A method for measuring time difference of digital satellite television signals

Abstract

The application provides a digital satellite television signal time difference measurement method, which increases a time value measurement process on the basis of the prior art. In the time value measurement process, a time interval Delta N between a data rising edge and a subsequent atomic clock output nominal frequency rising edge is measured by using a time-to-digital converter, so as to improve the time stamp precision. Time code data in the time value corresponding to the time stamp is added, so as to avoid errors above seconds in the time difference calculation process. The application can eliminate the influence of large errors and counter resolution on the measurement precision, improve the time difference measurement resolution to the order of tens of picoseconds, and has high measurement precision and good accuracy.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of communication technology, and particularly relates to a time difference measurement method. BACKGROUND

[0002] Digital satellite television high-precision time service is a time service means for transmitting high-precision time information by using an existing digital satellite television system. The satellite television system mainly consists of three parts: an uplink earth station, a satellite-borne transponder and a downlink receiving station. Among them, the geosynchronous satellite for transmitting television signals is about 36,000 kilometers away from the ground, and the time delay of the television signal from the uplink earth station to the satellite and then from the satellite transponder to the ground is about 250 milliseconds.

[0003] It is of great significance to accurately measure the time difference between two receiving stations and the satellite for transmitting television signals by using digital satellite television signals. The time difference measurement can provide key support for digital satellite television time service system and satellite orbit determination system, and can also be used as an information source for integrated PNT, which provides PNT service by fusing GNSS signals, eLoran signals or other opportunity signals, and is of great significance to the development of space-air-ground integration, military-civil integration and all-source navigation technology. At the same time, the measurement method makes full use of the existing digital satellite television signal resources, improves and widens the utilization efficiency and use of the broadcasting infrastructure.

[0004] In the patent "Digital satellite television time service system large loop time delay accurate measurement method" (ZL201410241032.0), a method for measuring the large loop time delay of the video signal by using time stamp is given. This method can also be used for time difference measurement, but there is no time code information in the time stamp, which will cause errors of seconds or even more when calculating the time difference. At the same time, the measurement accuracy is affected by the resolution of the counter. For example, the 10MHz frequency signal of the atomic clock is multiplied to 200MHz frequency signal, and each clock cycle is 5 nanoseconds. Then the counter value N is in the range of 0-199999999, and the time stamp resolution is 5 nanoseconds. SUMMARY

[0005] In order to overcome the shortcomings of the prior art, the present application provides a digital satellite television signal time difference measurement method. On the basis of the prior art, the measurement process of the time value is added to eliminate the influence of large errors and counter resolution on the measurement accuracy, and the time difference measurement resolution is improved to the order of tens of picoseconds, so that the measurement accuracy is high and the accuracy is good.

[0006] The technical solution adopted by the present application to solve its technical problems comprises the following steps:

[0007] (1) Signal synchronization

[0008] Seeking the byte with value 0x47 in the transport stream packet, if the corresponding byte every 187 bytes apart is 0x47, the 0x47 byte is considered as the synchronization byte of the transport stream packet, and the transport stream packet synchronization is completed;

[0009] (2) generating time stamp

[0010] 1) frequency signal of the atomic clock is multiplied to the frequency signal of f0 Hz, then every clock cycle is 1 / f0 second, and the counter counting value N ranges from 0 to f0-1; the value of the counter corresponding to the rising edge of the 1PPS signal of the local time is set to zero;

[0011] 2) the rising edge of the seventh bit to the eighth bit in the synchronization byte 0x47 is extracted, and the value N of the subsequent counter is recorded by using the rising edge; the rising edge of the subsequent frequency signal is used as the closing pulse of the time-to-digital converter, and the time interval ΔN is obtained;

[0012] 3) the time code T corresponding to the 1PPS of the local time is recorded, and the time value ΔT below the second below the second is calculated as N / f0-ΔN, then the time corresponding to the transport stream packet is The value is stored in register 1;

[0013] 4) the fifth byte to the eleventh byte after the packet header synchronization byte 0x47 is extracted and stored in register 2; the data in register 2 is converted into ASCII code and restored into the original transport data; the fourth bit data in register 2 is the program clock reference value flag bit, if it is zero, return to step 2); if it is 1, it indicates that the transport contains a program clock reference, and the time stamp is generated by the data of the second byte to the seventh byte in register 2 and the t code in register 1;

[0014] (3) time difference measurement

[0015] 1) the two ground receiving ends receive the downlink signal from the satellite through the satellite antenna respectively, demodulate and output 270 megabits per second asynchronous serial code stream signal, and repeat steps (1) and (2) to perform signal synchronization and high-precision time stamp generation;

[0016] 2) save the time stamps generated by the two ground receiving ends within a set time length, and obtain a plurality of time difference measurement values by subtracting the time values t corresponding to the same program clock reference value in the two ground receiving ends.

[0017] The beneficial effects of the present application are:

[0018] 1) in the process of measuring the time value, the time interval ΔN between the rising edge of the data and the subsequent rising edge of the nominal frequency output by the atomic clock is measured by using the time-to-digital converter, and the time stamp precision is improved;

[0019] 2) Added time code data to the timestamp corresponding to the time value to avoid errors of more than a second in the calculation of time difference. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the high-precision timestamp generation method in step (2) of the present invention.

[0021] Figure 2 This is a flowchart of the high-precision timestamp generation process of the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments. The present invention includes, but is not limited to, the following embodiments.

[0023] This invention includes the following steps:

[0024] (1) Signal synchronization

[0025] Using the same method as the signal synchronization method given in the patent "Precise Measurement Method of Large Loop Delay in Digital Satellite Television Timing System" (ZL201410241032.0), the first step is to find the byte with a value of 0x47 in the transmission stream packet. If multiple bytes with a spacing of 187 bytes are all 0x47, it is considered that the 0x47 byte is the synchronization byte of the transmission stream packet, and the transmission stream packet synchronization is completed.

[0026] (2) High-precision timestamp generation method

[0027] 1) Multiply the 10MHz frequency signal of the atomic clock to a frequency signal of f0 Hz. Then each clock cycle is 1 / f0 seconds, the counter count value N ranges from 0 to f0-1, and the value of the counter corresponding to the rising edge of the 1PPS signal of the local time is set to zero.

[0028] 2) Extract the rising edge from the seventh to the eighth bit of the packet header synchronization byte 0x47, and use this rising edge to record the value N of the subsequent counter; at the same time, this rising edge is used as the gate opening pulse of the time-to-digital converter, and the rising edge of the subsequent frequency signal is used as the gate closing pulse, so that the measured value ΔN seconds can be obtained.

[0029] 3) Record the time code T corresponding to 1 PPS of local time, calculate the time value ΔT = N / f0 - ΔN (below the second), then the time t corresponding to this transport stream packet is...

[0030]

[0031] Store the value in register 1.

[0032] 4) Extract bytes 5 to 11 after the packet header synchronization byte 0x47 and store them in register 2; convert the data in register 2 to ASCII code and restore the original transmission data; the fourth bit in register 2 is the program clock reference value flag bit. If it is zero, return to step 2); if it is 1, it indicates that the transmission contains a program clock reference. Generate a high-precision timestamp by encoding bytes 2 to 7 in register 2 with t in register 1.

[0033] (3) Time difference measurement

[0034] 1) The two ground receiving ends receive downlink signals from the satellite through satellite antennas, demodulate and output 270 megabits per second asynchronous serial code stream signals, perform signal synchronization and high-precision timestamp generation. The signal synchronization method in this step is exactly the same as the signal synchronization step (1), and the high-precision timestamp generation method is exactly the same as the high-precision timestamp generation method (2).

[0035] 2) Save the timestamps generated by the two ground receivers in step 1) for a period of time, and subtract the time values ​​t corresponding to the same program clock reference values ​​in the two ground receivers to obtain multiple time difference measurements.

[0036] The following example illustrates a method for measuring the time difference of digital satellite television signals based on an EDS-13100 module. The EDS-13100 module is a radio frequency module designed for digital satellite television signals. This module receives L-band digital satellite television signals from 950MHz to 2150MHz and outputs transport stream packets for specified programs. This embodiment includes the following steps:

[0037] (1) C-band satellite receiving antenna

[0038] C-band satellite receiving antennas receive television signals relayed by designated satellites, and after passing through the LNB, obtain L-band signals ranging from 950MHz to 2150MHz.

[0039] (2) EDS-13100 module

[0040] Set the program packet identifier to be decoded, including the video packet identifier and the program clock reference identifier. Connect the RF receiver of the EDS-13100 module to the L-band signal output by the C-band satellite receiving antenna. The EDS-13100 module outputs the transport stream packet of the specified program.

[0041] (3) Timestamp generation method

[0042] 1) Package synchronization

[0043] The 270 megabits per second (Mbps) bitstream signal is extracted from the serial asynchronous interface of the uplink ground station modulator in the digital satellite television system, and the synchronization process is as follows:

[0044] a) Clock recovery is performed using a serial digital receiver to achieve bit synchronization;

[0045] b) Place the bit-synchronized raw data into a 50-bit shift register sequentially. Check if there are two consecutive special control characters K28.5 as 0011111010 or 1100000101 in these 50 bits of data. If so, use this code as the boundary of the received data code byte, that is, group the subsequent data into one byte every 10 bits to complete the byte synchronization and proceed to step c). If not, repeat this step.

[0046] c) Each transport stream packet is a fixed length of 188 bytes. The synchronization byte in the packet header is 0x47, and the corresponding 10-bit encoding is "1110000101" or "0001110101". First, a 0x47 codeword is searched. If a codeword 187 bytes away is also 0x47, the next 0x47 codeword is considered to be the synchronization byte of the transport stream packet, and the transport stream packet synchronization is completed; otherwise, a 0x47 codeword is searched again.

[0047] 2) Design a counter

[0048] Assume the nominal frequency of the atomic clock output is f Hz, the counter count value range is 0 to f-1, the counter value starts from zero and is aligned with the second pulse of the time code, the counter value is incremented by 1 for each atomic clock nominal frequency period, and the counter value is reset to zero when it accumulates to f-1. The counter count period is 1 second and the resolution is 1 / f second.

[0049] 3) Time value measurement

[0050] Extract the rising edge from the seventh to the eighth bit of the packet header synchronization byte 0x47, and use this rising edge to record the value N of the counter. The time corresponding to this transmission stream packet is N / f seconds. At the same time, use a time-to-digital converter to measure the time interval between this rising edge and the subsequent rising edge of the atomic clock output nominal frequency, denoted as ΔN, in seconds. Calculate N / f-ΔN, and store the time code information and this value in register 1.

[0051] 4) Timestamp generation

[0052] Extract bytes 5 to 11 after the packet header synchronization byte 0x47 and store them in register 2; convert the data in register 2 to ASCII code to restore the original transmitted data; the fourth bit in register 2 is the program clock reference value status bit. If it is zero, return to step 2; if it is 1, it indicates that the transmission contains a program clock reference. Encode bytes 2 to 7 in register 2 and the data in register 1 according to a certain format to generate a timestamp.

[0053] (4) Time difference measurement method

[0054] Two sets of devices with timestamp generation function are used to obtain two sets of timestamp data within a certain period of time according to the methods of steps (1) to (3). Assuming that the time values ​​with the same program clock reference in the two sets of data are t1 and t2, then t2-t1 is the time difference between the two devices and the satellite that relays the TV signal at time t1.

Claims

1. A method for measuring the time difference of digital satellite television signals, characterized in that, Includes the following steps: (1) Signal synchronization Find the byte with a value of 0x47 in the transport stream packet. If the corresponding byte every 187 bytes is 0x47, consider the 0x47 byte to be the synchronization byte of the transport stream packet, and complete the transport stream packet synchronization. (2) Generate timestamps 2.1) Multiply the frequency signal of the atomic clock to... f A 0 Hz frequency signal means that each clock cycle is 1 / f 0 seconds, the counter value N ranges from 0 to... f 0-1; Set the value of the counter corresponding to the rising edge of the 1PPS signal of the local time to zero; 2.2) Extract the rising edges of bits 7 and 8 from the synchronization byte 0x47, and use these rising edges to record the subsequent counter value N; use these rising edges as the gate-on pulse of the time-to-digital converter, and the subsequent rising edge of the frequency signal as the gate-off pulse, to obtain the time interval. ; 2.3) Record the time code T corresponding to 1 PPS of local time, and calculate the time value in seconds. Then the time corresponding to the transmission stream packet ;Will t Stored in register 1; 2.4) Extract bytes 5 to 11 after the packet header synchronization byte 0x47 and store them in register 2; convert the data in register 2 to ASCII code to restore the original transmission data; the fourth bit in register 2 is the program clock reference value flag bit. If it is zero, return to step 2.2); if it is 1, it indicates that the transmission stream packet contains a program clock reference. Generate a timestamp by encoding bytes 2 to 7 in register 2 with the t in register 1. (3) Time difference measurement 3.1) The two ground receivers receive downlink signals from the satellite through satellite antennas, demodulate them, and output an asynchronous serial code stream signal of 270 megabits per second. Repeat steps (1) and (2) to synchronize the signal and generate a high-precision timestamp. 3.2) Save the timestamps generated by the two ground receivers within the set time period, and subtract the time values ​​t corresponding to the same program clock reference values ​​in the two ground receivers to obtain multiple time difference measurement values.

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