CN114244420A - Chirp signal tracking receiver for satellite communication - Google Patents
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Abstract
The invention discloses a Chirp signal tracking receiver for satellite communication, which relates to the field of satellite communication and mainly comprises: determining the signal form, time domain and frequency domain characteristics of satellite communication, and establishing a simulation model according to a tracking mode; designing intermediate frequency and sampling rate, and carrying out digital down-conversion, extraction and low-pass filtering on the signals after A/D sampling; designing a matched filter to obtain signal peak value distribution and carrying out frequency rough estimation on frequency offset of a Chirp signal; the method comprises the steps of storing a single-period Chirp signal, obtaining a frequency accurate estimation result through frequency scanning, and performing frequency correction; and locking and judging the signal after frequency correction according to the signal characteristics, synchronously demodulating the single-channel single-pulse signal, separating sum and difference signals, and sending the obtained error information to an antenna servo system for self-tracking. The invention utilizes a Chirp signal extraction combined single-channel single-pulse tracking mode, can enable the antenna system to achieve high-precision tracking performance, and is suitable for self-tracking of beacon-free satellite signals.
Description
Technical Field
The invention relates to a self-tracking system in the field of satellite communication, in particular to a method for tracking a satellite communication signal by using a Chirp signal as a frequency correction signal, wherein a single-channel single-pulse tracking mode is adopted, and the method is particularly suitable for designing a high-precision tracking receiver of a beacon-free satellite communication system.
Background
In the conventional satellite communication system, the antenna self-tracking technology ensures that a communication antenna beam assembled on a moving carrier such as an airplane, a ship, a vehicle and the like is always aligned with a satellite through a certain tracking means, so that stable communication is ensured. The antenna self-tracking system can be roughly divided into a program tracking system, a stepping tracking system, a cone scanning tracking system and a single-pulse tracking system, and several tracking modes have characteristics in the aspects of performance, implementation method and the like, but have respective advantages and disadvantages.
(1) The program tracking system calculates the difference between the angle value pointed by the antenna and the actual pointed angle value obtained by the current sensor according to the actual time, and controls the servo unit by using the difference value to enable the difference value to be zero. The tracking mode does not need signal feedback, the equipment is simplest, and in other tracking modes, the program guide is generally used as an initial acquisition means of a satellite communication antenna, but the tracking precision is low when the tracking mode is used alone, and the depth depends on high-precision inertial navigation in the tracking application of a mobile carrier, so that the tracking performance requirement of a system cannot be met generally.
(2) The maximum level of a received beacon signal when an antenna points to a satellite is the basis, and the maximum level belongs to maximum value tracking. The tracking mode uses signal feedback as the basis of antenna tracking closed loop, and needs to use a step tracking receiver to resolve signal power, but because the maximum value of the signal needs to be searched through step motion, the tracking speed is slow, so although the tracking precision is higher than that of program tracking, the tracking method still can not meet the requirements in some application occasions requiring fast and high-precision tracking.
(3) The basic idea of the cone scanning tracking system is that an antenna feed source is artificially deviated from the visual axis of an antenna by an angle and rotates around the visual axis of the antenna, when a satellite deviates from the visual axis, a beacon signal received by a receiving channel is added with amplitude modulation with the same rotating frequency as a wave beam compared with an original signal, a tracking receiver obtains azimuth and pitching error voltage through synchronous demodulation, a servo system drives the antenna to move towards the direction of reducing the error until the error is minimum, and the antenna is aligned with the satellite. The cone scanning tracking mode is the earliest difference self-tracking mode, the tracking precision is higher than that of stepping tracking, but the antenna feed source is always deviated from the visual axis, so that the gain is reduced, and meanwhile, the mechanical rotation of the antenna minor plane is required, so that the equipment quantity is increased.
(4) The monopulse tracking system is divided into amplitude-comparison monopulse and phase-comparison monopulse according to a comparison mode, and is divided into multimode monopulse tracking and multi-horn monopulse tracking according to a feed source mode. The principle is that the self-tracking is realized by utilizing the characteristic that an electric field directional diagram of the antenna is zero in the axial direction of the antenna and has polarity in the off-axis direction, and the self-tracking system is a high-precision difference self-tracking system and is particularly suitable for the tracking of a satellite antenna needing quick high-precision tracking.
However, in the above various tracking methods, except for program tracking, the conventional satellite communication system needs to use the beacon (single carrier) level received when the antenna beam points to the satellite as the basis in the conventional tracking methods, and obtain the maximum signal value, the azimuth and the pitch error information by means of maximum search, amplitude or phase modulation signal synchronous demodulation, etc., so that the antenna beam is aligned to the satellite direction.
With the use of satellite mobile communication systems such as the skynet series, new signal tracking requirements appear, and conventional beacon (single carrier) signals do not exist in satellite channels, so that the conventional beacon tracking mode loses judgment basis.
Disclosure of Invention
In view of the above, the invention provides a Chirp signal tracking receiver for satellite communication, which finds that a frequency correction Chirp signal in a channel can be used for antenna tracking by analyzing channel characteristics, and can enable an antenna system to achieve higher tracking performance by combining a single-channel single-pulse tracking mode, thereby solving the problem of fast and high-precision tracking of beacon-free satellite signals.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
a satellite communication chirp signal tracking receiver comprises an A/D sampling module, a first digital down-conversion module, a first extraction and filtering module, a data storage module, a frequency scanning and frequency deviation estimation module, a second digital down-conversion module, a second extraction and filtering module, a first matched filtering module, a signal peak value extraction module, an orthogonal demodulation module, a synchronous modulation signal generator, a low-pass filtering module, an azimuth difference and elevation difference signal normalization module, a digital phase correction module, a signal characteristic detection module, a locking detection module and a signal-to-noise ratio detection module;
the A/D sampling module receives an intermediate frequency signal at the radio frequency front end of the antenna, performs A/D sampling and outputs the signal to the first digital down-conversion module without distortion; the first digital down-conversion module carries out first-stage digital down-conversion on the digital signal and then outputs the digital signal to the second digital down-conversion module and the first extraction and filtering module respectively; the first extraction and filtering module extracts and low-pass filters the input signal and then stores the input signal into the data storage module; the frequency scanning and frequency deviation estimation module extracts stored data to perform matched filtering, frequency rough estimation and frequency fine estimation, a matched filtering result is output to the signal characteristic detection module, and a frequency deviation estimation result is output to the second digital down-conversion module; the signal characteristic detection module carries out signal characteristic detection on the signals after matching and filtering, and the frequency of output signals shifts to the locking detection module and the servo control unit; the second digital down-conversion module carries out secondary down-conversion on the signal output by the first digital down-conversion module and outputs the baseband signal subjected to frequency deviation correction to the second extraction and filtering module; the second decimation and filtering module performs decimation and low-pass filtering on the input signal and then outputs the input signal to the first matched filtering module; the first matched filtering module performs time domain matched filtering on the input signal according to the signal characteristics and outputs a result to the sum signal peak value extraction module; the signal peak value extraction module is used for carrying out peak value detection on the input signal and then respectively outputting a sum signal peak value to the signal-to-noise ratio detection module, the orthogonal demodulation module, the azimuth difference and pitch difference signal normalization module and the servo control unit; the signal-to-noise ratio detection module estimates the signal-to-noise ratio of the signal after peak detection and outputs the signal-to-noise ratio of the Chirp signal to the locking detection module and the servo control unit; the locking detection module performs locking judgment by utilizing the signal-to-noise ratio and the signal frequency offset and outputs a locking instruction to the servo control unit; the quadrature demodulation module carries out quadrature demodulation by using the sum signal peak value and the synchronous modulation signal output by the synchronous modulation signal generator, and outputs a quadrature demodulation result to the low-pass filtering module; after the low-pass filtering module filters high-frequency components, the azimuth difference signal and the pitch difference signal are output to the azimuth difference and pitch difference signal normalization module; the azimuth difference and pitch difference signal normalization module normalizes peak values of the azimuth difference signal, the pitch difference signal and the sum signal and outputs a normalization result to the digital phase correction module; the digital phase correction module receives the calibration phase sent by the servo control unit, performs phase correction on the azimuth difference and the pitch difference signals, removes cross coupling influence, and outputs the azimuth difference and the pitch difference to the servo control unit.
The frequency estimation function is completed by a data storage module and a frequency scanning and frequency offset estimation module together, wherein the data storage module comprises a data cache module, a DPRAM and a data selection module, and the frequency scanning and frequency offset estimation module comprises a frequency offset correction module, a second matched filtering module, a peak detection module, a frequency offset estimation module and a DDS module;
the data cache module receives the digital signals output by the first extraction and filtering module, stores the data into the DPRAM, and outputs the data scanning mark to the data selection module after the data is fully written; the data selection module reads the signals stored in the DPRAM according to the scanning marks and outputs the signals to the frequency deviation correction module; the frequency deviation correction module performs frequency correction according to the frequency control word output by the DDS module and outputs the corrected data to the second matched filtering module; the second matched filtering module performs time domain matched filtering on the corrected data according to the Chirp signal characteristics and outputs a result to the peak value detection module; the peak detection module carries out peak detection on the matched filtering output signal and outputs the time interval between peaks to the frequency offset estimation module; the frequency offset estimation module estimates a frequency difference delta f through a time interval delta t between peak values and outputs the updated frequency difference to the DDS module; the DDS module outputs frequency control words according to frequency difference control, and performs 4 frequency stepping scanning, wherein the maximum value of stepping is 8.192kHz, and the minimum value is 0.25 Hz.
The first matched filtering module and the second matched filtering module are obtained by cascading 8 groups of filters, and each group of filters is provided with a feedback port.
The synchronous modulation signal input by the orthogonal demodulation module needs to be synchronous with the modulation square wave, and the frequency of the modulation square wave is more than or equal to 16 kHz.
The DPRAM in the data cache module is 64ms in storage depth and is divided into four areas, each area comprises a non-overlapping data area of 8ms, an overlapping data area overlapping with other areas is respectively arranged in front of and behind the non-overlapping data area, and after data in a certain 24ms area is written to the full area, a data scanning mark is output to the data selection module.
Compared with the background technology, the invention has the following beneficial effects:
(1) the invention utilizes the Chirp signal in the satellite FCCH channel to track, is suitable for the non-beacon satellite signal tracking, and provides a solution for tracking time division and linear frequency modulation signals in the satellite channel.
(2) According to the invention, the frequency offset estimation is carried out on the Chirp signal by a full time domain matched filtering method, and meanwhile, the signal power is extracted by utilizing the peak level, so that the digital signal processing operand is reduced.
(3) According to the invention, the time division characteristics of signals in a satellite communication channel are utilized to perform high-speed signal processing in a time interval, and frequency scanning is performed on sampling data for multiple times, so that the frequency offset estimation precision is improved, a more accurate signal peak value is obtained, and the method can be used for initial signal capture and coordinate calibration correction in an uncalibrated state.
(4) According to the method, the tracking error signal levels of the azimuth direction and the pitching direction of the antenna are obtained by combining Chirp signal resolving with a single-pulse tracking mode, so that the automatic tracking of the mobile carrier is not dependent on high-precision inertial navigation, and the antenna tracking precision is improved.
(5) The tracking of the Chirp signal can be realized by utilizing a traditional satellite communication antenna tracking platform, the algorithm implementation mode is simple and feasible, the upgrading is convenient, and the system cost is effectively reduced.
In a word, the invention has the advantages of ingenious conception, clear thought and easy realization, not only solves the antenna self-tracking problem of the beacon-free satellite signal and improves the tracking precision, but also effectively saves the cost, and is an important improvement to the prior art.
Drawings
FIG. 1 is a schematic representation of an exemplary signal for a satellite broadcast channel of the present invention;
fig. 2 is a schematic block diagram of a Chirp signal tracking receiver of the present invention;
FIG. 3 is a schematic block diagram of frequency estimation according to an embodiment of the present invention;
FIG. 4 is a schematic block diagram of a matched filter in an embodiment of the present invention;
FIG. 5 shows the result of dual peaks of the matched filter output due to the presence of the Chirp signal frequency difference in the embodiment of the present invention;
fig. 6 shows a result that the Chirp signal frequency offset estimation is completed, and after frequency scanning and frequency offset estimation, matched filtering is output as a single peak in the embodiment of the present invention;
fig. 7 shows the result of synchronously demodulating the single-channel single-pulse signal to obtain the sum and difference signals and maintaining the sum and difference signals in the embodiment of the present invention.
Detailed Description
The invention is further described with reference to the following detailed description and accompanying drawings.
Before tracking a receiver, determining a signal form, time domain and frequency domain characteristics of satellite communication, analyzing a Chirp signal expression and establishing a simulation model;
fig. 1 is a schematic diagram of a typical signal of a satellite broadcast channel in an embodiment, where signal 1 is an FCCH (Chirp) signal, the time length is 7.5ms, the bandwidth is 15.36kHz, and the period is 480 ms; the signal 2 is a pi/4-CQPSK signal, the time is 15ms, the period is 480ms, the symbol rate is 16kBps, and the bandwidth is 21.6 kHz; other time signals vary with traffic. In the embodiment, a Chirp signal is selected as a tracking object, a simulation model is established in Matlab according to a signal expression, and fig. 2 is a schematic block diagram of a Chirp signal tracking receiver.
Then, the tracking mode is determined according to the requirement of tracking precision, and the form of the single-channel modulation signal is determined.
In the embodiment, a high-precision single-channel single-pulse tracking mode is used, a Chirp signal bandwidth is referred, and a single-channel converter in the radio-frequency front end of the antenna is designed to modulate a signal waveform into a square wave, wherein the frequency of the square wave is more than or equal to 16 kHz.
And then, an appropriate intermediate frequency and a sampling rate are designed, so that the signals enter a tracking receiver without distortion after A/D sampling.
In the embodiment, the intermediate frequency signal enters the tracking receiver after being subjected to A/D sampling to start a digital signal processing flow, and the embodiment selects a band-pass sampling mode to process because the signal bandwidth is smaller than the intermediate frequency.
As shown in fig. 2, an embodiment of a Chirp signal tracking receiver for satellite communication according to the present invention includes an a/D sampling module, a first digital down-conversion module, a first decimation and filtering module, a data storage module, a frequency scanning and frequency offset estimation module, a second digital down-conversion module, a second decimation and filtering module, a first matched filtering module, a signal peak extraction module, an orthogonal demodulation module, a synchronous modulation signal generator, a low-pass filtering module, an azimuth difference and pitch difference signal normalization module, a digital phase correction module, a signal characteristic detection module, a lock detection module, and a signal-to-noise ratio detection module;
the A/D sampling module receives an intermediate frequency signal at the radio frequency front end of the antenna, performs A/D sampling and outputs the signal to the first digital down-conversion module without distortion; the first digital down-conversion module carries out first-stage digital down-conversion on the digital signal and then outputs the digital signal to the second digital down-conversion module and the first extraction and filtering module respectively; the first extraction and filtering module extracts and low-pass filters the input signal and then stores the input signal into the data storage module; the frequency scanning and frequency deviation estimation module extracts stored data to perform matched filtering, frequency rough estimation and frequency fine estimation, a matched filtering result is output to the signal characteristic detection module, a frequency deviation estimation result is output to the second digital down-conversion module, the output of the matched filter is double peaks due to the existence of a Chirp signal frequency difference before frequency scanning and frequency deviation estimation, as shown in FIG. 5, and the output of the matched filtering is single peak after frequency scanning and frequency deviation estimation is completed, as shown in FIG. 6; the signal characteristic detection module carries out signal characteristic detection on the signals after matching and filtering, and the frequency of output signals shifts to the locking detection module and the servo control unit; the second digital down-conversion module carries out secondary down-conversion on the signal output by the first digital down-conversion module and outputs the baseband signal subjected to frequency deviation correction to the second extraction and filtering module; the second decimation and filtering module performs decimation and low-pass filtering on the input signal and then outputs the input signal to the first matched filtering module; the first matched filtering module performs time domain matched filtering on the input signal according to the signal characteristics and outputs a result to the sum signal peak value extraction module; the signal peak value extraction module is used for carrying out peak value detection on the input signal and then respectively outputting a sum signal peak value to the signal-to-noise ratio detection module, the orthogonal demodulation module, the azimuth difference and pitch difference signal normalization module and the servo control unit; the signal-to-noise ratio detection module estimates the signal-to-noise ratio of the signal after peak detection and outputs the signal-to-noise ratio of the Chirp signal to the locking detection module and the servo control unit; the locking detection module performs locking judgment by utilizing the signal-to-noise ratio and the signal frequency offset and outputs a locking instruction to the servo control unit; the quadrature demodulation module performs quadrature demodulation by using the sum signal peak value and the synchronous modulation signal output by the synchronous modulation signal generator, and outputs a quadrature demodulation result to the low-pass filtering module, wherein the quadrature demodulation result is shown in fig. 7; after the low-pass filtering module filters high-frequency components, outputting an azimuth difference signal and a pitch difference signal to an azimuth difference and pitch difference signal normalization module, wherein an input synchronous modulation signal needs to be synchronous with a modulation square wave, and the frequency of the modulation square wave is more than or equal to 16 kHz; the azimuth difference and pitch difference signal normalization module normalizes peak values of the azimuth difference signal, the pitch difference signal and the sum signal and outputs a normalization result to the digital phase correction module; the digital phase correction module receives the calibration phase sent by the servo control unit, performs phase correction on the azimuth difference and the pitch difference signals, removes cross coupling influence, and outputs the azimuth difference and the pitch difference to the servo control unit.
As shown in fig. 3, the frequency estimation function is performed by a data storage module and a frequency scanning and frequency offset estimation module together, where the data storage module includes a data cache module, a DPRAM and a data selection module, and the frequency scanning and frequency offset estimation module includes a frequency offset correction module, a second matched filter module, a peak detection module, a frequency offset estimation module and a DDS module;
the data cache module receives the digital signals output by the first extraction and filtering module, stores the data into a DPRAM (dual-port random access memory), the DPRAM is divided into four areas, each area comprises an 8ms non-overlapping data area, an overlapping data area which is overlapped with other areas is respectively arranged in front of and behind the non-overlapping data area, and after the data in a certain 24ms area is written, the data cache module outputs a data scanning mark to the data selection module; the data selection module reads the signals stored in the DPRAM according to the scanning marks and outputs the signals to the frequency deviation correction module; the frequency deviation correction module performs frequency correction according to the frequency control word output by the DDS module and outputs the corrected data to the second matched filtering module; the second matched filtering module performs time domain matched filtering on the corrected data according to the Chirp signal characteristics and outputs a result to the peak value detection module; the peak detection module carries out peak detection on the matched filtering output signal and outputs the time interval between peaks to the frequency offset estimation module; the frequency offset estimation module estimates a frequency difference delta f through a time interval delta t between peak values and outputs the updated frequency difference to the DDS module; the DDS module outputs frequency control words according to frequency difference control, and performs 4 frequency stepping scanning, wherein the maximum value of stepping is 8.192kHz, and the minimum value is 0.25 Hz.
Fig. 4 is a schematic block diagram of a matched filter, where the first matched filtering module and the second matched filtering module are cascaded by 8 sets of filters, and each set of filters has a feedback port.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and all simple modifications, variations and equivalent changes made to the above embodiment according to the technical spirit of the present invention still fall within the protection scope of the technical solution of the present invention.
Claims (5)
1. A satellite communication chirp signal tracking receiver is characterized by comprising an A/D sampling module, a first digital down-conversion module, a first extraction and filtering module, a data storage module, a frequency scanning and frequency offset estimation module, a second digital down-conversion module, a second extraction and filtering module, a first matched filtering module, a signal peak extraction module, an orthogonal demodulation module, a synchronous modulation signal generator, a low-pass filtering module, an azimuth difference and elevation difference signal normalization module, a digital phase correction module, a signal characteristic detection module, a locking detection module and a signal-to-noise ratio detection module;
the A/D sampling module receives an intermediate frequency signal at the radio frequency front end of the antenna, performs A/D sampling and outputs the signal to the first digital down-conversion module without distortion; the first digital down-conversion module carries out first-stage digital down-conversion on the digital signal and then outputs the digital signal to the second digital down-conversion module and the first extraction and filtering module respectively; the first extraction and filtering module extracts and low-pass filters the input signal and then stores the input signal into the data storage module; the frequency scanning and frequency deviation estimation module extracts stored data to perform matched filtering, frequency rough estimation and frequency fine estimation, a matched filtering result is output to the signal characteristic detection module, and a frequency deviation estimation result is output to the second digital down-conversion module; the signal characteristic detection module carries out signal characteristic detection on the signals after matching and filtering, and the frequency of output signals shifts to the locking detection module and the servo control unit; the second digital down-conversion module carries out secondary down-conversion on the signal output by the first digital down-conversion module and outputs the baseband signal subjected to frequency deviation correction to the second extraction and filtering module; the second decimation and filtering module performs decimation and low-pass filtering on the input signal and then outputs the input signal to the first matched filtering module; the first matched filtering module performs time domain matched filtering on the input signal according to the signal characteristics and outputs a result to the sum signal peak value extraction module; the signal peak value extraction module is used for carrying out peak value detection on the input signal and then respectively outputting a sum signal peak value to the signal-to-noise ratio detection module, the orthogonal demodulation module, the azimuth difference and pitch difference signal normalization module and the servo control unit; the signal-to-noise ratio detection module estimates the signal-to-noise ratio of the signal after peak detection and outputs the signal-to-noise ratio of the Chirp signal to the locking detection module and the servo control unit; the locking detection module performs locking judgment by utilizing the signal-to-noise ratio and the signal frequency offset and outputs a locking instruction to the servo control unit; the quadrature demodulation module carries out quadrature demodulation by using the sum signal peak value and the synchronous modulation signal output by the synchronous modulation signal generator, and outputs a quadrature demodulation result to the low-pass filtering module; after the low-pass filtering module filters high-frequency components, the azimuth difference signal and the pitch difference signal are output to the azimuth difference and pitch difference signal normalization module; the azimuth difference and pitch difference signal normalization module normalizes peak values of the azimuth difference signal, the pitch difference signal and the sum signal and outputs a normalization result to the digital phase correction module; the digital phase correction module receives the calibration phase sent by the servo control unit, performs phase correction on the azimuth difference and the pitch difference signals, removes cross coupling influence, and outputs the azimuth difference and the pitch difference to the servo control unit.
2. The satellite communication chirp signal tracking receiver of claim 1, wherein the frequency estimation function is performed by a data storage module and a frequency scanning and frequency offset estimation module together, wherein the data storage module comprises a data cache module, a DPRAM and a data selection module, and the frequency scanning and frequency offset estimation module comprises a frequency offset correction module, a second matched filter module, a peak detection module, a frequency offset estimation module and a DDS module;
the data cache module receives the digital signals output by the first extraction and filtering module, stores the data into the DPRAM, and outputs the data scanning mark to the data selection module after the data is fully written; the data selection module reads the signals stored in the DPRAM according to the scanning marks and outputs the signals to the frequency deviation correction module; the frequency deviation correction module performs frequency correction according to the frequency control word output by the DDS module and outputs the corrected data to the second matched filtering module; the second matched filtering module performs time domain matched filtering on the corrected data according to the Chirp signal characteristics and outputs a result to the peak value detection module; the peak detection module carries out peak detection on the matched filtering output signal and outputs the time interval between peaks to the frequency offset estimation module; the frequency offset estimation module estimates a frequency difference delta f through a time interval delta t between peak values and outputs the updated frequency difference to the DDS module; the DDS module outputs frequency control words according to frequency difference control, and performs 4 frequency stepping scanning, wherein the maximum value of stepping is 8.192kHz, and the minimum value is 0.25 Hz.
3. The satellite communication chirp signal tracking receiver of claim 1, wherein the first matched filtering module and the second matched filtering module are cascaded from 8 sets of filters, each set of filters having a feedback port.
4. The satellite communication chirp signal tracking receiver according to claim 1, wherein the quadrature demodulation module inputs a synchronous modulation signal synchronized with a modulation square wave, and the frequency of the modulation square wave is greater than or equal to 16 kHz.
5. The satellite communication chirp signal tracking receiver of claim 2, wherein the DPRAM storage depth in the data buffer module is 64ms, and is divided into four regions, each region includes an 8ms non-overlapping data region, the front and rear of the non-overlapping data region respectively have an overlapping data region overlapping with other regions, and when a 24ms region is full of data, the data scan flag is output to the data selection module.
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