Interference source identification and monitoring methods for the international search and rescue satellite system

Abstract

The invention discloses a method for identifying and monitoring interference sources of an international search and rescue satellite system. The method comprises: receiving a radio signal including a normal signal and an interference signal from a satellite by a ground receiving station, and then identifying and monitoring interference sources according to the following steps: Step 100, if the signal transmission time is continuously greater than 520ms±1%, it can be determined as an interference signal; Step 200, further determining the coding format: firstly, determining that the signal bit synchronization mark of the 1st to 15th bit and the frame synchronization mark of the 16th to 24th bit are not synchronized and consistent, it can be determined as an interference signal; secondly, if the 25th to 36th bit does not completely meet the above rules, it is determined as an interference signal; Step 300, determining the Doppler position of the interference signal. The invention adopts a globally deployed LUT ground reception to realize satellite reception data at different locations, with high coverage area and real-time performance, and accurate positioning accuracy, which is conducive to the search of interference sources, thereby reducing the interference of interference sources to the signals received by satellites and ground stations, and improving the positioning accuracy and positioning timeliness of distress beacons.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite communications, and in particular relates to an interference source identification and monitoring method based on the 406.0-406.1 MHz frequency band of an international search and rescue satellite system. Background Art

[0002] The International Search and Rescue Satellite System (COSPAS-SARSAT, Comicheskaya Sistema Poiska Avariynich Sudov-Search and Rescue Satellite) is an important part of the International Maritime Organization (IMO) Global Maritime Distress and Safety System (GMDSS). It is a system that uses satellites to provide distress alarm information services on a global scale.

[0003] See also Figure 1 This is the working principle diagram of the International Search and Rescue Satellite System. The International Search and Rescue Satellite System mainly includes three parts: beacon 1, satellite 2, and ground receiving station. The ground receiving station is divided into local user terminal 3 (LUT) and mission control center 4 (MCC). The distress beacon 1 is mainly equipped on ships, aircraft, or carried by individuals, and sends out alarm signals manually or automatically when in distress. Satellite 2 mainly includes multiple GNSS (Global Navigation Satellite System) satellites and low-orbit satellites distributed globally. LUT3 and MCC4 are distributed in 45 countries and regions around the world.

[0004] The main operation process of the international search and rescue satellite system is: when in distress, the beacon 1 sends an alarm signal to satellite 2, which is forwarded by satellite 2 to the local user terminal 3. The local user terminal 3 receives and decodes the beacon identification code and the relevant information of the location data and sends it to MCC4. MCC4 confirms the distress location and distress information and generates an alarm message (SIT-185), which is then distributed to the search and rescue center 5 to assist the search and rescue personnel to quickly carry out rescue work based on the distress location and information.

[0005] The International Search and Rescue Satellite Organization stipulates that all ships, aviation and land users around the world must be equipped with and use a unified 406MHz distress beacon. When a ship, aircraft, or individual is in danger of life, the distress beacon 1 will be automatically or manually turned on and transmit a 406.0-406.1MHz frequency band signal. Satellite 2 receives this signal, which is received and decoded by LUT3. MCC4 determines its distress coordinates and beacon information, and notifies the relevant search and rescue department 5 to conduct search and rescue. The 406.0-406.1MHz frequency band has been registered with the International Telecommunication Union (ITU) as a dedicated frequency for search and rescue and cannot be occupied at will. However, there is illegal use of the 406.0-406.1MHz frequency band around the world. Illegal transmissions in the 406.0-406.1MHz frequency band will interfere with the transmission of distress beacons and affect the reception of distress signals by ground stations, resulting in performance degradation, quality deterioration, reduced positioning accuracy, information errors or even loss. Interference with higher power may even mask the 406MHz distress beacon signal, affecting the timeliness of distress search and rescue and delaying the search and rescue of human lives.

[0006] Therefore, there is an urgent need for an interference source monitoring method to identify the interference source in the 406.0-406.1MHz frequency band and obtain its transmission position. Summary of the invention

[0007] Therefore, the purpose of the present invention is to provide an interference source identification and monitoring method for an international search and rescue satellite system, to identify the interference source in the 406.0-406.1 MHz frequency band and obtain its transmission position.

[0008] To achieve the above-mentioned object, the present invention provides an interference source identification and monitoring method for an international search and rescue satellite system, wherein the international search and rescue satellite system adopts a low-orbit search and rescue satellite system, the system includes at least one satellite and at least one ground receiving station, wherein each ground receiving station includes a local user terminal and a mission control center, the ground receiving station receives a radio signal containing a normal signal and an interference signal forwarded by the satellite, and then performs interference source identification and monitoring according to the following steps:

[0009] Step 100: If the signal transmission time is continuously greater than 520ms±1%, it is determined to be an interference signal, otherwise proceed to the next step;

[0010] Step 200, further determine the encoding format:

[0011] First, determine the bit synchronization flags of the 1st to 15th bits and the frame synchronization flags of the 16th to 24th bits of the signal. If they do not meet the requirements, they can be determined as interference signals.

[0012] Secondly, the 25th bit of a normal signal is the format identifier, the 26th bit is the protocol identifier 1, and the 27th to 36th bits are the country code. If the 25th to 36th bits do not fully comply with the above rules, it is determined to be an interference signal;

[0013] Step 300, determining the Doppler position of the interference signal:

[0014] Multiple satellites and multiple ground receiving stations are used for location positioning to locate the interference source; the mission control center distributes the monitored interference source information, including the emission time, emission frequency, emission coordinates, etc. of the interference source. More than 40 mission control centers around the world are connected through the FTP network, and multiple low-orbit ground receiving systems and data detected by different satellites are used for location positioning to confirm the location of the interference source.

[0015] The method further includes step 400, in which the interference source transmits a certain frequency:

[0016] If the interference source is detected at the same location within 72 hours, it can be identified as the same interference source and the detection count will be performed; if the location changes or it has not been monitored within 72 hours, it will be identified as another interference source and the detection count will be performed again.

[0017] The bit synchronization flags of the 1st to 15th bits and the frame synchronization flags of the 16th to 24th bits of the signal in step 200 are in synchronization with each other, which means that the first 24 bits of the signal are 1111111111111111000101111; or in self-test mode, they are 1111111111111111011010000. The 25th bit defines the message type, "0" indicates the short message format or "1" indicates the long message format.

[0018] The main formula used in step 300 to determine the Doppler position of the interference signal is:

[0019]

[0020] Where f is the observation frequency of the satellite that forwards the interference signal, Δv = v r -v s is the relative motion between the satellite and the local user terminal, v r is the speed of the local client, v s is the speed of the satellite, c is the speed of light 3×10 8 m / s, f0 is the carrier frequency transmitted by the satellite, and the frequency f versus time graph constitutes the Doppler curve.

[0021] If the location of the interference source cannot be determined in step 300, then useless interference information is generated and is not included in the interference source statistics.

[0022] The present invention adopts globally deployed ground receiving stations to realize satellite reception data in different locations, with high coverage area and real-time performance, and accurate positioning accuracy, which is conducive to finding interference sources, thereby reducing the interference of interference sources on satellite and ground station receiving signals, and improving the positioning accuracy and positioning timeliness of distress beacons.

[0023] The objectives of the present invention, as well as other objectives not listed here, are met within the scope of the independent technical solutions of the present application. The embodiments of the present invention are defined in the independent technical solutions, and the specific features are defined in the dependent technical solutions. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The technical solution of the present invention is described in detail below with reference to the accompanying drawings, wherein:

[0025] Figure 1 This is a diagram of the working principle of the International Search and Rescue Satellite System;

[0026] Figure 2 It is the coding format of the 406.0-406.1MHz distress beacon signal;

[0027] Figure 3 It is the short message coding format of 406MHz distress beacon signal;

[0028] Figure 4 It is the long message coding format of the 406MHz distress beacon signal;

[0029] Figure 5 A flow chart of a method for identifying and monitoring interference sources of an international search and rescue satellite system according to the present invention;

[0030] Figure 6 Plot of observed frequency versus time for a LEO satellite transmitting an interfering signal. DETAILED DESCRIPTION

[0031] The features and technical effects of the technical solution of the present invention are described in detail below with reference to the accompanying drawings and in combination with exemplary embodiments.

[0032] Generally speaking, any signal that causes a decrease in or damage to the reception quality of a radio signal receiving station due to illegal radio transmission can be called radio interference. The present invention mainly targets interference sources in the 406.0-406.1MHz frequency band, and uses a low-orbit satellite group and ground receiving stations in 45 countries and regions to monitor and locate interference sources in the 406.0-406.1MHz frequency band around the world, and distributes the monitored interference source information through the mission control center, including the emission time, emission frequency, emission coordinate position, etc. of the interference source. In this way, interference signals in the 406.0-406.1MHz frequency band can be identified, located, and confirmed.

[0033] The interference source of the present invention specifically refers to interference signals in the 406.0-406.1 MHz frequency band. Any signal that does not meet the requirements of the 406 MHz distress beacon is determined to be an interference signal.

[0034] The identification of interference signals in the 406.0-406.1MHz frequency band is achieved by judging whether the signal characteristics match those of a distress beacon. Real distress signals and interference signals cannot be distinguished when received by satellites, because both are sent in the 406.0-406.1MHz frequency band. The satellite is only responsible for forwarding the radio signal to the ground station, which will be responsible for distinguishing between real distress beacons and interference.

[0035] First, clarify the standard coding format used for the distress beacon coding, such as Figure 2 The figure shows the coding format of the 406MHz distress beacon signal. The figure shows the coding format of the message frame of the 406MHz distress beacon signal using two different protocols. The message type of each field is clearly defined. In the following steps, the interference signal will be judged based on these fixed flags.

[0036] The 406MHz distress beacon signal supports two messages with the same frame format but different total bit numbers, the short message is a 112-bit signal and the long message is 144 bits. Figure 3 It is the encoding format of short message; Figure 4 The encoding format for long messages.

[0037] The bits of the 406MHz distress beacon message frame are divided into five groups:

[0038] 1. Synchronization group: bits 1 to 24 are system bits, bits 1 to 15 are bit synchronization, and bits 16 to 24 are frame synchronization.

[0039] 2. PDF-1: Bits 25 to 85 are data bits. This set of data bits is called PDF-1 (Protected Data Field), and bit 25 defines the message type, "0" for short message format or "1" for long message format.

[0040] 3.BCH-1: The 86th to 106th bits are BCH error correction codes. This code can detect and correct up to 3-bit errors in PDF-1 and BCH-182 bits.

[0041] 4. The content of the fourth group depends on the message format.

[0042] Short message format: Bits 107 to 112 are not protected data bits. Figure 3 It is the encoding format of short message;

[0043] Long Message Format (PDF-2): Positions 107 to 132 are data bits protected by the second protected data field. Figure 3For short message encoding format; Figure 4 The encoding format for long messages.

[0044] 5. BCH-2: The last 12 bits are only applicable to long messages (bits 133 to 144), detecting and correcting errors of up to 2 bits out of 38 bits for BCH-2 and PDF-2.

[0045] The 406MHz distress beacon has a power output of 35-39dBm, and sends a 0.5s signal every 50s, which can be continuously transmitted for a minimum of 24 hours. The power output rise time should be less than 5ms from 10% to 90% power point. Assuming that the power output rises linearly from zero, it must be zero before 0.6ms of the rise time measurement. If it is not zero, the maximum acceptable level is -10dBm. Therefore, the 406MHz distress beacon determines the output power range.

[0046] Please also refer to Figure 1 The international search and rescue satellite ground receiving station uses the LEOSAR (Low Earth Orbit Search and Rescue) system to identify and monitor interference sources. The international search and rescue satellite system ground receiving station is divided into two parts: the local user terminal 3 (Local user terminal, LUT) and the mission control center 4 (Mission Control Center, MCC). The LUT is a satellite signal receiving station that is fixedly distributed in different regions of the world. It is used to receive the 406.0-406.1MHz signal sent by the satellite. After the signal is compiled and the ambiguity is analyzed, the interference source information is screened out and transmitted to the destination MCC through the network through the MCC system, and finally the interference source detection result is formed to complete the monitoring of the interference source in the 406.0-406.1MHz frequency band.

[0047] Please also see Figure 5 The present invention is a flow chart of the interference source identification and monitoring method of the international search and rescue satellite system; the search and rescue satellite system includes 1 to N satellites, and 1 to N LUTs and MCCs located on the ground. The system receives 406.0-406.1MHz frequency band signals forwarded by the satellites, including real distress signals and interference signals in this frequency band, and first distinguishes interference from real distress signals. The present invention determines that signals that do not meet all the above 406MHz distress beacon specifications are interference signals. First, the interference signal and the real distress signal are distinguished according to the following steps:

[0048] 1. Step 100, determine signal transmission rules

[0049] The 406MHz distress beacon bursts every 50s, and each transmission time is about 520ms±1%. When the 406MHz distress beacon is activated, a CW (consisting of an unmodulated carrier measured between the 90% power point and the start of modulation) preamble is generated before the coded data (message and error signal) is transmitted. The CW carrier lasts for about 160ms+1% (between 158.4ms-161.6ms), and then the long message is transmitted for 360ms (144 bits of information), and the short message is transmitted for 280ms (112 bits of information). The transmission bit rate is 400bps. Most interference signals in the 406MHz band usually last for a long time, and the interference signals can be distinguished by the signal transmission time. If the signal transmission time lasts for more than 520ms±1%, it can be determined as an interference signal, otherwise proceed to the next step.

[0050] 2. Step 200, determine the encoding format

[0051] The 406MHz distress beacon specifies that bits 1-24 are system bits, of which bits 1-15 have a bit synchronization identifier of 1111111111111111 (15 1s), and bits 15-24 have a frame synchronization identifier of 000101111 (011010000 in self-check mode). A distress beacon signal is identified only when the bit synchronization and frame synchronization are consistent (i.e., the first 24 bits of the signal are 111111111111111000101111), otherwise it will be identified as an interference signal. In self-check mode, the bit synchronization and frame synchronization are consistent, which are 1111111111111111011010000, otherwise it will be identified as an interference signal.

[0052] If the first 24 bits of the interference signal are exactly the same, it is necessary to verify through the 25th to 36th bit encoding. The 25th bit of the normal signal identifies the message type (0 for short message, 1 for long message), the 26th bit protocol identifier is 1, and 27-36 bits are the country code. If the 25th to 36th bits do not fully comply with the above rules, they will be judged as interference signals.

[0053] Signals judged to be normal will be processed normally as distress information. If judged to be interference information, they will be distributed to the corresponding mission control center for processing.

[0054] 3. Step 300, determining the Doppler position of the interference signal

[0055] The interference source positioning of the present invention is to use the interference signal itself to locate the interference source, which is called passive positioning. According to the Doppler effect between the satellite and the interference source, the relative speed between the interference source and the satellite is calculated. Combined with the measurement data of multiple low-orbit ground receiving systems and different satellites, the distance difference between the interference source and multiple satellites is obtained, and the hyperboloid intersection positioning principle is adopted to form two or more hyperboloids. The intersection point is formed by the intersection of the hyperboloids, so as to calculate the position of the interference source and realize the interference source positioning. It is impossible to determine the position of the interference source by providing information only by a single satellite, and multiple satellites are required to assist in realization. Therefore, it is necessary to connect the MCC, take the signals detected by multiple satellites and the ground receiving station for data reception and exchange, realize the interference source positioning, and distribute the monitored interference source information through the MCC, including the emission time, emission frequency, emission coordinate position, etc. of the interference source.

[0056] The location of the interference source is achieved by Doppler processing of the signal by the orbiting LEO (Low Earth Orbit Satellite) satellite. The LUT can independently process the search and rescue repeater downlink spectrum to determine the Doppler curves from the beacon and the interference source.

[0057] When the LEO satellite moves relative to the LUT, a frequency change relative to the LUT will occur. When the LEO satellite approaches the LUT, each carrier wave is transmitted from a closer position than the previous one. Therefore, the received wavelength becomes shorter and the frequency increases. Conversely, when the LEO satellite moves away from the LUT, each carrier wave is transmitted from a farther position than the previous one. Therefore, the received wavelength becomes longer and the frequency decreases. When the LEO satellite's relative speed Δv is much lower than the carrier wave's transmission speed, the observed frequency f of the LEO satellite transmitting the interference signal can be expressed as:

[0058]

[0059] Where Δv = v r -v s is the relative motion between the satellite and the LUT, v r is the speed of the LUT, v s is the speed of the satellite, c is the speed of light 3×10 8 m / s, f0 is the carrier frequency transmitted by the satellite.

[0060] The Doppler shift is the frequency difference between the transmitter and the receiver:

[0061] Δf=f-f0

[0062] Then we have:

[0063]

[0064] The relative speed between the satellite and the interference source can be expressed as:

[0065]

[0066] is the satellite's velocity vector, is the velocity vector of the interference source, so It is the relative speed of the satellite to the interference source. is the normalized vector from the interference source to the satellite, is the satellite’s position vector, is the position vector of the interference source, and θ is the propagation angle from the interference source to the satellite.

[0067] Due to the movement between the interferer and the satellite, the received frequency is highest when the satellite initially enters the field of view of an interferer and continues to decrease until the interferer is out of view. The resulting frequency versus time graph constitutes the Doppler curve. The LUT analyzes the Doppler curve to determine the inflection point in the curve (i.e., the point where the curve changes curvature). If the Doppler curve does not contain an inflection point, the LUT infers that the curve is either forward or backward in time to estimate the inflection point. At the inflection point, the Doppler shift is zero because the instantaneous relative velocity Δv between the interferer and the satellite is zero at this time. This is the time of closest approach (TCA) between the interferer and the satellite. The LUT can also determine the slope of the Doppler curve at the TCA. The slope at the TCA and the time are critical to determining the geographic location of the source.

[0068] For a frequency versus time curve, see Figure 6 , which includes the inflection point when the satellite moves towards the interference source, the frequency changes from the maximum value at time T1, passes through 0 at time T2, and becomes the minimum value at time T3. This is the Doppler shift effect.

[0069] By determining the inflection point of the Doppler curve, the time when the satellite is closest to the interference source is calculated. Combined with the satellite position, the satellite, the interference source position and the center of the earth form a triangle. Applying trigonometric functions, the distance equation between the satellite and the interference source can be obtained. Combining the measurement data of multiple satellites and LUT stations, multiple ranging equations can be formed for the same interference source. After solving the polynomial equation, the location of the interference source can be calculated.

[0070] Step 400, confirm the location and frequency of the interference source:

[0071] With the location information of the interference source in the 406.0-406.1MHz frequency band, the location of the interference source needs to be confirmed. More than 40 mission control centers around the world are connected through the FTP (File Transfer Protocol) network, using signals detected by multiple satellites and ground receiving stations to receive and exchange data to confirm the location of the interference source. The same interference source may continue to transmit. If the interference source is detected at the same location within 72 hours, it will be identified as the same interference source and detected and counted; once the location changes or has not been monitored within 72 hours, it will be identified as another interference source and the detection and counting will be performed again ( Figure 5 Interference detection flow chart).

[0072] Based on the interference source monitoring results, the MCC screens out the interference sources within the service area and distributes the information such as the interference source transmission time, transmission frequency, and transmission coordinate location to the MCCs in the service area. The MCC staff in the area submits the interference location data to the radio management department for investigation.

[0073] The advantage of this interference monitoring technology is that it uses globally deployed ground receiving stations to achieve satellite data reception in different locations, with high coverage area and real-time performance, and accurate positioning accuracy, which is conducive to the search for interference sources, thereby reducing the interference of interference sources on satellite and ground station receiving signals, and improving the positioning accuracy and timeliness of distress beacons.

[0074] Although the present invention has been described with reference to one or more exemplary embodiments, those skilled in the art will appreciate that various suitable changes and equivalents may be made to the device structure without departing from the scope of the present invention. In addition, many modifications that may be suitable for specific situations or materials may be made by the disclosed teachings without departing from the scope of the present invention. Therefore, the purpose of the present invention is not to be limited to the specific embodiments disclosed as the best mode for implementing the present invention, and the disclosed device structure and its manufacturing method will include all embodiments falling within the scope of the present invention.

Claims

1. A method for identifying and monitoring interference sources of an international search and rescue satellite system, wherein the international search and rescue satellite system adopts a low-orbit search and rescue satellite system, the system includes at least one satellite and at least one ground receiving station, wherein each ground receiving station includes a local user terminal and a mission control center, and the ground receiving station receives a radio signal containing a normal signal and an interference signal sent by the satellite, and then performs interference source identification and monitoring according to the following steps: Step 100: If the signal transmission time is continuously greater than 520ms±1%, it can be determined as an interference signal, otherwise proceed to the next step; Step 200, further determine the encoding format: First, if the bit synchronization mark of the 1st to 15th bit of the signal and the frame synchronization mark of the 16th to 24th bit are not synchronized, it can be determined as an interference signal; Secondly, the 25th bit of a normal signal identifies the message type, the 26th bit identifies the protocol as 1, and the 27th to 36th bits are the country code. If the 25th to 36th bits do not fully comply with the above rules, it is considered an interference signal. Step 300: If it is determined to be an interference signal, the Doppler position of the interference signal is measured and sent to a mission control center: Multiple satellites and multiple ground receiving stations are used for positioning to determine the location of the interference source; and the monitored interference source information is distributed through the mission control center, including the interference source's transmission time, transmission frequency, and transmission coordinate position.

2. The method according to claim 1, characterized in that: The method also includes step 400, in which the interference source location information is confirmed: By connecting more than 40 mission control centers around the world through a file transfer protocol network, the interference source location is confirmed by using signals detected by multiple satellites and low-orbit ground receiving stations to receive and exchange data. If an interference source is monitored at the same location within 72 hours, it can be identified as the same interference source and detected and counted. Once the location changes or has not been monitored within 72 hours, it is identified as another interference source and the detection and counting is repeated.

3. The method according to claim 1, characterized in that: The bit synchronization flag of the 1st to 15th bits and the frame synchronization flag of the 16th to 24th bits of the signal in step 200 are synchronously consistent, which means that the first 24 bits of the signal are 1111111111111111000101111; or 1111111111111111011010000 in self-test mode.

4. The method according to claim 1, characterized in that: The formula used to determine the Doppler position of the interference signal in step 300 is: Where f is the observation frequency of the satellite transmitting the interference signal, Δv = v r -v s is the relative motion between the satellite and the local user terminal, v r is the speed of the local client, v s is the speed of the satellite, c is the speed of light 3×10 8 m / s, f0 is the carrier frequency transmitted by the satellite, and the frequency f versus time graph constitutes the Doppler curve.

5. The method according to claim 1, characterized in that: If the location of the interference source cannot be determined in step 300, it is determined that there is no location interference information.

6. The method according to claim 1, characterized in that: In step 200, bit 25 defines the message type, "0" indicates a short message format or "1" indicates a long message format.

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