JPH11160422A - Radio altimeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radio altimeter which can increase the accuracy of measurement, while suppressing the increase in the scale of a circuit. SOLUTION: In a radio altimeter, an LPF 9 removes high-frequency waves from beats of radio waves transmitted by a transmission part and radio waves received by a microwave reception part, an A/D conversion circuit 10 converts the beat waves into a digital signal, a filter bank 35 breaks down in polyphase the beat waves, a DFT circuit 36 Fourier-transforms the analyzed beat waves in polyphase so as to be accumulated, their frequency is detected, and a CPU 12 computes the altitude on the basis of frequencies.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio altimeter mounted on an aircraft, and in particular, using simple hardware.
The present invention relates to a radio altimeter capable of performing highly accurate detection in a short processing time.

[0002]

2. Description of the Related Art Altimeters used on aircraft are currently classified into barometric altimeters and radio altimeters. The barometric altimeter utilizes a change in barometric pressure, and always requires correction of barometric pressure. BACKGROUND ART A radio altimeter is a device that measures an absolute altitude between an aircraft and the ground by emitting a radio wave from an aircraft toward the ground below and processing a signal returned by reflecting the radio waves on the ground.

The radio altimeter is characterized by indicating an absolute altitude, has a high accuracy, and is used for flight of a large aircraft or a helicopter at a low altitude or at the time of landing.

There are two types of radio altimeters: FM radio altimeters (for low altitude) and pulse type radio altimeters (can be used at high altitude). At altitude, barometric altimeters are mainly used. The FM type radio altimeter for low altitude is widely used.

First, the principle of the FM type radio altimeter will be described. Basically, it is based on the principle that the propagation speed of radio waves is constant, but when measuring at low altitudes, the propagation speed of radio waves is too fast for the measured altitude (vertical distance), so the pulse type In the method of calculating the altitude by measuring the time required for the round trip between the aircraft and the ground surface as described above, the measurement is extremely difficult and the accuracy is reduced.

Therefore, a frequency modulation (FM) system is used. FIG. 7 is a configuration block diagram of the FM radio altimeter. As shown in FIG. 7, the FM radio altimeter includes a transmitting unit 5 including a frequency modulator 1, a microwave oscillator (carrier oscillator) 2, a microwave receiving unit 3,
And a receiving unit 6 including a frequency detecting unit 4.

[0007] The microwave oscillator 2 of the transmitting section 5 has a frequency of several GH.
A sine wave having a frequency of about z is generated, and this is frequency-modulated by the frequency modulator 1 with a deviation of about several tens of MHz. In this case, there is no modulation signal in the normal FM modulation, and the signal changes linearly with time. At the same time as the modulated wave is emitted from the transmitting antenna, a part of the output of the microwave oscillator 2 is output to the microwave receiving unit 3.

The radio wave emitted from the transmitting antenna is reflected on the ground surface, the reflected wave is received by the receiving unit 6 via the receiving antenna, and the microwave receiving unit 3 calculates the frequency difference between the transmitted wave and the received wave. Combined for The frequency detector 4 detects and calculates how much the emission frequency has changed during the time required from the emission of the radio wave to the reception,
The altitude is displayed on the indicator based on the value.
The frequency modulator 1 emits a radio wave whose frequency changes at a frequency of about several tens to several hundreds per second.

Next, the change over time of the frequency of the radio wave transmitted and received by the radio altimeter will be described with reference to FIG. FIG. 8 is an explanatory diagram showing a temporal change in the frequency of a radio wave transmitted and received by the radio altimeter. In FIG. 8, a solid line indicates a frequency change of a transmitted radio wave with respect to time, and a dotted line indicates a frequency change of a received radio wave (reflected wave) with respect to time.

Assuming that a radio wave emitted at a certain time T1 is received at T2 and a time that the radio wave reciprocates T2-T1 = ΔT,
The frequency ΔF changed during this ΔT (ΔF = F2−F
Measure 1). When the beat frequency (frequency difference ΔF) between the transmission wave and the reception wave is calculated by the receiving unit 6, the beat frequency is
FIG. 9 shows a change with respect to time. FIG. 9 is an explanatory diagram illustrating a temporal change in the frequency of the beat wave.

In FIG. 9, since the beat frequency of the flat portion is proportional to the altitude, by measuring this frequency,
Altitude can be measured. The beat frequency fb that can be detected by the frequency detection unit 4 is represented by the following equation [Equation 1].

[0012]

(Equation 1)

Here, h is altitude, c is radio wave propagation speed, t
m represents the transmission FM modulation period, and Δf represents the transmission FM deviation width.

As a simple method of measuring the beat signal, a method of measuring a zero crossing of the beat signal is generally used. However, in order to provide a high measurement accuracy of about 1 ft (0.3 m) to several ft. Δf needs to be about 100 MHz.

Since Δf cannot be made too large due to restrictions on the device used, such as the frequency band used, etc.,
Methods for counting the beat frequency have been considered. As a method of detecting this frequency, there is a method of converting beat frequencies of a transmission wave and a reception wave into digital signals and performing FFT (fast Fourier transform).

Here, the frequency detector 4 of the conventional radio altimeter will be described with reference to FIG. FIG. 10 is a configuration block diagram of the frequency detection unit 4 of the conventional radio altimeter.
As shown in FIG. 10, the frequency detecting unit 4 of the conventional radio altimeter includes a frequency converting unit 8, a band limiting unit 9, an A / D converting unit 10, an FFT unit 11, and a CPU 12. .

Each component will be specifically described. Frequency conversion 8
Receives the signals from the transmitting unit 5 and the receiving unit 6 and detects the beat frequency. This beat frequency is several hundred k
Hz. The accuracy of altitude detection is required to be approximately 2 ft, and the frequency accuracy is required to be approximately 100 Hz to several 100 Hz. The band limiting unit (low-pass filter: LPF) 9
Aliasing is removed from the output of the frequency converter 8 by a low-pass filter, and the A / D converter 10 converts the analog signal into a digital signal.

The output from the A / D converter 10 is input to an FFT (Fast Fourier Transform) unit 11. The FFT unit 11
The CPU 12 that performs a fast Fourier transform (FFT) and includes a general-purpose microcomputer or the like
Controls the input and output of the signal and the control signal whose frequency has been detected. Here, the operation of the FFT is in principle D
It is the same as FT (Discrete Fourier Transform).

[0019]

However, in the frequency detecting section of the conventional radio altimeter described above, the frequency resolution is determined by the FFT configuration, the sampling frequency is fs, the number of data (F).
Fs / N when expressed using the number of FT points) N,
If the sampling frequency fs is increased in order to widen the band, the analysis capability is reduced. Therefore, it is necessary to increase the number N of data points of the FFT in proportion.
Is about several hundred kHz to 1 MHz, and N is 512 to 10
It is about 24, and increasing the number of data points N of the FFT means increasing the hardware, and there is a problem that if the hardware is reduced, the processing time increases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and in a discrete Fourier transform process for detecting a frequency difference between a received radio wave and a transmitted radio wave, a FFT sample frequency and a frequency resolution are improved. An object of the present invention is to provide a radio altimeter capable of relatively reducing the scale and processing time and improving the accuracy of detecting the frequency difference.

[0021]

According to a first aspect of the present invention, there is provided a signal processing apparatus which emits a radio wave from an aircraft to the ground, reflects the radio wave from the ground, and returns the signal. In the radio altimeter that measures the altitude between the aircraft and the ground by processing, a frequency modulator that changes the transmission frequency linearly with time and a part of the output of the frequency modulator for transmitting at a predetermined frequency And a transmitting unit provided with a power amplifier for amplifying the output of the carrier oscillator to a predetermined power, and a frequency difference between the received radio wave and the transmitted radio wave at the receiving unit that receives the reflected radio wave emitted from the transmitting unit. Is a discrete Fourier transform process that detects
It is characterized by providing a band division filter bank that performs band division by changing the sample rate for the purpose of improving the frequency detection capability, and using simple hardware, short processing time, high precision altitude detection It can be carried out.

According to a second aspect of the present invention, there is provided a radio altimeter according to the first aspect, further comprising: a frequency converter for detecting a frequency difference between a transmitter and a receiver; Band-pass filter or low-pass filter for band-limiting the output from the A / D converter, an A / D converter for converting an analog signal input from the filter into a digital signal, and a band for a signal from the A / D converter And a band splitting filter bank for performing splitting.Fast Fourier transforming the output from the band splitting filter bank, detecting the frequency difference, and measuring the altitude of the aircraft, using simple hardware. With short processing time,
Highly accurate altitude detection can be performed.

According to a third aspect of the present invention, there is provided a radio altimeter according to the second aspect of the present invention, wherein the band splitting filter performs a band split of a signal from an A / D converter of the radio altimeter. The bank includes a band division filter that performs band division on an input signal from the A / D converter,
It is characterized by performing fast Fourier transform or discrete Fourier transform processing on the output from the band division filter bank and performing frequency detection. Using simple hardware, short processing time and high precision altitude detection are performed. be able to.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A radio altimeter according to an embodiment of the present invention (this altimeter) includes a conventional frequency detecting unit for detecting a frequency by a DFT filter bank combining a filter bank by multi-rate signal processing and a DFT. It is used instead of the frequency detection unit.
Thus, the frequency resolution can be improved, the circuit scale can be significantly reduced, and the processing time can be reduced.

As shown in FIG. 1, the frequency detecting section 4 of the altimeter comprises a frequency converting section 8, a band limiting section 9, an A / D converting section 10, a DFT filter bank 30, a CPU 12
It is composed of DFT filter bank 3
0 comprises an M-divided filter bank 35 and a DFT unit 36. FIG. 1 shows the frequency detector 4 of the altimeter.
It is a block diagram of a structure of.

The M-divided parallel filter bank 35 will be described with reference to FIG. FIG. 2 shows an M-divided filter bank 3
FIG. 5 is a configuration block diagram of No. 5; M-divided filter bank 35
Is composed of a band division filter bank 19 (analyzer), a processing unit 15, a band synthesis filter bank 20 (synthesizer), and an adder 18, as shown in FIG.

The band division filter bank 19 is a division filter 1 obtained by polyphase decomposition of the reference low-pass filter.
3 and a downsampler (decimate) 14. The band synthesis filter bank 20 includes an upsampler (interpolate) 16 and a synthesis filter 17. The adder 18 includes a synthesis filter 17
Are sequentially added, summed, and output.

The characteristics of the filter bank shown in FIG. 1 are as shown in FIG. FIG. 3 is an explanatory diagram illustrating characteristics of the M-divided filter bank. In general, a filter bank can reconstruct a signal after performing processing, but the conditions for this reconstruction are severe and there is a problem in hardware implementation. Is the band splitting filter bank 14 in the left half of FIG.
You may use only.

The effect of the DFT filter bank 30 becomes more significant as the number of band divisions M increases, and M is 2
In the case of exponentiation, the FFT unit 11 similar to the conventional one can be used instead of the DFT unit 36.

The DFT filter bank 30 multiplies the input signal by a window function having a finite length, cuts out the signal, and performs a short-time Fourier transform process for performing a Fourier transform on the signal.
The type and length of the window function determine the frequency resolution and the time resolution.

The DFT filter bank 30 applicable to the altimeter will be described with reference to FIG. FIG.
5 is a configuration example of a DFT filter bank 30 applicable to the present altimeter. As shown in FIG. 4, the DFT filter bank 30 applicable to this altimeter includes M filters 21 and I filters.
It comprises a DFT (FFT) 22, a processing unit 23, and a combining unit 24.

The outline of the processing in the DFT filter bank 30 is as follows.

Further, the processing in the DFT filter bank 30 will be specifically described using the following mathematical expressions. M filters Pk (z) 21 (k = 0, 1, 2,..., M−
1) is to reduce the number of impulse responses h (n) of the original filter to M. That is, the impulse response of the filter 21 is totally represented by the following equation (Equation 2).

[0034]

(Equation 2)

The transfer function P _k (z) is expressed by the following equation (Equation 3).

[0036]

(Equation 3)

Here, the signal X _k (m) of the k channel is represented by the following equation (Equation 4).

[0038]

(Equation 4)

Then, the transfer function h _k (i) of the k-channel filter is 0 channel (reference LPF) filter h _0.
Since (i) has been shifted in frequency, the following equation (Equation 5) is obtained.
It is represented by

[0040]

(Equation 5)

However, W _M ^-ki is represented by the following equation (Equation 6).

[0042]

(Equation 6)

In the equation of [Equation 5], assuming that i = rM + j,
When the sum of r and j is calculated, the following equation is obtained.

[0044]

(Equation 7)

Further, the following equation is used.

[0046]

(Equation 8)

Then, since there is a relationship represented by the following equation [Equation 9],
[Equation 10] is obtained.

[0048]

(Equation 9)

[0049]

(Equation 10)

Here, the symbol * indicates that convolution operation is performed. That is, the k-channel signal XK
(M) is H0 of the 0 channel (reference LPF) filter.
(Z), the result of the impulse response pj (m) (j = 0, 1,..., M−1) of the polyphase filter 21 and the signal Xj (m) distributed by the switch are convolved. The result is generated by performing an inverse Fourier transform (or FFT) on the result by the IDFT 22.

A processing such as removing the influence of noise and other unnecessary signals is performed on the signal by the processing unit 23 and the signal is synthesized by the synthesizing unit 24, so that the frequency of the beat wave with high precision (high resolution) can be easily detected. Is what is done.

An example of the filter bank 35 of the altimeter will be described with reference to FIG. FIG. 5 is a configuration block diagram illustrating an example of the filter bank 35. The filter bank 35 shown in FIG. 5 is a two-divided filter bank (polyphase decomposition), and includes a plurality of first shift registers 25, a plurality of multipliers 26, a two accumulator 27, and a two accumulator 27. , And two second shift registers 28 provided correspondingly.

Here, since a filter bank of two divisions is taken as an example, the number of accumulators 27 and the number of second shift registers 28 are two. Arithmetic unit 27 and second shift register 28
Is equal to the number of divisions M, and the polyphase decomposition is M-
It can be realized by thinning out every one, and the total number of taps of the filter bank (FIR filter) is the same as the number of taps of the reference LPF.

Hereinafter, each part will be described in detail. The first shift register 25 receives a plurality of signals continuously and stores the input signals. When the next signal is input, the first shift register 25 converts the stored signal into the first signal of the next stage. , And stores the next signal. The multiplier 26 is connected to the first shift register 2
5 is multiplied by the transfer function of the filter corresponding to each of the signals input to 5.

The accumulator 27a receives an input of the signal from the odd-numbered multiplier 26, accumulates the signal, and outputs the second signal.
To the shift register 28a. Also,
The accumulator 27b receives the signal from the even-numbered multiplier 26, accumulates the signal, and outputs the accumulated signal to the second shift register 28b.

That is, the accumulator 27a and the accumulator 27b
, The outputs from the multipliers 26 are thinned out and accumulated every other output, and output to the corresponding second shift registers 28a and 28b in order from the signal input first.

Then, the second shift registers 28a, 28a
8b sequentially outputs signals input from the accumulator 27a and the accumulator 27b to the DFT unit 36, respectively.

The altimeter filter bank 35
The one shown in FIG. 6 may be used. FIG. 6 is a configuration block diagram showing another example of the filter bank 35 of the altimeter. The filter bank 35 of this altimeter
As shown in FIG. 6, a register circuit 61 and a multiplication circuit 62
, Accumulation circuit 63, and FIFO (First In First Out)
64, a control unit 65, and a RAM storing and storing the order in which the FIFO 64 reads data from the accumulation circuit 63.
66 may be used.

Hereinafter, each part will be described in detail. The register circuit 61 is provided in the first shift register 25 shown in FIG. 5, the multiplier circuit 62 is provided in the multiplier 26, and the accumulator circuit 63 is provided in the accumulator 27.
The FIFOs 64 respectively correspond to the second shift registers 28.

The register circuit 61 sequentially stores the input signals. The multiplying circuit 62 multiplies the signal sequentially stored in the register 61 by the transfer function of the corresponding filter in the same manner as the multiplier 26 of FIG.

The accumulation circuit 63 accumulates the signal input from the multiplication circuit 62 and outputs the accumulated signal to the RAM 66 and the FIFO 64, respectively. FIFO 64 is RAM 66
According to the reading order of the data stored in the DFT unit 36,
Is output to The control section 65 controls each section. RAM 66 is an accumulation circuit 63, FIFO 64
Is a memory for assigning the order of reading data.

According to such a filter bank 35,
Since the number of shift registers and multipliers is constant irrespective of the number of samples, there is an effect that the circuit scale can be reduced.

According to this altimeter, the DFT is
By using a filter bank, 512 points of F
A resolution of about 100 Hz to several 100 Hz (high resolution of several ft) can be obtained with a bandwidth of several MHz by using a DFT filter bank of 512 divisions by FT, and a circuit scale can be reduced by using an FFT filter bank that is polyphase-decomposed. There is an effect that highly accurate altitude measurement can be performed in a short processing time while reducing the size.

[0064]

According to the first aspect of the present invention, the radio wave is emitted from the aircraft toward the ground, and the signal reflected from the ground is processed to return the altitude between the aircraft and the ground. In a radio altimeter to be measured, a frequency modulator that changes the transmission frequency linearly with time, a carrier oscillator for transmitting a part of the output of the frequency modulator at a predetermined frequency, and an output of the carrier oscillator to a predetermined power. A transmitting section provided with a power amplifier to amplify, and a discrete Fourier transform process for detecting a frequency difference between a received radio wave and a transmitted radio wave at a receiving section that receives and reflects a radio wave emitted from the transmitting section and reflected, thereby achieving frequency detection capability. Since the radio altimeter is provided with a band division filter bank that performs band division by changing the sample rate for the purpose of improvement, simple hardware is used. Te, a short processing time, there is an effect that it is possible to perform a high detection precision.

According to the second aspect of the present invention, the frequency converter for detecting a frequency difference between the transmitter and the receiver, a band-pass filter or a low-pass filter for band-limiting an output from the frequency converter, An A / D converter for converting an analog signal input from the filter into a digital signal; and a band division filter bank for performing band division of a signal from the A / D converter. Fast Fourier transform the output, detect the frequency difference,
Since the radio altimeter according to the first aspect measures the altitude of an aircraft, there is an effect that highly accurate altitude detection can be performed in a short processing time using simple hardware.

According to the third aspect of the present invention, the band division filter bank that performs band division of the signal from the A / D converter of the radio altimeter performs band division on the input signal from the A / D converter. It is equipped with a band division filter, performs fast Fourier transform or discrete Fourier transform processing on the output from the band division filter bank, and performs frequency detection.Since the radio altimeter according to claim 2 is used, a simple hardware can be used. There is an effect that highly accurate altitude detection can be performed in the processing time.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram of a frequency detector 4 of an altimeter according to an embodiment of the present invention.

FIG. 2 is a configuration block diagram of an M-divided filter bank 35.

FIG. 3 is an explanatory diagram illustrating characteristics of an M-divided filter bank.

FIG. 4 is a DFT filter bank 3 applicable to the altimeter.
0 is a configuration example.

FIG. 5 is a configuration block diagram illustrating an example of a filter bank 35.

FIG. 6 is a configuration block diagram showing another example of the filter bank 35 of the altimeter.

FIG. 7 is a block diagram showing a configuration of an FM radio altimeter.

FIG. 8 is an explanatory diagram showing a temporal change of a frequency of a radio wave transmitted and received by the radio altimeter.

FIG. 9 is an explanatory diagram illustrating a temporal change in the frequency of a beat wave.

FIG. 10 is a configuration block diagram of a frequency detection unit 4 of a conventional radio altimeter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Frequency modulator, 2 ... Microwave oscillator, 3 ... Microwave reception part, 4 ... Frequency detection part, 5 ... Transmission part,
6 receiving section, 8 frequency converting section, 9 band limiting section, 10 A / D converting section, 11 FFT section, 12
... CPU, 13 ... Division filter, 14 ... Downsampler, 15 ... Processing unit, 16 ... Upsampler, 1
7: synthesis filter, 18: adder, 19: band division filter bank, 20: band synthesis filter bank,
21: filter, 22: IDFT, 23: processing unit,
24: synthesis unit, 25: first shift register, 2
6 Multiplier, 27 Accumulator, 28 Second shift register, 30 DFT filter bank, 35 Filter bank, 36 DFT unit, 61 Register circuit, 62 Multiplier circuit, 63 Accumulator circuit , 64 ... F
IFO, 65 ... Control unit, 66 ... RAM

Claims (3)

[Claims] 1. An aircraft launches radio waves toward the ground,
A radio altimeter that measures the altitude between an aircraft and the earth by processing the signal reflected from the earth and returning the signal, the frequency modulator that changes the transmission frequency linearly with time and the output of the frequency modulator A transmitting unit provided with a carrier oscillator for transmitting a part at a predetermined frequency and a power amplifier for amplifying an output of the carrier oscillator to a predetermined power,
Changing the sample rate for the purpose of improving frequency detection capability in a discrete Fourier transform process for detecting a frequency difference between a received radio wave and a transmitted radio wave in a receiving unit that receives a reflected radio wave emitted from a transmitting unit. A radio altimeter comprising a band division filter bank for performing band division by means of: 2. A frequency converter for detecting a frequency difference between a transmitter and a receiver, a band-pass filter or a low-pass filter for band-limiting an output from the frequency converter, and an analog signal input from the filter. A / D converter for converting the A / D converter into a digital signal, and a band division filter bank for performing band division of the signal from the A / D converter, fast Fourier transforming the output from the band division filter bank, 2. The radio altimeter according to claim 1, wherein the difference is detected to measure an altitude of the aircraft. 3. A band division filter bank for dividing a band from a signal from an A / D converter of a radio altimeter, comprising:
3. The apparatus according to claim 2, further comprising a band division filter that performs band division on an input signal from the converter, performing fast Fourier transform or discrete Fourier transform on an output from the band division filter bank, and performing frequency detection. Radio altimeter