A calibration method for wide-area distributed multi-node receiving links based on GNSS signals

Abstract

The present invention discloses a calibration method for a wide-area distributed multi-node receiving link based on GNSS signals, and belongs to the technical field of distributed radar detection. The node receiver processing program of the present invention processes the received GNSS signal to obtain the measured signal carrier phase difference and the satellite WGS-84 coordinates. The differential GPS is combined with the total station to obtain the antenna phase center WGS-84 coordinates, and the phase difference caused by space transmission is calculated with the satellite coordinates. After the measured carrier phase difference eliminates the phase difference caused by space transmission, the phase inconsistency of the receiving link is obtained. The local station center coordinate system established with the antenna phase center as the origin obtains the two-dimensional angle of the satellite relative to the origin, and corresponds to the phase inconsistency of the receiving link. GNSS data is continuously received to establish a two-dimensional angle-receiving link inconsistency associated fingerprint library. Thereafter, the corresponding receiving link inconsistency can be searched in the fingerprint library if the satellite coordinates are known. The present invention can be quickly deployed and can achieve the calibration requirements of "ready to use" and "use as needed".

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed radar detection, and in particular relates to a wide-area distributed multi-node receiving link calibration method based on GNSS signals. Background Art

[0002] In the working mode of the distributed radar system, it is necessary to realize the phase synthesis of the distributed received signal, which puts forward high phase accuracy requirements for the system's time and frequency source, baseband equipment, downlink channel, antenna reference point and phase center, etc. Therefore, it is very necessary to calibrate the receiving link.

[0003] There are currently two methods for calibrating the distributed node receiving link: the calibration tower solution and the drone solution:

[0004] 1. The working principle of the calibration tower solution is as follows: the beacon is placed on the top of the calibration tower, and the node receives the calibration signal transmitted by the beacon. According to the coordinates of the beacon and the node in the same coordinate system and the difference in the received calibration signal, the inconsistency of the node receiving link is inferred.

[0005] The calibration tower solution is limited by the height of the calibration tower. There are two problems: first, the calibration tower is relatively low relative to the node distribution area, and distant nodes may be blocked, and the working range cannot cover nodes distributed over a large area; second, the low elevation angle of the calibration tower relative to the node causes the multipath effect to seriously affect the calibration accuracy.

[0006] 2. The working principle of the drone solution is: place the beacon on the drone, the drone flies to the predetermined airspace and hovers, the node receives the calibration signal transmitted by the beacon, and based on the coordinates of the drone and the node in the same coordinate system and the difference in the received calibration signal, the inconsistency of the node receiving link is inferred.

[0007] The disadvantages of the drone solution are: due to the long baseline of the distributed nodes (much longer than the carrier wavelength), the coordinate measurement error of the drone-mounted antenna center is very sensitive to the results. Although high-precision RTK on drones can significantly improve the position measurement accuracy of the drone center, it still cannot overcome the coordinate drift problem of drones under the influence of high-altitude airflow. In addition, the drone calibration solution requires the cooperation of professional drone pilots and is greatly affected by the weather, and cannot meet the low-cost, all-day and all-weather calibration requirements.

[0008] GNSS (Global Navigation Satellite System) is a radio navigation system (RNSS) that uses satellites as reference bases. The reference bases are medium-orbit satellites and synchronous satellites in motion, and the orbits of these satellites are known. At present, the existing satellite navigation and positioning systems in the world mainly include the GPS system of the United States, the GLONASS system of Russia, the Beidou system of China, and the Galileo system of the European Union, which provide users with large-scale, high-precision and fast positioning services. The GNSS system modulates the navigation message including satellite ephemeris, satellite clock error correction parameters, ranging time mark and other information on the L2C (frequency 1227.6MHz, taking GPS as an example) carrier and broadcasts it to users on the ground. The distributed coherent system working frequency band covers the L2C carrier, and the software receiver can be used to perform steps including signal capture, tracking, navigation message demodulation, etc. on the GNSS signal, extract the satellite time-varying WGS-84 coordinates, and use the GNSS satellite as a "beacon" to complete the receiving link calibration work.

[0009] The use of a receiving link calibration solution based on GNSS signals has the following benefits: 1. Since the GNSS satellite is far away from the node (in the tens of thousands of kilometers), the measurement error of the satellite and node coordinates has little effect on the calibration results. In addition, the satellite has a high elevation angle relative to the receiving node, so the multipath effect has little effect; 2. GNSS signals transmitted by multiple satellites can calibrate the same receiving link at different directions at the same time to composite the results; 3. Since the relative position of the satellite and the node changes over time, this solution can calibrate the receiving link in different directions; 4. GNSS satellite signals are not affected by weather and can meet the requirements of all-day and all-weather calibration; 5. This calibration solution does not require the addition of any hardware, only the software receiver and subsequent signal processing program development work, and the cost is low. Summary of the invention

[0010] The technical problems to be solved by the present invention are: the calibration scheme based on the calibration tower cannot cover nodes in a wide area and the calibration signal is seriously affected by the multipath effect; the calibration accuracy of the calibration scheme based on the drone is easily affected by the drone's posture and the calibration cost is high.

[0011] The technical solution adopted by the present invention is:

[0012] A method for calibrating a wide-area distributed multi-node receiving link based on GNSS signals comprises the following steps:

[0013] Step 1: The wide-area distributed node receives the GNSS data from the satellite, and performs de-framing, capture and tracking to obtain the pseudo code phase, carrier frequency information, measured carrier phase difference and original navigation message information;

[0014] Step 2: Perform bit synchronization, subframe synchronization and parity check on the original navigation message information to obtain the navigation message, decode the navigation message, extract the satellite orbit elements, time parameters and Kepler parameters from it, and calculate the satellite WGS-84 coordinates by combining the pseudo code phase and carrier frequency information;

[0015] Step 3, using differential GPS and total station to obtain the WGS-84 coordinates of the center of the receiving node antenna, and according to the WGS-84 coordinates of the center of the satellite and the receiving node antenna, calculate the phase difference caused by the transmission path;

[0016] Step 4, calculating the phase inconsistency of the receiving link according to the measured carrier phase difference of the received signal obtained by the tracking loop and the phase difference caused by the transmission path;

[0017] Step 5: Establish a local station center coordinate system with the receiving node antenna phase center as the origin, and calculate the coordinates of the satellite in this coordinate system, and then obtain the two-dimensional angle of the satellite relative to the receiving node antenna phase center;

[0018] Step 6, looping steps 1-5 for a set number of times, correlating the phase inconsistency of the receiving link with the two-dimensional angle of the satellite relative to the center of the receiving node antenna, and establishing a correlation fingerprint library;

[0019] Step 7: When measuring again, according to the two-dimensional angle of the satellite relative to the center of the receiving node antenna, the corresponding receiving link phase inconsistency is searched in the associated fingerprint library.

[0020] Among them, the specific implementation process of step 3 is:

[0021] Step 3.1: Build three benchmark piers and accurately determine the coordinates of the three benchmark pier targets in the WGS-84 coordinate system;

[0022] Step 3.2: Use the total station to determine the coordinates of the three benchmark pier target points in the total station coordinate system;

[0023] Step 3.3: Calculate the conversion relationship between the WGS-84 coordinate system and the total station coordinate system based on the coordinates of the three benchmark pier targets in the WGS-84 coordinate system and the coordinates of the total station coordinate system;

[0024] Step 3.4: Use a total station to measure the coordinates of multiple targets on each receiving node in the total station coordinate system, and calculate the coordinates of the receiving node antenna center in the total station coordinate system based on the relative position relationship between the target and the receiving node antenna center;

[0025] Step 3.5: Based on the conversion relationship between the WGS-84 coordinate system and the total station coordinate system and the coordinates of the center of the receiving node antenna in the total station coordinate system, calculate the coordinates of the center of the receiving node antenna in the WGS-84 coordinate system;

[0026] Step 3.6: Calculate the phase difference caused by the transmission path based on the WGS-84 coordinates of the satellite and receiving node antenna centers

[0027]

[0028] Where rem(A,B) is the remainder of A divided by B, λ is the wavelength of the GNSS signal, (x sat ,y sat ,z sat ) is the satellite WGS-84 coordinate, (x0, y0, z0) is the WGS-84 coordinate of the receiving node antenna center.

[0029] Compared with the background technology, the present invention has the following advantages:

[0030] 1. Compared with the background technology, the wide-area distributed multi-node receiving link calibration method based on GNSS signals proposed in the present invention does not require additional hardware equipment, only software development is required, and the link calibration cost can be effectively reduced.

[0031] 2. The wide-area distributed multi-node receiving link calibration method based on GNSS signals proposed in the present invention has a low multipath effect in space transmission and will not affect the result because the elevation angle of the signal source (satellite) relative to the receiver (node) is relatively high.

[0032] 3. The wide-area distributed multi-node receiving link calibration method based on GNSS signals proposed in the present invention is easy and fast to deploy, is not affected by weather, and can achieve the calibration requirements of "ready to use", "use as soon as you collect", and "all day and all weather".

[0033] 4. The wide-area distributed multi-node receiving link calibration method based on GNSS signals proposed in the present invention has multiple signal sources (satellites), which can realize mutual comparison of signals to verify the calibration results.

[0034] 5. The wide-area distributed multi-node receiving link calibration method based on GNSS signals proposed in the present invention can calibrate the receiving link when the signal source is in different directions due to the movement of the satellite relative to the node. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of high-precision calibration of a wide-area distributed receiving node system using GNSS signals.

[0036] Figure 2 This is a schematic diagram of the GNSS signal capture principle of the present invention.

[0037] Figure 3 This is a schematic diagram of the GNSS signal tracking principle of the present invention.

[0038] Figure 4 This is a schematic diagram of the GNSS signal solution principle of the present invention.

[0039] Figure 5 It is the link phase inconsistency result of two receiving nodes after the fingerprint library is established. DETAILED DESCRIPTION

[0040] The present invention can obtain the time-varying phase inconsistency of the receiving link between the widely distributed nodes by using the outdoor measured GNSS data, and can improve the calibration efficiency and calibration accuracy of the receiving link of the widely distributed multi-node detection system. After the widely distributed nodes receive the GNSS data, the receiver processing program processes and solves the GNSS data to obtain the measured carrier phase difference and satellite WGS-84 coordinates of the received signal. The differential GPS is combined with the total station to obtain the WGS-84 coordinates of the phase center of the node antenna, and the phase difference caused by space transmission can be calculated with the satellite WGS-84 coordinates. After the measured carrier phase difference eliminates the phase difference caused by space transmission, the phase inconsistency of the receiving link can be obtained.

[0041] Furthermore, based on the WGS-84 coordinates of the satellite and node antenna phase centers, the coordinates of the satellite in the local station center coordinate system established with the receiving node antenna phase center as the origin can be obtained, and then the two-dimensional angle of the satellite relative to the node antenna phase center. By continuously and repeatedly receiving GNSS data and fitting the results, a two-dimensional angle-node inconsistency association fingerprint library is established based on the correspondence between the satellite two-dimensional angle and the receiving link inconsistency. After that, given any satellite WGS-84 coordinates, the corresponding receiving link inconsistency can be searched in the fingerprint library.

[0042] The present invention will be further explained below in conjunction with the accompanying drawings.

[0043] Step 1: Widely distributed nodes receive GNSS signals.

[0044] Establish a wide-area distributed node receiving system, as shown in the attached Figure 1 As shown. The system includes N receiving nodes and is linked to the data processing module. The pseudo code sent by each satellite has an independent PRN number. The node receives the signal sent by the GNSS satellite and sends it to the data processing module for subsequent processing. The working frequency band of the wide-area distributed node can cover the GNSS signal L2C carrier, so this technical solution does not require additional receiving antennas.

[0045] First, adjust the node antenna array to be flat to receive GNSS signals that may be transmitted from all directions and achieve maximum reception of GNSS signals.

[0046] Secondly, turn on the wide area distribution node and set it to receiving mode, and initialize it to standby state.

[0047] Subsequently, the GNSS signal is continuously collected for a duration of Ts, and operations such as signal deframing and useful information extraction are performed based on the signal’s frame structure.

[0048] Step 2: The receiver processor acquires the GNSS signal.

[0049] The GNSS signal x(t) received in step 1 with a duration of Ts is the superposition of signals from all n visible satellites. Different satellite pseudocodes are cross-correlated, and signals from other satellites can be suppressed through the correlation process. When satellite k is captured, the input signal x(t) is multiplied by the pseudocode of satellite k generated locally. In order to filter out the carrier of the received signal and avoid removing the useful signal component, the frequency of the local signal must be close to the carrier frequency of the received signal, and the code phase of the local signal must be aligned with the code of the received signal.

[0050] The present invention adopts the technical scheme of coarse capture first and fine capture later to obtain the code phase Φ and frequency value f k , complete the rough capture of the signal. Figure 2 shown.

[0051] Step 3: The receiver processor tracks the GNSS signal.

[0052] Since the relative motion between the node and the satellite causes the received signal to have frequency offset and code phase offset, it is necessary to design a tracking loop to track the changes in the signal Doppler frequency shift and code phase, so as to synchronize the carrier and pseudo code phase generated locally by the receiver with the received GNSS signal, thereby continuously demodulating the navigation information. The tracking loop includes a code tracking loop and a carrier tracking loop, which adjust each other until the code phase and carrier phase of the locally generated signal are aligned with the received signal. The working principle is as follows: Figure 3 shown.

[0053] The present invention uses a two-quadrant ATAN Costas phase detector, and the carrier phase difference between the output local reference signal and the input signal is After being filtered by the loop filter, it is used as the control signal of the carrier loop numerically controlled oscillator to adjust the local carrier frequency and finally stably track the carrier phase in the input signal.

[0054] Step 4: GNSS signal decoding and information extraction.

[0055] The specific implementation of step 4 includes the following sub-steps:

[0056] Step 4.1: After step 3, the GNSS pseudo code and carrier tracking loop remain locked, and the 1000bps original navigation message information can be output. After bit synchronization, subframe synchronization, and parity check of the 1000bps original navigation message, a 50Hz navigation message is obtained, as shown in the attached figure. Figure 4 shown.

[0057] Step 4.2: Decode the navigation message and extract information such as satellite orbit elements, time parameters (TOW, WN, etc.), Kepler parameters, etc., and combine the code and carrier tracking loop to output pseudo code phase and carrier frequency information to calculate the GNSS satellite position parameters (WGS-84 coordinates (x sat ,y sat ,z sat )).

[0058] Step 5: Establish the local station center coordinates of the receiving node and calculate the phase difference caused by the transmission path.

[0059] The specific implementation of step 5 includes the following sub-steps:

[0060] Step 5.1: Build three benchmark piers and accurately determine the coordinates of the three benchmark pier targets in the WGS-84 coordinate system.

[0061] Step 5.2: Use the total station to determine the coordinates of the three benchmark pier target points in the total station coordinate system.

[0062] Step 5.3: Use the software to calculate the conversion relationship between the WGS-84 coordinate system and the total station coordinate system based on the coordinates of the three benchmark pier targets in the WGS-84 coordinate system and the coordinates in the total station coordinate system.

[0063] Step 5.4: Use the total station to measure the coordinates of multiple targets at each node in the total station coordinate system, and use the software to calculate the coordinates of the phase center of the receiving antenna in the total station coordinate system based on the relative position relationship between the target and the phase center of the receiving antenna.

[0064] Step 5.6: Based on the conversion relationship between the WGS-84 coordinate system and the total station coordinate system and the coordinates of the phase center of the receiving antenna in the total station coordinate system, calculate the coordinates (x0, y0, z0) of the phase center of the receiving antenna in the WGS-84 coordinate system.

[0065] Step 5.7: In step 4.2, the satellite WGS-84 coordinates (x sat ,y sat ,z sat ), combined with the WGS-84 coordinates (x0, y0, z0) of the receiving antenna phase center, the phase difference caused by the transmission path can be calculated

[0066]

[0067] Where rem(A,B) is the remainder of A divided by B, and λ is the wavelength of the GNSS signal.

[0068] Step 6: Receive link phase inconsistency calculation.

[0069] In step 3.2, the phase detector outputs the measured carrier phase difference between the local reference signal and the received GNSS signal Includes phase inconsistency in the receiving link and phase difference caused by the transmission path Two parts (due to the large elevation angle of the satellite relative to the receiving node antenna, the multipath effect can be ignored). Then the receiving link phase inconsistency It can be calculated as:

[0070]

[0071] Step 7, establish a local station center coordinate system with the receiving node antenna phase center as the origin, and calculate the coordinates of the satellite in this coordinate system, and then obtain the two-dimensional angle of the satellite relative to the receiving node antenna phase center, and associate it with the receiving link phase inconsistency obtained in step 6;

[0072] Step 8, looping steps 1-7 for a set number of times, obtaining the correlation relationship between the two-dimensional angles of multiple satellites relative to the phase center of the receiving node antenna and the phase inconsistency of the receiving link, and establishing a correlation fingerprint library;

[0073] Step 9: When measuring again, search for the corresponding receiving link phase inconsistency in the associated fingerprint library based on the two-dimensional angle of the satellite relative to the receiving node antenna phase center. For the link inconsistency between two receiving nodes, the result is as shown in the attached figure. Figure 5 As shown. The two receiving nodes received satellite GNSS signals with PRNs of 5, 15, and 18, respectively. After the above steps were processed and the fingerprint library was established, about 1 minute of data was received to obtain the phase inconsistency results of the receiving link. According to the satellite GNSS signal of PRN5, the average link inconsistency was calculated to be 4.01°, PRN15 was 7.24°, and PRN18 was 7.65°. By compounding the three sets of results, the inconsistency of the receiving link between the nodes was obtained to be 6.3°.

Claims

1. A method for calibrating a wide-area distributed multi-node receiving link based on GNSS signals, characterized in that: The following steps are involved: Step 1: The wide-area distributed node receives the GNSS data from the satellite, and performs de-framing, capture and tracking to obtain the pseudo code phase, carrier frequency information, measured carrier phase difference and original navigation message information; Step 2: Perform bit synchronization, subframe synchronization and parity check on the original navigation message information to obtain the navigation message, decode the navigation message, extract the satellite orbit elements, time parameters and Kepler parameters from it, and calculate the satellite WGS-84 coordinates by combining the pseudo code phase and carrier frequency information; Step 3, using differential GPS and total station to obtain the WGS-84 coordinates of the center of the receiving node antenna, and according to the WGS-84 coordinates of the center of the satellite and the receiving node antenna, calculate the phase difference caused by the transmission path; Step 4: Calculate the phase inconsistency of the receiving link based on the measured carrier phase difference of the received signal obtained by the tracking loop and the phase difference caused by the transmission path.

2. The method for calibrating a wide-area distributed multi-node receiving link based on GNSS signals according to claim 1, characterized in that: The following steps are also included: Step 5: Establish a local station center coordinate system with the receiving node antenna phase center as the origin, and calculate the coordinates of the satellite in this coordinate system, and then obtain the two-dimensional angle of the satellite relative to the receiving node antenna phase center; Step 6, looping steps 1-5 for a set number of times, correlating the phase inconsistency of the receiving link with the two-dimensional angle of the satellite relative to the center of the receiving node antenna, and establishing a correlation fingerprint library; Step 7: When measuring again, according to the two-dimensional angle of the satellite relative to the center of the receiving node antenna, the corresponding receiving link phase inconsistency is searched in the associated fingerprint library.

3. The method for calibrating a wide-area distributed multi-node receiving link based on GNSS signals according to claim 1, characterized in that: The specific implementation process of step 3 is as follows: Step 3.1: Build three benchmark piers and accurately determine the coordinates of the three benchmark pier targets in the WGS-84 coordinate system; Step 3.2: Use the total station to determine the coordinates of the three benchmark pier target points in the total station coordinate system; Step 3.3: Calculate the conversion relationship between the WGS-84 coordinate system and the total station coordinate system based on the coordinates of the three benchmark pier targets in the WGS-84 coordinate system and the coordinates of the total station coordinate system; Step 3.4: Use a total station to measure the coordinates of multiple targets on each receiving node in the total station coordinate system, and calculate the coordinates of the receiving node antenna center in the total station coordinate system based on the relative position relationship between the target and the receiving node antenna center; Step 3.5: Based on the conversion relationship between the WGS-84 coordinate system and the total station coordinate system and the coordinates of the center of the receiving node antenna in the total station coordinate system, calculate the coordinates of the center of the receiving node antenna in the WGS-84 coordinate system; Step 3.6: Calculate the phase difference caused by the transmission path based on the WGS-84 coordinates of the satellite and receiving node antenna centers Where rem(A,B) is the remainder of A divided by B, λ is the wavelength of the GNSS signal, (x sat ,y sat ,z sat ) is the satellite WGS-84 coordinate, (x0, y0, z0) is the WGS-84 coordinate of the receiving node antenna center.

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