA0003103807350000021
the phase introduced for the sinusoidal variation of the laser signal frequency;
Figure FDA0003103807350000022
phase introduced for random variation of laser signal frequency, fr(t)～N(0，σfr 2) Random frequencies that are gaussian distributions; phi is ar(t)～N(0，σφr 2) Random phase noise that is gaussian distributed; a. theFAmplitude of sinusoidal variation of the signal frequency, fFFor the frequency of the sinusoidal variation of the signal frequency, σfrIs the standard deviation of the random frequency and,
Figure FDA0003103807350000023
is the standard deviation of random phase noise.
8. The synthetic aperture lidar signal coherence preservation method of claim 7, the signal processor (8) being configured to:
extracting local oscillator signal phase difference information contained in the phase of the electric signal obtained by sampling:
Figure FDA0003103807350000031
wherein the content of the first and second substances,
Figure FDA0003103807350000032
is the phase difference of the local oscillator signal, T is the time delay of the delay optical fiber, phi (T) is the phase introduced by the unstable frequency of the local oscillator signal, phir(t) is the random phase noise of the local oscillator signal,fmis the reference frequency of the frequency shifter;
cancellation in the digital domain
Figure FDA0003103807350000036
Phase 2 pi f introduced by medium nominal frequencycT and phase 2 pi f introduced by frequency shifter (4)mAnd T, assuming that the instantaneous frequency of the signal corresponding to phi (T) is unchanged in the time delay T of the time delay optical fiber (3), the estimated value is as follows:
Figure FDA0003103807350000033
performing integral processing on the instantaneous frequency to obtain a phase estimation result of the local oscillation signal:
Figure FDA0003103807350000034
wherein, FsFor AD sampling rate, M ═ Fs·TsIs the number of sampling points, TsThe time length of the local oscillation signal is;
will be provided with
Figure FDA0003103807350000035
And performing phase compensation on the target echo signal in the time length corresponding to the distance between the digital domain delay target and the radar.
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