Loran C signal detection system

Abstract

The invention belongs to the technical field of Roland C signal detection, and discloses a Roland C signal detection system, which comprises a tester host and a signal receiving module, the tester host comprises a space signal processing module, a direct acquisition signal processing module, a data statistical analysis module and a power supply module. The Roland C signal detection system has a space receiving measurement mode and a direct acquisition measurement mode, accurate detection of signal parameters can be achieved according to needs, the space receiving mode captures Roland C signals in the space through a receiving antenna, a coupler in the antenna carries out filtering, amplification and circuit matching processing on the received signals firstly, and then the Roland C signals in the space are collected through the direct acquisition measurement mode. The processed signal is transmitted to the space signal processing module through a cable; the module further simulates, filters and amplifies the signals and then converts the signals into digital signals through AD sampling, and station signal detection and standard zero crossing point identification are completed through the processes of searching and capturing, station identification, period identification and the like.

Description

Technical Field

[0001] This invention belongs to the field of Loran C signal detection technology, specifically a Loran C signal detection system. Background Technology

[0002] Loland C is a wide-area radio ground-based time synchronization method. Before the advent of GNSS systems such as BeiDou, it was my country's most accurate time synchronization technology. It employs a high-power ground-based transmission system, boasting advantages such as high system stability and strong anti-interference capabilities. Its time synchronization accuracy is unaffected by geographical location or weather conditions. In the complex electromagnetic environment of battlefield space, GNSS satellite navigation is extremely vulnerable to interference. Therefore, GNSS should never be used as the sole means of navigation, positioning, and timing; a reliable alternative backup system is essential, and Loland C is an ideal choice. The coding format used by Loland C reduces the probability of interference. For the enemy to suppress its signal, they would need to establish a transmitter and use high-power, long antennas, a task extremely difficult to accomplish in wartime. It is not only an independent radio navigation system but can also broadcast DGPS and GNSS integrity information to enhance the GNSS signal, a capability not possessed by other wide-area augmentation systems. If the Loland C transmitter is synchronized with UTC, it can act as a BDS / GPS satellite or pseudo-satellite in a GNSS / Loland C combined system, effectively compensating for the shortcomings of the GNSS satellite system. As a backup and redundancy measure for GNSS timing, Roland C can significantly improve system availability and reliability, working closely with GNSS to ensure the stability and reliability of the timing system.

[0003] Existing Loland C receivers can only complete basic processes such as signal acquisition and tracking, and obtain signal arrival time to achieve timing and positioning calculations. However, they cannot measure and analyze the signal parameters of Loland C timing or navigation station broadcasts. In particular, they lack the specific measurement capabilities for single-pulse group signal parameters, inter-pulse group signal parameters, and data modulation accuracy. They also cannot perform real-time analysis of transmitted signals, making it difficult to meet the need for in-depth control of Loland C signal quality. Summary of the Invention

[0004] The purpose of this invention is to provide a Roland C signal detection system to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a Roland C signal detection system, which includes a test instrument host and a signal receiving module; the test instrument host includes a spatial signal processing module, a direct sampling signal processing module, a data statistical analysis module, and a power supply module; the signal receiving module includes a receiving antenna, an attenuator, and a feeder cable;

[0006] The space signal processing module is used to perform analog signal processing, digital signal processing, and parameter measurement on the space Loran-C signal acquired by the receiving antenna; the direct acquisition signal processing module is used to perform analog-to-digital conversion, digital signal processing, and parameter measurement on the Loran-C signal directly input from the outside; the data statistical analysis module is used to perform statistical analysis, display, and reporting on the measurement data output by the two processing modules; and the power supply module provides stable power to each module of the system.

[0007] Preferably, the space signal processing module includes an analog signal processing unit, a digital signal processing unit, a parameter measurement unit, and a first power supply unit;

[0008] The analog signal processing unit is sequentially equipped with an impedance matching circuit, an active bandpass filter, an adjustable gain control chip, and a limiter, which are used to perform impedance matching, bandpass filtering, amplitude adjustment, and limiting processing on the signal transmitted by the receiving antenna.

[0009] The digital signal processing unit includes an A / D converter, a SOC processor chip, and a clock control circuit, used to realize analog-to-digital conversion of signals, digital filtering and noise reduction, signal search and acquisition, carrier phase tracking, period identification, and information demodulation and decoding.

[0010] The parameter measurement unit is used to calculate the signal-to-noise ratio, field strength, envelope difference, and timing difference of the Loran C signal; the first power supply unit provides power to the analog signal processing unit, the digital signal processing unit, and the parameter measurement unit.

[0011] Preferably, the digital signal processing unit employs anti-interference technology and envelope delay correlation detection technology during the digital signal processing process;

[0012] The anti-interference technology is based on an LMS adaptive notch filter. The LMS adaptive notch filter receives the original input signal containing narrowband single-frequency interference and a pure sinusoidal reference signal. It dynamically adjusts the weight vector coefficients through the LMS algorithm and outputs a signal that eliminates single-frequency interference. It also satisfies the convergence condition that the step size factor is less than the reciprocal of the largest eigenvalue of the autocorrelation matrix of the input signal.

[0013] When the signal contains frequency When subjected to sinusoidal disturbances, a notch filter with adaptive capabilities can effectively eliminate such interference; the original input signal From the expected signal Composed of narrowband single-frequency interference, and The reference signal is a pure sinusoidal signal associated with a single-frequency interference, and the two signals are out of phase. ,Right now:

[0014]

[0015] Combined with the LMS algorithm, the output signal for:

[0016]

[0017]

[0018]

[0019]

[0020] Combining the formula for the weight vector coefficients, we can obtain:

[0021]

[0022] In the formula Step size factor It is the output signal obtained after the adaptive notch filter has suppressed interference;

[0023] The LMS algorithm is a recursive operation that does not require prior knowledge of the statistical properties of the signal; it only uses their instantaneous estimates. The operation only yields estimates of the weighting coefficients, but as time increases, the weighting coefficients are gradually adjusted, and the estimates gradually improve, eventually leading to a convergent value. The convergence condition is:

[0024]

[0025] in It is the largest eigenvalue of the autocorrelation matrix of the input signal;

[0026] The envelope delay correlation detection technique obtains the same-direction branch signal and the orthogonal branch signal by mixing the received Loran C signal with the local orthogonal carrier and low-pass filtering. The signal envelope is obtained by summing the squares of the two signals. The signal envelope is then delayed by 1 GRI and correlation accumulation of length L is performed to enhance the desired signal envelope and suppress noise and cross-interference.

[0027] The envelope delay correlation detection method, at its core, obtains the correlation peak through delay correlation accumulation calculation, thereby realizing the detection of pulse signal groups. Assuming the received Loran C signal is:

[0028]

[0029] In the formula, To receive the envelope of the Loran C signal, If the signal carrier is the received Loran C signal, then the local quadrature carrier will be mixed with it.

[0030]

[0031] In the above mixing results, Given the phase difference between the local carrier and the received Loran C signal, the mixing result is low-pass filtered to obtain the in-phase branch signal and the quadrature branch signal, respectively:

[0032]

[0033] By summing the squares of the filtered signals from the same-direction branch and the orthogonal branch, we can obtain the signal envelope as follows:

[0034]

[0035] The above formula shows that the phase difference between the local carrier and the received signal is eliminated. This also eliminates the phase encoding in the pulse signal, resulting in a positive pulse signal;

[0036] Since the Loran C signal has the same repetition period (GRI) as its envelope and the correlation peaks, we delay the signal envelope by one GRI and then perform correlation accumulation with a cumulative length of L. The accumulated result is as follows:

[0037]

[0038] In the formula, For the relevant length; The desired station GRI is defined as follows: Since the Loran C signal repeats according to the GRI period, the envelopes of the desired GRI signals exhibit strong correlation, while noise signals show weak correlation. Cross-interference and noise also show weak correlation, thus enhancing the desired signal envelope. The results are expressed as follows:

[0039]

[0040] In the formula, This is the noise after correlation.

[0041] Preferably, the direct sampling signal processing module includes an analog-to-digital conversion unit, a second digital signal processing unit, a second parameter measurement unit, and a second power supply unit;

[0042] The analog-to-digital conversion unit uses the AD9268 high-precision, high-speed analog-to-digital converter from Analog Devices (ADI), which supports dual-channel, 16-bit, and signal acquisition at a maximum data rate of 125 Mbps.

[0043] The second digital signal processing unit, in conjunction with the externally input GTP signal, assists in the rapid periodic identification of the Loran C signal, directly performing carrier phase tracking, information demodulation and decoding, and timing signal generation, eliminating the need for digital filtering and automatic search steps;

[0044] The second parameter measurement unit is used to measure the single-pulse group signal parameters, inter-pulse group signal parameters, and data modulation accuracy; the second power supply unit supplies power to each unit of the direct acquisition signal processing module.

[0045] Preferably, the single-pulse group signal parameter measurement of the second parameter measurement unit includes half-cycle amplitude tolerance calculation, half-cycle amplitude overall tolerance calculation, pulse waveform amplitude stability calculation, pulse trailing edge calculation, reference zero-crossing time change calculation, reference zero-crossing stability calculation, radio frequency zero-crossing stability calculation, and peak point arrival time calculation.

[0046] The half-cycle amplitude tolerance calculation satisfies: |IN-SN| is less than or equal to 0.03 and |IN-SN| is less than or equal to 0.10, where IN is the normalized value of the relative pulse peak amplitude of the Nth half-cycle of the actual waveform leading edge, and SN is the normalized value of the relative pulse peak amplitude of the Nth half-cycle of the theoretical waveform leading edge.

[0047] The overall tolerance of the half-cycle amplitude is achieved by calculating the sum of the deviations between the actual and theoretical waveform leading edge of the first 8 half-cycle amplitude normalized values;

[0048] The overall tolerance for the half-cycle amplitude should meet the requirements of the following formula:

[0049]

[0050] In the formula:

[0051] I N —The normalized value of the relative pulse peak amplitude of the Nth half-cycle of the actual waveform leading edge;

[0052] S N —This is the normalized value of the relative pulse peak amplitude in the Nth half-cycle of the theoretical waveform leading edge;

[0053] The first eight half-cycles of the Rowland C pulse include the zero-crossing point of the third cycle. The overall tolerance of the half-cycle amplitude reflects the overall peak amplitude difference between the leading edge of the actual Rowland C pulse and the leading edge of the ideal Rowland C pulse. This is helpful in judging the waveform correctness of the Rowland C pulse and the stability of the zero-crossing point of the third cycle. The smaller the overall tolerance of the half-cycle amplitude, the smaller the difference between the actual and ideal Rowland C pulses, and the more stable the zero-crossing point of the third cycle.

[0054] The pulse waveform amplitude stability requirement is that, in a set of measurements with n≥20, the peak-to-peak variation of the peak amplitude in any half-cycle is less than 0.04A, and the relative variation (standard deviation) is less than 0.01A, where A is the pulse peak current;

[0055] When the pulse trailing edge satisfies t>500μs, i(t) is less than or equal to 0.0014A, and the percentage of the trailing edge current value relative to the envelope peak value is calculated;

[0056] The variation of the reference zero-crossing time is less than 60 ns when operating with a single repetition rate and less than 140 ns when operating with a dual repetition rate.

[0057] The reference zero-crossing stability is less than 50 ns peak-to-peak variation when operating at a single repetition rate and less than 140 ns peak-to-peak variation when operating at a dual repetition rate.

[0058] The RF zero-crossing stability is achieved by calculating the peak-to-peak time and standard deviation of the zero-crossing time within the range of τ+10 ≤ t ≤ τ+50. When operating at single repetition rate, the peak-to-peak variation is less than 50 ns and the standard deviation is less than 20 ns. When operating at dual repetition rate, the peak-to-peak variation is less than 150 ns and the standard deviation is less than 50 ns. The time to reach the peak point is calculated by Δtp = 65 + τ, where τ is the envelope difference.

[0059] Preferably, the pulse group signal parameter measurement of the second parameter measurement unit includes the calculation of the peak amplitude tolerance between pulses and the calculation of the envelope difference tolerance between pulses;

[0060] The peak amplitude tolerance between pulses is calculated by D=(Ipkmx-Ipkmin) / Ipkmx, where Ipkmx is the maximum peak value of the pulse within the group, and Ipkmin is the minimum peak value of the pulse within the group. The tolerance is less than 5% when operating at a single repetition rate and less than 10% when operating at a double repetition rate.

[0061] The inter-pulse packet error tolerance is calculated by D=ECDmx-ECDmin, where ECDmx is the maximum value of the pulse packet error within the group, ECDmin is the minimum value of the pulse packet error within the group, and the tolerance is less than 0.5μs.

[0062] Preferably, the data modulation accuracy measurement of the second parameter measurement unit includes Eurofix demodulation decoding and "ninth pulse" demodulation decoding;

[0063] The Eurofix demodulation uses a 100MHz clock to acquire the trigger signal MPT (transmitter signal detection unit signal), measures the arrival time of MPT by edge detection and analyzes the A and B periods, and decodes 7-bit information by combining the modulation pattern; Eurofix decoding performs CRC check and RS error correction by sliding every 30 7-bit information groups to achieve frame synchronization and valid message output;

[0064] The "ninth pulse" demodulation determines whether the ninth MPT of the pulse group exists, and decodes 5-bit information based on 32 delay offsets; the "ninth pulse" decoding performs co-set decoding and RS error correction by sliding every 24 5-bit information groups to achieve frame synchronization and effective message output.

[0065] Preferably, the data statistical analysis module includes an industrial control motherboard, host computer software, a touch screen, and input devices; the industrial control motherboard runs the Win10 operating system.

[0066] The host computer software adopts a modular design, including a spatial signal data statistical analysis module and a direct sampling signal data statistical analysis module;

[0067] The space signal data statistical analysis module is used to display the station signal acquisition, tracking, and locking status of the space signal processing module, as well as the real-time measurement values ​​and statistical results of mean, variance, and root mean square error; the direct acquisition signal data statistical analysis module is used to display the single pulse group, inter-pulse group, and data modulation accuracy measurement parameters of the direct acquisition signal processing module.

[0068] Preferably, the detection method includes two modes: spatial reception detection and direct sampling detection.

[0069] The spatial reception method detection includes the following steps:

[0070] S11: The receiving antenna receives the spatial Loran C signal, and the internal coupler of the antenna filters, amplifies, and matches the signal.

[0071] S12: The feeder cable transmits the processed signal to the space signal processing module, where the analog signal processing unit performs impedance matching, bandpass filtering, amplitude adjustment, and limiting.

[0072] S13: The digital signal processing unit performs AD sampling on the analog signal and realizes station signal detection and standard zero-crossing point identification through search acquisition, station identification, and period identification.

[0073] S14: The parameter measurement unit calculates the field strength, signal-to-noise ratio, envelope difference, and timing difference; the data statistical analysis module performs statistical analysis and reports the measurement data.

[0074] The direct sampling method for testing includes the following steps:

[0075] S21: Match and adjust the externally input Roland C signal and transmit it to the direct sampling signal processing module;

[0076] S22: The analog-to-digital conversion unit directly samples the signal using AD, and the second digital signal processing unit combines the external GTP signal to achieve period recognition;

[0077] S23: The second parameter measurement unit calculates the signal level, envelope difference, waveform parameters, zero-crossing stability, and modulation accuracy. The data statistical analysis module performs statistical analysis and reports on the measurement data.

[0078] Preferably, in the digital signal processing, the detection method employs an FIR bandpass filter for signal preprocessing. The FIR filter has a passband range of 75kHz-125kHz, an order of 90, and uses the equiripple optimal approximation method. The passband cutoff frequencies are 75kHz and 125kHz, and the stopband cutoff frequencies are 70kHz and 130kHz. The maximum passband attenuation is 1dB, and the minimum stopband attenuation is 60dB. Furthermore, in carrier frequency calculation, the least squares fitting method is used to determine the zero-crossing point of the Loran C signal. The specific steps are as follows:

[0079] S31: Select the two closest sampling points (one positive and one negative) to the x-axis and construct the prediction function f(x) = ax + b; S32: Calculate the parameters a and b using the least squares algorithm.

[0080] This minimizes the overall error L=Σ(f(xi)-yi)², where xi is the x-coordinate of the point to be fitted and yi is the y-coordinate of the point to be fitted.

[0081] S33: Based on the fitting function f(x)=ax+b, calculate the intersection point with the x-axis to obtain the zero-crossing time t;

[0082] S34: Calculate the zero-crossing point t6 from the 6th half-cycle to the zero-crossing point t12 of the 12th half-cycle. 12 The average of the three cycle lengths is then converted into a carrier frequency.

[0083] The beneficial effects of this invention are as follows:

[0084] 1. The Loran-C signal detection system of this invention has two measurement modes: spatial reception and direct sampling. It can accurately detect signal parameters as needed. In the spatial reception mode, the Loran-C signal in space is captured by a receiving antenna. The antenna's internal coupler first filters, amplifies, and matches the received signal, and then transmits the processed signal to the spatial signal processing module via cable. This module further filters and amplifies the signal, converts it into a digital signal through AD sampling, and completes station signal detection and standard zero-crossing point identification through processes such as search and capture, station identification, and period identification. Finally, key parameters such as field strength, signal-to-noise ratio, envelope difference, and time difference are obtained using parameter measurement methods. In the direct sampling mode, the Loran-C signal directly input from the outside is first matched and adjusted before being transmitted to the direct sampling signal processing module. The module directly acquires the digital signal through AD sampling, and combines it with the externally input GTP signal to complete the Loran-C signal period identification. Finally, parameter measurement methods are used to measure parameters such as signal level, envelope difference, waveform parameters, zero-crossing point stability, and modulation accuracy.

[0085] 2. This invention provides a highly efficient and accurate signal detection system comprising a test instrument host and a signal receiving module. The host features clearly defined spatial and direct acquisition signal processing modules, capable of processing both spatially received and externally acquired Roland C signals respectively, covering diverse detection needs and overcoming the limitations of single-reception methods. The spatial signal processing module employs multi-stage processing, combining anti-interference and envelope delay correlation detection technologies. An LMS adaptive notch filter eliminates narrowband single-frequency interference, while envelope delay correlation detection enhances the desired signal and suppresses noise, improving signal processing anti-interference capabilities and accuracy, ensuring precise subsequent parameter measurements. The direct acquisition signal processing module uses a high-precision AD9268 chip, combined with external GTP signal for rapid period identification, eliminating unnecessary steps and improving detection efficiency. It can also measure multiple key parameters, covering multi-dimensional signal characteristics. The data statistical analysis module, utilizing modular host computer software, clearly displays and analyzes measurement data, allowing users to intuitively grasp signal conditions. The two detection methods have clear workflows. FIR bandpass filtering preprocessing and least-squares fitting calculation of the carrier frequency further enhance detection accuracy and reliability, providing strong technical support for Roland C signal-related applications. Attached Figure Description

[0086] Figure 1 This is a schematic diagram illustrating the basic working principle of the Roland C receiver of the present invention;

[0087] Figure 2 This is a schematic diagram of the components of the Roland C signal detector of the present invention;

[0088] Figure 3 This is a schematic diagram of the spatial signal processing module of the present invention;

[0089] Figure 4 This is a schematic diagram of the analog signal processing principle of the present invention;

[0090] Figure 5 This is a schematic diagram of the LMS adaptive notch filter structure of the present invention;

[0091] Figure 6 This is a schematic diagram of the spatial signal processing module of the present invention;

[0092] Figure 7 This is a schematic diagram of the peak amplitude of the first 13 half-cycles of the Roland pulse of the present invention;

[0093] Figure 8 This is a schematic diagram of the peak amplitude of the first 8 half-cycles of the Roland pulse of the present invention;

[0094] Figure 9 This is a schematic diagram of the trailing edge of the Rowland pulse of the present invention;

[0095] Figure 10 This is a schematic diagram of the upper zero-crossing point used in this invention to determine the pulse period;

[0096] Figure 11 This is a schematic diagram showing the time it takes for the Roland C pulse to reach its peak value according to the present invention;

[0097] Figure 12 This is a schematic diagram of the software design of the present invention;

[0098] Figure 13 This is a schematic diagram of the FIR filter parameters of the present invention;

[0099] Figure 14 This is a schematic diagram of the measured FIR filtering effect of the present invention;

[0100] Figure 15 This is a schematic diagram showing the arrival time of the eLORAN pulse at its peak point according to the present invention;

[0101] Figure 16 This is a schematic diagram of the eLORAN pulse trailing edge of the present invention;

[0102] Figure 17 This is a schematic diagram of the front panel of the present invention;

[0103] Figure 18 This is a schematic diagram of the rear panel of the present invention. Detailed Implementation

[0104] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0105] like Figures 1 to 18 As shown, this embodiment of the invention provides a Roland C signal detection system, which includes a test instrument host and a signal receiving module; the test instrument host includes a spatial signal processing module, a direct sampling signal processing module, a data statistical analysis module, and a power supply module; the signal receiving module includes a receiving antenna, an attenuator, and a feeder cable;

[0106] The space signal processing module is used to perform analog signal processing, digital signal processing, and parameter measurement on the space Loran-C signal acquired by the receiving antenna; the direct acquisition signal processing module is used to perform analog-to-digital conversion, digital signal processing, and parameter measurement on the Loran-C signal directly input from the outside; the data statistical analysis module is used to perform statistical analysis, display, and reporting on the measurement data output by the two processing modules; and the power supply module provides stable power to all modules of the system.

[0107] The space signal processing module includes an analog signal processing unit, a digital signal processing unit, a parameter measurement unit, and a first power supply unit.

[0108] The analog signal processing unit is equipped with an impedance matching circuit, an active bandpass filter, an adjustable gain control chip, and a limiter in sequence, which are used to perform impedance matching, bandpass filtering, amplitude adjustment, and limiting processing on the signal transmitted by the receiving antenna.

[0109] The digital signal processing unit includes an A / D converter, a SOC processor chip, and a clock control circuit, which are used to realize analog-to-digital conversion of signals, digital filtering and noise reduction, signal search and acquisition, carrier phase tracking, period recognition, and information demodulation and decoding.

[0110] The parameter measurement unit is used to calculate the signal-to-noise ratio, field strength, envelope difference, and timing difference of the Loran C signal; the first power supply unit supplies power to the analog signal processing unit, the digital signal processing unit, and the parameter measurement unit.

[0111] Among them, the digital signal processing unit employs anti-interference technology and envelope delay correlation detection technology in the digital signal processing process;

[0112] The anti-interference technology is based on the LMS adaptive notch filter. The LMS adaptive notch filter receives the original input signal containing narrowband single-frequency interference and the pure sinusoidal reference signal. It dynamically adjusts the weight vector coefficients through the LMS algorithm and outputs a signal that eliminates single-frequency interference. It also satisfies the convergence condition: the step size factor is less than the reciprocal of the largest eigenvalue of the autocorrelation matrix of the input signal.

[0113] When the signal contains frequency When subjected to sinusoidal disturbances, a notch filter with adaptive capabilities can effectively eliminate such interference; the original input signal From the expected signal Composed of narrowband single-frequency interference, and The reference signal is a pure sinusoidal signal associated with a single-frequency interference, and the two signals are out of phase. ,Right now:

[0114]

[0115] Combined with the LMS algorithm, the output signal for:

[0116]

[0117]

[0118]

[0119]

[0120] Combining the formula for the weight vector coefficients, we can obtain:

[0121]

[0122] In the formula Step size factor It is the output signal obtained after the adaptive notch filter has suppressed interference;

[0123] The LMS algorithm is a recursive operation that does not require prior knowledge of the statistical properties of the signal; it only uses their instantaneous estimates. The operation only yields estimates of the weighting coefficients, but as time increases, the weighting coefficients are gradually adjusted, and the estimates gradually improve, eventually leading to a convergent value. The convergence condition is:

[0124]

[0125] in It is the largest eigenvalue of the autocorrelation matrix of the input signal;

[0126] Envelope delay correlation detection technology obtains the same-direction branch signal and the orthogonal branch signal by mixing the received Loran C signal with the local orthogonal carrier and low-pass filtering. The signal envelope is obtained by summing the squares of the two signals. The signal envelope is then delayed by 1 GRI and correlation accumulation of length L is performed to enhance the desired signal envelope and suppress noise and cross-interference.

[0127] The envelope delay correlation detection method, at its core, obtains the correlation peak through delay correlation accumulation calculation, thereby realizing the detection of pulse signal groups. Assuming the received Loran C signal is:

[0128]

[0129] In the formula, To receive the envelope of the Loran C signal, If the signal carrier is the received Loran C signal, then the local quadrature carrier will be mixed with it.

[0130]

[0131] In the above mixing results, Given the phase difference between the local carrier and the received Loran C signal, the mixing result is low-pass filtered to obtain the in-phase branch signal and the quadrature branch signal, respectively:

[0132]

[0133] By summing the squares of the filtered signals from the same-direction branch and the orthogonal branch, we can obtain the signal envelope as follows:

[0134]

[0135] The above equation shows that the phase difference between the local carrier and the received signal is eliminated. This also eliminates the phase encoding in the pulse signal, resulting in a positive pulse signal;

[0136] Since the Loran C signal has the same repetition period (GRI) as its envelope and the correlation peaks, we delay the signal envelope by one GRI and then perform correlation accumulation with a cumulative length of L. The accumulated result is as follows:

[0137]

[0138] In the formula, For the relevant length; The desired station GRI is defined as follows: Since the Loran C signal repeats according to the GRI period, the envelopes of the desired GRI signals exhibit strong correlation, while noise signals show weak correlation. Cross-interference and noise also show weak correlation, thus enhancing the desired signal envelope. The results are expressed as follows:

[0139]

[0140] In the formula, This is the noise after correlation.

[0141] The direct sampling signal processing module includes an analog-to-digital conversion unit, a second digital signal processing unit, a second parameter measurement unit, and a second power supply unit.

[0142] The analog-to-digital conversion unit uses ADI's AD9268 high-precision, high-speed analog-to-digital converter, which supports dual-channel, 16-bit, and signal acquisition at a maximum data rate of 125 Mbps.

[0143] The second digital signal processing unit, combined with the externally input GTP signal, assists in the rapid periodic identification of the Loran C signal, directly performing carrier phase tracking, information demodulation and decoding, and timing signal generation, eliminating the need for digital filtering and automatic search steps;

[0144] The second parameter measurement unit is used to measure the signal parameters of a single pulse group, the signal parameters between pulse groups, and the data modulation accuracy; the second power supply unit supplies power to each unit of the direct acquisition signal processing module.

[0145] Among them, the single-pulse group signal parameter measurement of the second parameter measurement unit includes half-cycle amplitude tolerance calculation, half-cycle amplitude overall tolerance calculation, pulse waveform amplitude stability calculation, pulse trailing edge calculation, reference zero-crossing time change calculation, reference zero-crossing stability calculation, radio frequency zero-crossing stability calculation, and peak point arrival time calculation.

[0146] The half-cycle amplitude tolerance calculation satisfies: |IN-SN| is less than or equal to 0.03 (1 is less than or equal to N and less than or equal to 8) and |IN-SN| is less than or equal to 0.10 (9 is less than or equal to N and less than or equal to 13), where IN is the normalized value of the relative pulse peak amplitude of the Nth half-cycle of the actual waveform leading edge, and SN is the normalized value of the relative pulse peak amplitude of the Nth half-cycle of the theoretical waveform leading edge.

[0147] The overall tolerance of the half-cycle amplitude is achieved by calculating the sum of the deviations between the actual and theoretical waveform's leading edge amplitude normalized values ​​for the first 8 half-cycles.

[0148] The overall tolerance for the half-cycle amplitude should meet the requirements of the following formula:

[0149]

[0150] In the formula:

[0151] I N —The normalized value of the relative pulse peak amplitude of the Nth half-cycle of the actual waveform leading edge;

[0152] S N —This is the normalized value of the relative pulse peak amplitude in the Nth half-cycle of the theoretical waveform leading edge;

[0153] The first eight half-cycles of the Rowland C pulse include the zero-crossing point of the third cycle. The overall tolerance of the half-cycle amplitude reflects the overall peak amplitude difference between the leading edge of the actual Rowland C pulse and the leading edge of the ideal Rowland C pulse, which is helpful in judging the waveform correctness of the Rowland C pulse and the stability of the zero-crossing point of the third cycle. The smaller the overall tolerance of the half-cycle amplitude, the smaller the difference between the actual Rowland C pulse and the ideal Rowland C pulse, and the more stable the zero-crossing point of the third cycle.

[0154] The pulse waveform amplitude stability requirement is that in a set of measurements with n≥20, the peak-to-peak variation of the peak amplitude in any half-cycle is less than 0.04A, and the relative variation (standard deviation) is less than 0.01A, where A is the pulse peak current;

[0155] When the pulse trailing edge satisfies t>500μs, i(t) is less than or equal to 0.0014A, and the percentage of the trailing edge current value relative to the envelope peak value is calculated;

[0156] The variation in the reference zero-crossing time is less than 60 ns when operating with a single repetition rate and less than 140 ns when operating with a dual repetition rate.

[0157] The reference zero-crossing stability is less than 50 ns peak-to-peak variation when operating at a single repetition rate and less than 140 ns peak-to-peak variation when operating at a dual repetition rate.

[0158] RF zero-crossing stability is achieved by calculating the peak-to-peak time and standard deviation of the zero-crossing point within the range of τ+10 ≤ t ≤ τ+50 (τ is the envelope difference). When operating at single repetition rate, the peak-to-peak variation is less than 50 ns and the standard deviation is less than 20 ns. When operating at dual repetition rate, the peak-to-peak variation is less than 150 ns and the standard deviation is less than 50 ns. The time to reach the peak point is calculated by Δtp = 65 + τ, where τ is the envelope difference.

[0159] Among them, the pulse group signal parameter measurement of the second parameter measurement unit includes the calculation of the peak amplitude tolerance between pulses and the calculation of the envelope difference tolerance between pulses;

[0160] The peak amplitude tolerance between pulses is calculated using D=(Ipkmx-Ipkmin) / Ipkmx, where Ipkmx is the maximum peak value of the pulse within the group, and Ipkmin is the minimum peak value of the pulse within the group. The tolerance is less than 5% when operating at single repetition rate and less than 10% when operating at double repetition rate.

[0161] The inter-pulse packet error tolerance is calculated by D=ECDmx-ECDmin, where ECDmx is the maximum intra-group pulse packet error and ECDmin is the minimum intra-group pulse packet error, with a tolerance of less than 0.5μs.

[0162] The data modulation accuracy measurement of the second parameter measurement unit includes Eurofix demodulation and decoding and "ninth pulse" demodulation and decoding.

[0163] Eurofix demodulation uses a 100MHz clock to acquire the trigger signal MPT, measures the arrival time of MPT by edge detection and analyzes the A and B cycles, and decodes 7-bit information by combining the modulation pattern; Eurofix decoding performs CRC check and RS error correction by sliding every 30 7-bit information in a group to achieve frame synchronization and valid message output;

[0164] The "ninth pulse" demodulation determines whether the ninth MPT of the pulse group exists and decodes 5-bit information based on 32 delay offsets. The "ninth pulse" decoding performs co-set decoding and RS error correction by sliding every 24 5-bit information groups to achieve frame synchronization and effective message output.

[0165] The data statistics and analysis module includes an industrial control motherboard, host computer software, touch screen, and input devices; the industrial control motherboard runs the Win10 operating system.

[0166] The host computer software adopts a modular design, including a spatial signal data statistical analysis module and a direct acquisition signal data statistical analysis module;

[0167] The space signal data statistical analysis module is used to display the station signal acquisition, tracking, and locking status of the space signal processing module, as well as the real-time measurement values ​​and statistical results of mean, variance, and root mean square error. The direct acquisition signal data statistical analysis module is used to display the measurement parameters of single pulse groups, inter-pulse groups, and data modulation accuracy of the direct acquisition signal processing module.

[0168] The detection method includes two modes: spatial reception detection and direct sampling detection.

[0169] Space reception method detection includes the following steps:

[0170] S11: The receiving antenna receives the spatial Loran C signal, and the internal coupler of the antenna filters, amplifies, and matches the signal.

[0171] S12: The feeder cable transmits the processed signal to the space signal processing module, where the analog signal processing unit performs impedance matching, bandpass filtering, amplitude adjustment, and limiting.

[0172] S13: The digital signal processing unit performs AD sampling on the analog signal and realizes station signal detection and standard zero-crossing point identification through search acquisition, station identification, and period identification.

[0173] S14: The parameter measurement unit calculates the field strength, signal-to-noise ratio, envelope difference, and timing difference; the data statistical analysis module performs statistical analysis and reports the measurement data.

[0174] The direct sampling method for testing includes the following steps:

[0175] S21: Match and adjust the externally input Roland C signal and transmit it to the direct sampling signal processing module;

[0176] S22: The analog-to-digital conversion unit directly samples the signal using AD, and the second digital signal processing unit combines the external GTP signal to achieve period recognition;

[0177] S23: The second parameter measurement unit calculates the signal level, envelope difference, waveform parameters, zero-crossing stability, and modulation accuracy. The data statistical analysis module performs statistical analysis and reports on the measurement data.

[0178] In this detection method, during digital signal processing, an FIR bandpass filter is used for signal preprocessing. The FIR filter has a passband range of 75kHz-125kHz, an order of 90, and uses the equiripple optimal approximation method. The passband cutoff frequencies are 75kHz and 125kHz, and the stopband cutoff frequencies are 70kHz and 130kHz. The maximum passband attenuation is 1dB, and the minimum stopband attenuation is 60dB. Furthermore, in carrier frequency calculation, the least squares fitting method is used to determine the zero-crossing point of the Loran C signal. The specific steps are as follows:

[0179] S31: Select the two closest sampling points (one positive and one negative) to the x-axis and construct the prediction function f(x) = ax + b; S32: Calculate the parameters a and b using the least squares algorithm.

[0180] This minimizes the overall error L=Σ(f(xi)-yi)², where xi is the x-coordinate of the point to be fitted and yi is the y-coordinate of the point to be fitted.

[0181] S33: Based on the fitting function f(x)=ax+b, calculate the intersection point with the x-axis to obtain the zero-crossing time t;

[0182] S34: Calculate the zero-crossing point t6 from the 6th half-cycle to the zero-crossing point t12 of the 12th half-cycle. 12 The average of the three cycle lengths is then converted into a carrier frequency.

[0183] Carrier frequency calculation

[0184] The ideal eLORAN pulse has a carrier frequency of 100kHz. After accurately calculating the pulse carrier frequency, it needs to be sent to the pulse control unit for transmitter tuning. Accurate calculation of the carrier frequency is crucial for the transmitter to transmit pulses.

[0185] The method for calculating the eLORAN pulse carrier frequency by the waveform quality monitoring unit is defined as follows (in microseconds):

[0186] Calculate the upper zero-crossing point t6 from the 6th half-cycle to the 12th half-cycle of the eLORAN pulse. 12 All zero-crossing points within this time period (4 in total), the time length between two adjacent zero-crossing points is 1 pulse period Δt (3 in total).

[0187] (2) Calculate the lengths Δt1, Δt2, and Δt3 of the three cycles respectively;

[0188] (3) After averaging the three cycle lengths, the result is converted into a carrier frequency, expressed as:

[0189] kHz

[0190] The pulse period Δt of an ideal eLORAN pulse is 10µs. The key to calculating the pulse carrier frequency is to accurately calculate the time difference Δt, and the key to calculating the time difference Δt is to accurately calculate the upper zero crossing point t of the eLORAN pulse.

[0191] The carrier frequency is calculated by determining the zero-crossing point of the pulse, then calculating the pulse period, and finally converting it into the carrier frequency. Therefore, the accuracy of the zero-crossing point calculation determines the accuracy of the carrier frequency calculation.

[0192] The zero-crossing point of the eLORAN pulse is calculated using the least squares fitting method. The accuracy of the obtained zero-crossing point of the eLORAN pulse depends on the accuracy of the ADC sampling points of the waveform quality monitoring unit.

[0193] When the eLORAN pulse signal is converted into a digital signal, it is affected by in-band noise, which causes the ADC to acquire the signal randomly, making it impossible to accurately acquire the position of the pulse zero crossing point.

[0194] However, there are always two sampling points (sampling point 1 and sampling point 2) that are closest to the x-axis, one positive and one negative. Now, for these two points, the least squares fitting method is used to accurately calculate the zero-crossing point of the eLORAN pulse.

[0195] Let f(x) i )=ax i +b is used as the predicted value, and the prediction function f(x) is calculated from the sampling points. i That is, to calculate the unknown parameters a and b. Now define (f(x)) i )-y i )² represents the single-point error, and the total error is:

[0196] L=2(f(x i )-y i )²(5-2)

[0197] Therefore, there always exists a pair of parameters a and b that minimizes the overall error L, and the prediction function at this point can be considered to be...

[0198] f(x) = ax + b provides the best fit and represents the true value.

[0199] method:

[0200] Target:

[0201]

[0202] By taking the partial derivatives of a and b with respect to L and setting them to 0, the minimum value can be found.

[0203]

[0204]

[0205] Find the value of b:

[0206]

[0207]

[0208] Find the value of a:

[0209]

[0210]

[0211] (5-10)

[0212]

[0213]

[0214] Ultimately, we can obtain:

[0215] in:

[0216]

[0217]

[0218] Based on the sampling points, the fitted linear function for the zero-crossing point of the Roland pulse is obtained, as shown in the following equation:

[0219]

[0220]

[0221]

[0222] In the formula:

[0223] x i The x-coordinate of the point to be fitted;

[0224] y i The ordinate of the point to be fitted;

[0225] The average value of the x-coordinates of the points to be fitted;

[0226] The average value of the ordinate of the point to be fitted.

[0227] According to the formula for fitting a straight line We can find its intersection with the x-axis using the following formula:

[0228]

[0229] The intersection point is the zero-crossing point t of the eLORAN pulse.

[0230] Based on this, the zero-crossing point t5 of the 6th half-cycle of the eLORAN pulse to the zero-crossing point t of the 12th half-cycle can be accurately calculated. 12 For all 4 points that cross zero during this time period, the pulse period Δt (3 points in total) can be calculated.

[0231] The carrier frequency can then be calculated using formula (5-1).

[0232] The broadcast monitoring unit uses a 50MHz ADC to sample the received eLORAN pulses. Due to in-band noise interference, the sampling point position of the ADC will be random, so the zero-crossing point of the calculated eLORAN pulse will have jitter error.

[0233] Calculation of time to reach peak

[0234] The pulse arrival time to peak point refers to the time between the pulse start point and the pulse peak point, and its calculation expression is:

[0235] △t p =65+τ

[0236] In the formula:

[0237] τ represents the envelope difference. The arrival time of the standard eLORAN pulse at the peak point is shown in the figure below.

[0238] In the waveform quality monitoring unit, the ADC is activated to acquire the eLORAN analog signal under the control of the exciter unit's MPT, PCSET, and PCRESET. The time between the acquired pulse start point and peak value is considered the pulse-to-peak time. The pulse-to-peak time reflects the length of the eLORAN pulse leading edge, which is helpful in determining the waveform's correctness.

[0239] Pulse delay

[0240] For a standard eLORAN signal, when t > 500 μs, the pulse delay should satisfy the following expression:

[0241] i(t) is less than or equal to 0.0014A

[0242] In the formula:

[0243] A – Pulse peak current.

[0244] We need to determine the percentage of the current value (maximum and average) relative to the peak envelope during the pulse delay (t>500μs):

[0245]

[0246]

[0247] In the formula:

[0248] T represents the duration of the eLORAN pulse.

[0249] The post-delay of a standard eLORAN pulse is shown in the figure below:

[0250] The pulse trailing edge indicator mainly reflects the trailing convergence of the eLORAN pulse signal, which is helpful in judging the correctness of the pulse waveform.

[0251] Pulse carrier frequency zero-crossing consistency calculation

[0252] Calculate all zero-crossing points between the first zero-crossing point t1 and the fifth zero-crossing point t5 of a certain eLORAN pulse, and thus calculate the length of the three cycles.

[0253] The zero-crossing time peak-to-peak value and standard deviation of the pulse carrier frequency within the range of τ+10 ≤ t ≤ τ+50 (τ is the envelope difference) are defined as follows:

[0254] △t 峰-峰 =max{△t}-min{△t}

[0255]

[0256] Pulse carrier frequency consistency is measured by the peak-to-peak value and standard deviation at the zero-crossing time.

[0257] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0258] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A Loran-C signal detection system characterized by: The system comprises a tester host and a signal receiving module; the tester host comprises a space signal processing module, a direct sampling signal processing module, a data statistical analysis module and a power module; the signal receiving module comprises a receiving antenna, an attenuator and a feeder cable; The space signal processing module is used for analog signal processing, digital signal processing and parameter measurement of the space Loran C signal obtained by the receiving antenna; the direct sampling signal processing module is used for analog-digital conversion, digital signal processing and parameter measurement of the externally directly input Loran C signal; the data statistical analysis module is used for statistical analysis, display and reporting of the measurement data output by the two processing modules; and the power module provides stable power supply for each module of the system.

2. A Loran-C signal detection system according to claim 1, characterised in that: The space signal processing module comprises an analog signal processing unit, a digital signal processing unit, a parameter measurement unit and a first power unit; The analog signal processing unit is sequentially provided with an impedance matching circuit, an active band-pass filter, an adjustable gain control chip and a limiter, and is used for impedance matching, band-pass filtering, amplitude adjustment and limiting of the signal transmitted by the receiving antenna; The digital signal processing unit comprises an A / D converter, a SOC processor chip and a clock control circuit, and is used for analog-digital conversion, digital filtering and denoising, signal searching and capturing, carrier phase tracking, period identification and information demodulation and decoding of the signal; The parameter measurement unit is used for calculating the signal-to-noise ratio, field strength, period difference and timing time difference of the Loran C signal; and the first power unit supplies power to the analog signal processing unit, the digital signal processing unit and the parameter measurement unit.

3. A Loran C signal detection system according to claim 1, characterized in that: Anti-interference technology and envelope delay correlation detection technology are adopted in the digital signal processing process of the digital signal processing unit; The anti-interference technology is realized based on an LMS adaptive notch filter; the LMS adaptive notch filter receives an original input signal containing narrowband single-frequency interference and a pure sine reference signal, dynamically adjusts the weight vector coefficient through an LMS algorithm, and outputs a signal in which the single-frequency interference is eliminated, and the convergence condition is that the step factor is less than the reciprocal of the maximum eigenvalue of the input signal autocorrelation matrix; When the signal contains sinusoidal perturbation with frequency , the adaptive notch filter can eliminate the disturbance reasonably; the original input signal is composed of the desired signal and narrowband single-frequency interference, and is a pure sinusoidal reference signal related to the single-frequency interference, and the two signals are , that is: In combination with the LMS algorithm, the output signal is: The formula of the weight vector coefficient can be obtained as follows: In the formula is a step factor, is the output signal after interference rejection by the adaptive notch filter; The LMS algorithm is a recursive operation, which does not need prior knowledge of the statistical characteristics of the signal, but only uses the instantaneous estimate value of the signal, and the weight coefficient is estimated, but with the increase of time, the weight coefficient is gradually adjusted, and the estimate value is gradually improved, and finally the convergence value is obtained, and the convergence condition is that: wherein is the largest eigenvalue of the input signal autocorrelation matrix; The envelope delay correlation detection technology obtains the co-directional branch signal and the quadrature branch signal by mixing the received Loran C signal with the local quadrature carrier and low-pass filtering, squares and sums the two signals to obtain the signal envelope, and then delays the signal envelope by 1 GRI and performs correlation accumulation with a length of L to enhance the expected signal envelope and suppress noise and cross interference; The core of the envelope delay correlation detection method is to obtain the correlation peak through delay correlation accumulation operation, so as to realize the detection of the pulse signal group; it is assumed that the received Loran C signal is as follows: wherein for receiving a Loran C signal envelope, for a signal carrier, the received Loran C signal is mixed with a local quadrature carrier: The mixing results are as follows: The phase difference between the local carrier and the received Loran C signal is denoted as φ. The in-phase branch signal and the quadrature branch signal obtained by low-pass filtering the mixing results are as follows: The signal envelope can be obtained by squaring and summing the co-directional branch signal and the quadrature branch signal obtained by filtering as follows: As can be seen from the above equation, the phase difference between the local carrier and the received signal is eliminated At the same time, the phase encoding in the pulse signal is also eliminated, and a positive pulse signal is obtained; Because the signal envelope and the correlation peak generated by correlation of the Loran C signal have the same repetition period, i.e., GRI, the signal envelope is delayed by 1 GRI and then accumulated for a correlation accumulation length L to obtain an accumulated result as follows: where is the correlation length; is the desired station GRI; since the Loran C signal repeats on a GRI cycle, there is a strong correlation between the desired GRI signal envelopes, a weak correlation between the noise, and a weak correlation between the cross interference and the noise, so the desired signal envelope is enhanced; the result is represented as: In the formula, is the noise after correlation.

4. A Loran C signal detection system according to claim 1, characterized in that: The direct-sampling signal processing module comprises an analog-to-digital conversion unit, a second digital signal processing unit, a second parameter measurement unit and a second power supply unit; The analog-to-digital conversion unit adopts a high-precision high-speed analog-to-digital converter ADI, supports double-channel, 16-bit and maximum 125M data rate signal acquisition; The second digital signal processing unit combines with the externally input GTP signal to assist in realizing fast cycle identification of the Loran C signal, directly performs carrier phase tracking, information demodulation and decoding and timing signal generation, and omits the steps of digital filtering and automatic search; The second parameter measurement unit is used to realize single-pulse group signal parameter measurement, pulse group signal parameter measurement and data modulation precision measurement; and the second power supply unit supplies power to each unit of the direct-sampling signal processing module.

5. A Loran C signal detection system according to claim 4, characterized in that: The single-pulse group signal parameter measurement of the second parameter measurement unit comprises half-cycle amplitude tolerance calculation, half-cycle amplitude overall tolerance calculation, pulse waveform amplitude stability calculation, pulse trailing edge calculation, reference zero-crossing point time variation calculation, reference zero-crossing point stability calculation, radio frequency zero-crossing point stability calculation and peak point time calculation; The half-cycle amplitude tolerance calculation satisfies: |IN-SN| is less than or equal to 0.03 and |IN-SN| is less than or equal to 0.10, wherein IN is a normalized value of an actual waveform front edge Nth half-cycle relative to a pulse peak value amplitude, and SN is a normalized value of a theoretical waveform front edge Nth half-cycle relative to a pulse peak value amplitude; The half-cycle amplitude overall tolerance is realized by calculating the deviation sum of the normalized values of the actual and theoretical waveform front edges of the first 8 half-cycles; The half-cycle amplitude overall tolerance should satisfy the following formula: In the formula: I N - the normalized value of the actual waveform front Nth half cycle relative to the peak amplitude of the pulse; S N - is the normalized value of the relative pulse peak amplitude for the Nth half cycle of the theoretical waveform front; The first 8 half-cycles of the Loran C pulse contain the third zero-crossing point, and the half-cycle amplitude overall tolerance reflects the overall peak value amplitude difference between the actual Loran C pulse front edge and the ideal Loran C pulse front edge, which is beneficial to judging the waveform correctness of the Loran C pulse and the stability of the third zero-crossing point, and the smaller the half-cycle amplitude overall tolerance is, the smaller the difference between the actual Loran C pulse and the ideal Loran C pulse is, and the more stable the third zero-crossing point is, The pulse waveform amplitude stability requires that, in a group of n≥20 measurements, the peak-peak variation of any half-cycle peak value amplitude is less than 0.04A, and the relative variation (standard deviation) is less than 0.01A, A being the pulse peak current; The pulse trailing edge satisfies i(t) less than or equal to 0.0014A when t>500μs, and the percentage of the trailing edge current value relative to the envelope peak value is calculated; The reference zero-crossing point time variation is less than 60ns when working at a single repetition rate and is less than 140ns when working at a double repetition rate; The reference zero-crossing point stability has a peak-peak variation less than 50ns when working at a single repetition rate and a peak-peak variation less than 140ns when working at a double repetition rate; The radio frequency zero-crossing stability is achieved by calculating the peak-to-peak value and the standard deviation of the zero-crossing time in the range of τ+10≤t≤τ+50, the peak-to-peak variation is less than 50 ns and the standard deviation is less than 20 ns when working at a single repetition rate, the peak-to-peak variation is less than 150 ns and the standard deviation is less than 50 ns when working at a double repetition rate; the time to peak is calculated by Δtp=65+τ, where τ is the pulse period difference.

6. A Loran C signal detection system according to claim 4, wherein: The pulse group signal parameter measurement of the second parameter measurement unit includes pulse-to-pulse peak amplitude tolerance calculation and pulse-to-pulse pulse period difference tolerance calculation; The pulse-to-pulse peak amplitude tolerance is calculated by D=(Ipkmx-Ipkmin) / Ipkmx, where Ipkmx is the maximum peak value of the pulses in the group, and Ipkmin is the minimum peak value of the pulses in the group, the tolerance is less than 5% when working at a single repetition rate, and the tolerance is less than 10% when working at a double repetition rate; The pulse-to-pulse pulse period difference tolerance is calculated by D=ECDmx-ECDmin, where ECDmx is the maximum pulse period difference value in the group, and ECDmin is the minimum pulse period difference value in the group, the tolerance is less than 0.5 μs.

7. A Loran C signal detection system according to claim 4, wherein: The data modulation accuracy measurement of the second parameter measurement unit includes Eurofix demodulation and decoding and "ninth pulse" demodulation and decoding; The Eurofix demodulation adopts a 100 MHz clock to collect a trigger signal MPT, measures the arrival time of MPT by edge detection, and analyzes A and B periods to decode 7-bit information in combination with a modulation pattern; the Eurofix decoding performs CRC check and RS error correction by sliding every 30 groups of 7-bit information to realize frame synchronization and effective text output; The "ninth pulse" demodulation judges whether the ninth MPT of the pulse group exists, decodes 5-bit information based on 32 delay offsets; the "ninth pulse" decoding performs companion set decoding and RS error correction by sliding every 24 groups of 5-bit information to realize frame synchronization and effective text output.

8. A Loran C signal detection system according to claim 1, characterized in that: The data statistical analysis module includes an industrial control mainboard, upper computer software, a touch screen and input devices; the industrial control mainboard runs a Win10 operating system; The upper computer software adopts modular design and includes a space signal data statistical analysis module and a direct sampling signal data statistical analysis module; The space signal data statistical analysis module is used to display the station signal capture, tracking and locking state of the space signal processing module, and to display real-time measurement values and statistical results of mean value, variance and root mean square error; the direct sampling signal data statistical analysis module is used to display the measurement parameters of a single pulse group, pulse group and data modulation accuracy of the direct sampling signal processing module.

9. A method of detecting a Loran C signal, characterized by: The detection method includes two modes of space receiving mode detection and direct sampling mode detection; The space receiving mode detection includes the following steps: S11: receiving a space Loran C signal by a receiving antenna, and filtering, amplifying and circuit matching the signal by an internal coupler of the antenna; S12: transmitting the processed signal to a space signal processing module through a feeder cable, and completing impedance matching, band pass filtering, amplitude adjustment and limiting by an analog signal processing unit; S13: The digital signal processing unit AD samples the analog signal, and realizes station signal detection and standard zero-crossing point identification through search capture, station identification and cycle identification; S14: The parameter measurement unit calculates the field strength, signal-to-noise ratio, packet period difference and timing time difference, and the data statistical analysis module statistically analyzes and reports the measurement data; The direct sampling mode detection includes the following steps: S21: The externally directly input Loran C signal is matched and adjusted, and is transmitted to the direct sampling signal processing module; S22: The analog-digital conversion unit directly AD samples the signal, and the second digital signal processing unit realizes cycle identification in combination with the external GTP signal; S23: The second parameter measurement unit calculates the signal level, packet period difference, waveform parameter, zero-crossing point stability and modulation accuracy, and the data statistical analysis module statistically analyzes and reports the measurement data.

10. A method of detecting a Loran-C signal as claimed in claim 9, characterized in that: In the digital signal processing process, the FIR band-pass filter is used for signal preprocessing, the FIR filter passband range is 75kHz-125kHz, the order is 90, the type is the equal-ripple best approximation method, the passband cutoff frequency is 75kHz, 125kHz, the stopband cutoff frequency is 70kHz, 130kHz, the passband maximum attenuation is 1dB, and the stopband minimum attenuation is 60dB; and in the carrier frequency calculation, the least square fitting method is used to determine the Loran C signal zero-crossing point, and the specific steps are: S31: Selecting the nearest one positive and one negative sampling point to the x axis, a prediction function f(x)=ax+b is constructed; S32: The parameters a and b are calculated through the least square algorithm, so that the total error L=Σ(f(xi)-yi)² is minimized, wherein xi is the horizontal coordinate of the to-be-fitted point, and yi is the vertical coordinate of the to-be-fitted point; S33: According to the fitting function f(x)=ax+b, the intersection with the x axis is calculated, and the zero-crossing point time t is obtained. S34: Calculate the length of 3 cycles between the zero-crossing point t6 on the 6th half cycle and the zero-crossing point t 12 on the 12th half cycle, convert the average value to the carrier frequency.

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