WO2003044559A1 - Procede pour utiliser un signal radar a onde entretenues interrompues a modulation de frequence (fmicw) afin d'effectuer une detection geophysique a distance - Google Patents

Procede pour utiliser un signal radar a onde entretenues interrompues a modulation de frequence (fmicw) afin d'effectuer une detection geophysique a distance Download PDF

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Publication number
WO2003044559A1
WO2003044559A1 PCT/FI2002/000899 FI0200899W WO03044559A1 WO 2003044559 A1 WO2003044559 A1 WO 2003044559A1 FI 0200899 W FI0200899 W FI 0200899W WO 03044559 A1 WO03044559 A1 WO 03044559A1
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Prior art keywords
signal
frequency
accordance
timeslots
gating
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PCT/FI2002/000899
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English (en)
Inventor
Aki Lilja
Henry Andersson
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Vaisala Oyj
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Priority to AU2002338985A priority Critical patent/AU2002338985A1/en
Publication of WO2003044559A1 publication Critical patent/WO2003044559A1/fr

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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
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/06—Systems determining position data of a target
    • G01S13/08—Systems for measuring distance only
    • G01S13/32—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
    • G01S13/34—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
    • 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
    • G01S13/951—Radar or analogous systems specially adapted for specific applications for meteorological use ground based
    • 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
    • Y02A90/10—Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation

Definitions

  • FMICW Frequency Modulated Interrupted Continuous Wave
  • the invention relates to a method according to the preamble of claim 1 for using a Frequency Modulated Interrupted Continuous Wave (FMICW) radar signal for geophysical remote sensing.
  • FMICW Frequency Modulated Interrupted Continuous Wave
  • This document describes a novel use of a modulation method to enhance the usability of wind profiler radar signal return.
  • the invention is related to a wind profiler radar, but the principle can be used in any remote sensing device measuring volume scattering or volume reflection profiles.
  • the invention is related to systems performing measurements of a geophysical volume, such as atmosphere, ocean, lake, soil, etc.
  • the invention is applicable to weather radar, wind profiler radar, or any other system that detects independent samples from a medium with a short time difference compared to the fading time of the detected phenomenon in the said medium.
  • the invention is particularly suitable for monostatic systems, but is not limited to those.
  • a typical wind profiler radar uses pulsed transmission. It sends a short pulse (of length ⁇ ) to the atmosphere and listens to the return of that pulse. Responses from different distances are received at different times in the receiver. If the scattering density in the atmosphere was a constant, the radar return would exhibit a 1/r 2 dependence on the distance. In practice, the scatterer density decreases slowly as a function of height in the free troposphere. This will cause the scatter to have 1/r" dependence on the distance, where n>2. The fact that return varies strongly as function of distance will call for a large dynamic range of the receiver.
  • the average returned power, as a function of distance from the radar can be written in the following form:
  • P r C * P t * ⁇ (r) / r/ where C incorporates all the constants, including those related to the antenna system, P t is the average transmitted power, ⁇ (r) is the average scattering density as a function of distance and r is the distance from the radar. In a pulsed mode, P t is constant. The formula is applicable in distances that are less than the round-trip time corresponding to the pulse interval:
  • T pu ⁇ se is the time between two successive pulse transmissions.
  • ⁇ (r) is a function that has an unknown form. In free troposphere, up to the heights approaching tropopause, ⁇ (r) is usually a decreasing function of distance.
  • Equation 1 When looking at Equation 1, it is understandable that wind profiler radars, but also weather radars, often detect scatterers up to a certain distance. If the receiver power P r falls below detectable value at a distance r 0 , a large increase in the function ⁇ (r), i.e., an increase that is larger than r. 2 , is needed to make the signal detectable. There are times when this happens, especially in the weather radars (if there is rain beyond r 0 ), but also in the wind profilers when there is layer of increased scatterer density beyond the distance r 0 .
  • a way to mitigate the detection limit r 0 has been to use a coded pulse transmission.
  • the coding used is typically binary phase coding.
  • the coded pulse consists of bauds that are as long as the original pulses in the previous discussion. If there are n bauds in the pulse, and if the pulse repetition period time is kept constant (to keep the same Nyquist range of radial velocities), the average transmitted power P t is increased n-fold.
  • the Equation 1 after updating the average transmitted power.
  • partial pulse decoding is more uncertain than using simple pulses has lead to a practice in which pulse profilers employ two separate modes: a low mode and a high mode.
  • a (simple) pulsed transmission is used in low mode to retrieve profiles up to a height r 0
  • a coded pulse transmission is used to extend the profile beyond r 0 .
  • Another technique used in some radar probing the atmosphere is the frequency- modulated continuous wave technique.
  • the transmitter and receiver are continuously on, and there are separate antennas for transmission and reception.
  • the average transmitted power equals the peak transmitted power, which will lead to better sensitivity of the system.
  • FMCW technique The shortcoming of FMCW technique is the need to isolate the transmitted waveform from the receiver. If the isolation is insufficient, the powerful transmitted signal and the weak scattered signal cannot be separated from each other due to the finite dynamic range of the receiver.
  • FMCW systems have turned out to be too complicated to be applied in commercial weather radars and wind profilers.
  • FMICW Frequency Modulated Interrupted Continuous Wave
  • the invention consists of a method to modulate a transmitted signal, a method to receive the scattered/reflected signal from a geophysical medium being measured and a feedback loop that is used to deduce from the received signal profile a new set of modulating parameters that helps to measure the relevant parts of the geophysical profile better than would be the case without changing the parameters or changing them in a predetermined way.
  • the modulation consists of a frequency sweep and a pulsing system.
  • the frequency deviation of the frequency sweep determines the range resolution of the geophysical profiling system.
  • the continuous frequency sweep is divided into a repeating chain of sub-blocks of transmission and reception.
  • the reception system receives the transmitted signals, scattered/reflected by the geophysical media under measurements.
  • the signal from the antenna is lead to the reception system only during times when the transmission is off. This makes it possible to use a single antenna, even though the duty cycle is high compared to a pulsed or a coded pulse modulation.
  • the goal of the invention is accomplished by changing the gating frequency in order to maximize the average signal power at the desired distance from the antenna.
  • the invention offers significant benefits over conventional techniques.
  • the invention provides an efficient means to measure geophysical backscatter profiles with a system that uses the same antenna aperture for both transmission and reception. This invention does not cause a need for separate transmission and reception apertures. However, the invention is applicable for dual aperture, or multistatic systems, if other considerations make the use of those techniques desirable.
  • the invention substantially decreases the peak power needed for a given continuous profiling range of a geophysical profiling system
  • the method in accordance with the invention makes it possible to generate a modulation that can be parameterized to give desired illumination to the geophysical medium. Further the invention offers a reception system that reconstructs the amount of scatter as a function of distance. The feedback loop of the invention sets the parameters of the modulation to give the optimum performance (optimum as determined by the user of the system).
  • Figure 1 shows a frequency/time graph of the principle of the FMICW-modulation.
  • Figure 2 shows another frequency/time graph of the principle of the FMICW- modulation.
  • Figure 3 shows as a power/altitude graph comparison with prior art and the invention.
  • Figure 4 shows as a power/altitude graph comparison with prior art and the invention.
  • Figure 5 shows as a power/altitude graph comparison with prior art and the invention.
  • Figure 6 is a frequency/time graph of the principle of the invention.
  • Figure 7 is a multiple graph presentation of the signals obtained by the invention.
  • Figure 8 is another multiple graph presentation of the signals obtained by the invention.
  • a solution to the known problems in the present state of the art is to use frequency- modulated interrupted continuous wave transmission (FMICW) with a feedback system that steers the parameters of the modulation.
  • FMICW frequency- modulated interrupted continuous wave transmission
  • a FMICW radar method comprises the following steps.
  • a signal with a frequency changing over a set bandwidth is generated.
  • This signal is gated on and off with a predetermined frequency (fg) thereby forming on-timeslots and off-timeslots.
  • This gated signal is sent to a desired target with help of a transmitter and an antenna.
  • the reflected or back-scattered signal is received during the off-timeslots of the gated signal, and desired properties from the received signal is detected and gathered during the off-timeslots of the gated signal.
  • FMICW transmission will be described here using parameters of a boundary layer wind profiler.
  • the distance resolution of the system be 150 meters, and the interesting range of distances be those above 150 meters, preferably to 3000 meters and above.
  • a linear frequency sweep 5 ms long and 1 MHz in deviation is generated.
  • the frequency sweep is connected to the transmitter, and the transmitter pulse modulates the signal by creating 20 ⁇ s long pulses with 20 ⁇ s long breaks.
  • Figure 1 illustrates this.
  • FIG. 1 illustrated in Figure 1 are the transmitted signal and the returns from 3 heights, 1200, 3000, and 4200 meters.
  • the return from 1200 meters is delayed 8 ⁇ s w.r.t. the transmission, and the return from 3000 meters is delayed 20 ⁇ s w.r.t. the transmission.
  • the return from 4200 meters is delayed 28 ⁇ s.
  • the signal is allowed to go into the receiver only during periods of no transmission.
  • the baseband signal is formed by mixing the received signal with the no-transmission parts of the frequency sweep.
  • the baseband return of the system from the three heights, 1200 meters, 3000 meters, and 4200 meters are obtained. The returns from these heights are illustrated in Figure 2.
  • the frequency sweep described in Figure 1 is continuous in phase throughout the full 5 ms cycle.
  • the fragments of 1.6 kHz (corresponding to 1200 meters), 4.0 kHz (corresponding to 3000 meters) and 5.6 kHz (corresponding to 4200 meters) signals shown in Figure 2 will also exhibit continuous phase throughout the frequency sweep, given the fact that the 5 ms sweep is much smaller in time than the decorrelation time of the scatterers in the volume studied.
  • Typical decorrelation times for the Bragg scatterers in clear air are 0.1 - 1.0 seconds.
  • each of the fragments in each of the frequencies in Figure 2 creates a sinusoidal signature that is broken in time.
  • the intermittent 1.6 kHz signal will have time to generate 8 cycles of intermittently broken sinusoid during the 5 ms sweep, the 4 kHz signal 20 cycles and the 5.6 kHz signal 28 cycles. These sinusoids get summed in the receiver, producing a voltage that contains various intermittent frequency components relating to various heights.
  • a time series of 125 points (if one sample is taken during one RX interval) is generated.
  • the various frequencies in this time series are separated by means of a Fourier transform.
  • the and 28 th sample in this Fourier transform correspond to the heights 1200, 3000 and 4200 meters.
  • the receiver is, as is preferably the case, a receiver capable of separating the in-phase and quadrature components in the base-band, the corresponding Fourier transform will be complex and the frequencies extracted in the given example are negative.
  • Equation 1 transforms into:
  • P t is 50 % of the peak power of the transmitter, and the transmit duty cycle of the system is considerably higher than for the pulsed or coded- pulse transmission systems.
  • the received power is proportional to 1/r, not 1/r 2 .
  • the power that is transmitted by the FMICW system is better utilized for the detection of the higher distances than with a pulsed system. This will increase the detection limit r 0 of the FMICW system. Above r opt the received power is:
  • a wind profiler with a power amplifier capable of producing 1000 Watts of output peak power Let us consider 3 types of wind profilers: one using uncoded pulses (typical values of 1 ⁇ s pulse with 40 ⁇ s pulse period), one using coded pulses (typical values of 6-baud pulses, 1 ⁇ s baud length and 40 ⁇ s pulse period) and a FMICW profiler targeting initially to detect signal up to 3 km.
  • a parameter that describes the useful power that the wind profiler is able to send as a function of height. This parameter will describe the ability of the system to detect atmospheric phenomena as a function of distance. The actual atmospheric scattering density and the factors related to antenna are removed from the parameter. The power amplifier and modulation characteristics of the system are kept in the parameter.
  • the parameter can be written Pt/r 2 for a pulsed system and as P t /(r*r op t) for the FMICW system. The proper P t , incorporating the duty cycle, must be used for each system.
  • line 1 corresponds to a pulsed wind profiler
  • line 2 to a coded-pulse wind profiler
  • line 3 to the FMICW wind profiler.
  • Line 2 starts from 900 meters, reflecting the fact that the transmitter power can be fully utilized, due to pulse decoding, starting from that height.
  • the value plotted in Figure 3 as a function of height is the average power within resolution cell divided by the distance squared. The value is normalized in such a manner that it equals the average power at 1 km height.
  • the scattering density ⁇ (r) has a constant value in the lowest kilometers of the atmosphere and that the constant value is such that 100 W of average power is needed at 1 km height to detect signal. It can be seen from Figure 3 that in such circumstances the pulsed WP, coded- pulse WP and FMICW WP detect the atmospheric signal up to height 500, 1200 and 1650 meters, respectively. Because the user was interested in the wind profile above 150 meters and preferably until 3000 meters, he set the system to detect with maximum average power at 3 km. In this case, the limited scatterer density prevents detection up to that height.
  • the system according to the present invention sets new pulsing parameters based on the detected profile. The system now sets to transmit 11 ⁇ s pulses with 11 ⁇ s breaks in between to maximize the illuminating power of the radar at the height where the signal disappears. As a result, the average illuminating power changes to correspond the situation in Figure 4.
  • the first illuminating power setting is shown with line 3 and the new setting with line 4.
  • the FMICW system is now capable detecting the atmospheric signal up to the height of 2000 meters.
  • the usefulness of the wind profiler signal was thus enhanced.
  • the enhancement is produced at the expense of detection power between 2.1 and 4.2 kilometers, but because the signal was undetectable at these heights with the first setting, there is no loss in usefulness of the signal compared to the first setting.
  • the system detects the signal up to a height of 2000 meters. This corresponds to a delay of 13.3 ⁇ s.
  • the system may now set its timing parameters to 14 ⁇ s to produce an illumination profile that better matches the height range where the signal is detectable.
  • Figure 5 illustrates the new modulation setting with line 5.
  • the FMICW profiler is now able to detect the atmospheric signal up to a height of 2.200 m, which is clearly higher than the original performance of up to 1650 meters.
  • the example described above illustrates a rough example of how the feedback to the FMICW pulsing parameters may be implemented.
  • Other methods such as computing the highest obtainable height from the first profile, based on a model of ⁇ (r), is part of the current invention.
  • the pulsing parameters may not be settable in a continuous fashion, but there may be a set of modes from which the system is able to select at the prevailing conditions. This is also part of the present invention.
  • One important application of the invention is the fact that the feedback is continuously on. In such case the system continuously updates the parameters of the FMICW modulation to achieve the maximum information from the currently prevailing geophysical condition.
  • the illumination curves consist of three segments, one with 1/r dependence, another with a 1/r 2 dependence and a third with a steep dependence (roughly 1/r 3 ). Referring to figure 6 in the following is described one advantageous embodiment of the invention.
  • each frequency component in the signal after integration will contain a signal contribution and a noise contribution.
  • the signal contribution results from the integrated voltage over the duration of each line shown in Figure 6.
  • the noise content in that frequency bin will correspond to the integration over the whole 20 ⁇ s period. While the signal contribution to the signal will remain coherent over several pulses, the noise contribution will not, and the noise will thus have an adverse effect to the signal via disturbing the phase and the voltage of the actual, wanted signal.
  • Figure 7 upmost row of plots, shows the time domain signal of a (simulated) target at
  • the right-hand plot shows the result of using a 5000-point fast
  • the signal-to-noise ratio for this target and method is 15.9 dB, measured over the resolved bandwidth of 200 Hz. It occurs at the 1.6 kHz baseband frequency, at the 8 th frequency bin, which corresponds to the 8 th height bin, 1200 meters.
  • the signal consists of 20 zeroes, corresponding to the transmission time during which the receiver is blanked, followed by 20 samples of signal and noise, followed by 20 zeroes, etc (see also Figure 8).
  • the time unit in left column of plots in Figure 4 is 1 ⁇ s
  • the frequency unit in the right column of plots is 1 MHz.
  • the blanking is achieved in the digital domain, making complete nulling of the transmission-time signal possible.
  • the 1-MHz bandwidth is larger than the pulse repetition frequency, 25 kHz, resulting in replicas of the signal ⁇ 25 kHz from the centermost frequencies.
  • FIG. 7 middle row of plots shows the effect of integrating over the period of 20 ⁇ s, the full reception time, and using a 125-point fast Fourier transform to extract the different range components from the signal.
  • the length of the Fourier transform was decreased without loss in signal-to-noise ratio, which is now 15.7 dB.
  • the pulsing effect is not anymore shown due to decreased Nyquist frequency in the baseband.
  • the integration has been realized as a sum of values over each of the reception periods, resulting in increased voltage amplitudes. It is noteworthy that the signal studied here, 1200 meters, corresponds to a rotating phasor that turns less than 5 degrees during one reception period. It is thus possible to sum all the samples from the reception period without losing significantly signal voltage due to summing rotating phasors.
  • Figure 7 and 8 can be regarded as belonging to the prior art.
  • Figure 7, last row shows the effect of integrating according to this invention.
  • a discrete Fourier transform which can also be implemented as using only one of the output components of the fast Fourier transform, is used to extract the frequency component corresponding to 1200-meter range.
  • an integrated signal in which the integration time has been matched with the two-way delay between the transmission and the said target's echo reception is used.
  • only the 8 first samples are integrated during each reception period to produce the time series shown in the left plot in the last row.
  • Eight-sample integration was chosen for this DFT, because it matches the amount of time at the beginning of the reception period with target-scattered signal. This can be seen in Figure 6, only the 8 first microseconds in the reception period contain scattered signal from the target at 1200 meters. The amount of noise in this time series is less than the noise in the previous time series, because the last 12 samples has been omitted in the integration, and the noise in those samples does not contribute to the output of the 8- point integrator. The signal itself remains intact, because all the information from that range has been used, as shown in Figure 6.
  • the signal-to-noise ratio resulting from partial reception time integration for this exemplary case is 19.5 dB, 3.8 dB higher than the SNR for the fully integrated reception period.
  • the essence of the invention described herein is to construct several digital integrators, preferably by accumulating a sum continuously, and use a variable integration time for each range.
  • 150-meter range would be detected by using 1 -point integration, i.e., the first sample, of each reception period with a DFT that resolves the first non-zero frequency from the 125-point time series consisting of said first samples.
  • 300-meter range would be detected by using 2-point integration, i.e., the first two samples summed, of each reception period, and subjecting the resulting 125-point time series to a DFT that resolves the second non-zero frequency component from the 125- point time series.
  • This scheme is continued up to the pulse matching height, 3000 meters in this case, for which the full 20-point integration result is used with a DFT resolving the 20 th non-zero frequency component.
  • the end part of the reception period is used in the integration, to include all the signal information and to minimize the amount of noise in the corresponding frequency component.
  • the 12 last samples during each of the 1-MHz sampled 20 ⁇ s reception period are used in conjunction with a DFT extracting the 28 th non-zero frequency component from the time series.
  • One preferred embodiment of the present invention would be to use a DSP or FPGA system to compute the range-resolved signals, using cumulative sum accumulator at a high-enough frequency to resolve the transmitted waveform and multiplying the contents of said accumulator with complex weights that correspond to the DFT coefficient for that pulse and that range.
  • the output of the multiplication would be input into another complex sum accumulator for each range, and the contents of those accumulators would be read at the end of each sweep. These values are the complex voltages corresponding to each height for that sweep.
  • the DFT accumulators are read and nulled at the end of each sweep, and the integrator accumulators are nulled at the beginning of each reception period.
  • a DFT to the range-matched subset of the original 1-MHz samples. This is useful in cases where targets at ranges commensurate with the sweep repetition period are detected. For example, in the exemplary case shown in this document, targets at several hundred kilometers would get damped if the summing integrator was used. This is due to summing complex voltages with a rotating phasor over a time in which the phasor rotates significantly. Thus, in the case of targets far away from the radar, the preferred method is to use a matched subset of the original samples as an input to a long DFT. In these cases summing the digital samples would result in loss of some of the signal amplitude due to summing a significantly rotating phasor.
  • the amount of detection improvement presented in the example corresponding to Figure 7, 3.8 dB, is due to the fact that the target being detected remains essentially coherent during the 5 ms sweep period, whereas the noise is fully incoherent, having no phase correlation between the samples.
  • noise power is additive during the reception period, and in the preferred integration method 8/20 of the noise power is received, instead of the 20/20 in the full integration.
  • the 8/20 factor corresponds to 4.0 dB, which essentially equivalent with the simulated result.
  • Figure 8 shows zoom-ins of the plots in Figure 7.

Abstract

La présente invention concerne un procédé pour utiliser un signal radar à ondes entretenues interrompues à modulation de fréquence (FMICW). Ce procédé consiste à produire un signal avec une fréquence qui varie sur une largeur de bande définie (B), à commander le déclenchement et le non déclenchement du signal avec une fréquence prédéfinie (fg), afin de former des intervalles de temps de déclenchement et des intervalles de temps de non déclenchement, à envoyer le signal à déclenchement commandé à une cible souhaitée, à l'aide d'un émetteur et d'une antenne, à recevoir un signal réfléchi lors des intervalles de temps de non déclenchement du signal à déclenchement commandé, puis à détecter et à collecter des caractéristiques souhaitées à partir du signal reçu lors des intervalles de temps de déclenchement du signal à déclenchement commandé. Selon cette invention, on fait varier la fréquence de commande de déclenchement (fg) afin de maximiser la puissance moyenne des signaux à la distance souhaitée de l'antenne.
PCT/FI2002/000899 2001-11-21 2002-11-13 Procede pour utiliser un signal radar a onde entretenues interrompues a modulation de frequence (fmicw) afin d'effectuer une detection geophysique a distance WO2003044559A1 (fr)

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AU2002338985A AU2002338985A1 (en) 2001-11-21 2002-11-13 A method for using a frequency modulated interrupted continuous wave (fmicw) radar signal for geophysical remote sensing

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FI20012273 2001-11-21
FI20012273A FI110965B (fi) 2001-11-21 2001-11-21 Menetelmä taajuusmoduloidun, katkotun jatkuva-aaltotutkasignaalin (FMICW) käyttämiseksi geofysikaalisessa kaukotunnistuksessa

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