JPH02143186A - Modulation index switching type fm-cw doppler radar - Google Patents

Abstract

PURPOSE:To output a modulated wave with a modulation index N by a frequency-modulating driver and to improve the efficiency of an oscillator by switching the amplitude of a frequency-modulated driving signal to N times as large as that in the case of a modulation index. CONSTITUTION:A frequency modulating driver 13 outputs a frequency-modulated wave with a modulation index m=N with a selection signal which sets the amplitude of the frequency-modulated driving signal as the output of the frequency modulating driver 13 to N times as that in case of a modulation index m=1. A Doppler radar is operated with modulation indexes on a time-division basis with a modulation index selection signal from a CPU 22 and the current S/N level is measured from the output signal of an S/N detector 21 to uses a modulation index with a higher S/N ratio principally for operation. For example, the modulation efficiency E is set to -31.9-(-41.5)=9.6 (dB) by switching the modulation index (m) from 1 to 3 and improved by about 10 dB.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention provides a modulation index switching method FM-CW having a mode for switching the modulation index m from m=1 to a larger value such as m=3 during low altitude flight. Regarding Doppler radar.

[Prior art and problems to be solved by the invention] In Doppler radar for measuring aircraft speed, typical transmission and reception methods include CW (continuous wave) method (no modulation) and FM-C method.
W (frequency modulation one continuous wave) method exists. In the latter case, a modulation index m is used to represent the extent to which frequency modulation 1 (FM) is applied.

In the conventional FM-CW method using a single modulation index, the signal strength is weaker than the CW method when the radio wave propagation distance is short, and the signal strength exceeds the noise strength at low altitudes above a calm sea surface where the radar reflection coefficient is small. This could easily lead to a significant deterioration of the S/N ratio.

A conventional FM-CW Doppler radar will be outlined using FIGS. 5 to 7.

Figure 5 is a block diagram of a conventional FM-CW Doppler radar, Figure 6 is the altitude characteristic curve of the first-order Bessel function, and Figure 7 is the block diagram of a conventional FM-CW Doppler radar.
The figure is a diagram showing altitude versus SN ratio.

In FIG. 5, a modulation signal with a modulation index m=1 is input to a frequency modulation driver 13, and the output of this modulation driver is input to a Gunn oscillator 14 to excite the Gunn oscillator. The Gunn oscillator output is radiated to the earth's surface as a transmission signal via the transmission antenna 16. A reflected signal from the ground surface is received by a receiving antenna 17 and sent to an RF (radio frequency) demodulator, and a portion of the output of the Gunn oscillator 14 is sent to an RF demodulator 18 via a coupler 15 for demodulation. , its demodulated output is sent to a filter amplifier 19. Filter amplification 2S
19 filters and amplifies only a specific frequency, and its output is sent to a frequency tracker 20.

This one output is sent to the S/N detector 21 and the S/N ratio is detected. The output of the S/N detector 21 is CP
Sent to U22. The other output of the frequency tracker 20 is sent to the CPU 22 as a Doppler frequency output. The CPU controls these inputs to obtain Doppler frequency output.

Here, in the FM-CW Doppler radar shown in FIG.
If the modulation angular frequency is Wl, the modulation efficiency is E, and the first Bessel function is Jl (M), then S/N CC = lφl
og J, (M)′ ・” (A)τ However, it is expressed as M−2n+sjn(Wm ・).

From equation (A), it can be seen that when the round trip time τ of radio waves to the ground is small, that is, when the altitude is low, M approaches O. The Doppler radar using this first-order Bessel function J, (M) and the high FM-CW method has weak signal strength at low altitudes. That is, the altitude vs. S/N characteristic curve in FIG. 7 shows this, especially the low altitude region is indicated by diagonal lines. In this low altitude region, if the ground surface has a small radar reflection coefficient, such as a calm sea surface, a sufficient S/N ratio may not be obtained and speed detection may not be possible. Furthermore, the conventional device shown in FIG. 5 has the disadvantage that the CPU output is not fed back and the modulation index cannot be switched.

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, it is an object of the present invention to increase the received signal strength of Doppler radar when flying at low altitudes and ensure a sufficient S/N ratio. An object of the present invention is to provide a Doppler radar that can reliably detect speed during low-altitude flight over a surface.

[Means for Solving the Problems] According to the present invention, a frequency divider that divides a constant frequency, a frequency modulation driver excited by the output of the g1 division period, a Gunn oscillator driven by the frequency modulation driver, and A transmitting/receiving antenna unit that transmits the output of the Gunn oscillator and receives reflected waves from the target, a demodulation circuit that receives the output of the reception antenna and demodulates it using the output of the Gunn oscillator, and allows only a specific frequency to pass through the output of the demodulation circuit. (modulation frequency 10 Doppler frequency), a means for detecting the S/N ratio from the output of the frequency tracker to measure the S/N level, and selecting a modulation index with a better S/N ratio, Provided is a modulation index switching type FM-CW Doppler radar, characterized in that a modulated wave with a modulation index of N is output from a frequency modulation driver by switching the amplitude of a modulation drive signal to N times that in the case of a modulation index of 1. .

The feature of the present invention is that regarding the modulation index in the FM-CW system, the modulation index m = 1 that has been conventionally used.
A larger modulation index, for example m=3, is also used, and means are provided to select the optimal modulation index by comparing the signal-to-noise ratios of the signals measured for modulation indexes m=1 and m=3, respectively. It was designed to be provided.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the Doppler radar of the present invention.

The crystal oscillator 11 oscillates at a constant frequency. This output is frequency-divided by a frequency divider 12 and input to a frequency modulation driver 13. This input signal is a stable TT signal with a stable frequency.
The modulation driver 13, which is the L level signal 5(1),
The modulation frequency is input to the Gunn oscillator 14 by the selection signal from the Pt122, but this input signal is a signal with stable amplitude and frequency. By adding a selection signal, it is possible to supply a signal whose amplitude is fixed to an even larger value.

A signal 5(3) in the microwave region frequency-modulated by the Gunn oscillator 14 is radiated into space via the transmitting antenna 16. Further, a part of the transmitted wave 5(3) is extracted from the output of the Gunn oscillator 14 by the coupler 15 and used as a local signal for the RF demodulation circuit 18 of the receiving section.

The received wave 5 (4) reflected from the ground surface is sent to the receiving antenna 17
is received via. The received wave 5 (4) is a signal in which the Doppler frequency ``, which is proportional to the speed of the aircraft, is added to the transmitted frequency ft, and is delayed with respect to the transmitted wave.This received wave 5 (4) and the local received wave S ( 3'), and by mixing, a single sideband (SSB) Doppler signal is sent to the filter amplifier 19 as a demodulated wave 5 (5).In this filter amplifier 19, only the Doppler signal associated with the sideband C is mixed. Filtered, amplified filter amplified output 5 (6)
is sent to the frequency tracker 20, and the frequency tracker 20
sends a signal at the center frequency of the Jl Doppler signal spectrum to the CPU 22. On the other hand, from the wave number tracker 20, S
A signal is also sent to the /N detector 21 to detect the SN ratio. The output of the S/N detector 21 is sent to the CPU 22.

The modulation index switching function, which is the object of the present invention, is performed as follows. That is, the amplitude of the frequency modulation drive signal as the output of the frequency modulation driver 13 is determined by the modulation index m=
A frequency modulated wave with a modulation index m=N is output from the frequency modulation driver 13 by a selection signal N times that in the case of 1.

The Doppler radar is time-divisionally operated with a plurality of modulation indices according to the modulation index selection signal from the CPU 22, and the S/N level in that case is measured from the output signal of the S/N detector 21, and the modulation with the better S/N ratio is selected. The index will be used as the main component. In the Doppler radar system of the present invention, the CPU
Measurement and determination of the SN ratio and generation of the modulation index selection signal are performed by software No. 22.

The Doppler radar system according to the present invention will be explained as follows.

If the modulated wave S(]) input to the frequency modulation driver 13 is expressed by ea = Ea cosω-1, C, , : amplitude ω, and two modulation angular frequencies, then the modulated wave 5(2) of each amplitude is expressed by the following formula. It is expressed by

em = E, cos ωai 't Modulated wave with amplitude E1 N-e*=N-εaCO5ωa + t Modulated wave with amplitude NE (1) In this case, e or N-e is the modulation index selection signal s < 7 ) and frequency modulates the next carrier wave eL'.

8t' = EtSin(dt-L However, E: amplitude ωL: Carrier wave angular frequency In this case, the instantaneous angular frequency ω is expressed by the following equation.

ω12ωt (-NE to 1+, cosω〦・1) = ωt±
Δωt·cosω1 where K is a unit coefficient where ΔωL=−N−E,·ω is called the maximum angular frequency deviation. The output of Gunn oscillator 14 is transmitted to transmitting antenna 16
Sent from. The transmitted wave 5(3)et of this FM-CW Doppler radar is eL=Etsin(ω@rt +
m5iriω,t)'' (2) where m-Δω,/ω, and the value of m is called the modulation index.

The output of Gunn oscillator 14 is also sent to RF demodulator 18 via coupler 15. When this signal S(3') is input to the RF demodulator 18, eta=Lsin(a+, -t+m5inω, -t)
-(3) and etg=ELcos(ω, -t+m5inω, t) with a 90° phase shift
”It is divided into (4).

Next, the radio waves radiated from the transmitting antenna 16 are reflected from the ground surface, and the received waves e are received by the receiving antenna 17.
is expressed by the following formula.

er=Ersin [(ωt+ωa)(t −r)+m
5inωm(t−r))...[5] Here, E is the amplitude, ω is the Doppler angular frequency, and τ is the propagation delay time.

The received wave demodulated by the signal of formula [3] in the RF demodulator 18 is as follows: eaA=etA' er TomEz! ! In(ωL't+m5snωa-t)'
Er5in ((ωt + (1) a) (t-τ)
+m5inω, (t-τ))ζ-Ec4rcos (ω
4. − cos ω, (1 − τ/2)) + sin ω, −
t ・sin (Mcosωa(t − r /2) )
However, M = 2m sin (ω* r /2)] ... [6] In the [6] formula, the Bessel function cos (xcosy) = Jl (X) -2Jx
(x) cos2y + 2Ja (x) cos4y
sin(xcosy)=2J+(x)cosy+2Jz
Expanding using (x) cas3y+..., the received wave demodulated by is eo song e, ・(-e,) +2Js(M)cos5ω, (t-)=tsuka...[8
] (7) Formula J.

Jl of the sum and difference component of equation [8] for the sideband, J of the output of the RF demodulator 18 due to the sum and difference for the sideband, and the component for the sideband is ea tss+u +-eaa++eam+J, other sidebands except Similarly to []1], the single sideband (55
B), and the Jo and all sideband signals can be expressed as e4 = ea* + ea <sts> + + 8=
(!!ml 2 + ...10 ea (ssm)n ... is expressed.

The filter amplifier 19 is ed (3311
1, that is, only signal 5 (6) is allowed to pass through.

Next, the frequency tracker 20 calculates (f) in the formula [11].

(10f hate).

Furthermore, the S/N detector 21 calculates the signal level in formula [11], that is, amplitude = Et-H2J+(M) m (1
3) Detect. Here, J+(M) is a first-order Bessel function.

From equation [6], [12] f,: Doppler frequency...[14] Substituting equation [14] into equation []3 and setting the speed of light to C in Figure 4,...[15] In Equation (5), E, , ω-9ω are constant values, so for the amplitude of the modulated wave in Equation [1], N = 1 to N
-3, that is, by multiplying the modulation index m by three times m=
1 to m=3. Therefore, the signal level in equation [13] increases in accordance with the value of the modulation index.

Next, the beam projection direction of the microwave transmitted from the antenna is shown in FIG.
+ σ. ,ψ. When the roll angle R and the pitch angle P are given, the beam projection angle shown in FIG.
sP-sinR-cos a .

+cos P - cos R - cos ψ 0) The propagation delay time τ at the altitude is obtained from the following formula [16].

Therefore, in the example case, the modulation efficiency E is -31,9-(-41,5) =9.6(
dB), indicating an improvement of about 10 dB.

〔Effect of the invention〕

According to the present invention, at the BIT timing which is the self-diagnosis period of the Doppler control CPU 22, the modulation index m is set to m=
If the SN ratio at the BIT timing decreases by switching from 1 to m==3 or vice versa, the modulation index during operation is left unchanged, and when the SN ratio increases, the modulation index is switched to the modulation index during operation. In this case, Doppler radar! !
When the aircraft is flying at a low altitude, the modulation index m is switched from 1 to 3, and as a result, the S/N ratio is approximately 1, as seen from equation (A).
Increase by 0dB. This improvement in the SN ratio eliminates the disadvantage that speed detection may not be possible when an aircraft equipped with a Doppler radar flies at a low altitude over the ground surface where the radar reflection coefficient is small.

Note that m=3 is a limit value determined from the efficiency of the Gunn oscillator, and if the oscillator efficiency improves, it is possible to take a corresponding value.

As a modification of this case, it is also possible to obtain altitude information from an altitude sensor and switch the modulation index based on the altitude information.

Furthermore, it is also possible to implement a method in which a Doppler radar with altitude measurement ability measures its own altitude and changes the modulation index based on the altitude information.

[Brief explanation of the drawing]

FIG. 1 shows the modulation index switching method FM-
A block diagram of the CW Doppler radar. Figure 2 shows the altitude vs. SN ratio characteristic curve showing the degree of improvement in the SN ratio in the case of m = l and m = 3 in the device shown in Figure 1. Figure 3 shows the transmission from the antenna. FIG. 4 is a diagram showing the relationship between altitude and propagation delay time. FIG. 5 is a block diagram showing the conventional FM-CW Doppler radar at m=1. 6 shows the Jl(M) altitude characteristic curve of the apparatus shown in FIG. 5, and FIG. 7 shows the altitude versus SN ratio characteristic curve of the apparatus shown in FIG. Figure Figure No.

Claims (1)

[Claims] 1. A frequency divider that divides a constant frequency, a frequency modulation driver that is excited by the output of the frequency divider, a Gunn oscillator that is driven by the frequency modulation driver, and a device that transmits the output of the Gunn oscillator. and a transmitting/receiving antenna section that receives the reflected wave from the target, a demodulating circuit that receives the output of the receiving antenna and demodulates it using the Gunn oscillator output, and a demodulating circuit that passes only a specific frequency from the output of the demodulating circuit (modulation frequency + Doppler frequency). ), from the frequency tracker output S
By detecting the N ratio, measuring the S/N level, and selecting a modulation index with a better S/N ratio, and switching the amplitude of the frequency modulation drive signal to N times that in the case of a modulation index of 1. Modulation index switching method F characterized in that a modulated wave with a modulation index N is output from a frequency modulation driver.
M-CW Doppler radar. 2. The apparatus of claim 1, further comprising feeding back the modulation index selection signal to the frequency modulation driver.