A frequency domain measurement method for instrument landing systems based on software radio

Abstract

This invention relates to the field of civil air traffic control system technology, and discloses a frequency domain measurement method for instrument landing systems based on software radio. The method includes the following steps: The digital baseband signal obtained from AM demodulation is divided into three frequency ranges using the CZT algorithm in the spectrum refinement algorithm for refinement analysis, resulting in three refined spectrum segments centered at 90Hz, 150Hz, and DC. By calculating the maximum value within each frequency range, the amplitude of the desired frequency is obtained, and then the modulation index and modulation difference are calculated. Compared with traditional time-domain demodulation methods and methods using Fourier transform, this method can effectively save hardware resources while better identifying the signal frequency domain amplitude.

Description

Technical Field

[0001] This invention relates to the field of civil air traffic control system technology, and in particular to a frequency domain measurement method for instrument landing systems based on software radio. Background Technology

[0002] The Instrument Landing System (ILS) is a radio navigation system for aircraft landing. It originated after World War II and was recognized by ICAO as a standard approach and landing equipment for aircraft in 1949. The function of the Instrument Landing System is to provide an approaching aircraft with a fixed glide path and 2-3 distance checkpoints to ensure that the aircraft lands safely in weather conditions with visibility of not less than 400 meters and cloud height of not less than 30 meters.

[0003] The ILS consists of two parts: ground beacon equipment and airborne receiving equipment. The ground equipment consists of glide slope beacons (GS), localizers (LOC), and marker beacons (MKR). These beacons radiate specific electromagnetic waves into the air to establish a virtual path from the runway to the sky, guiding the aircraft to a safe landing.

[0004] Taking a localizer as an example, it establishes a flight path by transmitting a localizer signal (LOC) consisting of two highly directional VHF amplitude-modulated (AM) waves along the runway centerline. The beam to the left of the runway centerline is an AM wave modulated by a 90Hz single-frequency signal, and the beam to the right is an AM wave modulated by a 150Hz single-frequency signal, both with a modulation depth of 20% at transmission. On the left side of the runway centerline, the 90Hz modulation is dominant, while on the right side, the 150Hz modulation is dominant. The degree to which an aircraft deviates from the runway centerline is represented by the modulation difference (DDM) between the two single-tone signals. If DDM = M90 - M150, the modulation difference is zero on the runway centerline, and the absolute value of DDM is symmetrical on both sides of the runway centerline, varying with the angle of aircraft deviation.

[0005] The glide slope transmitter works similarly to the directional transmitter; it transmits a glide slope signal (GS) consisting of two UHF (ultra-high frequency) amplitude-modulated waves at a certain elevation angle.

[0006] The runway and glide slope together form a narrow "tunnel" leading from the runway into the air. Once the ILS system is operational, it can provide landing guidance information, including heading angle, glide slope angle, and the distance between the aircraft and the runway threshold. The aircraft receives the heading and glide slope signals through the onboard ILS receiver and calculates the DDM value to determine its relative position to the "tunnel." This allows the aircraft to fly in the correct direction towards the runway and descend smoothly, ultimately achieving a safe landing.

[0007] Currently, the measurement of ILS signals in China mainly uses imported field testing equipment purchased from navigation equipment manufacturers. This equipment mainly uses analog components to realize the signal processing, resulting in disadvantages such as large size, heavy weight, and poor scalability, making it difficult to install on UAVs for field testing.

[0008] Software-defined radio (SDR) technology, based on a modular, standardized, and universal hardware support platform, enables radio stations to achieve multi-functionality through software programming. The fundamental idea of ​​SDR is to integrate the A / D module with the antenna module, leveraging the advantages of software to diversify radio characteristics. This allows communication systems to be freed from hardware constraints, achieving diverse software functions while maintaining hardware universality and system stability. Researching ILS signal measurement methods based on SDR technology can result in smaller, more flexible receivers that are easier to maintain and optimize later.

[0009] Currently, there are two main algorithms for measuring ILS signals based on general-purpose software radio platforms, both domestically and internationally:

[0010] Based on time-domain measurement, this paper describes a data processing method for an airborne instrument landing system (ILS) based on software-defined radio. The method involves bandpass filtering the ILS baseband signal to extract specified frequency signals, followed by rectification to obtain the audio signal. The DC signal amplitude is obtained through a DC estimation module, and the modulation index is then calculated. The bandpass filter in this paper is designed using an IIR digital filter. However, due to the feedback structure of the IIR filter, cumulative errors are introduced into the output signal. Furthermore, the filter coefficients, when implemented on a software-defined radio platform, often introduce quantization errors, resulting in suboptimal filtering and difficulty in accurately calculating the required modulation index.

[0011] Algorithms based on frequency domain measurements, considering the high design requirements of bandpass filters for signal separation in the time domain and the unavoidable influence of DC components, which increases the instability of modulation calculations, currently employ FFT on the ILS baseband signal in the frequency domain to analyze its frequency components and obtain the average power at 90Hz and 150Hz. However, to accurately distinguish between the 90Hz and 150Hz signals, given a fixed sampling rate, the sampling time needs to be increased as much as possible to reduce the frequency resolution and avoid the picket fence effect. Therefore, this method has a long computation time and consumes many resources under certain conditions, and also has a significant error in frequency identification.

[0012] In summary, existing methods for measuring ILS signals suffer from problems such as high resource consumption and large audio signal estimation errors, and cannot accurately calculate modulation. Summary of the Invention

[0013] To overcome the shortcomings of existing technologies, the present invention aims to provide a frequency domain measurement method for instrument landing system receivers based on software radio. This method improves upon traditional instrument landing system demodulation methods and, by incorporating software radio theory, solves problems such as high resource consumption and large audio signal estimation errors when measuring navigation equipment signals using current field testing equipment.

[0014] The present invention is achieved by at least one of the following technical solutions.

[0015] A frequency domain measurement method for an instrument landing system based on software radio includes the following steps:

[0016] 1) The digital baseband signal obtained by AM demodulation is divided into multiple spectral intervals for refined analysis, resulting in three refined spectra of the baseband signal;

[0017] 2) The specified frequency range can be refined by setting the start and end frequencies of the range and the number of refinement points in the frequency domain;

[0018] 3) Detect the maximum amplitude within multiple frequency ranges;

[0019] 4) Obtain the modulation in the instrument landing system.

[0020] Furthermore, the digital baseband signal is a dual-tone signal with a first frequency of 90 Hz and a second frequency of 150 Hz, and includes a DC component.

[0021] Furthermore, the CZT algorithm in the spectrum refinement algorithm is used to divide the spectrum into multiple spectrum intervals for refinement analysis.

[0022] Furthermore, the CZT algorithm is as follows:

[0023]

[0024] In the above formula, n is the number of elements in the input sequence x[n], and A = exp(j2π·f start / f s ) represents the Z-plane sampling point under the initial phase angle, W = exp(-j2π·(f stop -f start ) / m·f s ) represents the Z-plane span of the refined frequency band, j is the imaginary unit, m is the number of refinement points in the frequency domain, and f start and f stop f represents the start and end frequencies of the interval. s Where is the signal sampling rate, and CZT(x[n]) is the frequency domain amplitude corresponding to x[n].

[0025] Furthermore, in the refinement process of step 2), the frequency resolution is:

[0026] (f stop -f start ) / m

[0027] Where f start and f stop Here, m represents the starting and ending frequencies of the interval, and m is the number of refinement points in the frequency domain.

[0028] Furthermore, let the bandwidth of the frequency range be B, the number of refinement points be m, and the frequency ranges be centered at 90Hz, 150Hz, and 0Hz respectively.

[0029] Further, in step 3), with 90Hz as the center, a frequency range of (90–B / 2)Hz to (90+B / 2)Hz is selected, and an m-point linear frequency modulation Z-transform is performed to obtain the corresponding refined spectrum, where B is the bandwidth of the frequency range.

[0030] Further, in step 3), with 150Hz as the center, a frequency range of (150–B / 2)Hz to (150+B / 2)Hz is selected, and an m-point linear frequency modulation Z-transform is performed to obtain the corresponding refined spectrum, where B is the bandwidth of the frequency range.

[0031] Further, in step 3), with 0Hz as the center, a frequency range of (0–B / 2)Hz to (0+B / 2)Hz is selected, and an m-point linear frequency modulation Z-transform is performed to obtain the corresponding refined spectrum, where B is the bandwidth of the frequency range.

[0032] Furthermore, the modulation difference and modulation sum in the instrument landing system are calculated using the following formulas:

[0033] Modulation difference: DDM = (P 150 -P 90 ) / P DC

[0034] Modulation and: SDM = (P 150 +P 90 ) / P DC

[0035] Where P 90 P 150 P DC The expression represents the maximum amplitude within the three frequency spectrum intervals.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] 1. The present invention provides a frequency domain measurement method for an instrument landing system receiver based on software radio. Compared with traditional time domain demodulation methods, it adopts a bandpass filter design, which effectively avoids the problem of excessive multiplier consumption or unstable modulation output caused by filter design.

[0038] 2. This invention improves upon the method using Fourier transform by proposing the CZT transform from a frequency domain refinement algorithm. This algorithm refines the analysis of a selected frequency range, achieving higher frequency resolution. It effectively saves hardware resources while better identifying the signal's frequency domain amplitude, thus accurately calculating the modulation index. Attached Figure Description

[0039] Figure 1 This is a signal processing module diagram of a frequency domain measurement method for an instrument landing system based on software radio, as illustrated in an embodiment.

[0040] Figure 2 As an example, a frequency domain measurement method for an instrument landing system based on software radio utilizes a refined 90Hz spectrum obtained from CZT transformation;

[0041] Figure 3 As an example, a frequency domain measurement method for an instrument landing system based on software radio utilizes a refined 150Hz spectrum diagram after CZT transformation;

[0042] Figure 4 As an example, a frequency domain measurement method for an instrument landing system based on software radio utilizes a refined 0Hz spectrum obtained from CZT transformation. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0044] Traditional spectrum analysis methods typically employ the Fast Fourier Transform (FFT) algorithm to perform panoramic spectrum analysis of a signal. While this method quickly and conveniently implements digital spectrum analysis, considering the calculation of ILS signal modulation, it is only necessary to focus on the 90Hz, 150Hz, and 0Hz frequency components in the baseband signal, and then perform detailed observation and analysis on the three spectral intervals containing these frequencies. This necessitates improving the frequency resolution of the selected intervals.

[0045] Example 1

[0046] A frequency domain measurement method for instrument landing systems based on software-defined radio (SDR) involves dividing the digital baseband signal obtained from AM demodulation into three frequency bands using the CZT algorithm, a spectrum refinement algorithm, to obtain three refined spectrum segments centered at 90Hz, 150Hz, and DC. By calculating the maximum value within each frequency band, the amplitude of the desired frequency is obtained, and subsequently, the modulation index and modulation difference are calculated. Compared to traditional time-domain demodulation methods and methods employing Fourier transform, this method effectively saves hardware resources while providing better identification of the signal's frequency domain amplitude. The specific steps include:

[0047] 1) The digital baseband signal obtained by AM demodulation is divided into multiple spectral intervals for refined analysis to obtain three refined spectra of the baseband signal; the digital baseband signal is a dual-tone signal with a first frequency of 90 Hz and a second frequency of 150 Hz, and contains a DC component.

[0048] By setting the start and end frequencies of the interval and the number of frequency domain refinement points, the CZT algorithm in the spectrum refinement algorithm is used to divide the spectrum into multiple intervals for refinement analysis.

[0049] 2) Refine the specified frequency range;

[0050] 3) Detect the maximum amplitude within multiple frequency ranges;

[0051] 4) Calculate the modulation in the instrument landing system.

[0052] Example 2

[0053] This embodiment provides a frequency domain measurement method for an instrument landing system receiver based on software radio, such as... Figure 1 As shown, it includes the following steps:

[0054] 1) The digital baseband signal obtained by AM demodulation is divided into three spectral intervals for refinement analysis using the CZT (Chirpz-transform) algorithm in the spectrum refinement algorithm. In this embodiment, the spectral intervals are centered at 90Hz, 150Hz and 0Hz respectively, to obtain three refined spectra of the baseband signal.

[0055] Specifically, the formula for the CZT algorithm used can be simplified as follows:

[0056]

[0057] In the above formula, n is the number of elements in the input sequence x[n], and A = exp(j2π·f start / f s ) represents the Z-plane sampling point under the initial phase angle, W = exp(-j2π·(f stop -f start ) / m·fs ) represents the Z-plane span of the refined frequency band, j is the imaginary unit, m is the number of refinement points in the frequency domain, and f start and f stop f represents the start and end frequencies of the interval. s Where is the signal sampling rate, and CZT(x[n]) is the frequency domain amplitude corresponding to x[n].

[0058] Furthermore, the CZT algorithm operation can be divided into the following steps:

[0059] 1.1 Setting f start f stop Let m be the start and end frequencies of the interval and the refinement factor, then the frequency resolution is (f stop -f start ) / m.

[0060] 1.2 will and Perform 2 separately H Perform point FFT and multiply, where H is log2(n+m-1) rounded up.

[0061] 1.3 Perform an IFFT on the multiplied signal and then combine it with... The first m sequences are multiplied sequentially to obtain the refined spectrum of the specified interval.

[0062] The modulation difference and modulation sum in the instrument landing system are calculated using the following formulas:

[0063] Modulation difference: DDM = (P 150 -P 90 ) / P DC

[0064] Modulation and: SDM = (P 150 +P 90 ) / P DC .

[0065] Specifically, let the frequency range bandwidth be B and the number of refinement points be m, then

[0066] Select the frequency range of (90–B / 2)Hz to (90+B / 2)Hz, perform m-point CZT, and obtain its refined frequency spectrum;

[0067] Select the frequency range of (150–B / 2)Hz to (150+B / 2)Hz, perform m-point CZT, and obtain its refined spectrum;

[0068] Select the spectral range of (0–B / 2)Hz to (0+B / 2)Hz, perform m-point CZT, and obtain its refined spectrum.

[0069] Example 3

[0070] This embodiment of a frequency domain measurement method for an instrument landing system based on software radio includes the following steps:

[0071] 1) The digital baseband signal obtained by AM demodulation is divided into multiple spectral intervals for refined analysis, resulting in three refined spectra of the baseband signal;

[0072] 2) By setting the start and end frequencies and the number of refinement points, a specified frequency range is refined; during the refinement process, the frequency resolution is:

[0073] (f stop -f start ) / m

[0074] Where f start and f stop Here, m represents the starting and ending frequencies of the interval, and m is the number of refinement points in the frequency domain.

[0075] 3) Detect the maximum amplitude within multiple frequency ranges;

[0076] 4) Obtain the modulation index in the instrument landing system (ILS), and calculate the modulation index difference and modulation index sum in the ILS using the following formulas:

[0077] Modulation difference: DDM = (P 150 -P 90 ) / P DC

[0078] Modulation and: SDM = (P 150 +P 90 ) / P DC

[0079] Where P 90 P 150 P DC The expression represents the maximum amplitude within the three frequency spectrum intervals.

[0080] In this embodiment, a refined frequency range of 80Hz to 100Hz was selected, and 128-point CZT was performed to obtain its refined spectrum, as shown below. Figure 2 The maximum value within the refined interval is detected to obtain the spectral amplitude P at 90Hz. 90 ;

[0081] Example 4

[0082] This embodiment of a frequency domain measurement method for an instrument landing system based on software radio includes the following steps:

[0083] 1) The digital baseband signal obtained by AM demodulation is divided into multiple spectral intervals for refined analysis, resulting in three refined spectra of the baseband signal;

[0084] 2) By setting the start and end frequencies and the number of refinement points, the specified frequency range is refined; the CZT algorithm in the spectrum refinement algorithm is used to divide the spectrum into multiple segments for refinement analysis. The CZT algorithm is as follows:

[0085]

[0086] In the above formula, n is the number of elements in the input sequence x[n], and A = exp(j2π·f start / f s ) represents the Z-plane sampling point under the initial phase angle, W = exp(-j2π·(f stop -f start ) / m·f s ) represents the Z-plane span of the refined frequency band, j is the imaginary unit, m is the number of refinement points in the frequency domain, and f start and f stop f represents the start and end frequencies of the interval. s Where is the signal sampling rate, and CZT(x[n]) is the frequency domain amplitude corresponding to x[n].

[0087] 3) Detect the maximum amplitude within multiple frequency ranges;

[0088] 4) Obtain the modulation in the instrument landing system.

[0089] In this embodiment, a refined frequency range of 140Hz to 160Hz was selected, and a 128-point CZT was performed to obtain its refined spectrum, as shown below. Figure 3 The maximum value within the refined interval is detected to obtain the spectral amplitude P at 150Hz. 150 ;

[0090] Example 5

[0091] This embodiment of a frequency domain measurement method for an instrument landing system based on software radio includes the following steps:

[0092] 1) The digital baseband signal obtained by AM demodulation is divided into multiple spectral intervals for refined analysis, resulting in three refined spectra of the baseband signal;

[0093] 2) The specified frequency range can be refined by setting the start and end frequencies of the range and the number of refinement points in the frequency domain;

[0094] 3) Detect the maximum amplitude within multiple frequency ranges;

[0095] 4) Obtain the modulation in the instrument landing system.

[0096] In this embodiment, a refined spectrum of -10Hz to 10Hz was selected, and 128-point CZT was performed to obtain the refined spectrum, as shown below. Figure 4The maximum value within the refined interval is detected to obtain the spectral amplitude P at 0 Hz. DC .

[0097] At this point, the frequency resolution is (20 / 128) = 0.15625. 90Hz, 150Hz, and DC signals are all integer multiples of this resolution, allowing for accurate identification of the required frequency amplitude.

[0098] The modulation in the instrument landing system is calculated using the following formula:

[0099] Modulation difference: DDM = (P 150 -P 90 ) / P DC

[0100] Modulation and: SDM = (P 150 +P 90 ) / P DC

[0101] The frequency domain measurement method was verified by computer simulation. The error of the obtained modulation difference data did not exceed 0.000076%, and the error of the modulation and data did not exceed 0.000011%.

[0102] The method was deployed on a software-defined radio hardware platform and tested using an ILS signal generator. The error in the obtained modulation difference data did not exceed 0.046%, and the error in modulation and data did not exceed 0.021%. This invention improves upon the method using Fourier transform by proposing the CZT transform from a frequency domain refinement algorithm. This algorithm refines the analysis of a selected frequency range, achieving higher frequency resolution. It effectively saves hardware resources while better identifying the signal frequency domain amplitude, thus accurately calculating the modulation.

[0103] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A frequency domain measurement method for an instrument landing system based on software-defined radio, characterized in that, Includes the following steps: 1) The digital baseband signal obtained by AM demodulation is divided into multiple spectral intervals for refined analysis, resulting in three refined spectra of the baseband signal; The CZT algorithm, a spectrum refinement algorithm, is used to divide the spectrum into multiple spectrum intervals for refinement analysis. The CZT algorithm is as follows: In the above formula, n is the number of elements in the input sequence x[n], and A = exp(j2π·f start / f s ) represents the Z-plane sampling point under the initial phase angle, W = exp(-j2π·(f stop -f start ) / m·f s ) represents the Z-plane span of the refined frequency band, j is the imaginary unit, m is the number of refinement points in the frequency domain, and f start and f stop f represents the start and end frequencies of the interval. s Where C is the signal sampling rate, and CZT(x[n]) is the frequency domain amplitude corresponding to x[n]. 2) The specified frequency range can be refined by setting the start and end frequencies of the range and the number of refinement points in the frequency domain; 3) Detect the maximum amplitude within multiple frequency ranges; 4) Obtain the modulation in the instrument landing system.

2. The frequency domain measurement method for an instrument landing system based on software radio according to claim 1, characterized in that, The digital baseband signal is a dual-tone signal with a first frequency of 90 Hz and a second frequency of 150 Hz, and includes a DC component.

3. The frequency domain measurement method for an instrument landing system based on software radio according to claim 1, characterized in that, In the refinement process of step 2), the frequency resolution is: (f stop -f start ) / m Where f start and f stop Here, m represents the starting and ending frequencies of the interval, and m is the number of frequency domain refinement points.

4. A frequency domain measurement method for an instrument landing system based on software radio according to any one of claims 1 to 3, characterized in that, The linear frequency modulation Z-transform algorithm in the spectrum refinement algorithm is used to refine the spectrum into three spectrum intervals. Let the bandwidth of the frequency interval be B, the number of refinement points be m, and the spectrum intervals be centered at 90Hz, 150Hz and 0Hz respectively.

5. The frequency domain measurement method for an instrument landing system based on software radio according to claim 4, characterized in that, In step 3), with 90Hz as the center, a frequency range of (90-B / 2)Hz to (90+B / 2)Hz is selected, and an m-point linear frequency modulation Z-transform is performed to obtain the corresponding refined spectrum, where B is the bandwidth of the frequency range.

6. The frequency domain measurement method for an instrument landing system based on software radio according to claim 4, characterized in that, In step 3), with 150Hz as the center, a frequency range of (150-B / 2)Hz to (150+B / 2)Hz is selected, and an m-point linear frequency modulation Z-transform is performed to obtain the corresponding refined spectrum, where B is the bandwidth of the frequency range.

7. The frequency domain measurement method for an instrument landing system based on software radio according to claim 4, characterized in that, In step 3), with 0Hz as the center, a frequency range of (0-B / 2)Hz to (0+B / 2)Hz is selected, and an m-point linear frequency modulation Z-transform is performed to obtain the corresponding refined spectrum, where B is the bandwidth of the frequency range.

8. The frequency domain measurement method for an instrument landing system based on software radio according to claim 4, characterized in that, The modulation difference and modulation sum in the instrument landing system are calculated using the following formulas: Modulation difference: DDM = (P 150 -P 90 ) / P DC Modulation and: SDM = (P 150 +P 90 ) / P DC Where P 90 P 150 P DC The expression represents the maximum amplitude within the three frequency spectrum intervals.

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