JP2006226711A - Radar - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-altitude high-resolution rainfall radar having a distance resolution of several meters for highly-accurate rainfall observation. <P>SOLUTION: This radar, which is a wide-band bistatic small-sized low-output rainfall pulse radar using a full-digital pulse compression technology, is constituted of an optional signal generator (AWG), a transmitter/receiver, an A/D conversion board, a GPS, a transmitting/receiving antenna and a plurality of amplifiers for amplifying signals. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a radar apparatus, and more particularly to a broadband bistatic rain radar using a pulse compression technique.

  The observation of precipitation intensity by radar has the advantage of being able to observe a three-dimensional space over a wide range, but when the rain gauge installed on the ground is assumed to be a true value, there are various errors between the radar rainfall and the true value. There are factors. For example, advection of raindrops by horizontal wind, evaporation of raindrops during fall, insufficient beam filling, etc. will affect as errors. Among them, one of the important error factors is that a low altitude and high resolution profile cannot be obtained. The conventional radar in Japan, especially in mountainous areas, can obtain a profile only several hundred meters or several kilometers from the ground due to the effects of clutter and monostatic antennas. It was difficult to accurately estimate the amount of rainfall that falls on the surface of the earth without evaluating the advection and evaporation.

  Further, in a radar having a distance resolution of about a few hundred tens of meters, there is a possibility that the radar equation assuming the uniformity of rainfall within the pulse volume may not be established. In order to suppress such beam filling errors and perform highly accurate rainfall observation, a radar having a high range resolution is required, and it is desired that the radar output be practically low.

In the pulse radar, in order to improve the maximum detection distance and the distance resolution, which are the basic performance, the peak transmission output has been increased and the pulse width has been narrowed. However, there is a limit to the peak transmission output, and narrowing the pulse width leads to a decrease in the S / N ratio because the transmission signal has a wide band, resulting in a short detection distance. In order to solve such a trade-off, a method of increasing the transmission pulse width and modulating the bandwidth in a wide range within the transmission pulse has been devised as a pulse compression method (for example, Non-Patent Document 1). Pulse compression technology has been used mainly for military radars for a long time, but there are few applications for weather radars. The reason is the problem of the range side lobe. The range side lobe is a false echo that exists before and after the time axis of the compression pulse, and has the problem of affecting the grasp of the rainfall distribution and the estimation of precipitation.
Yoji Furudate, Kenichi Okamoto, Harunobu Masuko “Microwave Remote Sensing Using Artificial Satellites” IEICE, Tokyo 1997

  Recent advances in digital technology have enabled high-speed digital sampling and pulse compression on software using an arbitrary signal generator, and range sidelobe is a modulation method used when creating a transmission waveform with an arbitrary signal generator. The possibility that it can be reduced by devising this has become apparent. In addition, it has become possible to realize an inexpensive system configuration.

  The present invention has been made in view of such circumstances, and has a high distance resolution in a low altitude range of about 0 to 1200 m as compared with a conventional rain radar by adopting a full digital pulse compression system. A bistatic small low-power radar device is provided.

The present invention relates to an arbitrary signal generator that generates an original signal for observation, a transmission unit that outputs an RF wave by up-converting and amplifying the original signal by pulse compression, and an RF wave from the transmission unit in the atmosphere An antenna that receives the RF wave emitted from and reflected by the object, a receiving unit that amplifies the received RF wave and down-converts using the phase-synchronized oscillator, and an IF signal There is provided a radar apparatus including an A / D converter that performs A / D conversion, and a reference clock providing unit that provides a reference clock that synchronizes the signal generator and the A / D converter.
The arbitrary signal generator may be configured to generate a modulated pseudo noise signal and a chirp signal.
The transmission unit may further include a phase-locked oscillator, and may be configured such that the original signal is up-converted using a signal from the phase-locked oscillator, and the reception unit is down-converted using the signal from the phase-locked oscillator.
Furthermore, the transmission unit may be configured to output an RF wave having a bandwidth in the range of 20 MHz to 100 MHz and a center frequency in the range of 1 GHz to 20 GHz.
The antenna may be composed of a transmission antenna for transmission and a reception antenna for reception.
Furthermore, the antenna may receive RF waves reflected by raindrops or ice in the clouds and water.
The receiving unit may be configured to down-convert the received RF wave into an IF wave having a center frequency of 10 MHz to 100 MHz.

  Since the center frequency of the pulse radar according to the present invention is high, it is possible to obtain a vertical profile having a high distance resolution in a low altitude region as compared with a conventional rainfall radar. That is, it is possible to obtain detailed behavior of the rain by observing the falling of the rain with higher resolution. Therefore, it is possible to accurately estimate the amount of rain falling on the ground surface.

Hereinafter, the present invention will be described in more detail based on embodiments shown in the drawings.
<Configuration of radar device>
FIG. 1 is a block diagram showing an embodiment of a radar apparatus (hereinafter referred to as broadband radar) according to the present invention. This radar mainly consists of an arbitrary signal generator (AWG), a transceiver, an A / D conversion board, GPS, a transmission Cassegrain antenna, and a receiving pyramid horn antenna.
The arbitrary signal generator can generate an arbitrary transmission signal. In this rainfall observation, a chirp signal with a pulse width of 128 μs, a center frequency of 60 MHz, and a band of 80 MHz is generated and output. In addition, the trigger signal output in synchronization with the rising edge of the transmission pulse from the arbitrary signal generator is used as a trigger to start data acquisition on the A / D conversion board.

  In the transmitter of the transceiver, the signal from the arbitrary signal generator is up-converted to a center frequency of 15.75 GHz by two PLOs (Phase Locked Oscillators) and further amplified to about 110 mW by an amplifier.

  The transmitting and receiving antennas are both installed vertically, and the transmitting Cassegrain antenna has a diameter of 45 cm, a beam width of 2.5 °, and a gain of 33 dB. The receiving pyramid horn antenna has a beam width of 15 ° and a gain of 20 dB.

  Scattered signals from rainfall received by the horn antenna are input to the receiver of the transceiver and down-converted to a center frequency of 60 MHz. The IF signal is digitally sampled directly by the A / D conversion board. The A / D conversion board has a vertical resolution of 12 bits, a sampling frequency of 400 MS / s, and a memory of 4 Mpoints. The sampled data is stored in the connected PC and subjected to pulse compression processing by software.

  In this radar system, the A / D converter board and the internal clock of the arbitrary signal generator are synchronized using a high-precision 10MHz reference signal output from GPS. It also synchronizes the time between three PCs, a PC for data acquisition using GPS, a Disdrometer, and a MicroRainRadar used for comparison.

<Pulse compression method>
There are mainly frequency modulation method and code modulation method for pulse compression, and frequency modulation method is linear or non-linear modulation method, and code modulation method is Barker code, complementary code, M sequence, etc. There are various methods. Broadband radar uses a linear frequency modulation method that transmits Chirp waveforms.

  In addition, there are analog compression methods using SAW filters and software methods for pulse compression processing, but broadband radar performs digital sampling of IF signals directly with an A / D conversion board, and software Pulse compression was performed above.

  A flowchart of the pulse compression process is shown in FIG. Applying pulse compression digitally means cross-correlation between the received signal and the reference signal, and this radar uses fast Fourier transform (FFT) for correlation processing. The conjugate complex number of the transmission signal was used as a matched filter. In addition, real data sampled by the A / D conversion board uses the Hilbert transform to create a complex analysis signal consisting of I / Q signals and compresses it.

The envelope f ^(t) of the waveform after pulse compression can be expressed by the following equation where T: pulse width and B: bandwidth.
This waveform includes a sinc function, and side lobes generated at both ends of the main lobe of the sinc function appear in the rain observation as a false echo, or mask weak scattering near strong scattering. In order to reduce this range side lobe, the Blackman Harris window function was used as amplitude modulation in the pulse of the transmission signal. Also, the amplitude is
The distance resolution Δt obtained by applying pulse compression is
It becomes.

<Distance resolution and operation specifications>
If the theoretical distance resolution is calculated from the equation (2) using the specifications of this radar, it is 1.9m. However, in this radar, in order to reduce the range side lobe generated by the pulse compression, a waveform obtained by multiplying the chirp pulse by the Blackman Harris window function in the arbitrary signal generator is used. As a result, the width of the main lobe was widened and the distance resolution was deteriorated, but the distance resolution obtained from the main lobe measured in the experiment was about 4 m. This is a sufficiently high range resolution compared to the weather radar currently in operation.

  The arbitrary signal generator used in this radar can generate a chirp signal with an almost arbitrary pulse width. In determining the pulse width, considering that the raindrops to be observed have speed, the pulse width is set such that the phase change within the pulse can be sufficiently ignored. For example, if the drop speed of raindrops is 5 m / s and the pulse width is 1 ms, the wavelength is about 2 cm, whereas the raindrops move 5 mm (about 1/4 of the wavelength) within the pulse. It will be. If the pulse width is 128 μs, the movement of the raindrop is 0.64 mm (about 1 / 31.11 deg of the wavelength), and the phase change is considered to be negligible. The arbitrary signal generator can generate signals with shorter pulse widths, but in order to obtain a sufficient pulse compression ratio, it was decided to be 128 μs.

As shown in FIG. 3, the pulse repetition time is 64 pulses transmitted in one sequence, and the interval between each sequence is 2 s. This was determined by the performance of the A / D conversion board. In this radar, each pulse is subjected to pulse compression processing on the PC, converted into dB, and then averaged 64 times as a 2-second value and 1920 times as a minute value.

<Broadband radar calibration>
Next, absolute calibration of broadband radar is described.
As an absolute calibration procedure of the radar, first, a radar equation using a radar constant C ₀ obtained from a system design value is derived. Then, using the radar equation, the equivalent radar reflection factor Zm including the effect of rain attenuation at a distance of 40 m is calculated from the received power, and the Ze observed by the disdrometer installed close to the broadband radar is calculated. The F value is calculated as a disdrometer, because it is a measuring instrument installed on the ground, there is a spatial and temporal inconsistency with broadband radar, but the altitude difference is only 40 m, and raindrop evaporation and Errors such as rainfall attenuation are considered negligible. Also, although there is a time lag of several seconds, it can be said that this error can be sufficiently ignored when considering comparisons with 1-minute values. Furthermore, after pulse compression, a sharp peak due to leakage between the transmitting and receiving antennas appears at an altitude of 0 m. A comparison was made at 40m as the minimum altitude that is not affected.

  In this radar, antennas with different gain and beam width are used for transmitting and receiving antennas. For this reason, the radar equation applicable to the bistatic radar shown below is derived and used.

First, the radar equation for a single scatterer is expressed as follows:
Here, P _r and P _t are reception power and transmission power, λ is the wavelength of the transmission wave, G _t and G _r are antenna gains on the transmission side and reception side, R _t and R _r are distances to the scatterers, and σ Is the radar cross section of the scatterer. The following assumption formula regarding the antenna gain is applied to this expression.
Here, G ₀ is the maximum gain in the beam center direction (θ = 0), and 2θ _h is the beam width. Then, the received power is integrated with respect to the scattering volume V from the equations (3) and (4).
In Expression (6), G _t0 and G _r0 are transmission and reception antenna gains, h is a pulse space length, and θ _t0 and θ _r0 are transmission and reception antenna beam widths.

In addition, this radar calibration incorporates the effects of pulse compression processing into the radar equation. The output after pulse compression is applied to the received echo can be regarded as an echo when a pulse-compressed transmission signal is transmitted. In other words, the transmission power and the pulse space length in Expression (6) are not the values that are actually transmitted, but the signal power and the pulse space length that have been subjected to pulse compression in advance on the software. The relational expressions of Zm, R, and Pt obtained in this way are expressed in dB notation as follows:
Next, FIG. 4 shows a scatter diagram of Ze observed with a disdrometer at the same time as the observed value Zm of broadband radar at R = 40 m obtained using Equation (7). The broken line in the figure represents the function of, and the solid line represents a linear equation that approximates the least squares with the slope fixed to 1 with respect to the scatter diagram. At this time, the rainfall observed with the disdrometer is 20 dBZ or less, and the broadband radar assumes that it cannot be detected due to insufficient transmission power, and approximates it linearly. The linear equation is shown below.
The intercept 0.72 dB in the equation (8) is the F value in dB.

<Measurement example>
Next, a description will be given of the results of rainfall observation performed with the above-mentioned operation specifications. Furthermore, the results of comparing and verifying the observation results (Zm) of the broadband radar with the values of the disdrometer (Ze) and the values of the microrain radar (Zm) installed in the vicinity will be described. The micro rain radar is a K-band FM-CW radar with a distance resolution of 40 m and a time resolution of 1 minute.

  First, we compared Zm of broadband radar, Ze observed by disdrometer, and Zm of microrain radar in time series. Figures 5 and 6 show a comparison between the broadband radar and the microrain radar at 40m altitude, and the disdrometer at the ground level.

  From the figure, it can be seen that there is a very good agreement, and further, when the correlation coefficient with broadband radar was obtained for both the disdrometer and the microrain radar, the correlation coefficient was 0.98 for both. A good correlation was obtained.

  Next, Fig. 7 shows the vertical profile of Zm (2-second value) observed by broadband radar. FIG. 8 shows a vertical profile of Zm (1 minute value) observed by the microrain radar. In both figures, the horizontal axis is time and the vertical axis is altitude, and the observation time is from 21:00 to 22:00. From this figure, it can be seen that the falling of rain can be observed with higher resolution by broadband radar than MRR.

  As described above, the radar device was designed and created based on the present invention, and the vertical profile of rainfall was observed using the created radar device. Then, the radar was calibrated and compared with each other using a disdrometer and a micro rain radar installed in close proximity. As a result, the observation results showed a very good agreement with both, indicating the effectiveness of rainfall observation. In addition, from the observation results of the vertical profile, it was possible to obtain detailed behavior of rainfall that could not be observed with the resolution of the microrain radar.

The radar device of the present invention is useful as a radar for various applications including a rain radar. The following are examples of particularly useful applications.
First, in order to monitor the cumulonimbus cloud around the airport and take measures against downburst, if 3 to 4 radars according to the present invention are arranged on each runway, it is possible to accurately warn the landing airplane. become. This is because the current airport is equipped with a topler radar, but it cannot perform local observations in the low altitude region like the radar of the individual invention.

  Furthermore, in order to accurately predict the rainfall in a limited area, the observation data by the radar of the present invention is added to the current algorithm that predicts the precipitation based on the cloud observation data of the satellite. , Enabling more accurate predictions. Since the current radar observes rain over 1-2 km, it takes 20-40 minutes for the observed rain to reach the ground, and there is a great discrepancy from the situation on the ground. Since it is possible to observe lower altitude regions, for example, it is possible to realize a more accurate prediction by observing the situation several minutes before the rain reaches the ground. Therefore, a method such as using it for basic data of flood warning is conceivable.

  It can also be used to collect data that could not be observed as observation equipment for meteorological research. For example, observation of a bright band, that is, an ice melting layer can be obtained by observing a difference in dielectric constant between ice and water in a cloud. This makes it possible to conduct research from a new point of view on how rain falls, for example, changes in rainfall over time.

It is a block diagram which shows the one aspect | mode of the implementation of the radar apparatus by this invention. It is a flowchart which shows the pulse compression process in the radar apparatus of FIG. It is explanatory drawing which shows the repetitive pulse waveform and its time in the radar apparatus of FIG. It is a graph which shows the correspondence of the observation value Zm of the radar apparatus of this invention, and the observation value Ze of the disdrometer which is a prior art apparatus.

The observation value Zm (solid line) of the radar device of the present invention, the observation value Ze (dashed line) of the disdrometer which is a conventional device, and the observation value Zm (one-dot chain line) of a microrain radar which is a different device of the prior art It is a graph which shows each time-sequential change in a time slot | zone. Diagram of the observed value Zm (solid line) of the radar device of the present invention, the observed value Ze (dashed line) of the disdrometer which is a conventional device, and the observed value Zm (one-dot chain line) of a microrain radar which is a device different from the prior art 5 is a graph showing each time-series change in a time zone different from FIG. It is a graph which shows the vertical profile of Zm observed with the radar apparatus of this invention. A vertical profile of Zm (1 minute value) observed by a microrain radar which is a device of the prior art is shown.

Explanation of symbols

1 Arbitrary signal generator, AWG
2 Transceiver 3 A / D converter, A / D conversion board 4 GPS
5 Transmitting antenna, Cassegrain antenna 6 Receiving antenna, pyramid horn antenna 7 PC, personal computer 11, 12 Phase synchronized oscillator, PLO, Phase Locked Oscillator
13, 14, 15, 16, 17, 18 AMP, amplifier

Claims (7)

An arbitrary signal generator for generating an original signal for observation;
A transmitter that outputs an RF wave by up-converting and amplifying the original signal by pulse compression;
An antenna for emitting the RF wave from the transmission unit to the atmosphere and receiving the RF wave reflected by the object;
A receiver that amplifies the received RF wave and obtains an IF signal by down-converting using the phase-synchronized oscillator;
An A / D converter for A / D converting the IF signal;
A radar apparatus comprising: a reference clock providing unit that provides a reference clock for synchronizing a signal generator and an A / D converter.   The radar apparatus according to claim 1, wherein the arbitrary signal generator generates a modulated pseudo noise signal and a chirp signal. The transmission unit further includes a phase-locked oscillator, up-converts the original signal using a signal from the phase-locked oscillator,
The radar apparatus according to claim 1, wherein the receiving unit performs down-conversion using a signal from the phase-locked oscillator.   The radar device according to claim 1, wherein the transmission unit outputs an RF wave having a bandwidth of 20 MHz to 100 MHz and a center frequency of 1 GHz to 20 GHz.   The radar apparatus according to claim 1, wherein the antenna includes a transmission antenna for transmission and a reception antenna for reception.   The radar apparatus according to claim 1, wherein the antenna receives an RF wave reflected by raindrops or ice in a cloud and water.   The radar apparatus according to claim 1, wherein the receiving unit down-converts the received RF wave into an IF wave having a center frequency of 10 MHz to 100 MHz.