Satellite communication terminal crystal oscillator calibration method, storage medium and satellite communication terminal

Abstract

The application relates to the technical field of satellite communication, in particular to a satellite communication terminal crystal oscillator calibration method, which comprises the following steps: triggering a crystal oscillator calibration process when a terminal is started or in a communication idle period; tuning the terminal to the frequency point of a geosynchronous orbit satellite, searching for and capturing the synchronization signal of the geosynchronous orbit satellite; performing correlation operation on the received synchronization signal and a reference signal generated by a local crystal oscillator of the terminal, and calculating the frequency deviation value between the synchronization signal and the reference signal; and generating a crystal oscillator control signal according to the frequency deviation value, and finely adjusting the frequency of the local crystal oscillator. The application realizes accurate, robust and verifiable calibration of the terminal crystal oscillator without an external high-precision reference source.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, specifically to a crystal oscillator calibration method for a satellite communication terminal, a storage medium, and a satellite communication terminal. Background Technology

[0002] In satellite wireless communication systems, terminal devices need to maintain precise frequency synchronization with the satellite to ensure the reliability of the communication link. Terminal devices typically rely on a local crystal oscillator to generate their operating clock, but the crystal oscillator is susceptible to frequency deviation due to factors such as temperature and aging. The traditional calibration method involves performing correlation calculations between the satellite synchronization signal received by the terminal and the signal generated by the local crystal oscillator to obtain the frequency deviation value, and then adjusting the crystal oscillator to keep the frequency deviation value within an acceptable range.

[0003] However, for low Earth orbit (LEO) satellite communication systems, the high-speed motion of the satellite relative to the ground terminal (approximately 7.8 km / s) causes a rapid change in the frequency of the received signal due to the Doppler effect. This change is superimposed on the frequency offset of the crystal oscillator itself, making it difficult for the terminal to accurately distinguish which frequency offset components originate from the crystal oscillator itself and which from the Doppler effect. Therefore, directly using the LEO satellite's synchronization signal as a calibration reference will lead to inaccurate crystal oscillator calibration, affecting the normal operation of the terminal.

[0004] In existing technologies, there are also schemes that use highly stable external reference sources (such as GPS signals and ground base station signals) for calibration. However, these schemes cannot be used in scenarios with no GPS coverage or weak base station signals, and they increase the complexity and cost of the system.

[0005] Therefore, there is a need for a method that can accurately calibrate the terminal crystal oscillator without an external high-precision reference source, while also possessing multipath robustness and calibration self-verification capability. Summary of the Invention

[0006] In order to overcome the above-mentioned defects, this application is proposed to solve or at least partially solve the problem of inaccurate crystal oscillator calibration caused by the Doppler effect of low-orbit satellite signals, and to achieve accurate, robust and verifiable calibration of terminal crystal oscillators without the need for an external high-precision reference source.

[0007] According to one aspect of the present invention, a method for calibrating a crystal oscillator in a satellite communication terminal is provided, comprising the following steps: The crystal oscillator calibration process is triggered when the terminal is powered on or during a communication idle period. The terminal is tuned to the frequency of a geostationary orbit satellite to search for and capture the synchronization signal of the geostationary orbit satellite; wherein the geostationary orbit satellite is substantially stationary relative to the ground terminal; The received synchronization signal is correlated with the reference signal generated by the local crystal oscillator of the terminal to calculate the frequency deviation value between the synchronization signal and the reference signal. Based on the frequency deviation value, a crystal oscillator control signal is generated to fine-tune the frequency of the local crystal oscillator.

[0008] The above technical solution may also include: Determine whether the frequency deviation value meets the preset tolerance requirement; if not, return to recalculate the frequency deviation value to form closed-loop control until the frequency deviation value meets the preset tolerance requirement. When the frequency deviation value meets the preset tolerance requirement, save the current crystal oscillator adjustment parameters and exit the calibration process.

[0009] In any of the above technical solutions, before searching for the synchronization signal, the following may also be included: Based on pre-stored satellite orbit parameters or historical signal quality records, select the geostationary orbit satellite with the best signal quality from multiple geostationary orbit satellites as the calibration reference satellite; The signal quality includes at least one of signal strength, historical acquisition success rate, and historical frequency offset stability.

[0010] In any of the above technical solutions, the step of tuning the terminal to the frequency of a geostationary orbit satellite and searching for and capturing the synchronization signal of the geostationary orbit satellite includes: Tune the terminal's radio frequency front end to the frequency of the calibration reference satellite; Synchronization signal capture is performed within the preset frequency offset search range; If capture fails, the next geostationary orbit satellite will be selected for capture until the geostationary signal is successfully captured.

[0011] In any of the above technical solutions, the step of performing correlation calculation between the received synchronization signal and the reference signal generated by the local crystal oscillator of the terminal includes: The received synchronization signal is down-converted to obtain an analog baseband signal; The analog baseband signal is sampled to obtain a digital baseband signal; The digital baseband signal is digitally correlated with the reference clock generated by the local crystal oscillator, and the rate of change of the phase difference between the two over time is calculated to obtain the frequency deviation value.

[0012] The above-mentioned technical solutions also include: N frequency deviation measurements are performed consecutively to obtain N frequency deviation measurement values; where N is an integer greater than or equal to 2. The N frequency deviation measurements are averaged arithmetically, and the resulting average value is taken as the final frequency deviation value.

[0013] In any of the above technical solutions, the step of generating a crystal oscillator control signal based on the frequency deviation value and fine-tuning the frequency of the local crystal oscillator includes: Based on the sign and magnitude of the frequency deviation value, generate the corresponding analog control voltage or digital frequency modulation word; The analog control voltage or digital frequency modulation word is sent to the voltage control terminal or digital frequency modulation interface of the local crystal oscillator to fine-tune the output frequency of the local crystal oscillator.

[0014] In any of the above technical solutions, saving the current crystal oscillator adjustment parameters when the frequency deviation value meets the preset tolerance requirement includes: The crystal oscillator adjustment parameters used in this calibration, the identification information of the geosynchronous orbit satellite, and the calibration timestamp are stored in a non-volatile memory. The crystal oscillator adjustment parameters include control voltage values ​​or digital frequency modulation words. The method further includes: After exiting the calibration process, the terminal is switched to the communication frequency of the target low-Earth orbit satellite to conduct normal communication services; wherein, the target low-Earth orbit satellite is a satellite that moves at high speed relative to the ground terminal.

[0015] In any of the above technical solutions, before switching the terminal to the communication frequency of the target low-Earth orbit satellite, the method further includes: When the frequency deviation value meets the preset tolerance requirement, the radio frequency front end is controlled to alternately listen to the pilot signals of the geostationary orbit satellite and the target low-Earth orbit satellite in a time-division or frequency-division manner, and to obtain the observation frequency of the geostationary orbit satellite and the observation frequency of the low-Earth orbit satellite respectively under the same time reference. Calculate the frequency difference between the observation frequency of the geostationary orbit satellite and the observation frequency of the low orbit satellite, and determine the Doppler shift observation value of the target low orbit satellite based on the calibrated frequency reference of the geostationary orbit satellite; Obtain the ephemeris data of the target low-Earth orbit satellite, and calculate the theoretical Doppler shift prediction value of the target low-Earth orbit satellite based on the terminal position and the ephemeris data; The Doppler shift observation value is compared with the theoretical Doppler shift prediction value. If the difference between the two is within the verification tolerance range, the geostationary orbit satellite calibration is determined to be valid, and the radial velocity of the terminal relative to the target low-Earth orbit satellite is determined based on the Doppler shift observation value. If the difference between the two exceeds the verification tolerance range, recalibration or an alarm is triggered.

[0016] According to another aspect of the present invention, a computer-readable storage medium is also provided, wherein a plurality of program codes are stored, the program codes being adapted to be loaded and run by a processor to perform the satellite communication terminal crystal oscillator calibration method described in any of the above technical solutions.

[0017] According to another aspect of the present invention, a satellite communication terminal is also provided, wherein the satellite communication terminal crystal oscillator calibration method described in any of the above technical solutions is performed.

[0018] This invention effectively avoids the interference of the Doppler effect of low-Earth orbit satellites on crystal oscillator calibration by introducing geostationary orbit satellites as a stable frequency reference, and realizes autonomous and accurate calibration under the condition of no external high-precision reference source. Through the GEO and LEO dual-frequency overlapping monitoring and ephemeris verification mechanism, it breaks the traditional gap between calibration and communication switching, realizes real-time self-verification of calibration effectiveness and pre-acquisition of low-Earth orbit satellite Doppler information, thereby significantly improving the efficiency of establishing low-Earth orbit satellite communication links and the overall reliability of the system. Attached Figure Description

[0019] The disclosure of this application will become more readily understood with reference to the accompanying drawings. It will be readily understood by those skilled in the art that these drawings are for illustrative purposes only and are not intended to limit the scope of protection of this application. Wherein: Figure 1 This is a flowchart of a satellite communication terminal crystal oscillator calibration method according to an embodiment of this application; Figure 2 This is a flowchart of a satellite communication terminal crystal oscillator calibration method according to another embodiment of this application. Detailed Implementation

[0020] Some embodiments of this application are described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of this application and are not intended to limit the scope of protection of this application.

[0021] In the description of this application, "module" and "processor" can include hardware, software, or a combination of both. A module can include hardware circuitry, various suitable sensors, communication ports, memory, and may also include software components, such as program code, or a combination of software and hardware. A processor can be a central processing unit, microprocessor, image processor, digital signal processor, or any other suitable processor. The processor has data and / or signal processing capabilities. The processor can be implemented in software, in hardware, or a combination of both. Computer-readable storage media includes any suitable medium capable of storing program code, such as magnetic disks, hard disks, optical disks, flash memory, read-only memory, random access memory, etc. The term "A and / or B" means all possible combinations of A and B, such as only A, only B, or A and B. The terms "at least one A or B" or "at least one of A and B" have a similar meaning to "A and / or B" and can include only A, only B, or A and B. The singular terms "a" or "this" can also include plural forms.

[0022] Reference Figure 1 This embodiment provides a method for calibrating a crystal oscillator in a satellite communication terminal, which may include the following steps: Step 102: Calibration Trigger. The terminal powers on and completes its self-test, then determines if it is currently in a communication idle state (e.g., no service connection has been established). If yes, the calibration process begins; otherwise, it can be delayed until the next idle period.

[0023] Step 104: GEO Satellite Selection. The terminal reads pre-stored GEO satellite orbital parameters (such as satellite identifier, frequency, and historical signal strength) from non-volatile memory and selects the GEO satellite with the strongest and most stable signal as the calibration benchmark. If no historical data is available, the default frequency list can be tried sequentially.

[0024] In this embodiment, the pre-stored list of GEO satellites includes, for example, ChinaSat 10 (110.5°E, C-band), AsiaSat 6C (134°E, Ku-band), and ChinaSat 6B (115.5°E, C-band). The terminal prioritizes the satellite with the highest historical signal strength based on the previous communication record.

[0025] Step 106: Synchronization Signal Search. The terminal tunes its RF front-end to the frequency of the selected GEO satellite and acquires the synchronization signal within a preset frequency offset search range (e.g., ±50ppm). If the acquisition is successful, proceed to step 108; if it fails, return to step 104 to select the next GEO satellite.

[0026] Step 108: Frequency Offset Calculation. The terminal down-converts and samples the received GEO synchronization signal to obtain a digital baseband signal. Simultaneously, the local crystal oscillator generates a local reference clock through a phase-locked loop (PLL). The baseband signal and the local reference clock are digitally correlated to calculate the rate of change of their phase difference over time, thus obtaining the frequency offset value Δf.

[0027] To improve accuracy, N=10 consecutive measurements were performed, and the average value was taken as the final frequency offset value. Step 110: Tolerance determination. Compare |Δfavg| with the preset tolerance value Δfmax. If |Δfavg| ≤ Δfmax, the crystal oscillator frequency is sufficiently accurate, and proceed to step 114; otherwise, proceed to step 112.

[0028] Step 112: Crystal Oscillator Adjustment. Based on the sign and magnitude of Δfavg, generate the corresponding digital frequency modulation word and send it to the digital frequency modulation interface of the local DCXO to fine-tune the crystal oscillator output frequency. After adjustment, return to step 108 to remeasure the frequency deviation, forming a closed-loop control until the tolerance requirements are met.

[0029] Step 114: Parameter Saving and Exit. Save the final digital frequency modulation word, GEO satellite identifier, and calibration timestamp used in this calibration to non-volatile memory. Exit calibration mode; the terminal can then normally switch to the target LEO satellite frequency for communication.

[0030] By utilizing GEO satellite signals as a stable frequency reference, interference from the LEO satellite Doppler effect on frequency offset measurements is avoided. This allows the crystal oscillator frequency offset to be calibrated from the initial ±20ppm to within ±0.1ppm, meeting the frequency offset tolerance requirements of LEO communication. Furthermore, no additional GPS receiver module is required, reducing system complexity and cost.

[0031] Reference Figure 2 This embodiment addresses the issue of gaps in calibration and LEO communication switching by providing a GEO-LEO dual-frequency collaborative verification and frequency offset propagation method.

[0032] The above embodiment directly switches to LEO after calibration, making it impossible to verify the calibration validity or obtain the initial LEO Doppler information. This embodiment achieves calibration self-verification and LEO Doppler pre-compensation through dual-frequency overlapping monitoring.

[0033] Step 202: GEO calibration convergence. Following the method in Example 1, crystal oscillator calibration is performed based on the GEO satellite, bringing the frequency deviation to within ±100Hz.

[0034] Step 204: Dual-frequency overlapping monitoring. During GEO calibration convergence, the RF front-end is controlled to alternately monitor the GEO and target LEO satellites (Starlink satellites) in a time-division manner.

[0035] The terminal operates in TDD (Time Division Duplex) mode with a frame period of 10ms, wherein: Downlink time slot (0-5ms): Maintain communication link with LEO satellites and extract instantaneous frequency measurements of LEO pilots; Guard interval (5-5.5ms): Rapid switching of the RF front end; GEO monitoring time slot (5.5-9.5ms): Switch to GEO frequency, perform short-time integration (4ms), and obtain the GEO observation frequency f. GEO ; Guard interval (9.5-10ms): Switch back to LEO frequency.

[0036] By using an interpolation algorithm, the rates f obtained at different times are... GEO and f LEO Align to the same time t0: Step S303: Frequency offset propagation calculation. Calculate the frequency difference: Based on the calibrated GEO frequency reference, determine the Doppler frequency shift observations for LEO: Among them, f residual The residual frequency offset of the GEO signal (characterizing the residual error of the crystal oscillator and the error of the receiving channel, typically <10Hz).

[0037] Step 206: Ephemeris Verification. Obtain ephemeris data from LEO satellites (via broadcast channels of GEO satellites or pre-stored ephemeris), and calculate the theoretical Doppler shift prediction value of LEO based on the terminal's approximate position and ephemeris data.

[0038] The approximate location of the terminal is obtained through the following methods: Method 1: Based on the previous GPS positioning result (39.9°N, 116.4°E), combined with IMU motion integral (velocity 10m / s, direction northeast), calculate the current approximate location; Method 2: Using f doppler The two-satellite positioning solution is calculated based on the azimuth of the GEO satellite (azimuth 180°, elevation 45°).

[0039] According to the ephemeris, the position of the LEO satellite at time t0 is (x s y s , zs The approximate location of the terminal is (x u y u , z u Then the theoretical Doppler frequency shift is: Step 208: Self-consistency judgment. Compare observed values ​​with predicted values: If ϵ < 500 Hz (verification tolerance), then the GEO calibration is considered valid, and based on f dopple Determine the radial velocity of the terminal relative to the LEO. , used for initial Doppler pre-compensation in LEO communication.

[0040] If ϵ≥500HZ, the calibration is considered abnormal, triggering recalibration or reporting an alarm.

[0041] Step 210: LEO communication switching. After successful verification, the terminal switches to the LEO frequency point with pre-compensated Doppler information, reducing the frequency search range from ±150kHz to ±1kHz, significantly shortening the synchronization time.

[0042] By employing dual-frequency overlapping monitoring and ephemeris verification, real-time self-verification of calibration validity was achieved, significantly improving the calibration error detection rate. Simultaneously, by acquiring initial LEO Doppler information through frequency offset propagation, the LEO communication link establishment time was reduced from the traditional 2-5 seconds to <100ms, significantly improving communication continuity in high-speed mobile scenarios such as vehicle-mounted and airborne applications. Furthermore, the dual-satellite positioning capability provides a coarse positioning backup for the terminal, enhancing autonomous navigation capabilities in GPS-free scenarios.

[0043] It should be understood that the process of determining whether the frequency deviation value meets the preset tolerance requirements in any of the above embodiments, as well as the process of selecting geostationary orbit satellites, are not necessary and can be selected and configured according to the needs of the scenario.

[0044] It should be noted that although the steps in the above embodiments are described in a specific order, those skilled in the art will understand that in order to achieve the effect of this application, different steps do not necessarily have to be executed in such an order. They can be executed simultaneously (in parallel) or in other orders. These adjusted solutions are equivalent to the technical solutions described in this application and therefore will also fall within the protection scope of this application.

[0045] This invention also provides a computer-readable storage medium storing multiple lines of program code adapted to be loaded and executed by a processor to perform the satellite communication terminal crystal oscillator calibration method described in any of the above embodiments. For ease of explanation, only the parts related to the embodiments of this application are shown; for specific technical details not disclosed, please refer to the method section of the embodiments of this application. This computer-readable storage medium can be a storage device comprising various electronic devices. Optionally, in the embodiments of this application, the computer-readable storage medium is a non-transitory computer-readable storage medium.

[0046] Similarly, embodiments of the present invention also provide a satellite communication terminal, which performs the satellite communication terminal crystal oscillator calibration method described in any of the above embodiments. This satellite communication terminal may be a satellite phone, vehicle / shipborne satellite communication equipment, portable satellite broadband terminal, etc.

[0047] Another aspect of this application provides an electronic device.

[0048] In an embodiment of an electronic device according to this application, the electronic device may include at least one processor; and a memory communicatively connected to the at least one processor; wherein the memory stores a computer program that, when executed by the at least one processor, implements the method described in any of the above embodiments.

[0049] In summary, this invention effectively avoids the interference of the Doppler effect of low-Earth orbit satellites on crystal oscillator calibration by introducing geostationary orbit satellites as a stable frequency reference, and achieves autonomous and accurate calibration without an external high-precision reference source. Through the GEO and LEO dual-frequency overlapping monitoring and ephemeris verification mechanism, it breaks the traditional gap between calibration and communication switching, realizes real-time self-verification of calibration effectiveness and pre-acquisition of low-Earth orbit satellite Doppler information, thereby significantly improving the efficiency of establishing low-Earth orbit satellite communication links and the overall reliability of the system.

[0050] Those skilled in the art will understand that all or part of the processes in the method of the above-described embodiment can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable file, or some intermediate form. The computer-readable storage medium can include any entity or device capable of carrying the computer program code, a medium, a USB flash drive, a portable hard drive, a magnetic disk, an optical disk, a computer memory, a read-only memory, a random access memory, an electrical carrier signal, a telecommunication signal, and a software distribution medium, etc.

[0051] The technical solution of this application has been described above with reference to one embodiment shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of this application is obviously not limited to these specific embodiments. Without departing from the principles of this application, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of this application.

Claims

1. A method for calibrating a crystal oscillator in a satellite communication terminal, characterized in that, Includes the following steps: The crystal oscillator calibration process is triggered when the terminal is powered on or during a communication idle period. The terminal is tuned to the frequency of a geostationary orbit satellite to search for and capture the synchronization signal of the geostationary orbit satellite. The received synchronization signal is correlated with the reference signal generated by the local crystal oscillator of the terminal to calculate the frequency deviation value between the synchronization signal and the reference signal. Based on the frequency deviation value, a crystal oscillator control signal is generated to fine-tune the frequency of the local crystal oscillator.

2. The method according to claim 1, characterized in that, Also includes: Determine whether the frequency deviation value meets the preset tolerance requirement; If the condition is not met, the frequency deviation value is recalculated to form a closed-loop control until the frequency deviation value meets the preset tolerance requirement. When the frequency deviation value meets the preset tolerance requirement, save the current crystal oscillator adjustment parameters and exit the calibration process.

3. The method according to claim 1, characterized in that, Before searching for the synchronization signal, the process also includes: Based on pre-stored satellite orbit parameters or historical signal quality records, select the geostationary orbit satellite with the best signal quality from multiple geostationary orbit satellites as the calibration reference satellite; The signal quality includes at least one of signal strength, historical acquisition success rate, and historical frequency offset stability.

4. The method according to claim 3, characterized in that, The step of tuning the terminal to the frequency of a geostationary orbit satellite and searching for and capturing the synchronization signal of the geostationary orbit satellite includes: Tune the terminal's radio frequency front end to the frequency of the calibration reference satellite; Synchronization signal capture is performed within the preset frequency offset search range; If capture fails, the next geostationary orbit satellite will be selected for capture until the geostationary signal is successfully captured.

5. The method according to claim 1, characterized in that, The step of performing correlation calculations between the received synchronization signal and the reference signal generated by the local crystal oscillator of the terminal includes: The received synchronization signal is down-converted to obtain an analog baseband signal; The analog baseband signal is sampled to obtain a digital baseband signal; The digital baseband signal is digitally correlated with the reference clock generated by the local crystal oscillator, and the rate of change of the phase difference between the two over time is calculated to obtain the frequency deviation value. N frequency deviation measurements are performed consecutively to obtain N frequency deviation measurement values; where N is an integer greater than or equal to 2. The N frequency deviation measurements are averaged arithmetically, and the resulting average value is taken as the final frequency deviation value.

6. The method according to claim 1, characterized in that, The step of generating a crystal oscillator control signal based on the frequency deviation value and fine-tuning the frequency of the local crystal oscillator includes: Based on the sign and magnitude of the frequency deviation value, generate the corresponding analog control voltage or digital frequency modulation word; The analog control voltage or digital frequency modulation word is sent to the voltage control terminal or digital frequency modulation interface of the local crystal oscillator to fine-tune the output frequency of the local crystal oscillator.

7. The method according to claim 2, characterized in that, When the frequency deviation value meets the preset tolerance requirement, saving the current crystal oscillator adjustment parameters includes: The crystal oscillator adjustment parameters used in this calibration, the identification information of the geosynchronous orbit satellite, and the calibration timestamp are stored in a non-volatile memory. The crystal oscillator adjustment parameters include control voltage values ​​or digital frequency modulation words. The method further includes: After exiting the calibration process, the terminal is switched to the communication frequency of the target low-Earth orbit satellite to conduct normal communication services; wherein, the target low-Earth orbit satellite is a satellite that moves at high speed relative to the ground terminal.

8. The method according to claim 7, characterized in that, Before switching the terminal to the communication frequency of the target low-Earth orbit satellite, the process also includes: When the frequency deviation value meets the preset tolerance requirement, the radio frequency front end is controlled to alternately listen to the pilot signals of the geostationary orbit satellite and the target low-Earth orbit satellite in a time-division or frequency-division manner, and to obtain the observation frequency of the geostationary orbit satellite and the observation frequency of the low-Earth orbit satellite respectively under the same time reference. Calculate the frequency difference between the observation frequency of the geostationary orbit satellite and the observation frequency of the low orbit satellite, and determine the Doppler shift observation value of the target low orbit satellite based on the calibrated frequency reference of the geostationary orbit satellite; Obtain the ephemeris data of the target low-Earth orbit satellite, and calculate the theoretical Doppler shift prediction value of the target low-Earth orbit satellite based on the terminal position and the ephemeris data; The Doppler shift observation value is compared with the theoretical Doppler shift prediction value. If the difference between the two is within the verification tolerance range, the geostationary orbit satellite calibration is determined to be valid, and the radial velocity of the terminal relative to the target low-Earth orbit satellite is determined based on the Doppler shift observation value. If the difference between the two exceeds the verification tolerance range, recalibration or an alarm is triggered.

9. A computer-readable storage medium storing a plurality of program codes, characterized in that, The program code is adapted to be loaded and run by a processor to perform the satellite communication terminal crystal oscillator calibration method according to any one of claims 1 to 7.

10. A satellite communication terminal, characterized in that, The satellite communication terminal performs the satellite communication terminal crystal oscillator calibration method according to any one of claims 1 to 7.

Images