CN111045013A - Multi-frequency differential absorption radar system for measuring sea surface air pressure - Google Patents

Multi-frequency differential absorption radar system for measuring sea surface air pressure Download PDF

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CN111045013A
CN111045013A CN201911258607.9A CN201911258607A CN111045013A CN 111045013 A CN111045013 A CN 111045013A CN 201911258607 A CN201911258607 A CN 201911258607A CN 111045013 A CN111045013 A CN 111045013A
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sea surface
frequency
radar
signal
air pressure
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董晓龙
朱迪
胥鑫
云日升
徐国庆
徐星欧
马建英
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National Space Science Center 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
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/95—Radar or analogous systems specially adapted for specific applications for meteorological use
    • 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
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/36—Means for anti-jamming, e.g. ECCM, i.e. electronic counter-counter measures
    • 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
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
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Abstract

The invention discloses a multi-frequency differential absorption radar system for measuring sea surface air pressure, which comprises: the device comprises an antenna, a transmitting-receiving front end, a transmitter, a receiver and a sea surface air pressure calculation module; the antenna is used for transmitting a dual-frequency radar signal and receiving a radar echo signal; the receiving and transmitting front end is used for isolating the transmitting channel and the receiving channel and generating a calibration signal of the receiver; the transmitter is used for generating a dual-frequency linear frequency modulation signal, amplifying the dual-frequency linear frequency modulation signal to a required power level, up-converting a baseband signal to a working frequency, amplifying the power and then sending the amplified baseband signal to an antenna; the receiver is used for amplifying the radar echo signal, converting the radar echo signal into a baseband through down conversion, and sending the baseband to the sea surface air pressure calculation module; and the sea surface air pressure calculation module is used for acquiring the power ratio of the radar echo signals of two or more channels in the strong oxygen absorption band from the radar echo signals to realize the inversion of the sea surface air pressure.

Description

Multi-frequency differential absorption radar system for measuring sea surface air pressure
Technical Field
The invention relates to the field of radars, in particular to a multi-frequency differential absorption radar system for measuring sea surface air pressure.
Background
Sea surface air pressure is a very critical meteorological factor, and plays an important role in numerical weather forecast, climatology and atmospheric dynamics research, tropical cyclone moving path and strength forecast, tropical cyclone structure and dynamic characteristic analysis. Sea surface pressure is one of the most important meteorological parameters for predicting the intensity and trajectory of tropical cyclones, including tropical low pressure, tropical storms, hurricanes and super typhoons. The sea level central air pressure of tropical cyclones is the most common measure of hurricane intensity. For example: the classification of tropical storms and hurricanes on the Saffir-Simpson hurricane scale (SSHS) is based on sea-surface wind speeds, which is a direct result of the interaction of the tropical storm central air pressure with the surrounding air pressure field, and it is therefore important to measure the surface air pressure of tropical storms.
In the ocean, the sea surface air pressure is usually observed on site for a limited number of times by using a buoy station and an oil merchant ship platform, and the observation means have small coverage area, poor space-time resolution and inconsistent detection precision and are difficult to meet the application requirements of weather forecast and the like. For example, the NOAA Ocean Observation System (NOOS) of the National Data Buoy Center (NDBC) in the united states east coast and gulf of mexico has only about 40 buoys available for measuring sea surface air pressure. The tropical atmosphere ocean program (TAO) has only 10 sites to measure barometric pressure. For severe weather conditions (e.g., tropical storms and hurricane weather), buoy systems typically cannot provide regional field measurements due to either the lack of buoy data on storm tracks or buoy failure due to severe weather itself.
The only currently available on-site measurement is by landing sonde technology piloting the aircraft. The falling type sonde technology has the problems that in the storm process, due to the limitation of the number and the spatial distribution of the falling type sondes and the problems that the falling type sondes are difficult to accurately position and cannot be repeatedly used, most storm areas cannot be measured.
Large spatial coverage and high frequency sub-observations of the barometric pressure data are essential to improve the prediction and prediction of tropical storm intensities and trajectories. This need cannot be met by on-site buoy and drop-off sonde technology. The only hope for barometric pressure measurements on large spatial and temporal scales at sea comes from remote sensing technologies, including manned aircraft, Unmanned Aerial Vehicles (UAVs) and remote sensing observations on satellite platforms.
Over the past thirty years, airborne, satellite-borne sea surface pressure telemetry for large-scale, global scale has significantly lagged behind other important meteorological parameters (such as temperature and humidity). Barton and Scott proposed in 1985 to measure sea surface barometric pressure using active and passive methods, respectively, over the a band. However, the designed lidar instrument has high dependence on the stability of the space platform and is complex to operate, so that the lidar instrument is difficult to realize technically. While passive methods are limited to measurements in daylight and low cloud coverage areas (Barton, Scott 1986).
Disclosure of Invention
The invention aims to overcome the defects of lack of sea surface air pressure observation means, small coverage area, poor space-time resolution and inconsistent detection precision at present, and provides a multi-frequency differential absorption radar system for measuring the sea surface air pressure, wherein the differential absorption air pressure radar system effectively eliminates the microwave absorption effect and the influence of sea surface reflection caused by liquid water, atmospheric water vapor and the like in the atmosphere by utilizing the ratio of the reflected radar signal power of two or more channels in a strong oxygen absorption band (50-70GHz), so as to obtain the total oxygen amount information; meanwhile, the inversion of sea surface air pressure is realized by utilizing the constant proportion of oxygen in air and the proportional relation between the total amount of the gas column of the uniform mixed gas and the surface air pressure.
In order to achieve the above object, the present invention discloses a multi-frequency differential absorption radar system for measuring sea surface barometric pressure, the system comprising: the device comprises an antenna, a transmitting-receiving front end, a transmitter, a receiver and a sea surface air pressure calculation module;
the antenna is used for transmitting a dual-frequency radar signal and receiving a radar echo signal;
the receiving and transmitting front end is used for isolating the transmitting channel and the receiving channel and generating a calibration signal of the receiver;
the transmitter is used for generating a dual-frequency linear frequency modulation signal, amplifying the dual-frequency linear frequency modulation signal to a required power level, up-converting a baseband signal to a working frequency, amplifying the power and then sending the amplified baseband signal to an antenna;
the receiver is used for amplifying the radar echo signal, converting the radar echo signal into a baseband through down conversion, and sending the baseband to the sea surface air pressure calculation module;
and the sea surface air pressure calculation module is used for acquiring the power ratio of the radar echo signals of two or more channels in the strong oxygen absorption band from the radar echo signals to realize the inversion of the sea surface air pressure.
As an improvement of the above system, the specific implementation process of the sea surface barometric pressure calculation module is as follows:
pressure P of sea surfaceoAs a function of radar power ratio:
Po＝C0(λ1,λ2)+C1(λ1,λ2)loge(Pr(λ1)Pr -1(λ2))
wherein, C0(λ1,λ2) And C1(λ1,λ2) A wavelength correlation coefficient which is a relationship between a radar power ratio and a ground air pressure; can be obtained by utilizing the constant proportion of oxygen in the air and the proportional relation between the total amount of the gas column of the uniform mixed gas and the surface pressure, Pr(λ1) And Pr(λ2) For using a wavelength of λ1And λ2Two radar channels ofThe radar received power of the frequency channel.
As an improvement of the above system, the system further comprises: and the modular switching power supply adopting an integrated circuit is used for supplying power to the whole radar system.
As an improvement of the above system, the antenna is an active phased array antenna with a 3dB beamwidth gain of no less than 50dB at a 10 degree scan angle.
As an improvement of the system, the receiver adopts a matched filter, the signal-to-noise ratio is not lower than 10dB, and the noise coefficient is 6 dB.
As an improvement of the above system, the system further comprises: the frequency synthesizer is used for generating a required local oscillation frequency for transmitting the double-frequency signal and generating a required local oscillation frequency for the receiver; an oven controlled crystal oscillator with high stability is adopted.
As an improvement of the above system, when the tolerable time deviation Δ τ of the receiving window is 2 μ s, the farthest and closest distances detected by the radar system are respectively: rmax＝550km，Rmin＝500km。
As an improvement of the above system, the frequency range of the radar system is: 50-55 GHz.
The invention has the advantages that:
1. the radar system of the invention adopts satellite-borne microwave remote sensing to realize the sea surface air pressure measurement with large space, large scale and high coverage in the global range;
2. the radar system of the invention utilizes the multi-frequency measurement of the strong oxygen absorption band to effectively eliminate the microwave absorption effect and the influence of sea surface reflection caused by liquid water in the atmosphere, water vapor in the atmosphere and the like, and has higher precision.
Drawings
FIG. 1 is a block diagram of a multi-frequency differential absorption radar system for measuring sea surface air pressure in accordance with the present invention;
FIG. 2 is a schematic diagram of intermediate frequency signal generation;
FIG. 3 is a differential absorption baroradar observation geometry;
FIG. 4 illustrates a differential absorption barometric pressure radar scan mode;
FIG. 5 is a graph of 50-55GHz frequency versus single pass atmospheric attenuation;
FIG. 6 is a schematic diagram of a pulse timing sequence;
FIG. 7 is a PRF constraint of a pulse timing model;
FIG. 8 is a graph of pulse width versus signal-to-noise ratio for different frequencies for given system parameters;
FIG. 9 is a schematic diagram of simulation;
FIG. 10 is a schematic diagram of a scanning trace of a differential absorption air pressure radar;
FIG. 11 is a graph of differential absorption index versus surface air pressure for a 50.3GHz-53.9GHz frequency pair;
FIG. 12 is a graph of differential absorption index versus surface air pressure for a 50.3GHz-55.0GHz frequency pair;
FIG. 13 is a graph of antenna gain and beam width versus operating frequency;
FIG. 14 is a plot of 50GHz sea backscatter coefficient versus angle of incidence.
Detailed Description
The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.
As shown in fig. 1, the present invention provides a multi-frequency differential absorption radar system for measuring sea surface air pressure, which can realize global sea surface air pressure measurement by active remote sensing, and the system comprises: an antenna, a transceiver front-end, a transmitter, a receiver, a frequency synthesizer, a central electronic system, a signal processor and a power supply.
The antenna sequentially transmits dual-frequency radar signals and simultaneously receives radar echoes from the sea surface, swing scanning is realized by adopting an active phased array antenna to obtain a wider swath, and the gain of the 3dB wave beam width at a scanning angle of 10 degrees is not less than 50 dB;
the transmitting and receiving front section is linked with a switch or a scaling network between an antenna and a transmitter and a receiver, a transmitting channel and a receiving channel are isolated, the receiver is prevented from being damaged by a transmitting signal, the interference of the transmitting signal leakage to a receiver signal is avoided, and a signal for scaling the radar receiver is generated according to the requirement;
the transmitter sequentially generates and amplifies the dual frequency chirp signal to the required power level, up-converts the baseband signal to the operating frequency, and amplifies the power to 100w for feeding to the antenna. Mainly comprises three modules: the device comprises an intermediate frequency signal generating module, a transmitting up-conversion module and a power amplifier, wherein the intermediate frequency signal generating module generates signals in a digital mode as shown in figure 2. The power amplifier adopts a solid power amplifier, and has the characteristics of long service life, high reliability, good linearity, wide band and the like.
The receiver mainly amplifies weak echo signals from the sea surface and down-converts the weak echo signals to a baseband, and in order to improve the spatial resolution bandwidth to be 500k, a matched filter is adopted, the signal-to-noise ratio is not lower than 10dB, and the noise coefficient is 6 dB.
The frequency synthesizer is used for generating various frequencies, such as a local oscillator frequency required by a transmitting double-frequency signal, a local oscillator frequency required by a receiver, various clock frequencies required by a central electronic system and the like, in order to ensure the coherence of the differential absorption air pressure radar system, the constant temperature crystal oscillator with high stability is adopted, the output frequency of the oscillator is ensured to be kept stable within a certain temperature range, and meanwhile, a temperature compensation measure is adopted.
The power supply provides various required power supplies with different specifications for the whole radar system, and the modularized switching power supply of the integrated circuit is adopted, so that the volume and the weight of the power supply are reduced.
The central electronic system is used for controlling the work of the whole radar system, sampling, storing and transmitting output signals of the receiver and preprocessing the output signals, the central electronic system is constructed by adopting a technical framework of FPGA + DSP + ADC + data transmission, and real-time calculation and storage of the on-satellite differential absorption index are realized by utilizing log ratio calculation of double-frequency echo power, so that the transmission data volume and the transmission time are reduced.
The sea surface air pressure calculation module (part of a central electronic system) is used for eliminating the microwave absorption effect and the sea surface reflection influence caused by liquid water, atmospheric water vapor and the like in the atmosphere by utilizing the ratio of the reflected radar signal power of two or more channels in a strong oxygen absorption band, and acquiring total oxygen amount information; meanwhile, the inversion of sea surface air pressure is realized by utilizing the constant proportion of oxygen in air and the proportional relation between the total amount of the gas column of the uniform mixed gas and the surface air pressure.
The multifrequency differential absorption radar system for measuring sea surface air pressure is carried on a satellite platform, the observation geometry is shown in figures 3 and 4, h is the orbit height of the satellite platform, the included angle between the irradiation center pointing direction of a beam antenna and a satellite point is α, the local incident angle is theta, and due to the influence of ground curvature, the local incident angle theta is larger than a pitch angle α, RaBeing the radius of the earth, then the geometric relationship of θ to α can be expressed as:
Figure BDA0002310988700000051
the distance d of the antenna footprint to the nadir point may be expressed as:
d＝Ra·η＝Ra·(θ-α)
the skew distance r can be expressed as:
Figure BDA0002310988700000052
if the wave beam widths of the differential absorption baroradar in the pitching direction and the azimuth direction are β dB respectivelyelAnd βazThen the distance of the ground footprint is towards the width deltaelAnd the width delta in the azimuth directionazCan be respectively expressed as:
Figure BDA0002310988700000053
δaz＝r·βaz
using linear frequency-modulated signals to differentially absorb the sub-satellite vertical resolution delta of the air pressureRComprises the following steps:
Figure BDA0002310988700000054
where B is the system fm signal bandwidth.
The resolution is preferably 1km or less. There is also 4km (cloudsat cpr, trmm pr); but the stability requirements for the antenna position and platform will be higher when the beam is narrowed. Meanwhile, the illumination area is too small, which is more sensitive to the smoothness of the sea surface (the sea surface is used as a reflecting surface on the premise that the sea surface is flat), so that the illumination area cannot be smaller than 200 m.
The frequency used by the differential absorption barometric pressure radar is in a strong absorption band of oxygen, which means that the atmospheric oxygen can greatly attenuate the propagation of the signal, thereby affecting the reception of the signal and the accuracy of the inversion of the barometric pressure. The key to the trade-off is to precisely study the atmospheric absorption attenuation to obtain the optimal solution for frequency selection. Therefore, the absorption attenuation of the atmosphere needs to be studied in detail. The model in question is mainly derived from the microwave remote sensing of the Urabh, as shown in FIG. 5.
The differential absorption barometric radar eliminates the microwave absorption effect and sea surface reflection influence caused by liquid water, atmospheric water vapor and the like in the atmosphere by utilizing the ratio of the reflected radar signal power of two or more channels in a strong oxygen absorption band (50-70GHz), and obtains total oxygen amount information; meanwhile, the inversion of sea surface air pressure is realized by utilizing the constant proportion of oxygen in air and the proportional relation between the total amount of the gas column of the uniform mixed gas and the surface air pressure.
For wavelength lambda, antenna gain G, transmit power PTThe radar of (1), the returned power Δ P after reaching a small surface Δ a at a distance R from the viewing angle θs：
Figure BDA0002310988700000061
Where T is the atmospheric transmission rate of the radar wave at that frequency. Reception of radar receiver from Δ PsPower Δ P ofrComprises the following steps:
Figure BDA0002310988700000062
wherein A iseIs an effect of an antennaPore diameter equivalent to λ2G/4π，σ0Is the backscattering coefficient of the surface. Total power received by the receiver (view angle)
Figure BDA0002310988700000063
) Comprises the following steps:
Figure BDA0002310988700000064
therein about
Figure BDA0002310988700000065
Is along the antenna illumination angle, the view angle θ and the distance R can be considered constant when the radar beam is sufficiently narrow, so:
Figure BDA0002310988700000066
Figure BDA0002310988700000067
equations (3) and (4b) are generalized radar equations that simplify the area extension target of the atmospheric radiation delivery process. Since only the parameters T, σ are present in (4b)0And R is related to environmental conditions, and others are related to radar system design, so (4b) can be further simplified as:
Figure BDA0002310988700000068
wherein C (λ) ═ PTAeThe/4 pi will vary with different radar system parameters at different wavelengths. At the sub-star point (nadir, θ ═ 0) can be further simplified as:
Figure BDA0002310988700000069
under the condition of no precipitation, the influence of atmospheric scattering on radar signal propagation can be ignored, and main atmospheric media for attenuating radar signals are oxygen, cloud liquid water and water vapor. Thus:
T(λ)＝exp(-τo-τL-τV)
＝exp(-αoO-αLL-αVV) (7)
in which τ is wavelength dependentiAnd αiIs the atmospheric optical thickness and effective absorption coefficient of the atmospheric medium i (i ═ O, L, or V), O, L, V are the atmospheric column oxygen, cloud liquid water, and water vapor amounts, respectively, note αOThe relationship to atmospheric pressure and temperature is small, assuming these values are a function of wavelength only, to simplify the present discussion, and this and other radar signal propagation effects will be taken into account in the simulation.
In the atmosphere, oxygen is generally uniformly mixed with other gases. The quantity of oxygen being proportional to the mass of air, e.g. O-MOA, wherein MOIs the proportion of oxygen in the mixed air and A is the mass of the air column. Because A is P0/g，P0And g are the surface air pressure and the acceleration of the earth's gravity, respectively, so equation (6) can be expressed as:
Figure BDA0002310988700000071
when two radar channels with sufficiently close wavelengths are used (such as those listed in table I), the surface radar backscattering coefficient, the absorption decay of liquid water and the absorption decay of water vapor are very similar. The ratio of the radar received power of the two frequency channels at this time is:
Figure BDA0002310988700000072
this ratio is determined primarily by the surface atmospheric pressure. The temperature and pressure dependence of the effective oxygen absorption coefficient has a secondary effect on the spectral power ratio. Rearranging (9), we take the surface air pressure as a function of the radar power ratio:
Figure BDA0002310988700000073
or simply written as:
Po＝C0(λ1,λ2)+C1(λ1,λ2)loge(Pr(λ1)Pr -1(λ2)) (11)
＝C0(λ1,λ2)+C1(λ1,λ2)Ri(λ1,λ2)
wherein C is0(λ1,λ2) And C1(λ1,λ2) The wavelength correlation coefficient, which is the relationship between the radar power ratio and the ground pressure, can be estimated from radar measurements or theoretical calculations in radar system design. Ri (lambda)1,λ2) Is the wavelength lambda1,λ2The logarithm of the lower radar power ratio (hereinafter referred to as the differential absorption index).
Frequency selection: the range of 50-55 GHz; as an active instrument, a differentially absorbing barometric radar needs to control its transmission and avoid radio wave contamination potentials to passive microwave satellite measurements, such as advanced microwave detection units (AMSUs) operating in the oxygen band. The research is mainly characterized in that the system optimization and the frequency selection of the differential absorption air pressure radar instrument provide guidance for the design work of the differential absorption remote sensing instrument system in the future.
The ratio of the radar received power of the two channels is:
Figure BDA0002310988700000074
if C (λ)1) And C (lambda)2) Quite closely, it can be deduced from equation (9):
Figure BDA0002310988700000081
namely:
Figure BDA0002310988700000082
τ＝-0.5Ln(η)
where τ is the O2 Differential Absorption Optical Depth (DAOD) between frequencies 1 and 2, η is equal to P1/P2, which is the key signal that needs to be measured by the differential absorption barometric radar system, indicating that for a differential absorption radar wavelength pair in the 50-55GHz band, the first requirement for frequency selection is a very small frequency shift (within 3 GHz) to minimize the variation of continuous absorption (this O2 band is already very far from the weak 22.2GHz absorption line). the second consideration is the signal strength of the radar return power.
Assume δ τ and δ η are the measurement uncertainties in τ and η, respectively, according to equation (2) to:
δη＝e-2τ2δτ＝-2ηδτ (13)
as expected, the results indicate that the uncertainty of the DAOD (or oxygen level and surface barometric pressure) measurement is directly determined by the uncertainty of the differential absorption barometric pressure radar power measurement-2τThe product of (a) is proportional. The compound represented by formula (13):
γ＝τ/δτ＝2τ/2δτ＝-2τη/δη＝-2τe-2τ/δη (14)
for general measurement environments and oxygen band differential absorption barometric pressure radar systems, the uncertainty of the measurement signal or δ η can be assumed to be at a constant level-2τ. To obtain the highest SNR, the maximum SNR of K is evaluated over the differential absorption optical depth τ:
Figure BDA0002310988700000083
therefore, τ is 0.5. Around this τ value, the second derivative of K:
Figure BDA0002310988700000084
when τ < 1. Therefore, when the bi-directional DAOD between frequencies 1 and 2 is equal to 1, there is the highest SNR. Based on equation (14), the SNR of the DAOD equals the SNR of the radar return signal power ratio at that value of τ. That is, a designed differentially-aspirated barometric radar system will provide the best oxygen DAOD return from the estimation of radar power ratio. The results also show that in this two-way DAOD, not only is there a high enough difference between the radar echoes of the two channels to measure the amount of O2, but also the atmospheric O2 absorption is not too high to cause the radar to transmit excessive attenuated power. Therefore, the optimal DAOD SNR would be obtained directly from the optimal power measurement.
Table 1: recommended frequency selection combination
Frequency (GHz) Atmospheric opacity (Np) Oxygen opacity (Np) Water vapor opacity (Np)
50.011 0.3516 0.3108 0.03932
52.341 0.8542 0.8106 0.04193
53.018 1.3567 1.3123 0.04272
53.396 1.7497 1.7048 0.04319
54.595 4.8262 4.7797 0.04479
54.643 5.3242 5.2776 0.04486
Through analytical studies, an optimal selection procedure for the frequency of the differential absorption barometric radar is obtained. One of the key criteria is that the differential absorption optical depth between the radar frequency pairs should be about 0.5. The optimization method and general conclusion obtained by the work can be used for the design of the current differential absorption air pressure radar instrument and can also be widely applied to other differential absorption remote sensing systems.
The choice of pulse timing determines the pulse duty cycle and the average transmit power, which affects the signal-to-noise ratio of the system. The pulse timing is required to meet the range ambiguity due to contamination of the echo power of the previous and subsequent radar pulses at a certain pulse reception time. For satellite-borne radar, the range-blurred image becomes more severe since a few pulse periods may be required from pulse transmission to pulse reception.
The range ambiguity requires that the pulse PRF be small enough that a common pulse timing design is shown in FIG. 5. Wherein, TpFor the pulse repetition period, τpIs the pulse width, RmaxAnd RminΔ τ is the echo time shift due to the range uncertainty that can be tolerated by the receive window, for the farthest and closest range, respectively, that the radar is designed to detect.
In fig. 5, the rectangle represents the transmission pulse, the trapezoid represents the echo signal, and the parameters in fig. 5 should satisfy the following relations:
Figure BDA0002310988700000091
Figure BDA0002310988700000092
on the premise of meeting the distance ambiguity requirement, a higher PRF can be realized by adopting a pulse cluster form.
Figure BDA0002310988700000101
Under the condition of constant transmission power, the higher the PRF of a transmission signal is, the shorter the pulse time is, the lower the signal-to-noise ratio is, and the more serious the thermal noise decorrelation is. While the lower the PRF, the longer the interference time interval, the more severe the spatial decorrelation.
In order to select the optimal pulse timing, when the tolerable time deviation delta tau of the receiving window is 2 mus, the farthest distance R and the nearest distance R detected by the radar designmax＝550km，RminAt 500km, the simulation simulates the PRF constraint of the pulse timing model. As shown in fig. 6.
The left graph of fig. 7 is the relationship between the PRF and the pulse time width, and the right graph of fig. 7 is the relationship between the PRF and the duty ratio, and it can be seen from the graph that as the PRF of the reflected signal increases, the maximum time width of the transmitted signal gradually decreases, and the usable PRF is discontinuous, and the timing constraint cannot be met at some PRFs. Under the condition of meeting the spatial decorrelation, the interval of the sea surface region measured by the two-channel pulse of the differential absorption barometric radar is reasonable, in order to meet the requirement of the signal to noise ratio as much as possible, when the PRF is 428Hz, the pulse width is about 998us, and the duty ratio can also reach the maximum 0.429, as shown in FIG. 8.
As shown in fig. 9 and 10, simulation was performed under the designed system parameter conditions (table 2) using the 31-orbit atmospheric profile data (table 1) of the atmospheric radiation transmission model (Liebe, MPM89) and the CloudSat satellite data ECMWF-aux.p _ R05(2018 month 1), the differential absorption index was calculated, and the root mean square error was calculated with the corresponding sea surface barometric pressure, so as to obtain the simulation result.
Table 2: simulation system parameters
Height of track 500km
Frequency of operation 50～55GHz
Effective antenna aperture 1m*1m
Antenna gain ～50dB
Horizontal resolution 3km(nadir)
Vertical resolution 300m(nadir)
Signal bandwidth B 500k
Transmitting power 100W
Noise figure 6dB
System reference temperature 290K
Simulation results show that there is a very simple near linear relationship between the surface barometric pressure in the strong oxygen absorption band and the differential absorption exponent (logarithm of the radar power ratio) obtained from the radar echo data, where in the frequency pair (52.0GHz and 54.5GHz) of fig. 11, since the snr of the received signal at 52.0GHz is about 35dB and the snr of the received signal at 54.5GHz is about 5dB, the root mean square error of the sea surface barometric pressure estimation is high due to the noise interference. In fig. 12, the signal-to-noise ratio of the received signal at 50.1GHz is about 45dB, the signal-to-noise ratio of the received signal at 53.9GHz is about 15dB, the frequency pair has a higher signal-to-noise ratio, the root mean square error of the sea surface barometric pressure estimation can be within 4hPa, and the method has great potential in practical application, which provides reference and basis for the design and implementation of the surface barometric pressure of the satellite-borne remote sensing detection.
By optimizing the design of the differential absorption air pressure radar system and further researching error analysis and noise influence, the optimal frequency combination is selected to further reduce the root mean square error of sea surface air pressure estimation, so that the sea surface air pressure information with high space-time resolution can be obtained by a remote sensing means, and the method has very important significance for weather prediction, atmospheric dynamics research, especially tropical cyclone structure and dynamic characteristic analysis and the like.
The differential absorption gas pressure radar system design is shown in table 3:
table 3: index of system parameter
Figure BDA0002310988700000111
Figure BDA0002310988700000121
The high signal-to-noise ratio requirements of differential absorption barometric pressure radars make the antenna have a high-gain, low-sidelobe directional diagram, and at the same time, have a flexible scanning form to expand the search range. Antenna gain G and antenna effective area A under general experienceeWavelength λ and efficiency η have the following relationships:
Figure BDA0002310988700000122
it can be seen that the larger the aperture of the antenna, the larger the gain, and at the same time, the higher the operating frequency and the larger the gain, and at the same time, the 3dB beamwidth β of the antenna can be expressed as:
Figure BDA0002310988700000123
when the working frequency of the antenna is higher and the aperture is larger, the beam width of the main lobe of the antenna is narrowed, so that the irradiation area of the differential absorption air pressure radar is reduced, and the echo power is influenced. However, since the contribution of the antenna gain to the echo power is larger than that of the irradiation area, the increase of the effective area of the antenna directly affects the gain of the antenna and further affects the signal-to-noise ratio.
The relationship between the antenna gain and the beam width and the operating frequency obtained by the differential absorption barometric radar simulation is shown in fig. 13. It can be seen that the aperture of the antenna is the main influence factor of the gain, and the larger the aperture of the antenna is, the larger the gain is. But a larger aperture antenna means a higher manufacturing technology and cost. The aperture of the antenna with the diameter of about 1m multiplied by 1m is selected by comprehensive consideration, and whether the antenna with the larger aperture needs to be selected or not is judged through the subsequent calculation of the signal to noise ratio.
The differential absorption air pressure radar adopts a phased array antenna, and a high gain, wide swath and flexible scanning mode are obtained.
The signal-to-noise ratio of a pulse compressed single pulse echo can be expressed as:
Figure BDA0002310988700000124
wherein the echo energy E and the noise power spectral density N0Respectively as follows:
Figure BDA0002310988700000125
N0＝k·Fn·T0
in the formula PtPeak power (W), G antenna gain, λ wavelength (m), δatδelAs the area of illumination (km × km), τpIs the pulse width (ms), σ0Is the backscattering coefficient, R is the distance (m), LTRFor transmit-receive channel loss, LatmK is the Boltzmann constant (W/K/Hz), F is the atmospheric one-way lossnAs noise coefficient, T0Is the reference temperature (K).
The signal-to-noise ratio influencing factor: the scan angle affects the slope/double wall atmospheric loss/illuminated area/backscatter coefficient as shown in fig. 14.
Table 4: frequency-affected double-city atmospheric loss/antenna gain/illuminated area
Figure BDA0002310988700000131
Figure BDA0002310988700000141
And (3) simulation results:
influence of cloud
In order to investigate the influence of cloud on the measurement accuracy of the differential absorption barometric radar, end-to-end simulation was performed using cloud classification data of 2017, 1 month, 1 day to 5 days of 2B-CLDCCLASS _ GRANULE _ P1_ R05 of Cloudsat. The cloud is classified into rolling cloud, high-layer cloud, high-lying cloud), layer cloud, lying cloud, layer cloud, disordered layer cloud and the like according to the information of cloud layer height and the like. Under the condition of clear sky, the water condensate detection information and the rainfall data are utilized to eliminate the condition of rain,
simulation results show that the differential absorption index and sea surface air pressure have an obvious linear relation under the cloud condition, the root mean square error of the sea surface air pressure under different cloud conditions is between 2.67mbar and 4.13mbar, and the root mean square error RMSE represents the error of the surface air pressure and the sea surface air pressure obtained by inverting the differential absorption index. Where lighter areas indicate a greater density of points, which means that the differentially absorbing baroradar can still obtain information on the sea surface barometric pressure in cloudy conditions. The root mean square error in a clear sky is 2.46mbar, which is mainly due to the effects of different background environments, especially temperature. In addition to the effect of temperature, water condensation is also the main factor of influence in the increase of error in cloudy conditions compared to in clear sky conditions.
Influence of temperature and humidity
Although the absorption coefficient of oxygen has a major relationship with radar frequency, temperature also affects the attenuation of electromagnetic waves by oxygen. The relationship between the sea surface air pressure and the differential absorption index under different temperature profiles is shown in the figure, and it can be seen that the temperature has a large influence on the sea surface air pressure inversion accuracy of the differential absorption air pressure radar, and different humidity in clear air has almost no influence on the sea surface air pressure inversion accuracy, so that the influence of the air humidity can be ignored as long as a frequency combination with a proper signal-to-noise ratio is selected. Although the change of the temperature profile can affect the inversion accuracy of the sea surface air pressure, the change of the sea surface temperature is slow in space, and as long as the measured sea surface temperature condition is roughly estimated, the change of the influence is greatly reduced, which can be optimized in the subsequent inversion process.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and are not limited. Although the present invention has been described in detail with reference to the embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (8)

1. A multi-frequency differential absorption radar system for measuring sea surface barometric pressure, the system comprising: the device comprises an antenna, a transmitting-receiving front end, a transmitter, a receiver and a sea surface air pressure calculation module;
the antenna is used for transmitting a dual-frequency radar signal and receiving a radar echo signal;
the receiving and transmitting front end is used for isolating the transmitting channel and the receiving channel and generating a calibration signal of the receiver;
the transmitter is used for generating a dual-frequency linear frequency modulation signal, amplifying the dual-frequency linear frequency modulation signal to a required power level, up-converting a baseband signal to a working frequency, amplifying the power and then sending the amplified baseband signal to an antenna;
the receiver is used for amplifying the radar echo signal, converting the radar echo signal into a baseband through down conversion, and sending the baseband to the sea surface air pressure calculation module;
and the sea surface air pressure calculation module is used for acquiring the power ratio of the radar echo signals of two or more channels in the strong oxygen absorption band from the radar echo signals to realize the inversion of the sea surface air pressure.
2. The multi-frequency differential absorption radar system for measuring sea surface barometric pressure of claim 1, wherein the sea surface barometric pressure calculation module is implemented by:
pressure P of sea surfaceoAs a function of radar power ratio:
Po＝C0(λ1，λ2)+C1(λ1，λ2)loge(Pr(λ1)Pr -1(λ2))
wherein, C0(λ1，λ2) And C1(λ1，λ2) A wavelength correlation coefficient which is a relationship between a radar power ratio and a ground air pressure; can utilize the constant occupation of oxygen in airThe ratio and the proportional relation between the total gas column amount of the uniformly mixed gas and the surface gas pressure are obtained, Pr(λ1) And Pr(λ2) For using a wavelength of λ1And λ2Two radar channels, the radar of the two channels receives power.
3. The multi-frequency differential absorption radar system for measuring sea surface barometric pressure of claim 1, further comprising: and the modular switching power supply adopting an integrated circuit is used for supplying power to the whole radar system.
4. The multi-frequency differential absorption radar system for sea surface barometric pressure measurements of claim 1, wherein the antenna is an active phased array antenna with a 3dB beamwidth gain of no less than 50dB for a 10 degree scan angle.
5. The multi-frequency differential absorption radar system for sea surface barometric pressure measurements of claim 1, wherein the receiver employs a matched filter, signal to noise ratio is no less than 10dB, and noise figure is 6 dB.
6. The multi-frequency differential absorption radar system for measuring sea surface barometric pressure of claim 1, further comprising: the frequency synthesizer is used for generating a required local oscillation frequency for transmitting the double-frequency signal and generating a required local oscillation frequency for the receiver; an oven controlled crystal oscillator with high stability is adopted.
7. A multifrequency differential absorption radar system for measuring sea surface barometric pressure according to claim 1, wherein when the tolerable time deviation Δ τ of the receiving window is 2 μ β, the farthest and closest distances detected by the radar system are respectively: rmax＝550km，Rmin＝500km。
8. The multi-frequency differential absorption radar system for measuring sea surface barometric pressure of claim 1, wherein a frequency range of the radar system is: 50-55 GHz.
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