A method for compressing and transmitting RTK differential data based on DMR narrowband channel

Abstract

The present application relates to a kind of RTK difference data compression transmission methods based on DMR narrowband channel, belong to railway wireless communication and satellite positioning technology field, comprising: S1 message simplification step, parse RTCM3.x format RTK difference data message, only keep the necessary observation field under marshalling station application scenario, delete redundant information field and unnecessary satellite system observation data block;S2 difference encoding step, the change of each observation is calculated adjacent epoch instead of absolute value transmission;S3 quantization compression step;S4 frame structure design step;S5 adaptive rate control step.The present application compresses RTCM difference message to the range that DMR narrowband channel can bear, after compression data rate is less than 4.8kbps, positioning accuracy loss is less than 5cm, end-to-end transmission delay is less than 500ms, it is applicable to the real-time transmission of high-precision positioning data under the limited bandwidth scene such as railway marshalling station.

Description

Technical Field

[0001] This invention relates to the field of railway wireless communication and satellite positioning technology, and in particular to an RTK differential data compression transmission method based on a DMR narrowband channel. Background Technology

[0002] RTK (Real-time Dynamic Differential Positioning) technology can achieve centimeter-level positioning accuracy, which is of great value for tracking the position of shunting locomotives and personnel in marshalling yards. The RTK system requires the base station to send differential correction data to the rover station in real time. The differential data message length of the standard RTCM3.x format is about 200-500 bytes, the update frequency is 1-10Hz, and the data rate is about 2-25kbps. In contrast, the DMR system has a single time slot data carrying capacity of only about 9.6kbps (including overhead) and needs to share channel resources with voice services, so it cannot directly carry standard RTCM data.

[0003] In existing technologies, RTCM 3.x standard messages (such as message types 1004, 1005, and 1012) use fixed field length encoding, which contains a large number of redundant information fields in specific application scenarios. For example, the standard 1004 message (GPS observations) contains complete observation data slots for 32 satellites, but in actual marshalling yard scenarios, the number of visible satellites is usually 8-15, resulting in about 50%-75% of the fields being empty. In addition, RTCM messages contain observation data from multiple systems such as GLONASS, Galileo, and BeiDou, while marshalling yard scenarios only require GPS+BDS dual systems to meet positioning needs.

[0004] Therefore, there is an urgent need for a transmission method that can effectively compress RTCM differential messages to the range that DMR narrowband channels can carry, while ensuring that the positioning accuracy meets the application requirements of marshalling yards. Summary of the Invention

[0005] This invention provides an RTK differential data compression and transmission method based on a DMR narrowband channel, which solves the problems mentioned in the background technology above, compresses RTCM differential messages to the range that the DMR narrowband channel can carry, and at the same time ensures that the positioning accuracy meets the application requirements of marshalling yard.

[0006] The solution to the above-mentioned technical problems of the present invention is as follows: an RTK differential data compression and transmission method based on DMR narrowband channel, including the following steps: S1 message simplification step, parsing RTCM3.x format RTK differential data message, retaining only the observation fields necessary for marshalling station application scenarios, deleting redundant information fields and unnecessary satellite system observation data blocks. The function of this step is to eliminate redundant information unrelated to marshalling station positioning, significantly reducing the amount of data to be transmitted from the source, laying the foundation for subsequent efficient compression. Its advantage is that it can minimize data redundancy while retaining all the core information required for RTK calculation.

[0007] The S2 differential coding step uses differential coding on each retained observation, calculates the change between adjacent epochs instead of the absolute value for transmission, and forces the transmission of a key frame carrying complete observation data after every N differential frames, where N is a positive integer. The function of this step is to take advantage of the strong temporal correlation of RTK observations, converting large absolute values ​​into smaller changes to save transmission bits. At the same time, the periodic insertion of key frames prevents the accumulation of decoding errors caused by wireless packet loss. Its advantage is that it balances compression efficiency and transmission reliability, and can maintain data synchronization even when there are bit errors in the wireless channel.

[0008] The S3 quantization compression step quantizes the difference or absolute value of each observation into a fixed-point integer according to a preset quantization bit and quantization step size, forming a compressed data payload. The function of this step is to further compress the number of bits required for data representation through the conversion from floating-point numbers to fixed-point integers. Its advantage is that it can maximize the compression of data volume while keeping the quantization error within the allowable range of RTK positioning accuracy.

[0009] The S4 frame structure design steps involve designing a dedicated data frame format adapted to the DMR general data transmission time slot. The data frame includes at least a frame header, compressed data payload, and a cyclic redundancy check field. The function of this step is to enable the compressed data to be reliably transmitted within the standard DMR protocol framework. Its advantage is that it enhances the practicality and standard compatibility of the method, ensuring that the data can be directly deployed in existing DMR systems.

[0010] The S5 adaptive rate control step dynamically adjusts the data update frequency and compression mode based on the quality indicators of the DMR channel. The function of this step is to enable the system to intelligently adjust the transmission strategy according to real-time channel conditions. Its advantages are that it can provide high-precision and high-frequency services when the channel is good, and ensure the continuity and robustness of data transmission when the channel is bad, thereby improving the system's adaptability to the complex electromagnetic environment of the marshalling yard.

[0011] Furthermore, in the message simplification step, the field retention determination adopts the following rules: Rule 1: If the field is a direct input parameter of the RTK solution algorithm, including pseudorange, carrier phase, satellite number and base station coordinates, it must be retained. Rule 2: If the value of this field is fixed in the marshalling yard application scenario, including reference frame, code type and system flag, then the field is deleted and the receiving end restores it in a pre-configured manner. Rule 3: If the field is supplementary information to other fields and does not affect positioning accuracy, then bit compression shall be performed; Rule 4: Only retain observation data from GPS system PRN1-32 and BDS system PRN1-63, and delete data blocks from GLONASS, Galileo, and QZSS systems; Rule 5: Traverse the satellite data blocks. If the pseudorange and carrier phase fields of a satellite are both zero or filled, delete the entire block and only transmit the valid satellite data.

[0012] The function of the above rules is to provide a systematic and quantifiable judgment standard for the message simplification process, so that the simplification operation has clear consistency and repeatability. Its advantage is that it avoids the uncertainty caused by subjective judgment, and ensures that the simplified data can not only meet the integrity requirements of RTK calculation, but also achieve the maximum compression efficiency. At the same time, through the dual screening of rules four and five, only the satellite data that is actually visible and valid in the marshalling yard scenario is transmitted, further reducing the proportion of invalid data.

[0013] Furthermore, the differential coding includes the following steps: The coordinate difference of the base station is given by P(n) for the coordinates of the base station in the nth epoch. If P(n) = (X(n), Y(n), Z(n)), then the difference value ΔP(n) = P(n) - P(n-1), n ​​≥ 2. The first frame uses ΔP(1) = P(1) - P_ref, where P_ref is the reference coordinate. The pseudorange observation difference is given by ρ_k(n) for the k-th satellite in the n-th epoch. The difference value is Δρ_k(n) = ρ_k(n) - ρ_k(n-1). Carrier phase differential: Let the carrier phase of the k-th satellite in the n-th epoch be φ_k(n), then the differential value Δφ_k(n) = φ_k(n) - φ_k(n-1); The signal-to-noise ratio difference is ΔSNR_k(n) = SNR_k(n) - SNR_k(n-1). The differential coding fault tolerance mechanism uses a default interval N between the key frame and the differential frame, which is 10. When the receiver receives a differential frame, it restores the previous frame as a reference. If the cyclic redundancy check fails, the frame and all subsequent differential frames are discarded until the next key frame is received and then resynchronized.

[0014] The function of the aforementioned differential coding scheme is to adopt appropriate differential strategies for four different physical characteristics of observations: reference station coordinates, pseudorange, carrier phase, and signal-to-noise ratio, so that each observation can obtain the optimal compression effect. Its advantages are that the reference station coordinate differential takes advantage of the fixed installation of the reference station and the zero displacement between epochs, the pseudorange differential takes advantage of the continuity of satellite motion in a short period of time, the carrier phase differential takes advantage of the stability of the phase change rate, and the signal-to-noise ratio differential takes advantage of the slow change of signal strength. The combination of the four achieves the best compression of various observations. The fault tolerance mechanism is to periodically reset the decoding reference through key frames. Its advantage is that it effectively curbs the spread of bit errors in wireless transmission and ensures that the receiver can quickly restore data synchronization after encountering continuous packet loss.

[0015] Furthermore, in the quantization compression step, the quantization parameters for each observation are set as follows; Base station coordinate difference ΔP: 16-bit quantization, range [-327.67mm, +327.67mm], quantization step size 0.01mm; Pseudorange differential Δρ: 20 bits quantization, range [-5242.87m, +5242.87m], quantization step size 0.01m; Carrier phase differential Δφ: 20 bits quantization, value range [-524.288 Hz, +524.288 Hz], quantization step size 0.001 Hz; Signal-to-noise ratio differential ΔSNR: 4 bits quantization, value range [-4dB-Hz, +4dB-Hz], quantization step size 0.5dB-Hz; The absolute value of the pseudorange in the keyframe: quantization bit depth 24 bits, value range [0m, 29999999.99m], quantization step size 0.01m; Absolute carrier phase value in keyframes: 20 bits quantization, value range [-524288 Hz, +524287 Hz], quantization step size 1 Hz; The quantization formula is Q=round((V-V_min) / step), and the dequantization formula is V'=Q×step+V_min, where V is the original floating-point value, V_min is the lower limit of the range, step is the quantization step size, and Q is the quantized integer value.

[0016] The function of the above quantization parameter settings is to accurately match the quantization bit width and step size for each observation value. The coordinate difference uses 16-bit quantization, with a maximum quantization error of only 0.005mm, and the accuracy loss is negligible. The pseudorange difference uses 20-bit quantization, which can cover a dynamic range of ±5242m, which is sufficient to cope with pseudorange changes caused by satellite motion and receiver clock errors. The carrier phase difference uses 20-bit quantization, which can cover a wide range of ±524 cycles and can accommodate abnormal situations such as cycle slips. The signal-to-noise ratio difference uses 4-bit quantization to transmit slowly changing signal quality information at the lowest bandwidth cost. The overall scheme minimizes the data volume while meeting the RTK millimeter-level positioning accuracy requirements, achieving an optimized balance between compression ratio and positioning accuracy.

[0017] Furthermore, the frame format in the frame structure design step includes the following fields: The synchronization word, which is 2 bytes long, is used for frame synchronization detection at the receiving end. Frame Type, 1 bit long, used to distinguish between differential frames and keyframes; Compression Level, 3 bits in length, indicates the current compression mode. Station ID, 1 byte in length; Epoch Time, 20 bits in length, represents time within GPS seconds with a resolution of 1ms; Sat Count, 5 bits in length, indicates the number of satellites contained in this frame; GPS satellite visibility bitmap (Sat Flag Bitmap), 4 bytes in length, uses PRN1-32 to indicate whether each GPS satellite has data; BDS Flag Bitmap, 8 bytes in length, identifies whether each BDS satellite has data according to PRN1-63; The coordinate differences ΔX / ΔY / ΔZ are each 16 bits long, for a total of 48 bits; The satellite data block is of variable length and is arranged sequentially for each satellite. A single satellite data block in a differential frame contains 16 bits of L1 pseudorange differential value, 20 bits of L1 carrier phase differential value, and 4 bits of L1 signal-to-noise ratio differential value, and selectively includes 16 bits of L2 pseudorange differential value, 20 bits of L2 carrier phase differential value, and 4 bits of L2 signal-to-noise ratio differential value, depending on the compression level. A single satellite data block in a key frame contains 24 bits of L1 pseudorange absolute value, 20 bits of L1 carrier phase absolute value, 6 bits of L1 signal-to-noise ratio absolute value, 4 bits of L1 signal lock time indicator, 24 bits of L2 pseudorange absolute value, 20 bits of L2 carrier phase absolute value, 6 bits of L2 signal-to-noise ratio absolute value, and 4 bits of L2 signal lock time indicator. Cyclic Redundancy Check (CRC-16) is used, with a length of 2 bytes, covering the frame type field to the end of the data payload.

[0018] The aforementioned frame format design functions to structurally encapsulate compressed data according to the DMR standard time slot requirements. The frame structure is highly compact, with each differential frame requiring only 10 bytes per satellite. For 20 satellites, the entire frame is approximately 68 bytes, corresponding to a transmission rate of about 550 bps, far below the DMR single time slot's carrying capacity limit of 4.8 kbps. This reserves ample channel resources for voice and other data services. The synchronization word ensures frame boundary detection at the receiver. The frame type field enables rapid identification of differential frames and key frames. The compression level field supports multi-mode decoding with adaptive rate control. The GPS and BDS dual visibility bitmap mechanism allows the receiver to flexibly parse non-uniformly distributed satellite data without allocating fixed slots for each satellite, further saving bandwidth. CRC-16 checksums cover the entire data payload, ensuring the integrity of the transmitted data.

[0019] Furthermore, in the adaptive rate control step, the received signal strength index (RSSI) and bit error rate (BER) provided by the DMR system are used to comprehensively evaluate the channel quality; Define the channel quality index CQI as α × normalize(RSSI) + (1-α) × normalize(1 / BER), where α = 0.6 and normalize() is the normalization function; RSSI normalization is as follows: RSSI ≥ -70dBm is mapped to 1.0, RSSI ≤ -110dBm is mapped to 0.0, with intermediate linear interpolation; BER normalization is as follows: BER ≤ 10 -4 Mapped to 1.0, BER≥10 -2 Mapped to 0.0, intermediate logarithmic interpolation; The channel is divided into five levels according to the CQI value: Level 0 is [0.8, 1.0] corresponding to excellent, Level 1 is [0.6, 0.8) corresponding to good, Level 2 is [0.4, 0.6) corresponding to average, Level 3 is [0.2, 0.4) corresponding to poor, and Level 4 is [0, 0.2) corresponding to very poor. The update frequency and compression mode corresponding to each level are as follows: Level 0 uses a 10Hz update frequency, with one keyframe inserted every 10 frames of differential frames and keyframes; Level 1 uses a 5Hz update frequency, with one keyframe inserted every 5 frames of differential frames and keyframes; Level 2 uses a 2Hz update frequency, with one keyframe inserted every 5 frames of L1 differential frames and keyframes; Level 3 uses a 1Hz update frequency, with one keyframe inserted every 3 frames of L1 simplified differential frames and keyframes; Level 4 uses a 0.5Hz update frequency, with only keyframes transmitting complete L1+L2 data. The level switching introduces an anti-jitter mechanism: downgrade switching requires the CQI to remain in the lower level range for T_down seconds before execution, with T_down being 3s, 3s, 2s, and 1s for each level; upgrade switching requires the CQI to remain in the higher level range for T_up seconds before execution, with T_up being 5s for all levels; T_up must be greater than T_down; if the CQI suddenly drops below 0.1, it will immediately downgrade to Level 4. The adaptive rate control scheme described above achieves real-time and accurate quantification of DMR wireless channel quality through weighted fusion of RSSI and BER, and dynamically selects the optimal transmission strategy based on the quantization results. RSSI reflects signal strength attenuation, while BER reflects demodulation error rate. The complementarity of the two allows CQI to comprehensively characterize the actual transmission conditions of the channel. The five-level fine classification enables the system to smoothly switch to the most suitable combination of transmission parameters under different channel conditions. The asymmetric design of the anti-jitter mechanism, with fast degradation response and slow upgrade response, can quickly protect the continuity of data transmission when the channel deteriorates, and avoid frequent switching caused by instantaneous fluctuations when the channel improves briefly. The emergency degradation rule ensures that the system can immediately enter the most robust transmission mode when sudden strong interference occurs, significantly improving the stability of the system in the complex electromagnetic environment of the marshalling yard.

[0020] Furthermore, during each level switch, the transmitting end continuously sends three data frames carrying the new level identifier to ensure synchronization of the receiving end; the receiving end decodes based on the compression level field in the most recently received data frame; if the receiving end does not receive a frame with valid cyclic redundancy check within 2 seconds, it automatically lowers the level and attempts to decode. The aforementioned synchronization mechanism ensures the consistency of compression modes between the transmitting and receiving ends during level switching. It uses a redundant transmission strategy to ensure reliable notification of level switching, so that even under channel conditions with a high packet loss rate, the receiving end still has a high probability of receiving the new level identifier. The receiving end uses the compression level field in the most recently received valid frame as the standard for decoding, avoiding decoding confusion caused by out-of-order delivery or delay. The automatic downgrade attempt mechanism further enhances the system's self-recovery capability. Even in the extreme case of losing multiple frames consecutively, the decoder can still attempt to parse the data by gradually reducing the expected level, avoiding prolonged communication interruptions.

[0021] Furthermore, the method is applied to an RTK differential data transmission system consisting of a base station and a mobile station. The GNSS receiver at the base station outputs raw RTCM3.x data to the compression module for message simplification, differential coding, and quantization compression. The compressed data is then sent to the DMR base station for transmission via a wireless channel. The DMR terminal at the mobile station receives the data, decompresses it to restore the RTCM data, and then sends it to the GNSS receiver for RTK calculation. The compression module is integrated into the DMR base station equipment or connected to the DMR base station via Ethernet as a standalone device.

[0022] The above system architecture solution establishes a complete data transmission link from the base station GNSS receiver to the rover GNSS receiver. The two deployment methods of the compression module provide flexible options for different scenarios: integrated deployment is suitable for new systems, enabling a compact equipment structure and the lowest transmission latency; independent equipment deployment is suitable for the transformation of existing DMR systems in marshalling yards, without the need to replace base station hardware, and can be accessed via Ethernet, which greatly reduces the difficulty of engineering implementation and system upgrade costs, and has good promotional value.

[0023] The beneficial effects of this invention are as follows: This invention provides an RTK differential data compression and transmission method based on a DMR narrowband channel, which has the following advantages: 1. The compressed data rate does not exceed 4.8kbps, which can be reliably carried in a single DMR time slot. The measured average rate is about 0.5 to 5.5kbps, which is dynamically adjusted according to the channel quality. Sufficient channel resources are reserved for voice services and other data services, solving the bottleneck problem that standard RTCM data cannot be directly transmitted in the DMR narrowband channel.

[0024] 2. The positioning accuracy loss is less than 5cm. Compared with the original RTCM data, the measured difference is less than 1cm. The error introduced by quantization is far less than the RTK positioning accuracy requirements, which fully meets the application requirements of marshalling yard horizontal accuracy better than 0.5m and vertical accuracy better than 0.8m.

[0025] 3. The end-to-end transmission latency is less than 500ms, covering the entire process of compression processing, DMR wireless transmission and decompression and restoration, meeting the real-time operation requirements of mobile carriers such as marshalling locomotives in marshalling yards for positioning data.

[0026] 4. It supports GPS and BDS dual-system differential. The compressed frame length under dual system is about 68 to 76 bytes, covering the typical number range of visible satellites in the marshalling yard scenario. At the same time, through the dual filtering mechanism of rule four and rule five, it effectively removes non-essential system data such as GLONASS, Galileo, QZSS and invalid satellite data.

[0027] 5. It is compatible with the DMR intelligent channel sharing system, which can dynamically switch with voice services, make full use of the channel resource scheduling capability of the DMR system, and support seamless integration with the existing railway wireless communication system.

[0028] 6. Adaptive rate control comprehensively evaluates channel quality based on RSSI and BER, finely adjusts five-level channel levels, effectively avoids frequent switching through an asymmetric anti-jitter mechanism, and ensures data transmission continuity under sudden interference through emergency degradation rules, achieving an intelligent balance between compression efficiency and transmission reliability.

[0029] 7. Differential coding and keyframe mechanism balance compression efficiency and fault tolerance. The keyframe interval is configurable, with a default of 10 seconds, which effectively suppresses the spread of errors in the wireless channel. The receiver can quickly recover decoding synchronization after encountering continuous packet loss.

[0030] 8. The compression module supports both integrated and standalone deployment. The standalone deployment solution does not require replacement of existing DMR base station hardware and can be accessed via Ethernet. It is suitable for the transformation of existing DMR systems in marshalling yards and has good engineering promotion value and economic benefits.

[0031] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it according to the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Specific embodiments of the present invention are given in detail below with reference to the accompanying drawings. Attached Figure Description

[0032] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings: Figure 1 A flowchart of a compression transmission method for RTK differential data compression transmission based on a DMR narrowband channel is provided in an embodiment of the present invention. Figure 2A comparison diagram of the RTCM message structure and simplified scheme of an RTK differential data compression transmission method based on a DMR narrowband channel provided in an embodiment of the present invention; Figure 3 A diagram of the DMR data frame format of an RTK differential data compression and transmission method based on a DMR narrowband channel provided in an embodiment of the present invention; Figure 4 A compression ratio-accuracy relationship curve of an RTK differential data compression and transmission method based on a DMR narrowband channel provided in an embodiment of the present invention; Figure 5 This is a system integration architecture diagram of an RTK differential data compression and transmission method based on a DMR narrowband channel, provided as an embodiment of the present invention. Detailed Implementation

[0033] The following is in conjunction with the appendix Figure 1-5 The principles and features of the present invention are described below. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. The invention is described more specifically in the following paragraphs by way of example with reference to the accompanying drawings. The advantages and features of the invention will become clearer from the following description. It should be noted that the drawings are in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of the invention.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0035] Example 1: This example provides a specific hardware implementation scheme for an RTK differential data compression and transmission method based on a DMR narrowband channel; The base station configuration is as follows: The GNSS receiver uses a Trimble BD990 or u-blox ZED-F9P to output RTCM3.x raw differential data. The compression module uses an embedded processor STM32H743, which is based on the ARM Cortex-M7 core, has a main frequency of 480MHz, and runs the FreeRTOS real-time operating system. The compressed data is sent to the DMR base station through the UART / SPI interface and transmitted through the DMR wireless channel (400-470MHz band, 12.5kHz channel bandwidth).

[0036] The mobile station is configured as follows: The DMR terminal uses Hytera PD980 or Motorola MOTOTRBO series products to receive DMR wireless signals. The decompression module is integrated in the DMR terminal MCU and uses an STM32F407 processor. The decompressed RTCM data is sent to the mobile station GNSS receiver for RTK calculation through the UART interface.

[0037] Example 2: The compression module supports two deployment methods; In the integrated deployment method, the compression module is integrated inside the DMR base station equipment, and the compression algorithm runs as part of the base station firmware. This method is suitable for newly built DMR systems and can achieve a compact equipment structure and the lowest transmission latency.

[0038] In the standalone equipment deployment method, the compression module is connected to the DMR base station via Ethernet as a standalone device. This method is suitable for the transformation of existing DMR systems in marshalling yards. It does not require replacement of base station hardware and can be accessed via Ethernet, which greatly reduces the difficulty of engineering implementation and the cost of system upgrade.

[0039] Example 3: Specific implementation of message simplification steps; This embodiment details the implementation process of the S1 message simplification step. For a comparison of the corresponding RTCM message structure and simplification scheme, please refer to [link / reference needed]. Figure 2 As shown.

[0040] After the compression module is powered on and initialized, it first receives the RTCM3.x format data stream output by the GNSS receiver through the UART interface. The compression module performs frame synchronization detection on the received data stream, identifies the RTCM3.x protocol frame header (0xD3), parses out each message type (such as 1004, 1005, 1006, etc.), and performs simplification processing for each RTCM message frame according to the following field retention judgment rules: Table 1. Simplification rules for RTCM 1004 message (GPS observation) fields;

[0041] Table 2. Simplification Rules for Fields in RTCM 1005 / 1006 Messages (Base Station Coordinates);

[0042] The rules for determining field retention are as follows: Rule 1 (Necessity Determination): If the field is a direct input parameter of the RTK solution algorithm (pseudorange, carrier phase, satellite number, base station coordinates), it must be retained; Rule 2 (Fixed Value Judgment): If the value of this field remains fixed in the marshalling yard application scenario (such as reference frame, code type, system flag), then the field is deleted and the receiving end restores it in a pre-configured manner; Rule 3 (Redundancy Determination): If the field is supplementary information to other fields and does not affect positioning accuracy (such as a high-precision representation of signal lock time), then bit compression is performed; Rule 4 (Satellite Selection): Only retain observation data from the GPS system (PRN 1-32) and the BDS system (PRN 1-63), and delete data blocks from other systems such as GLONASS, Galileo, and QZSS; Rule 5 (Empty Satellite Removal): Traverse the satellite data blocks. If the pseudorange and carrier phase fields of a satellite are both zero or filled, delete the entire block and only transmit valid satellite data. After simplification, approximately 50%-75% of the redundant fields in the original RTCM message are removed, leaving only the core observation data necessary for RTK calculation.

[0043] Example 4: This example details the specific implementation process of the S2 differential coding step; After the compression module completes message simplification, it performs differential coding on the retained observations. Differential coding utilizes the strong temporal correlation of RTK observations to convert large absolute values ​​into smaller changes, thus saving transmission bits. The specific coding process is as follows: 1. Base station coordinate differential encoding: Let the coordinates of the base station in the nth epoch be P(n) = (X(n), Y(n), Z(n)), then the differential value is: ΔP(n) = P(n) - P(n-1) (n≥2) ΔP(1) = P(1) - P_ref (The first frame uses reference coordinates P_ref) Since the base station is a fixed station, the coordinate change within an adjacent second is usually <1mm, and the differential value range is [-5mm, +5mm]; 2. Differential encoding of pseudorange observations: Let ρ_k(n) be the pseudorange of the k-th satellite at epoch n. Then the difference value is: Δρ_k(n) = ρ_k(n) - ρ_k(n-1) For a 1Hz update rate, the pseudorange variation rate caused by satellite motion is approximately ±800m / s, but the variation within a 1s sampling interval is mainly affected by the receiver clock error, and the actual differential range is [-20m, +20m]. 3. Carrier phase differential coding, the unit of carrier phase observation is cycle, L1 frequency 1575.42MHz corresponds to wavelength λ1=0.1903m, let the carrier phase of the k-th satellite in the nth epoch be φ_k(n), then the differential value is: Δφ_k(n) = φ_k(n) - φ_k(n-1) Under normal circumstances, the variation is ±10 cycles / second, and a 20-bit signed integer representation can cover large differential values ​​under abnormal conditions such as cycle slips; 4. Signal-to-noise ratio differential coding, ΔSNR_k(n) = SNR_k(n) - SNR_k(n-1) SNR changes slowly, with a differential range of [-5dB-Hz, +5dB-Hz]; 5. Differential coding fault tolerance mechanism: After every N differential frames are transmitted, a key frame carrying complete observation data is forcibly sent. N is 10 by default (i.e., one key frame every 10 seconds). When the receiver receives a differential frame, it restores the data based on the previous frame. If a CRC error is detected, the frame and all subsequent differential frames are discarded until the next key frame is received and resynchronization is performed. The key frame is identified by the Frame Type field (1 bit) in the frame header: 0 = differential frame, 1 = key frame.

[0044] Example 5: This example details the specific implementation process of the S3 quantization compression step.

[0045] The compression module quantizes the differentially encoded floating-point correction value into a fixed-point integer according to a preset quantization bit depth and quantization step size. The quantization formula is as follows: Q = round((V - V_min) / step); The inverse quantization formula is: V' = Q × step + V_min; Where V is the original floating-point value, V_min is the lower limit of the value range, step is the quantization step size, and Q is the quantized integer value.

[0046] The quantization parameters for each observation are set as shown in the table below: Table 3. Quantification parameters for each observation;

[0047] Taking 16-bit coordinate differential quantization as an example, with a step size of 0.01mm and a maximum quantization error of 0.005mm, which is much smaller than the RTK positioning accuracy (on the order of 10mm), the error introduced by quantization is negligible. According to actual tests, the difference between the RTK positioning results of the compressed data after 16-bit quantization and the original RTCM data is <1mm, which meets the sub-meter positioning requirements of marshalling yards.

[0048] Example 6: This example details the specific implementation process of the S4 frame structure design steps. Please refer to the corresponding DMR data frame format. Figure 3 As shown.

[0049] After the compression module completes the quantization compression, it encapsulates the compressed data payload according to the designed DMR data frame format, with the frame structure adapted to the general data transmission time slot (Data Packet, DP Type 1 / 2) of the DMR TS 102 361 standard.

[0050] Table 4. Compression Frame Structure Definition Table;

[0051] The structure of a single satellite data block in a differential frame is shown in Table 5. Table 5. Structure of a single satellite data block (differential frame);

[0052] The structure of a single satellite data block in a keyframe is shown in Table 6. Table 6. Structure of a single satellite data block (key frame);

[0053] Example of frame length calculation (default mode GPS+BDS dual system, 12 GPS + 8 BDS = 20 satellites): Differential frame: 2 + 0.5 + 6 + 8 + 4 + 20×10 / 8 + 2 = 2 + 0.5 + 6 + 8 + 4 + 25 + 2 = 67.5 bytes → rounded down to 68 bytes; Keyframe: 2 + 0.5 + 6 + 8 + 4 + 20×13.5 / 8 + 2 = 2 + 0.5 + 6 + 8 + 4 + 33.75 + 2 ≈ 76 bytes; Based on a 1Hz update rate: differential frames are 68×8=544bps, key frames are 76×8=608bps, and the average is about 550bps, which is far lower than the DMR single-slot carrying capacity of 4.8kbps. After encapsulation, the compression module sends the complete DMR data frame to the DMR base station via the UART / SPI interface, and the DMR base station transmits it through the wireless channel (400-470MHz band, 12.5kHz channel bandwidth) according to the TS 102 361 standard protocol.

[0054] Example 7: This example details the specific implementation process of the S5 adaptive rate control step; The compression module continuously monitors the quality indicators of the DMR channel during operation and dynamically adjusts the data update frequency and compression mode according to the channel conditions. 1. Channel quality assessment uses two metrics provided by the DMR system: Received Signal Strength Indicator (RSSI) and Bit Error Rate (BER), to comprehensively evaluate channel quality. The Channel Quality Index (CQI) is defined as follows: CQI = α × normalize(RSSI) + (1-α) × normalize(1 / BER) Where α = 0.6 (RSSI weight), and normalize() is a normalization function that maps the index to the [0,1] interval.

[0055] RSSI normalization is as follows: RSSI ≥ -70dBm is mapped to 1.0, RSSI ≤ -110dBm is mapped to 0.0, with linear interpolation in between. BER normalization is as follows: BER ≤ 10 -4 Mapped to 1.0, BER≥10 -2 Mapped to 0.0, intermediate logarithmic interpolation.

[0056] 2. Channel Classification and Adjustment Strategy The channel is divided into five levels based on the CQI value. The update frequency and compression mode corresponding to each level are shown in Table 7. Table 7 Adaptive Rate Triggering Conditions and Adjustment Strategies;

[0057] 3. Level switching anti-jitter mechanism: To avoid frequent switching due to channel quality fluctuations, a hysteresis mechanism is introduced during level switching. Downgrade switching: CQI performs downgrade after T_down seconds in the low-level range (T_down is 3s / 3s / 2s / 1s depending on the level). Upgrade switching: CQI will perform the upgrade after T_up seconds in the high-level range (T_up is 5s / 5s / 5s / 5s depending on the level). The design of T_up > T_down ensures fast degradation response (protecting data transmission) and slow upgrade response (avoiding frequent switching when the channel quality is briefly improved). Emergency Degradation: If the CQI suddenly drops below 0.1 (e.g., due to sudden channel interference), the system will immediately downgrade to Level 4 without any delay. 3. The receiver adopts adaptive synchronization. When the level changes, the transmitter sends three frames carrying the new level identifier to ensure the receiver is synchronized. The receiver uses the Compression Level field in the most recently received frame as the standard for decoding. If the receiver does not receive a valid frame within 2 seconds (CRC check fails), it will automatically lower the expected level by one level and try to decode.

[0058] Example 8: This example illustrates the complete data transmission process from the base station GNSS receiver to the rover GNSS receiver. Please refer to the corresponding compressed transmission flowchart. Figure 1 As shown, please refer to the system integration architecture. Figure 5 As shown.

[0059] The base station processing flow is as follows: Step 1: The GNSS receiver (such as Trimble BD990 or u-blox ZED-F9P) outputs raw differential data in RTCM3.x format at a frequency of 1-10Hz; Step 2: The compression module (STM32H743 embedded processor, running FreeRTOS real-time operating system) receives RTCM data through the UART interface; Step 3: The compression module performs message simplification (S1), removing redundant fields according to the field simplification rules shown in Tables 1 and 2, and retaining only the core observation data of the GPS and BDS systems; Step 4: The compression module performs differential coding (S2) to differentially encode the base station coordinates, pseudorange, carrier phase, and signal-to-noise ratio to generate differential values. Step 5: The compression module performs quantization compression (S3), quantizing the floating-point differential value into a fixed-point integer according to the quantization parameters shown in Table 3; Step 6: The compression module encapsulates the compressed data according to the frame structure shown in Tables 4, 5 and 6, and adds fields such as synchronization word, frame type, compression level, and CRC-16. Step 7: The compressed DMR data frame is sent to the DMR base station through the UART / SPI interface and transmitted through the DMR wireless channel (400-470MHz band, 12.5kHz channel bandwidth).

[0060] The mobile site processing flow is as follows: Step 1: The DMR terminal (such as Hytera PD980 or Motorola MOTOTRBO series) receives the DMR wireless signal and demodulates the data frame; Step 2: The decompression module (integrated in the DMR terminal MCU, such as STM32F407) performs CRC-16 verification on the received data frame. If the verification fails, the frame and all subsequent differential frames are discarded until the next key frame is received and resynchronization is performed. Step 3: The decompression module determines the frame type (differential frame or key frame) based on the Frame Type field in the frame header, and determines the compression mode based on the Compression Level field; Step 4: The decompression module performs a dequantization operation to restore the fixed-point integers to floating-point observations; Step 5: The decompression module performs differential decoding to restore the absolute values ​​of each observation based on the key frame or the previous frame; Step 6: The decompression module sends the restored RTCM data to the mobile station GNSS receiver via the UART interface for RTK calculation to obtain centimeter-level positioning results.

[0061] Example 9: This example illustrates the software implementation of the compression algorithm; The core code of the compression algorithm is written in C language, with a total code size of about 3,000 lines. It can achieve real-time compression on ARM Cortex-M7 processor (compression time per frame <1ms). Key algorithm parameters (such as keyframe interval N, quantization step size, CQI calculation weight α, level switching delay time, etc.) are managed through configuration files and can be adjusted on-site without recompiling the firmware.

[0062] The software modules are divided as follows; RTCM parsing module: responsible for RTCM3.x protocol frame synchronization, message type identification, and field extraction; Message simplification module: Performs field filtering and removal according to five field retention rules; Differential coding module: performs differential coding of base station coordinates, pseudorange, carrier phase, and signal-to-noise ratio; Quantization compression module: Performs floating-point to quantitative conversion according to the quantization parameter table; Frame encapsulation module: Assembles complete data frames according to the DMR frame structure definition; Adaptive rate control module: monitors RSSI and BER, calculates CQI, and performs channel level switching; CRC Calculation Module: Calculates the CRC-16-CCITT checksum. Example 10: This example illustrates the system integration scheme of the method of the present invention in a marshalling yard scenario; Base station end: The GNSS receiver is installed at a fixed location with known coordinates within the marshalling yard (such as the roof of the signal building), and the antenna is erected in an open area without obstruction. The GNSS receiver is connected to the compression module via a serial port, and the compression module is connected to the DMR base station via Ethernet or serial port. The DMR base station antenna is erected at a high position to cover the entire marshalling yard operation area. Mobile station end: The DMR terminal is installed in the cab of the shunting locomotive, and the GNSS receiver antenna is installed on the roof in an unobstructed position. The DMR terminal is connected to the mobile station GNSS receiver via a serial port. When the system is working, the GNSS receiver of the base station continuously outputs RTCM differential data, which is compressed by the compression module and broadcast by the DMR base station. All mobile station DMR terminals in the marshalling yard can receive the broadcast data, which is then decompressed and used by their respective GNSS receivers for RTK calculation, so as to realize real-time high-precision positioning of all shunting locomotives and operators in the marshalling yard. The compression module can be deployed in two ways: integrated into the DMR base station or connected via Ethernet as a standalone device. The choice can be made flexibly based on the existing facilities of the marshalling yard. The standalone device solution is suitable for the retrofitting of existing DMR systems in marshalling yards without replacing the base station hardware, which greatly reduces the difficulty of engineering implementation and the cost of system upgrades.

[0063] Example 11: This example illustrates the verification results of positioning accuracy using the method of the present invention at different compression ratios. Please refer to the corresponding compression ratio-accuracy relationship curve. Figure 4 As shown.

[0064] In a typical marshalling yard scenario (GPS+BDS dual system, 8-15 visible satellites), the relationship between compression ratio and positioning accuracy was tested and verified. The test used the raw RTCM3.x data output by the Trimble BD990 receiver as the benchmark. After being processed by the compression method of this invention, the original RTCM data and the raw RTCM data were respectively subjected to RTK calculation, and the difference in positioning results between the two was compared.

[0065] Test results show that: In differential frame mode, a single satellite data block is 10 bytes, with a compression ratio of approximately 1 / 5 to 1 / 8 of the original RTCM data, and a positioning accuracy loss of less than 1 cm. In keyframe mode, a single satellite data block is 13.5 bytes, and the positioning accuracy is basically consistent with the original RTCM data, with a difference of less than 0.5 cm; The positioning accuracy corresponding to different compression levels (Level 0 to Level 4) all meet the application requirements of marshalling yards for horizontal accuracy better than 0.5m and vertical accuracy better than 0.8m; The contribution of quantization error to positioning accuracy is less than 0.005 mm, which is negligible in the overall positioning error.

[0066] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Content not described in detail in this specification is prior art known to those skilled in the art.

[0067] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Those skilled in the art can readily implement the present invention based on the accompanying drawings and the above description. However, any modifications, alterations, or variations made by those skilled in the art without departing from the scope of the present invention, utilizing the disclosed technical content, are equivalent embodiments of the present invention. Furthermore, any equivalent changes, alterations, or variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A method for RTK differential data compression and transmission based on a DMR narrowband channel, characterized in that, Includes the following steps: The S1 message simplification process involves parsing RTK differential data messages in RTCM3.x format, retaining only the observation fields necessary for marshalling yard applications, and removing redundant information fields and unnecessary satellite system observation data blocks. The S2 differential coding step uses differential coding for each retained observation, calculates the change between adjacent epochs instead of the absolute value for transmission, and forces the transmission of a key frame carrying complete observation data after every N differential frames are transmitted, where N is a positive integer. The S3 quantization compression step quantizes the difference or absolute value of each observation into a fixed-point integer according to the preset quantization bits and quantization step size, forming a compressed data payload. The S4 frame structure design steps involve designing a dedicated data frame format adapted to the DMR general data transmission time slot. The data frame includes at least a frame header, compressed data payload, and a cyclic redundancy check field. The S5 adaptive rate control step dynamically adjusts the data update frequency and compression mode based on the quality indicators of the DMR channel.

2. The RTK differential data compression and transmission method based on a DMR narrowband channel according to claim 1, characterized in that, In the S1 message simplification step, the field retention determination adopts the following rules; Rule 1: If this field is a direct input parameter of the RTK solution algorithm, including pseudorange, carrier phase, satellite number and base station coordinates, it must be retained; Rule 2: If the value of this field is fixed in the marshalling yard application scenario, including reference frame, code type and system flag, then delete the field and restore it at the receiving end in a pre-configured manner; Rule 3: If the field is supplementary information to other fields and does not affect positioning accuracy, then bit compression shall be performed; Rule 4: Only retain observation data from GPS system PRN1-32 and BDS system PRN1-63, and delete data blocks from GLONASS, Galileo, and QZSS systems; Rule 5: Traverse the satellite data blocks. If the pseudorange and carrier phase fields of a satellite are both zero or filled, delete the entire block and only transmit the valid satellite data.

3. The RTK differential data compression and transmission method based on a DMR narrowband channel according to claim 1, characterized in that, The S2 differential coding includes the following steps; S201 Base Station Coordinate Difference: Let the coordinates of the base station in the nth epoch be P(n); If P(n) = (X(n), Y(n), Z(n)), then the difference value ΔP(n) = P(n) - P(n-1), n ​​≥ 2. The first frame uses ΔP(1) = P(1) - P_ref, where P_ref is the reference coordinate. S202 Pseudorange Observation Difference: Let the pseudorange of the k-th satellite in the n-th epoch be ρ_k(n), then the difference value Δρ_k(n) = ρ_k(n) - ρ_k(n-1); S203 Carrier Phase Differential: Let the carrier phase of the k-th satellite in the nth epoch be φ_k(n), then the differential value Δφ_k(n) = φ_k(n) - φ_k(n-1); S204 signal-to-noise ratio difference: ΔSNR_k(n) = SNR_k(n) - SNR_k(n-1); The S205 differential coding fault tolerance mechanism uses a default interval N of 10 between the key frame and the differential frame. When the receiver receives a differential frame, it restores the previous frame as a reference. If the cyclic redundancy check fails, the frame and all subsequent differential frames are discarded until the next key frame is received and then resynchronized.

4. The RTK differential data compression and transmission method based on a DMR narrowband channel according to claim 1, characterized in that, In the S3 quantization compression step, the quantization parameters for each observation are set as follows; The base station coordinate difference ΔP: 16 bits of quantization, value range [-327.67mm, +327.67mm], quantization step size 0.01mm; Pseudorange differential Δρ: 20 bits quantization, range [-5242.87m, +5242.87m], quantization step size 0.01m; Carrier phase differential Δφ: 20 bits quantization, value range [-524.288 Hz, +524.288 Hz], quantization step size 0.001 Hz; Signal-to-noise ratio differential ΔSNR: 4 bits quantization bit depth, value range [-4dB-Hz, +4dB-Hz], quantization step size 0.5dB-Hz; The absolute value of the pseudorange in the keyframe: quantization bit depth 24 bits, value range [0m, 29999999.99m], quantization step size 0.01m; Absolute carrier phase value in keyframes: 20 bits quantization, value range [-524288 Hz, +524287 Hz], quantization step size 1 Hz; The quantization formula is Q=round((V-V_min) / step); The dequantization formula is V'=Q×step+V_min, where V is the original floating-point value, V_min is the lower limit of the range, step is the quantization step size, and Q is the quantized integer value.

5. The RTK differential data compression and transmission method based on a DMR narrowband channel according to claim 1, characterized in that, The frame format in the S4 frame structure design step includes the following fields: The synchronization word, which is 2 bytes long, is used for frame synchronization detection at the receiving end. Frame Type, 1 bit long, used to distinguish between differential frames and keyframes; Compression Level, 3 bits in length, indicates the current compression mode. Station ID, 1 byte in length; Epoch Time, 20 bits in length, represents time within GPS seconds with a resolution of 1ms; Sat Count, 5 bits in length, indicates the number of satellites contained in this frame; GPS satellite visibility bitmap (Sat Flag Bitmap), 4 bytes in length, uses PRN1-32 to indicate whether each GPS satellite has data; BDS Flag Bitmap, 8 bytes in length, identifies whether each BDS satellite has data according to PRN1-63; The coordinate differences ΔX / ΔY / ΔZ are each 16 bits long, for a total of 48 bits; The satellite data block is of variable length and is arranged sequentially for each satellite. A single satellite data block in a differential frame contains 16 bits of L1 pseudorange differential value, 20 bits of L1 carrier phase differential value, and 4 bits of L1 signal-to-noise ratio differential value, and selectively includes 16 bits of L2 pseudorange differential value, 20 bits of L2 carrier phase differential value, and 4 bits of L2 signal-to-noise ratio differential value, depending on the compression level. A single satellite data block in a key frame contains 24 bits of L1 pseudorange absolute value, 20 bits of L1 carrier phase absolute value, 6 bits of L1 signal-to-noise ratio absolute value, 4 bits of L1 signal lock time indicator, 24 bits of L2 pseudorange absolute value, 20 bits of L2 carrier phase absolute value, 6 bits of L2 signal-to-noise ratio absolute value, and 4 bits of L2 signal lock time indicator. Cyclic Redundancy Check (CRC-16) is used, with a length of 2 bytes, covering the frame type field to the end of the data payload.

6. The RTK differential data compression and transmission method based on a DMR narrowband channel according to claim 1, characterized in that, In the S5 adaptive rate control step: the received signal strength index RSSI and bit error rate BER provided by the DMR system are used to comprehensively evaluate the channel quality; Define the Channel Quality Index (CQI); CQI = α × normalize(RSSI) + (1 - α) × normalize(1 / BER), where α = 0.6, and normalize() is the normalization function; RSSI normalization is as follows: RSSI ≥ -70dBm is mapped to 1.0, RSSI ≤ -110dBm is mapped to 0.0, with intermediate linear interpolation; BER normalization is as follows: BER ≤ 10 -4 Mapped to 1.0, BER≥10 -2 Mapped to 0.0, intermediate logarithmic interpolation; The channel is divided into five levels according to the CQI value: Level 0 is [0.8, 1.0] corresponding to excellent, Level 1 is [0.6, 0.8) corresponding to good, Level 2 is [0.4, 0.6) corresponding to average, Level 3 is [0.2, 0.4) corresponding to poor, and Level 4 is [0, 0.2) corresponding to very poor. The update frequency and compression mode corresponding to each level are as follows: Level 0 uses a 10Hz update frequency, with one keyframe inserted every 10 frames of differential frames and keyframes; Level 1 uses a 5Hz update frequency, with one keyframe inserted every 5 frames of differential frames and keyframes; Level 2 uses a 2Hz update frequency, with one keyframe inserted every 5 frames of L1 differential frames and keyframes only; Level 3 uses a 1Hz update frequency, with one keyframe inserted every 3 frames of L1 simplified differential frames and keyframes only. Level 4 uses a 0.5Hz update frequency and transmits complete L1+L2 data only in key frames; Level switching introduces an anti-jitter mechanism: downgrade switching requires the CQI to remain in the lower level range for T_down seconds before execution, with T_down being 3s, 3s, 2s, and 1s for each level; upgrade switching requires the CQI to remain in the higher level range for T_up seconds before execution, with T_up being 5s for all levels; T_up must be greater than T_down; if the CQI suddenly drops below 0.1, it will immediately downgrade to Level 4.

7. The RTK differential data compression and transmission method based on a DMR narrowband channel according to claim 6, characterized in that, During each level switch, the transmitting end continuously sends three data frames carrying the new level identifier to ensure synchronization at the receiving end. The receiving end decodes the data based on the compression level field in the most recently received data frame. If the receiving end does not receive a frame with valid Cyclic Redundancy Check within 2 seconds, it will automatically downgrade to a lower level and attempt decoding.

8. The RTK differential data compression and transmission method based on a DMR narrowband channel according to claim 1, characterized in that, The method is applied to an RTK differential data transmission system consisting of a base station and a mobile station. The GNSS receiver at the base station outputs raw RTCM3.x data, which is then sent to the compression module to perform message simplification, differential coding, and quantization compression. The compressed data is then sent to the DMR base station for transmission via a wireless channel. The mobile station DMR terminal receives data, and after the RTCM data is restored by the decompression module, it is sent to the GNSS receiver for RTK calculation. The compression module is integrated inside the DMR base station equipment or connected to the DMR base station via Ethernet as a standalone device.

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