CN102590837B - A method for correcting phase error of GPS signal source - Google Patents

Abstract

The invention relates to the technical field of simulating and generating a GPS signal source, in particular to a method for correcting the phase error of the GPS signal source. The technical scheme of the present invention uses the input Doppler frequency parameter to calculate the frequency control word in floating point format, then converts it to a fixed point and calculates the total error generated in the correction time interval, and then uses the frequency control word after the fixed point to control the phase Accumulation, if the correction time is reached, the accumulation result will be added to the total error and saved and output, otherwise it will be directly saved and output. If the parameter update time interval is reached, recalculate the frequency control word in floating-point format according to the Doppler frequency at the current moment and repeat the above steps, otherwise continue to perform phase accumulation. The technical scheme of the present invention compromises the high precision of floating-point implementation and the simplicity and low cost of fixed-point implementation, and can correct the limited word length effect in the process of fixed-point implementation.

Description

A method for correcting phase error of GPS signal source

technical field

The invention relates to the technical field of simulating and generating a GPS signal source, in particular to a method for correcting the phase error of the GPS signal source.

Background technique

The GPS signal source can provide repeatable analog signals for various researches, simulations and tests in the field of satellite navigation, which is helpful for the advancement of research work. At the same time, in environments where it is not convenient to install antennas, such as remote mountainous areas and areas with harsh climates, the GPS signal source can also ensure the normal operation of the navigation terminal, greatly reducing the construction and maintenance costs of installing antennas.

In the process of simulating the generation of GPS signals by the GPS signal source, it is necessary to calculate the frequency control word of the intermediate frequency carrier, the frequency control word of the C/A code and the frequency control word of the navigation message, and then use these three frequency control words to control the phase respectively. Accumulated to obtain the phases of IF carrier, C/A code and navigation message respectively.

The method for calculating the above-mentioned frequency control word and performing phase accumulation mainly includes the following steps:

Step 1: Calculate the frequency control word of the intermediate frequency carrier, the frequency control word of the C/A code and the frequency control word of the navigation message by using the Doppler frequency parameter;

Step 2: According to the sampling interval, use the frequency control word obtained in Step 1 to perform phase accumulation to obtain the phases of the intermediate frequency carrier, C/A code and navigation message.

Among them, the formula for calculating the frequency control word FCW_Carrier_Float of the intermediate frequency carrier by using the Doppler frequency parameter is:

$\mathrm{<span\; class="notranslate">\; FCW</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Carrier</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Float</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}$

f _I represents the intermediate frequency, f _d represents the Doppler frequency, and f _s represents the signal sampling frequency.

The formula for calculating the frequency control word FCW_Code_Float of the C/A code using the Doppler frequency parameter is:

$\mathrm{<span\; class="notranslate">\; FCW</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Code</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Float</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}$

f _C is the code rate of the C/A code, and f _T is the carrier frequency of the GPS signal.

The formula for calculating the frequency control word FCW_Data_Float of the navigation message by using the Doppler frequency parameter is:

$\mathrm{<span\; class="notranslate">\; FCW</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Data</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Float</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}$

f _D represents the code rate of the navigation message.

In step 2, when using the frequency control word for phase accumulation, if the Doppler frequency used is calculated for the first time for this satellite, it should be accumulated from the initial phase of the GPS signal, otherwise it should be accumulated at the last time Continue to accumulate based on the obtained phase accumulation value.

It can be seen that for the calculation of the phase of the intermediate frequency carrier, C/A code and navigation message of the GPS signal source, the Doppler frequency and the initial phase of the GPS satellite signal are its basic input parameters. These two parameters are generally It is calculated and generated by other modules of the GPS signal source and output to the phase calculation module.

When implementing the above steps on a digital platform, either a fixed-point digital platform or a floating-point digital platform can be used. After analysis, we found that no matter which platform is used to implement, there are the following difficulties: if a fixed-point platform is used to implement, it means that a finite length binary string is used to represent the IF carrier frequency control word and the C/A code frequency control word. When it is used with the frequency control word of the navigation message, there is a representation error, that is, there is a deviation between the value represented by the fixed-point method and the ideal value. This creates a gap between the simulated GPS signal and the real one. And, as time increases, this gap may gradually become larger, leading to further deterioration of performance. If the data is represented and processed on a floating-point platform, the error caused by the finite word length effect can be greatly reduced, but this will inevitably increase the difficulty of implementation, the cost of the hardware platform, and the power consumption of the entire system.

Judging from the published literature at home and abroad, we have not found a technical solution that can effectively solve the above two technical problems at the same time.

Contents of the invention

A kind of method for correcting the phase error of GPS signal source that the present invention proposes is characterized in that comprising the following steps successively:

Step 1: Set the word length of the digital platform to be L _W , the maximum tolerable phase error of the IF carrier, C/A code and navigation message signal to be Max_Phase_Err, the parameter update time interval T _u , and the time interval for correction of the finite word length effect T _e , signal sampling time interval T _s ;

Step 2: use the Doppler frequency parameter at the current moment to calculate the frequency control word in floating point format;

Step 3: Convert the frequency control word in floating-point format obtained in step 2 into a frequency control word in a fixed-point format with a word length of L _W , record the error generated by a single conversion, and calculate the time interval T for correcting the finite word length effect The sum of the errors generated in _e , and then save the sum of errors in fixed-point format;

取模后保存并输出，然后进入步骤5，如果没有达到，则直接将相位累加值对

取模后保存并输出该结果，然后进入步骤5；Step 4: Use the frequency control word in the fixed-point format obtained in step 3, take the sampling time interval T _s of the signal as the time beat, accumulate the phase of the signal, and then judge whether the time of the accumulation process reaches the time interval of the correction of the finite word length effect T _e , if the interval is reached, add the sum of errors obtained in step 3 to the phase accumulation value to obtain the phase error correction value, and compare the phase error correction value to

After taking the modulus, save and output, and then go to step 5, if not, directly add the phase accumulation value to

After taking the modulus, save and output the result, and then go to step 5;

Step 5: Judging whether the parameter update time interval T _u is reached, if so, return to step 2; if not, return to step 4.

The parameter updating time interval T _u is greater than or equal to the time interval T _e of finite word length effect correction.

The parameter update time interval T _u is an integer multiple of the time interval T _e for finite word length effect correction.

The frequency control word is one of the intermediate frequency carrier frequency control word, the C/A code frequency control word and the navigation message frequency control word.

The time interval T _e for correcting the finite word length effect is determined by the following method:

1) For each of the frequency control words, calculate the required minimum number of accumulations Min_N=Max_Phase_Err/Max_ΔFCW according to the maximum tolerable phase error Max_Phase_Err respectively, wherein, Max_ΔFCW is the floating point in the step 3 The absolute value of the maximum error that may be caused when the frequency control word in the fixed-point format is converted into the frequency control word in the fixed-point format. For the conversion method that uses the decimal place rounding method, Max_ΔFCW=0.5, and for the conversion method that directly removes the decimal place, Max_ΔFCW=1 ;

2) For each of the frequency control words, calculate T _e =T _s ×Min_N respectively, where T _s is the sampling time interval of the signal;

3) Take the minimum value among the three T _e obtained in step 2), which is the time interval T _e for correction of the finite word length effect.

Said step 3 converts the frequency control word of the floating-point format into the fixed-point format whose word length is L _W and includes:

4) multiply the frequency control word in the floating-point format by

5) The value obtained in step 1) is rounded off to an integer or the value obtained in step 1) is directly removed from the decimal place.

The sum of the errors generated in the time interval T _e of the finite word length effect correction described in step 3 = the error generated by the single conversion×T _e /T _s .

The method for accumulating the phase of the signal in the step 4 is to execute the following formula cyclically: phase accumulation value=phase accumulation value+the frequency control word in the fixed-point format obtained in the step 3, if for the current simulated GPS satellite If it is the first time to calculate its Doppler frequency parameter, the initial value of the phase accumulation value is the initial phase value of the signal; otherwise, the initial value of the phase accumulation value is the phase accumulation value saved in step 4.

The technical scheme of the present invention makes a good compromise between the floating-point implementation and the fixed-point implementation, combines the accuracy of the floating-point implementation and the simplicity of the fixed-point implementation, and considers the limited word Errors introduced by long effects were corrected for. Before step 3 of this technical solution, the calculation frequency of the parameters is relatively low (equal to the parameter update interval _Tu ), and the floating-point implementation is used to calculate the average rate control word more accurately according to the Doppler frequency of the satellite signal at the current moment. value; after step 3, the calculation frequency is relatively high (mainly the phase accumulation calculation is carried out according to the sampling interval T _s ), and the fixed-point implementation can be used to generate satellite signals more simply and quickly. At the same time, the phase accumulation error caused by the finite word length effect is also calculated, and corrected at the parameter update interval _Te of the fixed-point implementation part, which ensures the accuracy of the fixed-point implementation.

Description of drawings

Accompanying drawing 1 is the overall flowchart of a method for correcting the phase error of the GPS signal source proposed by the present invention.

Detailed ways

The implementation manner of the technical solution of the present invention will be described in detail below.

Step 1: Initialization, specifically including: setting parameters, including GPS signal carrier frequency f _T =1575.42MHz, C/A code rate f _C =1.023MHz, navigation message code rate f _D =50Hz, sampling frequency f _s =16.384MHz , the sampling interval T _s =1/16.384us, the intermediate frequency f _I =5.12MHz, the word length L _W of the digital platform =32bit, the maximum phase error tolerance value caused by the three frequency control word errors is 8192, and the rounding method is adopted Realize fixed-point, that is, Max_ΔFCW=0.5, the finite word length effect correction interval T _e =1ms obtained from this, the number of data points in the correction interval N _e =T _e /T _s =16384, in addition, set the parameter update interval T _u = 2 ms, N _u =T _u /T _e =2.

Step 2: Calculate the frequency control word in floating point format according to the Doppler frequency at the current moment

(a) Calculate the frequency control word FCW_Carrier_Float of the intermediate frequency carrier, namely

$\mathrm{<span\; class="notranslate">\; FCW</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Carrier</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Float</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 5.12</span>}\mathrm{<span\; class="notranslate">\; MHz</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}}{\mathrm{<span\; class="notranslate">\; 16.384</span>}\mathrm{<span\; class="notranslate">\; MHz</span>}}$

(b) Calculate the frequency control word FCW_Code_Float of the C/A code, namely

$\mathrm{<span\; class="notranslate">\; FCW</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Code</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Float</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1.023</span>}\mathrm{<span\; class="notranslate">\; MHz</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1.023</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 1575.42</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 16.384</span>}\mathrm{<span\; class="notranslate">\; MHz</span>}}$

(c) Calculate the frequency control word FCW_Data_Float of the navigation message, namely

$\mathrm{<span\; class="notranslate">\; FCW</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Data</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Float</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 50</span>}\mathrm{<span\; class="notranslate">\; Hz</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 50</span>}\mathrm{<span\; class="notranslate">\; Hz</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 1575.42</span>}\mathrm{<span\; class="notranslate">\; MHz</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 16.384</span>}\mathrm{<span\; class="notranslate">\; MHz</span>}}$

where f _d is the Doppler frequency.

Step 3: Perform fixed-point processing on the frequency control word, and record the error caused by the finite word length effect, specifically including the following steps:

Step 3-1: Multiply the IF frequency control word FCW_Carrier_Float in floating point format by Afterwards, the decimal place is rounded to obtain the intermediate frequency frequency control word FCW_Carrier in the fixed-point format, namely

FCW_Carrier＝round{FCW_Carrier_Float×2 ³² }

Calculate the intermediate frequency carrier phase cumulative error ΔPhase_Carrier caused by the finite word length effect within the correction interval T _e of the finite word length effect, which is still expressed in a fixed-point format, namely

ΔPhase_Carrier=round{(FCW_Carrier_Float×2 ³² -FCW_Carrier)×16384}

后进行对小数位的四舍五入，得到定点格式的C/A码频率控制字FCW_Code，即Step 3-2: Multiply the C/A code frequency control word FCW_Code_Float in floating point format by

Carry out the rounding to decimal places afterward, obtain the C/A code frequency control word FCW_Code of fixed-point format, namely

FCW_Code＝round{FCW_Code_Float×2 ³² }

Calculate the C/A code phase cumulative error ΔPhase_Code caused by the finite word length effect within the correction interval T _e of the finite word length effect, which is still expressed in a fixed-point format, namely

ΔPhase_Code＝round{(FCW_Code_Float×2 ³² -FCW_Code)×16384}

后进行对小数位的四舍五入，得到定点格式的导航电文频率控制字FCW_Data，即Step 3-3: Multiply the navigation message frequency control word FCW_Data_Float in floating point format by

Afterwards, the decimal place is rounded to obtain the frequency control word FCW_Data of the navigation message in fixed-point format, namely

FCW_Data＝round{FCW_Data_Float×2 ³² }

Calculate the cumulative error ΔPhase_Data of the navigation message phase caused by the finite word length effect within the correction interval T _e of the finite word length effect, which is still expressed in a fixed-point format, namely

ΔPhase_Data=round{(FCW_Data_Float×2 ³² -FCW_Data)×16384}

The round{} operation in the above formula indicates the rounding operation of decimal places. In addition to using the method of rounding decimal places to complete the conversion from floating-point format to fixed-point format, the decimal place can also be discarded directly, that is, only integer places are reserved. Convert the frequency control word into a fixed-point format by using this method. If this method is used, Max_ΔFCW=1, and other parameters related to Max_ΔFCW should also be changed accordingly.

Step 4: Use the frequency control word in fixed-point format to carry out phase accumulation, specifically including the following steps:

Step 4-1: Use the frequency control word of the intermediate frequency carrier to perform phase accumulation, namely

Phase_Carrier = Phase_Carrier + FCW_Carrier

Step 4-2: Use the frequency control word of the C/A code to perform phase accumulation, namely

Phase_Code＝Phase_Code+FCW_Code

Step 4-3: Use the frequency control word of the navigation message to perform phase accumulation, namely

Phase_Data = Phase_Data + FCW_Data

Before starting to accumulate the phase with the frequency control word, it is necessary to judge whether the adopted Doppler frequency is calculated for the first time for this satellite. If so, the initial phase of the above three accumulation formulas The value is the initial phase of the satellite signal; if not, the initial value of the phase in the above three accumulation formulas is the accumulated phase value saved after the last accumulation.

Step 5: If the finite word length effect correction interval T _e is reached, use the accumulated error amount to correct the finite word length effect, and save the accumulated result after correction, specifically including the following steps:

Step 5-1: Correct the IF carrier phase, that is

Phase_Carrier = Phase_Carrier + ΔPhase_Carrier

Step 5-2: Correct the C/A code phase, namely

Phase_Code＝Phase_Code+ΔPhase_Code

Step 5-3: Correct the navigation message phase, namely

Phase_Data = Phase_Data + ΔPhase_Data

If the finite word length effect correction interval T _e is not reached, the accumulation result is directly saved.

Step 6: Take the modulus of the three phase accumulation values saved in step 5 according to ²³² and output them, specifically including the following steps:

Step 6-1: Take the modulus of the carrier phase of the intermediate frequency, namely

Phase_Carrier = mod{Phase_Carrier, 2 ³² }

Step 6-2: Take the modulus of the C/A code phase, namely

Phase_Code = mod{Phase_Code, 2 ³² }

Step 6-3: Take the modulus of the phase of the navigation message, namely

Phase_Data = mod{Phase_Data, 2 ³² }

In the fixed-point implementation process, the modulus is simply to take the low bit, that is, to take the modulo of ²³² , that is, to simply take out the low 32 bits of the phase accumulation value.

Step 7: Judging whether the parameter update interval T _u has been reached, if so, return to step 2, otherwise return to step 4.

Step 2 of this specific embodiment is implemented in a floating-point manner, and steps 3, 4, 5, and 6 are implemented in a fixed-point manner. As can be seen from the above step description, only when the parameter update interval _Tu is reached, the floating-point method is required. The platform recalculates each frequency control word, and the value of the parameter update interval T _u is generally longer than the finite word length effect correction interval T _e , which is a good balance between accuracy and system cost, making high cost and high power consumption The amount of floating-point operations is relatively small, and a large number of operations are implemented on fixed-point platforms. Therefore, in the specific implementation of this technical solution, it is recommended to separate the floating-point implementation part from the fixed-point implementation part and place them on different platforms. For example, if the "DSP+FPGA" mode is adopted, DSP is responsible for floating-point related calculations, and FPGA is responsible for phase Accumulate and look up the table to complete the task of simulating GPS signals together.

Claims (6)

1. A method for correcting a phase error of a GPS signal source, characterized in that it comprises the steps successively: Step 1: Set the word length of the digital platform as L _w , the maximum tolerable phase error of the intermediate frequency carrier, C/A code and navigation message signal as Max_Phase_Err, the parameter update time interval T _u , and the time interval for correction of the finite word length effect T _e , signal sampling time interval T _s ; Step 2: use the Doppler frequency parameter at the current moment to calculate the frequency control word in floating point format; Step 3: Convert the frequency control word in floating-point format obtained in step 2 into a frequency control word in fixed-point format with a word length of _Lw , record the error generated by a single conversion, and calculate the time interval T for correcting the finite word length effect The sum of the errors generated in _e , and then save the error sum in fixed-point format; Step 4: Use the frequency control word in the fixed-point format obtained in step 3, take the sampling time interval T _s of the signal as the time beat, accumulate the phase of the signal, and then judge whether the time of the accumulation process reaches the time interval of the correction of the finite word length effect T _e , if the interval is reached, add the sum of errors obtained in step 3 to the phase accumulation value to obtain the phase error correction value, and compare the phase error correction value to

After taking the modulus, save and output, and then go to step 5, if not, directly add the phase accumulation value to

After taking the modulus, save and output the result, and then go to step 5; Step 5: Judging whether the parameter update time interval T _u is reached, if so, return to step 2; if not, return to step 4; Wherein, the frequency control word is one of the intermediate frequency carrier frequency control word, the C/A code frequency control word and the navigation message frequency control word; the time interval Te of the finite word length effect correction is determined by the following method: 1) For each of the frequency control words, calculate the required minimum number of accumulations Min_N=Max_Phase_Err/Max_ΔFCW according to the maximum tolerable phase error Max_Phase_Err respectively, wherein, Max_ΔFCW is the floating point in the step 3 The absolute value of the maximum error that may be caused when the frequency control word in the fixed-point format is converted into the frequency control word in the fixed-point format. For the conversion method that adopts the rounding method of decimal places, Max_ΔFCW=0.5, and for the conversion method that directly removes decimal places, Max_ΔFCW=1 ; 2) For each of the frequency control words, calculate Te=Ts×Min_N respectively, where Ts is the sampling time interval of the signal; 3) Take the minimum value among the three Te obtained in step 2), which is the time interval Te for correction of the finite word length effect. 2. A method for correcting a phase error of a GPS signal source according to claim 1, characterized in that: the parameter updating time interval T _u is greater than or equal to the time interval T _e of finite word length effect correction. 3. A method for correcting a phase error of a GPS signal source according to claim 1, characterized in that: the parameter update time interval T _u is an integer multiple of the time interval T _e of finite word length effect correction. 4. A kind of method for correcting the phase error of GPS signal source according to claim 1, is characterized in that: described step 3 converts the frequency control word of floating-point format into word length and is the fixed-point format of _Lw Steps include: 1) multiply the frequency control word in the floating-point format by

; 2) The value obtained in step 1) is subjected to an integer operation of rounding off decimal places, or the value obtained in step 1) is subjected to a rounding operation of directly removing decimal places. 5. A kind of method for correcting the phase error of GPS signal source according to claim 1, it is characterized in that: refer to the summation of the error that produces in the time interval _Te that finite word length effect is corrected described in step 3 = Error produced by said single conversion x T _e /T _s . 6. A kind of method that is used to correct the phase error of GPS signal source according to claim 1, is characterized in that: the method that described step 4 is accumulated to the phase of signal is to carry out following formula cyclically: phase accumulative value= Phase accumulation value + the frequency control word in fixed-point format obtained in step 3, if it is the first time to calculate its Doppler frequency parameter for the currently simulated GPS satellite, the initial value of the phase accumulation value is the signal The initial phase value, otherwise, the initial value of the accumulated phase value is the accumulated phase value saved in step 4.

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