Communications: directive radio wave systems and devices (e.g. – Radar for meteorological use
Reexamination Certificate
2002-06-14
2003-11-11
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar for meteorological use
C342S137000
Reexamination Certificate
active
06646587
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a Doppler radar apparatus for observing the Doppler velocity and intensity of meteorological echoes by use of an FMICW modulated signal with a high pulse repetition frequency.
FIG. 13
is a diagram showing a system of related-art Doppler radar apparatus, for example, disclosed in the Unexamined Japanese Patent Application Publication No. 2000-275329. In
FIG. 13
, the system includes: a first highly stable local oscillator
1
for generating a signal at a frequency f
1
-fif; a second highly stable local oscillator
2
for generating a signal at a frequency f
2
-fif; a switching circuit
3
for switching the signals from the first and second highly stable local oscillators
1
and
2
alternately every pulse; a mixer
4
for mixing an output signal from the switching circuit with a signal at a frequency fif; an IF local oscillator
5
for generating a signal at the frequency fif; a high pass filter
6
for passing only signals at frequencies f
1
and f
2
; a pin modulator
7
for pulse-modulating a signal; a transmitting tube
8
; and a polarized-wave switching circuit
9
for switching a transmission path in accordance with a polarized wave.
The system further includes circulators
10
and
11
; a mixer
12
for mixing the signals from the local oscillators
1
and
2
; a low pass filter
13
for extracting a signal at a frequency fclk; a control circuit
14
; TR limiters
15
and
27
for protecting a reception circuit from transmitted waves leaking into the reception circuit; high-frequency amplifiers
16
and
28
; mixers
17
and
29
; polarized component extracting filters
18
and
30
each for extracting a reception signal at the frequency fif; 90-degree shifters
19
and
31
each for providing a phase difference of 90 degrees; mixers
20
,
21
,
32
and
33
; filters
23
,
24
,
34
and
35
each for extracting a Doppler signal; and A/D converters
25
,
26
,
36
and
37
each for converting an analog signal into a digital signal.
In meteorological radars for observing rainfall or snowfall, the velocity of the wind of an echo (raindrop or snowflake) can be gauged by use of the Doppler effect. When the wind is measured with a Doppler radar, however, the number of pulses shot per second (Pulse Repetition Frequency (PRF)) cannot be increased sufficiently in relation between the maximum observable range Rmax and the maximum observable velocity Vmax so that the measurement of the Doppler velocity is limited by phenomena as follows. One is a problem that folding of the Doppler velocity is generated, and the other is a limit in the observable range for suppressing the generation of secondary echoes.
C-band meteorological radars usually observe intensity in an observable range up to about 250 km. The PRF is limited by this maximum range Rmax of 250 km in the following relation.
PRF≦C
/(2
×R
max) (
C
represents light velocity) (1)
Therefore, the PRF cannot be made higher than 3×10
8
m/s/(2×250 km)=600 Hz. On the other hand, in Doppler observation, the maximum velocity Vmax is limited in the following relational expression.
(&lgr; designates wavelength, which is about 5.6 cm when the frequency is in the C-band ranging from 5,250 MHz to 5,350 MHz)
V
max≦|&lgr;/2
×PRF/
2| (2)
Normal meteorological radars need to be not lower than 40 m/s as their observable range of Doppler velocity. When the PRF is about 600 Hz, the observable range is, however, limited to 8.4 m/s in accordance with the expression (2). Accordingly, when Doppler observation is carried out, the PRF is increased to about 1,000 Hz to obtain the maximum velocity Vmax of 14 m/s, while observation is carried out a plurality of times with different PRFs. Thus, “processing of folding of Doppler velocity” is carried out to secure ±40 m/s or higher while the observable range is set to about 150 km.
(Detailed Description of Folding of Doppler Velocity)
In radars, a reception signal is obtained in every period corresponding to the PRF. The phase shift between the discrete signals received thus is used for the work of estimating an original continuous wave after measuring some points of the continuous wave. Therefore, as shown by a sampling theorem, the measuring limit of Doppler velocity f
d
is expressed by PRF/2 so that folding of Doppler velocity occurs in a frequency higher than PRF/2. Doppler velocity shown in the following expression (3) is called “folded velocity (Nyquist velocity) V
nyq
. Actually, the wind velocity higher than V
nyq
really exists as described above. Thus, outputted (folded) Doppler velocity V
r
in the case of velocity V
0
higher than V
nyq
is expressed by:
V
r
=V
0
±n×V
nyq
(
n=
2, 4, 6 . . . ) (3)
FIG. 14
shows a phenomenon of velocity folding. Since normal meteorological radars use approximately &lgr;=5.6 cm, PRF=896 Hz, and V
nyq
=12.5 m/s, for example, actual Doppler velocity obtained when the wind velocity is 20 m/s is 20−(2×12.5)=−5.0 (m/s). Accordingly, in actual Doppler radars, correction processing for canceling folding by use of two kinds of frequencies as PRF is carried out to expand the wind space measurable range into about three times as wide as V
nyq
.
(Detailed Description of Secondary Echo)
For such a reason, in Doppler radars, the PRF is set to be relatively high to obtain V
nyq
as high as possible. As a result, the Doppler radars cannot help making the Doppler observable range R
max
narrower than the intensity observable range in accordance with the expression (1). The Doppler observable range R
max
becomes 167 km at the PRF of 896 Hz. In this case, there appears a false target (called a secondary echo) if an intensive echo exists in a range exceeding the radius of 167 km which is the observable range. It has been therefore necessary to take separate measures to remove the secondary echo.
Next, description will be made on the operation of the related-art apparatus. In the related-art apparatus, two kinds of frequencies are used to double the observable range of Doppler velocity. The operation at that time will be described below.
Signals at frequencies f
1
-fif and f
2
-fif are outputted from the first and second local oscillators
1
and
2
respectively. These signals are switched alternately in a transmission repetition period (the reciprocal of the repetition frequency PRF) by the switching circuit
3
, and transmitted with their polarized waves changed, respectively. The repetition period with which two reception systems corresponding to those polarized waves receive the signals is twice as long as the transmission repetition period (that is, the repetition frequency is half as high as the transmission repetition frequency). Accordingly, the maximum velocity Vmax of each received signal becomes halved in each of the two reception systems in accordance with the expression (2). However, since the signals are transmitted and received alternately, Doppler velocity can be measured at the initial repetition frequency by use of both the received signals.
For example, assume that the repetition frequency is 1,000 Hz. In this apparatus, signals are transmitted at the frequencies f
1
and f
2
with their polarized waves changed. Thus, the signals are independent of each other, and the repetition frequencies of their own become half, that is, 500 Hz, respectively. Accordingly, the maximum range Rmax observable in each of the respective reception systems is 300 km. On the other hand, Doppler velocity can be also observed up to velocity equivalent to 500 Hz×2=1,000 Hz by continuous processing of signals received alternately. Thus, the observable range of Doppler velocity is doubled to have the relation Rmax=C/PRF. Incidentally, since the periods of alternate transmission and reception are overlapped, the polarized waves of signals for transmission and reception are changed to prevent the signals from interfering with each other in
Andrea Brian
Mitsubishi Denki & Kabushiki Kaisha
Tarcza Thomas H.
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