CN102375146A - Method and system for simulating GPS (Global Positioning System) digital medium-frequency signal - Google Patents

Abstract

The present invention provides a method and system for simulating GPS digital intermediate frequency signals using software methods. The method includes: Step 1: Read the configuration file written by the user according to the specified format, extract system configuration parameters, and configure the simulated signal characteristics and system work requirements. Input; step 2, extract the RINEX file header information and the ephemeris data groups of all satellites in the defined time period; step 3, encode the ephemeris data extracted in step 2 to form a binary navigation message code stream; step Fourth, the message code stream obtained in step three is modulated with a carrier wave and a spreading code to synthesize intermediate frequency digital signal data. The system includes: signal generation module, RINEX management module and auxiliary module. The invention uses a software method to simulate the GPS digital intermediate frequency signal as a test stimulus for the GPS prototype receiver system and its sub-modules, simplifies the problem of signal collection in the test, and quickly satisfies the needs of testers for the richness of signal features.

Description

Simulation method and system of GPS digital intermediate frequency signal

technical field

The invention relates to a method and a system in the field of satellite navigation, in particular to a method and a system for simulating GPS digital intermediate frequency signals using a software method.

Background technique

A full-featured GPS receiver needs three functional modules: the first one is the radio frequency receiving module, the specific function is to receive the radio frequency signal transmitted by the GPS satellite through the antenna, and then convert the radio frequency signal into a lower frequency intermediate frequency through mixing and filtering The signal, after being sampled by the ADC, becomes a digital signal and is processed by the baseband processing module; the second one is the baseband processing module, which is responsible for capturing and tracking the digital signal output by the radio frequency receiving module, and obtaining the navigation message information and the propagation time information of each satellite ; The third is the navigation calculation module, which is responsible for processing the information output by the baseband processing module through certain mathematical operations to obtain the PVT (position, speed, time) information of the receiver, and realize GPS positioning.

Therefore, the GPS receiver chip needs a GPS signal source as a test stimulus in the early design verification process, and the design and verification personnel need to master certain prior information of the signal in order to verify the correctness and robustness of the receiver. There are two commonly used solutions. One is to establish a real receiving environment, that is, purchase a third-party RF receiving module and connect it with your own test system, and obtain test statistical results through on-site signal reception and measurement, and require a reference receiver to operate at the same time at the same location and under the same conditions , to obtain comparative test data; the other is to build an independent radio frequency receiving and sampling system, which needs to be connected to a storage device to store the signals collected in the field in the form of a file as a test stimulus. Both solutions are limited by high environmental costs and very harsh experimental conditions. For the first solution, on the one hand, the real-time test environment can only complete system-level testing, and it is difficult to implement module testing or high-level model testing; on the other hand, it needs to be carried out outdoors, requiring a lot of extra labor costs and using vehicles , portable power supply and other additional auxiliary equipment and tools. For the second solution, additional labor and equipment are also required, and the stored signal has no prior knowledge, and additional software tools are needed to analyze it. Only when reliable information is available can it be used for testing, and the test results The reliability of the algorithm depends on whether the mining of signal information is correct.

Contents of the invention

Aiming at the shortcomings and deficiencies in the above-mentioned technologies, the present invention proposes a method and system for simulating GPS intermediate-frequency digital signals realized by software based on a general-purpose computer platform. This method uses real ephemeris data, through software coding and simulation of real carrier and spread spectrum modulation, and through inverse calculation to adjust the time difference between the arrival of signals from different satellites at the receiver, and synthesize a given time, place and signal-to-noise ratio Visible satellite signals, stored as binary files, are ready for test stimulus. The system simulates the output signal of the ADC in the RF module of the GPS navigation receiver. The software method ensures the convenience and flexibility of use, and the signal can be generated at any time and meet the requirements of any signal characteristics.

According to one aspect of the present invention, a method for simulating a GPS digital intermediate frequency signal is proposed, including the following steps: Step 1, read the configuration file written by the user according to the specified format, extract the system configuration parameters, and configure the simulated signal characteristics and system working conditions Need to input; step 2, extract the RINEX file header information and the ephemeris data group of all satellites within the defined time period; step 3, encode the ephemeris data extracted in step 2 to form a binary navigation message stream; Step 4: Modulate the carrier wave and the spread spectrum code with the message code stream obtained in Step 3 to synthesize the intermediate frequency digital signal data.

According to another aspect of the present invention, there is also provided an analog system for GPS digital intermediate frequency signals, including: a signal generation module, a RINEX management module, and an auxiliary module, wherein the auxiliary module mainly includes submodules including a navigation calculation module, a time system Management module, geographic coordinate conversion module, macro module.

More specifically, the present invention is achieved through the following technical solutions:

The software GPS intermediate frequency digital signal simulation method based on general computer platform that the present invention relates to comprises the following steps:

Step 1: Read the configuration file written by the user according to the specified format, extract the system configuration parameters, and configure the simulated signal characteristics and input required for system work. The system configuration parameters include:

RINEX导航星历文件路径。RINEX是一种存储导航相关数据的通用文本文件格式，其中的一种导航星历文件用于存储一段时间内某观测站采集的卫星星历数据。星历数据用于计算卫星运行参数（速度、位置、钟差等）。从该格式文件提取星历、并且按照官方给定星历编码格式和规范进行编码，能够还原指定卫星在一定时间段内发射的导航数据流。

RINEX navigation ephemeris file path. RINEX is a common text file format for storing navigation-related data, and one of the navigation ephemeris files is used to store satellite ephemeris data collected by an observation station over a period of time. The ephemeris data is used to calculate satellite operating parameters (velocity, position, clock error, etc.). Extract the ephemeris from this format file, and encode it according to the officially given ephemeris encoding format and specification, which can restore the navigation data stream transmitted by the specified satellite within a certain period of time.

接收机位置、速度、加速度信息。这些参数用于配置接收机模型的运动状态。这些和运动状态相关的参数最终会作用到模拟出的GPS信号中。

Receiver position, velocity, acceleration information. These parameters are used to configure the motion state of the receiver model. These parameters related to the motion state will eventually be applied to the simulated GPS signal.

所模拟的信号的接收起始时刻、时间跨度、星历使用组数信息。

The reception start time, time span, and ephemeris of the simulated signal use group number information.

This step reads the configuration parameters listed above from the configuration file to configure each module of the system, and sets the current operating mode and data generation mode of the system.

Step 2, check the validity of the RINEX navigation ephemeris file, extract the header information of the RINEX ephemeris file and the ephemeris data of all satellites within the defined time period.

The RINEX ephemeris file header information includes: RINEX version information, observation station location information, signal receiving period information, ionospheric correction model parameters, UTC time information, etc. The more important ones are: version information, which determines the format of the file; ionospheric correction model parameters and UTC time information, these parameters also exist in the navigation message broadcast by the satellite, and are used to correct the ionospheric delay and UTC time in GPST respectively. Convert between. Any type of information occupies several lines in the file header and is distinguished by a descriptor. This step first caches the file header of the RINEX file, and then the user can search for the corresponding information in the cache through a specific descriptor as needed, and use it in signal generation.

The function of the ephemeris is explained in step 1, which is used to describe satellite orbit information and satellite motion information. GPS satellites update and broadcast ephemeris every two hours to ensure the accuracy of the information. Observation stations that make RINEX files will receive satellite signals continuously, and store multiple sets of ephemeris of all satellites that can be observed within a time period (such as one day) in a RINEX navigation ephemeris file in chronological order.

In this step, after parsing the RINEX file header, all the ephemeris data in the signal simulation time period are extracted and stored in a dynamic two-dimensional array to form an ephemeris data group. The type of the ephemeris data set is a self-defined structure, which stores a set of ephemeris of a satellite. The reason for the dynamic array is that when there are theoretical ephemeris updates in the time span of the signal simulation, multiple sets of ephemeris are required per satellite. Each row of the two-dimensional array corresponds to a GPS satellite, a total of 32 rows, and each column corresponds to a set of ephemeris data, different columns correspond to different time periods, and the number of columns is determined by .

This step searches the RINEX navigation ephemeris file for all satellite ephemeris corresponding to the time period according to the configuration information of the signal start time and the time span, and calculates whether there is an ephemeris update, and a total of several updates. For example, when it is calculated that the signal to be generated contains an ephemeris update, each satellite needs to extract two sets of time-continuous ephemeris, which are consistent with the configuration information of the start time and length.

Step 3: Encode the ephemeris data extracted in step 2 to form a binary navigation message stream.

Encoding the ephemeris data extracted in step 2 to form a binary navigation message stream refers to: according to the simulated signal start time and time span, and the stipulations of the GPS official document on the format of the message, each The ephemeris of a satellite is encoded to form a binary navigation message code stream, and the parity bit is calculated at the same time, and finally the code stream of each satellite is stored in an independent file. The navigation message is in binary format, with 30 seconds as a frame, and the ephemeris data extracted in step 2 corresponds to the information contained in one frame. A frame contains 5 subframes, numbered from 1 to 5, and each subframe contains 300 bits of data, so the width of each bit of data is 20 ms. Each satellite starts to broadcast the navigation message from the early hours of Sunday morning of Universal Coordinated Time (UTC), continuously loops from subframe 1 to subframe 5, and updates at two-hour intervals. The algorithm for encoding all satellite messages is the same, and the process is as follows:

1. Calculate the time of week TOW according to the information of the signal start time;

2. Take the remainder of the quotient obtained by dividing TOW by 6 to 5, (remainder + 1) is the subframe number k of the subframe at the starting moment of the signal (theoretically, the time for each satellite to transmit the subframe is synchronous, the above The calculated subframe number can be used as the starting subframe of the navigation message code stream generated by all satellites);

3. Calculate the number of subframes that each satellite needs to generate according to the signal time span n (in seconds): N = n/6+1;

 i＜N）个子帧的子帧号： 4. For satellite p, the i-th (0

The subframe number of i<N) subframes:

，若计算得sf为0，

；

, if the calculated sf is 0,

;

Calculate the ephemeris data in which ephemeris group should be selected according to the time of the week corresponding to the subframe:

Find the 2-hour ephemeris update time within the simulated signal time span, judge the time period when itow appears, and judge the ephemeris data group that should be selected. According to the subframe format corresponding to the sf subframe and the ephemeris data of satellite p in the selected ephemeris data group, the i-th subframe is encoded; repeat this step to complete the encoding of all subframes, and obtain the message code stream of satellite p.

Step 4: Modulating the carrier wave and the spreading code for the navigation message, and synthesizing the intermediate frequency digital signal data. The present invention simulates the output signal of the ADC in the radio frequency module of the GPS navigation receiver, and the characteristics of the signal are: 1. digital signal, 2. the carrier frequency is an intermediate frequency ranging from several megabytes to tens of megabytes, and the intermediate frequency is considered to be the receiver The radio frequency module obtains by down-converting the radio frequency. According to these two features, this step completes the final analog synthesis function of the signal. The core idea is to calculate the signal sample value by sampling point, that is, to calculate at each sampling point which satellites the signal received by the antenna at this moment is from. Signal synthesis, what kind of message value, spread spectrum code phase and carrier phase, frequency, and considering factors such as frequency folding and ADC quantization order when the antenna down-converts the signal, accurately simulate the behavior of the receiving end on the signal Impact. The algorithm steps are as follows:

1. Calculate the GPS spreading code sequence. The generation method is implemented according to the definition of the official GPS interface document. The generated sequence is stored in a two-dimensional array CA_Code, and its row vector represents the CA code sequence of a GPS satellite. The period of the CA code is 1 ms and contains 1023 chips, so the length of the row vector is 1023, which stores a period sequence value of the CA code.

。其中n为信号的时间跨度（单位为秒），

为采样率（单位为Hz）。 Calculate the sample number Nsample of the digital intermediate frequency signal to be generated. The calculation method is

. where n is the time span of the signal (in seconds),

is the sampling rate (in Hz).

When i is less than Nsample, start to calculate the value of the i-th (initial value is 0) sampling point. Go to step 4. Otherwise, go to step 6.

Determines whether i corresponds to the number of samples in whole milliseconds. The algorithm extrapolates the propagation time of the signal from each satellite (the signal travels from the satellite antenna to the receiver antenna) in whole milliseconds, and assumes that this propagation time is constant for the next 1 millisecond, i.e. arrives at the receiver in the next 1 millisecond The propagation time of the satellite signal is constant. This assumption ensures a very small error and avoids the estimation of the propagation time of each satellite signal corresponding to each sampling point. Under the premise that the sampling rate is more than 1 trillion , which increases the algorithm efficiency by more than 1000 times. If i is the number of samples in whole milliseconds, perform the following steps 1) to 5) for each satellite (within the range of all 32 GPS satellites), otherwise skip to step 5:

1) According to i, find the satellite ephemeris data group corresponding to the current moment, extract the ephemeris of the current satellite p from it, and check the validity of the ephemeris. If it is invalid, the satellite will be invisible, and skip to the next satellite and repeat step 4 process;

秒之前的卫星钟差校正值clk和钟漂校正值clk_drift；传播时间 使用上一次整毫秒计算时得到的值，即暂时认为在过去的1ms内传播时间没有发生变化，在初始化时定义为0.07秒； 2) Calculate the propagation time of the satellite signal received at the current moment (corresponding to i) according to the description of the official GPS interface document

Satellite clock correction value clk and clock drift correction value clk_drift before seconds; propagation time Use the value obtained in the last whole millisecond calculation, that is, temporarily consider that the propagation time has not changed in the past 1ms, and define it as 0.07 seconds during initialization;

3) Calculate the launch time of the satellite signal received at the current moment (corresponding to i), and the launch time is the start time of the signal (in a week), ms is the number of milliseconds currently recorded (in seconds);

4) Loop and iteratively calculate the satellite PVT information, the distance between the satellite and the receiver and its change rate information at the time when the currently calculated signal is transmitted. The specific method is:

i. Recalculate propagation time:

range is the distance between the satellite and the receiver calculated in step 4 last time, LIGHTSPEED is the speed of light;

ii. Recalculate the signal emission time tow:

；

;

iii. Calculate the PVT information of the satellite at tow time according to the method defined in the official GPS interface document;

； iv. Calculate the distance range and distance change rate between the satellite and the receiver at tow time in the ECEF coordinate system

;

和

。 v. Go back to step i and repeat this process, and iterate 4 times to get the exact range,

and

.

) Calculate the satellite elevation angle and azimuth angle at tow time, if the elevation angle is greater than 0, then calculate the Doppler frequency shift:

Where WAVELENGTHL1 is the GPS L1 signal carrier wavelength. The negative sign here is because it is believed that the RF receiving module inverts the frequency spectrum during down-conversion, so the sign of the doppler is inverted;

Then calculate the carrier phase step size at the current sampling rate:

Where IF is the intermediate frequency.

Finally calculate the exact travel time:

clk is the satellite clock error calculated in step 2). Return to step 1) after the calculation is complete, and enter the launch time and corresponding PVT information of the signal that the next satellite arrives at the current whole millisecond time.

If the elevation angle is less than 0, the satellite is set to be invisible, skip other calculations in this step, and return to step 1) to execute the above steps for the next satellite.

Calculate the value of the i-th sample point for the signal of each currently visible satellite. The satellite's visibility has already been determined in step 4 and remains constant until the next time step 4 is processed (i.e. the next whole millisecond is reached). The algorithm steps of this step are as follows:

1) Calculate the navigation message value corresponding to the i-th sampling point of the currently visible satellite q. Its pseudo code is as follows:

表示信号起始时刻所在子帧的起始时刻，则

表示当前采样点距离该子帧起点的时间距离。计算

的原因是，在步骤三中编码得到的每颗卫星的导航电文码流均以该子帧为起点，因此寻找当前采样点对应的电文数值也需以此为起点。设该子帧为

。 in,

Indicates the start time of the subframe where the start time of the signal is located, then

Indicates the time distance from the current sampling point to the start of the subframe. calculate

The reason is that the navigation message code stream of each satellite encoded in step 3 starts from this subframe, so the search for the message value corresponding to the current sampling point also needs to use this as the starting point. Let the subframe be

.

为一个二维数组，每一行对应一颗卫星的导航电文码流，该码流在步骤三中得到。一行中的每个元素在软件中为无符号整型，其低30位存放一个长度为30比特的导航字。以上伪代码找到当前采样点所在的码流中的第几个字

的第几个比特

，然后从中找出对应的电文数值

。

is a two-dimensional array, each row corresponds to the navigation message code stream of a satellite, and the code stream is obtained in step 3. Each element in a row is an unsigned integer in the software, and its lower 30 bits store a 30-bit navigation word. The above pseudo code finds the first word in the code stream where the current sampling point is located

the first few bits of

, then from Find the corresponding message value in

.

) Calculate the spreading code and carrier value corresponding to the i-th sampling point of the currently visible satellite q. Its pseudo code is as follows:

起始对应的码相位为0，并且CA码的doppler效应忽略不计。因此codephase即为当前采样点对应的扩频码码相位。 Since the CA code has a cycle of 1ms and a cycle contains 1023 chips, the algorithm assumes

The code phase corresponding to the start is 0, and the doppler effect of the CA code is negligible. Therefore, codephase is the code phase of the spreading code corresponding to the current sampling point.

is the carrier phase, its step size is calculated in step 4, and remains constant within 1ms. The synthesized data data is the sum of the products of the navigation message, CA code and carrier value of each satellite at the current sampling point.

) superimpose Gaussian white noise on data, and its signal-to-noise ratio is configured by the user in step 1. Store data in a pre-opened binary file.

After all the sampling points are calculated, step 4 is exited, and the file of the GPS digital intermediate frequency signal is obtained at the same time.

The GPS digital intermediate frequency signal simulation system involved in the present invention includes a signal generation module, a RINEX management module and an auxiliary module, wherein the auxiliary module mainly includes a navigation calculation module, a time system management module, a geographical coordinate conversion module, and a macro module.

Described signal generation module provides the interface of following function, comprises: the extraction of system configuration information; Call RINEX management module to extract the ephemeris data group of required quantity, this ephemeris data group corresponds to all satellites of all GPS satellites within two hours valid period ephemeris, and the group number of the ephemeris group is determined by the start time and time span of the simulated signal, how many ephemeris updates occur in the time span, it is necessary to extract the corresponding number of ephemeris data groups; for the ephemeris The data is coded and the check digit is added to obtain the code stream of the navigation message; the carrier wave and the spread spectrum code are modulated on the code stream of the navigation message; Gaussian noise simulation; the overall algorithm flow of combining the above functional components to realize the signal simulation is provided.

Described RINEX management module provides the interface of following function, comprise: obtain the RINEX ephemeris file of user configuration; Analyze file name check file usability and extract the information that file name contains, comprise base station name abbreviation, signal collection time; Cache RINEX ephemeris file file header data; extract the RINEX version information, base station location information, signal receiving period information, ionospheric correction parameters, UTC time information, etc. in the file header of the RINEX ephemeris file; extract a set of ephemeris data according to a time reference point, This function module is repeatedly called by the interface for extracting ephemeris data groups in the aforementioned signal generation module to obtain the required number of ephemeris data groups.

The navigation solution module provides an interface for the following functions, including: calculation of satellite clock difference and clock drift; calculation of satellite position and velocity information; calculation of satellite, receiver distance, and distance change rate; satellite elevation angle relative to the receiver and azimuth calculations.

The time system management module is responsible for data conversion between different time systems.

The geographic coordinate conversion module is responsible for the conversion of coordinate values between different geographic coordinate systems.

The macro module is responsible for the macro management of the system, including the management of global constants and the management of user-defined mathematical operation macros.

Description of drawings

Fig. 1 is the input and output schematic diagram of GPS digital intermediate frequency signal simulation system of the present invention;

Fig. 2 is a software top-level flowchart of the present invention;

Fig. 3 is the schematic diagram of the input configuration file format of the system of the present invention;

Fig. 4 is the process schematic diagram that RINEX file header information of the present invention extracts;

Fig. 5 is a schematic diagram of the process of satellite ephemeris data group extraction in the present invention;

Fig. 6 is a schematic diagram of the satellite message code stream generation process of the present invention;

Fig. 7 is the algorithm flowchart of the signal synthesis process of the present invention;

Fig. 8 is a diagram of the software logic hierarchy structure of the present invention.

Detailed ways

The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings: this embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating procedures are provided, but the protection scope of the present invention is not limited to the following the described embodiment.

As shown in Figure 1, it is the IO mode of the system provided according to the present invention, the input of the system is the configuration text file written by the user according to the requirements of the generated signal and the format requirements of the system for the configuration file, and the output is stored digital intermediate frequency Signal binary.

As shown in FIG. 2 , it is a flow chart of the top layer of software of the system provided by the present invention. This figure has shown the GPS digital intermediate frequency signal simulation method provided by the present invention, comprises the following concrete steps:

Step 1: Read the configuration file written by the user according to the specified format, and extract parameters such as receiver location, RINEX ephemeris file path, and signal time information.

）加大写英文字母标识，分别是接收机坐标信息、RINEX星历文件路径信息和时间信息。每组信息中的每个单元信息之间用空格区分。具体说明参见下表： As shown in Figure 3, it is a schematic diagram of the system input configuration file format. The configurable parameters are divided into three categories, and the tilde symbol (

) are marked with capitalized English letters, which are receiver coordinate information, RINEX ephemeris file path information and time information. Each unit information in each group of information is separated by spaces. For details, see the table below:

Table 1 Configuration file format description table

Step 2: After the configuration parameters are successfully extracted, open the RINEX ephemeris file, buffer the header information, and extract the corresponding data according to the requirements. After the extraction is completed, find the required number of navigation ephemeris data groups for subsequent message encoding.

As shown in FIG. 4 , in this embodiment, the parameters of the ionospheric delay correction model exist in the third and fourth lines of the file header. After the file header is buffered in a one-dimensional string array, the search is performed with the newline character as the end character and the line unit, and the descriptor (descriptor) corresponding to the ionospheric correction model parameters is searched in each search unit. The search unit is extracted and put into a temporary array, and then the next search unit is checked, and so on until the end of the file header buffer. All temporary arrays form a new buffer in sequence, and finally extract the corresponding data from the buffer in accordance with the official format of RINEX.

As shown in Figure 5, the satellite ephemeris is extracted according to the configured signal start time, time span, and number of updated ephemeris data sets. The body part of the RINEX file stores the ephemeris of each visible satellite by time period, and the length of this time period is the update period of the GPS satellite ephemeris—2 hours. The RINEX file that present embodiment adopts has stored the ephemeris of one day, so its text is divided into 12 sections. The simulated signal starts at 1:58 and has a time span of 330 seconds, so there is an ephemeris update, and two sets of ephemeris data need to be extracted. Search for the time period where the starting moment of the signal is located, extract this time period and a subsequent time period, a total of two sets of ephemeris data sets, and put them into a two-dimensional array. Each column of the two-dimensional array corresponds to a set of ephemeris data of a time period, and the length of the column is the total number of GPS satellites; each row corresponds to the same satellite, and its data structure contains flags to identify the ephemeris of the satellite at the corresponding time Whether the segment is valid (exists).

Step 3: Calculate the time of week corresponding to the start time of the signal, and encode the current subframe every 6 seconds from this time until the frame data not less than the signal duration is completed. The message code stream of each satellite is stored in a text file.

As shown in Figure 6, the starting time of the signal configured in step 1 is converted to the time of week TOW, which is 93520s in this embodiment. Since the GPS subframe length is 6s, the navigation message is encoded subframe by subframe from 93516s . First calculate the subframe number of the starting subframe:

； Then the subsequent subframe sequence is

;

The total number of subframes that need to be generated at the same time is

That is, each satellite needs to encode 56 consecutive subframes starting from the 2nd subframe.

，其应该生成的子帧号

，若sf等于0，则令其为5。 For the ith subframe of satellite p

, which should generate the subframe number

, if sf is equal to 0, let it be 5.

Calculate which ephemeris group should be selected according to its corresponding week time. In this embodiment, the integral multiple times of 2 hours in the signal time span occur at 93600s. Therefore, to calculate how long the i-th subframe exists:

When it is less than 93600, select the ephemeris of satellite p in the first ephemeris group, otherwise select the ephemeris of satellite p in the second ephemeris group.

Then encode the ephemeris according to the GPS message format to obtain the code stream of the corresponding subframe.

After all subframes of satellite p are encoded, they are stored in a single file. All satellites carry out the above steps.

Step 4, modulate the carrier wave and spread spectrum code for the navigation message obtained in Step 3, and synthesize the intermediate frequency digital signal data.

As shown in FIG. 7 , it is an algorithm flow chart of the signal synthesis process.

Calculate the GPS spreading code sequence. The generation method is implemented according to the definition of the official GPS interface document, and the generated sequence is stored in a two-dimensional array In , its row vector represents the CA code sequence of a GPS satellite. The period of the CA code is 1 ms and contains 1023 chips, so the length of the row vector is 1023, which stores a period sequence value of the CA code.

。其中5.714285e6为采样率。 Calculate the number of samples

. Among them, 5.714285e6 is the sampling rate.

Calculate the sample value by sample point. Set temporary variable i=0. When i is less than Nsample, calculate the value of the i-th sample point. Go to step 4. Otherwise, go to step 6.

是否为0，其中5714取1ms采样点数的约数。若i是否对应于整毫秒的样点数，则对全部GPS卫星进行如下1）到5）的步骤，否则跳过该步骤： judge

Whether it is 0, where 5714 is the divisor of the number of 1ms sampling points. If i corresponds to the number of samples in whole milliseconds, then perform the following steps 1) to 5) for all GPS satellites, otherwise skip this step:

1) Find the satellite ephemeris data group corresponding to the current moment according to i, extract the ephemeris of the current satellite p from it, check the validity of the ephemeris, if it is invalid, set the satellite p to invisible, and skip to the next satellite and repeat step 4 process;

秒之前的卫星钟差校正值clk和钟漂校正值

；传播时间

使用上一次整毫秒计算时得到的值，即暂时认为在过去的1ms内传播时间没有发生变化，在初始化时定义为0.07秒； 2) Calculate the propagation time of the satellite signal received at the current moment according to the description of the official GPS interface document

Satellite clock correction value clk and clock drift correction value before seconds

; propagation time

Use the value obtained in the last whole millisecond calculation, that is, temporarily consider that the propagation time has not changed in the past 1ms, and define it as 0.07 seconds during initialization;

； 3) Calculate the launch time of the satellite signal received at the current moment (corresponding to i)

;

4) Loop and iteratively calculate the satellite PVT information, the distance between the satellite and the receiver and its change rate information at the time when the currently calculated signal is transmitted. The specific method is:

i. Recalculate propagation time:

range is the distance between the satellite and the receiver calculated in step 4 last time, and LIGHTSPEED is the speed of light.

Recalculate the signal emission time tow:

iii. Calculate the PVT information of the satellite at tow time according to the method defined in the official GPS interface document;

； iv. Calculate the distance range and distance change rate between the satellite and the receiver at tow time in the ECEF coordinate system

;

。 v. Go back to step i and repeat this process, and iterate 4 times to get the exact range, and

.

) Calculate the satellite elevation angle and azimuth angle at tow time, if the elevation angle is greater than 0, then calculate the Doppler frequency shift:

Where WAVELENGTHL1 is the GPS L1 signal carrier wavelength. The negative sign here is because it is believed that the RF receiving module inverts the frequency spectrum during down-conversion, so the sign of the doppler is inverted;

Then calculate the carrier phase step size at the current sampling rate:

Where IF is the intermediate frequency.

Finally calculate the exact travel time:

clk is the satellite clock error calculated in step 2). Return to step 1) after the calculation is complete, and enter the launch time and corresponding PVT information of the signal that the next satellite arrives at the current whole millisecond time.

If the elevation angle is less than 0, the satellite is set to be invisible, skip other calculations in this step, and return to step 1) to execute the above steps for the next satellite.

Calculate the value of the i-th sample point for the signal of each currently visible satellite. The satellite's visibility has already been determined in step 4 and remains constant until the next time step 4 is processed (i.e. the next whole millisecond is reached). The algorithm steps of this step are as follows:

1) Calculate the navigation message value corresponding to the i-th sampling point of the visible satellite q:

2) Calculate the spreading code and carrier value corresponding to the i-th sampling point of satellite q. Its pseudo code is as follows:

3) Gaussian white noise is superimposed on the data, and its signal-to-noise ratio is configured by the user in step 1. Store data in a pre-opened binary file.

After all the sampling points are calculated, step 4 is exited, and the file of the GPS digital intermediate frequency signal is obtained at the same time.

As shown in FIG. 8 , it is a software logic hierarchical structure diagram of the GPS intermediate frequency signal simulation system provided by the present invention. The software implementation of the present invention includes three logical levels: a signal generation module, a RINEX management module, and an auxiliary module.

Described signal generation module provides the interface of following function, comprises: the extraction of system configuration information; Call RINEX management module to extract the ephemeris data group of required quantity, this ephemeris data group corresponds to all satellites of all GPS satellites within two hours valid period ephemeris, and the group number of the ephemeris group is determined by the start time and time span of the simulated signal, how many ephemeris updates occur in the time span, it is necessary to extract the corresponding number of ephemeris data groups; for the ephemeris The data is coded and the check digit is added to obtain the code stream of the navigation message; the carrier wave and the spread spectrum code are modulated on the code stream of the navigation message; Gaussian noise simulation; the overall algorithm flow of combining the above functional components to realize the signal simulation is provided.

Described RINEX management module provides the interface of following function, comprise: obtain the RINEX ephemeris file of user configuration; Analyze file name check file usability and extract the information that file name contains, comprise base station name abbreviation, signal collection time; Cache RINEX ephemeris file file header data; extract the RINEX version information, base station location information, signal receiving period information, ionospheric correction parameters, UTC time information, etc. in the file header of the RINEX ephemeris file; extract a set of ephemeris data according to a time reference point, This function module is repeatedly called by the interface for extracting ephemeris data groups in the aforementioned signal generation module to obtain the required number of ephemeris data groups.

The navigation solution module provides an interface for the following functions, including: calculation of satellite clock difference and clock drift; calculation of satellite position and velocity information; calculation of satellite, receiver distance, and distance change rate; satellite elevation angle relative to the receiver and azimuth calculations.

The time system management module is responsible for data conversion between different time systems.

The geographic coordinate conversion module is responsible for the conversion of coordinate values between different geographic coordinate systems.

The macro module is responsible for the macro management of the system, including the management of global constants and the management of user-defined mathematical operation macros.

Claims (10)

1. A method for simulating a GPS digital intermediate frequency signal is characterized by comprising the following steps:

reading a configuration file written by a user according to a specified format, extracting system configuration parameters, and configuring simulated signal characteristics and input required by system work;

step two, extracting RINEX file header information and ephemeris data groups of all satellites in a defined time period;

thirdly, encoding the ephemeris data extracted in the second step to form a binary navigation message code stream;

step four, modulating the carrier wave and the spread spectrum code by the text code stream obtained in the step three, and synthesizing intermediate frequency digital signal data.

2. The analog method of GPS digital intermediate frequency signals according to claim 1, wherein the system configuration parameters include: a RINEX navigation ephemeris file path, receiver position information, a reception start time of the simulated signal, a time span, and ephemeris use set number information; the method comprises the following steps that an RINEX navigation ephemeris file used by ephemeris file path designation signal simulation is used, and ephemeris in the file is used as a source of text encoding; the receiver location information specifies a reception location of the analog signal; receiving the starting time, the time span and ephemeris using set number information which describe the time information and ephemeris updating information of the simulated signal; step one comprises the substeps of: and extracting all the input required by the work of the information configuration system and the operation parameters of each module.

3. The simulation method of GPS digital intermediate frequency signals according to claim 1, wherein the RINEX header information contains RINEX version information, observation station location information, signal reception period information, ionosphere correction model parameters, UTC time information, and the like; wherein, RINEX version information determines the format of the file; ionospheric correction model parameters and UTC time information, which also exist in navigation messages broadcast by satellites, are used for correcting ionospheric delay and converting between GPST time and UTC, respectively; any type of information occupies several lines in the header and is distinguished by a descriptor from which corresponding information can be searched for and extracted from the cached RINEX header for use in signal generation.

4. The method for simulating a GPS digital intermediate frequency signal according to claim 1, wherein the ephemeris data set comprises a satellite ephemeris at a starting time of the simulated signal and an updated ephemeris occurring within a simulation time span, and is stored in a two-dimensional array form, each column of the two-dimensional array corresponds to a set of ephemeris data of a time period, and the length of each column is the total number of GPS satellites; each row corresponds to the same satellite and its data structure contains a flag bit to identify whether the ephemeris for that satellite is valid for the corresponding time period.

5. The method for simulating a GPS digital intermediate frequency signal according to claim 1, wherein the ephemeris data encoding is: according to the simulated signal starting time and time span and the regulation of the message format by the GPS official document, the ephemeris of each satellite is coded to form a binary navigation message code stream, the parity check bit is calculated at the same time, and finally the code stream of each satellite is stored in an independent file.

6. The method for simulating a GPS digital intermediate frequency signal according to claim 1, wherein the modulating a carrier and a spreading code on the navigation message to synthesize intermediate frequency digital signal data includes: and finishing the final analog synthesis function of the signal, wherein the final analog synthesis function comprises calculating signal sample values by sampling points, namely calculating the text numerical value, the spread spectrum code phase, the carrier phase and the frequency of the signal received by the antenna at the moment which is synthesized by the signals of the satellites at each sampling point, and accurately simulating the influence of the behavior of a receiving end on the signal by considering the factors of frequency folding, the quantization order of an ADC and the like when the antenna performs frequency reduction on the signal.

7. An analog system for GPS digital intermediate frequency signals, comprising: the system comprises a signal generation module, a RINEX management module and an auxiliary module, wherein the auxiliary module mainly comprises a navigation calculation module, a time system management module, a geographic coordinate conversion module and a macro module.

8. The simulation system of claim 7, wherein the signal generation module provides an interface for functions comprising: extracting system configuration information; calling a RINEX management module to extract a required number of ephemeris data sets, wherein the ephemeris data sets correspond to all ephemeris of all GPS satellites in two-hour validity period, the number of the ephemeris data sets is determined by the starting time and the time span of the simulated signals, and the ephemeris data sets with corresponding number need to be extracted when the ephemeris is updated for a plurality of times in the time span; encoding ephemeris data and adding check bits to obtain a navigation message code stream; modulating carrier waves and spread spectrum codes for the navigation message code stream; simulating Gaussian noise; and providing an overall algorithm flow combining the functional components to realize signal simulation.

9. The simulation system of claim 7, wherein the RINEX management module provides an interface for functions comprising: acquiring a RINEX ephemeris file configured by a user; analyzing the file name to check the file availability and extracting information contained in the file name, wherein the information comprises a base station name abbreviation and signal acquisition time; caching file header data of a RINEX ephemeris file; extracting RINEX version information, base station position information, signal receiving period information, ionosphere correction parameters, UTC time information and the like in a file header of a RINEX ephemeris file; and extracting a group of ephemeris data according to a time reference point, and repeatedly calling the functional module by the interface for extracting the ephemeris data group from the signal generation module to acquire the required number of ephemeris data groups.

10. The simulation system of claim 7, wherein the navigation solution module provides an interface for functions including: calculating the clock error and the clock drift of the satellite; calculating satellite position and speed information; calculating the distance of the satellite and the receiver and the distance change rate; calculation of the elevation and azimuth of the satellite relative to the receiver;

the time system management module is responsible for data conversion among different time systems;

the geographic coordinate conversion module is responsible for converting coordinate values among different geographic coordinate systems;

the macro module is responsible for finishing macro management of the system, including management of global constants and management of user-defined mathematical operation macros.

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