CN102590837A - Method for calibrating phase errors of global positioning system (GPS) signal source - Google Patents

Method for calibrating phase errors of global positioning system (GPS) signal source Download PDF

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CN102590837A
CN102590837A CN2011103942190A CN201110394219A CN102590837A CN 102590837 A CN102590837 A CN 102590837A CN 2011103942190 A CN2011103942190 A CN 2011103942190A CN 201110394219 A CN201110394219 A CN 201110394219A CN 102590837 A CN102590837 A CN 102590837A
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phase
frequency control
control word
correct
time interval
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CN102590837B (en
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林静然
李玉柏
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University of Electronic Science and Technology of China
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Abstract

The invention relates to the technical field of simulative generation of GPS signal source, in particular to a method for calibrating phase errors of a GPS signal source. The technical scheme of the method includes calculating a frequency control word of a floating point format by utilizing an input Doppler frequency parameter; subjecting the frequency control word to a fix-point process and calculating an overall error produced within a calibrating time internal; controlling phase accumulation by using the frequency control word after the fix-point process; if the calibrating time is reached, adding the overall error into the accumulation result, storing and outputting the final result, and storing and outputting the accumulation result directly otherwise; and if a parameter updated time interval is reached, recalculating the frequency control word of the floating point format according to the Doppler frequency at the moment and repeating the above steps, and proceeding phase accumulation otherwise. According to the technical scheme of the method for calibrating the phase errors of the GPS signal source, the high accuracy of floating point accomplishment and simplicity and low cost of fixed point accomplishment are balanced and combined, and the finite word-length effect in the fixed point accomplishment process can be calibrated simultaneously.

Description

A kind of method that is used to proofread and correct the phase error in gps signal source
Technical field
The present invention relates to simulate the technical field that produces the gps signal source, be specifically related to a kind of method that is used to proofread and correct the phase error in gps signal source.
Background technology
The gps signal source can provide repeatably simulating signal for studies, emulation and the test in satellite navigation field, helps the propelling of research work.Simultaneously, in the environment that is not easy to erect a television antenna, like the area of remote mountain areas and awful weather, the gps signal source also can guarantee the operate as normal of navigation terminal, greatly reduces the construction cost and the maintenance cost that erect a television antenna.
Simulation produces in the process of gps signal in the gps signal source; Need calculate the frequency control word of IF carrier; The frequency control word of C/A sign indicating number and the frequency control word of navigation message; And then go control phase to add up with these three frequency control words respectively, obtain the phase place of IF carrier, C/A sign indicating number and navigation message respectively.
Calculate the said frequencies control word and carry out phase-accumulated method and mainly comprise the steps:
Step 1: utilize the frequency control word of Doppler frequency calculation of parameter IF carrier, the frequency control word of C/A sign indicating number and the frequency control word of navigation message;
Step 2: according to the SI, the frequency control word that utilizes step 1 to obtain carries out phase-accumulated, obtains the phase place of IF carrier, C/A sign indicating number and navigation message.
Wherein, utilize the formula of the frequency control word FCW_Carrier_Float of Doppler frequency calculation of parameter IF carrier to be:
FCW _ Carrier _ Float = f I + f d f s
f IThe expression IF-FRE, f dThe expression Doppler frequency, f sThe expression signal SF.
Utilize the formula of the frequency control word FCW_Code_Float of Doppler frequency calculation of parameter C/A sign indicating number to be:
FCW _ Code _ Float = f C + f d × ( f C / f T ) f s
f CBe the code check of C/A sign indicating number, f TBe the gps signal carrier frequency.
The formula that utilizes the Doppler frequency calculation of parameter to calculate the frequency control word FCW_Data_Float of navigation message is:
FCW _ Data _ Float = f D + f d × ( f D / f T ) f s
f DExpression navigation message code check.
Step 2 is being carried out with frequency control word when phase-accumulated; If used Doppler frequency is to calculate for the first time as far as this satellite; Then should begin from the initial phase of gps signal to add up, otherwise just on adding up the phase-accumulated value basis that obtains last time, continue to add up.
This shows; For the phase place of the IF carrier, C/A sign indicating number and the navigation message that calculate the gps signal source; The initial phase of Doppler frequency and this gps satellite signal is its basic input parameter, and these two parameters generally are to be calculated by other modules in gps signal source to generate and export to the phase calculation module.
When on a digital platform, specifically realizing above-mentioned steps, both can adopt the digital platform of fixed point, also can adopt the digital platform of floating-point.We find through analyzing; No matter adopt which kind of platform to realize merely; All there is following difficult point: if adopt the fixed point platform to realize; Mean then when representing IF carrier frequency control word, C/A code frequency control word and navigation message frequency control word that just there is the expression error in this, that is: have deviation between the value represented of its fixed point mode and the ideal value with a time-limited binary string.This makes simulates the gps signal that produces and has gap between the signal really.And along with the increase of time, this gap possibly become greatly gradually, causes performance further to worsen.If on the floating-point platform, represent and deal with data, can reduce the error that finite word length effect causes greatly, but this will increase the difficulty of realization, the cost of hardware platform and the power consumption of total system.
From domestic and international disclosed document, we do not find the technical scheme that can effectively solve above-mentioned two aspect technical matterss simultaneously.
Summary of the invention
A kind of method that is used to proofread and correct the phase error in gps signal source that the present invention proposes is characterized in that in turn including the following steps:
Step 1: the word length of setting digital platform is L W, IF carrier, C/A sign indicating number and the navigation message signals phase error of largest tolerable separately are Max_Phase_Err, parameter update time interval T u, the time interval T that finite word length effect is proofreaied and correct e, signals sampling time interval T s
Step 2: utilize the Doppler frequency parameter of current time, calculate the frequency control word of floating-point format;
Step 3: it is L that the frequency control word of the floating-point format that step 2 is obtained converts word length to WThe frequency control word of fixed point format, the error that the record single conversion produces, and calculate the time interval T that proofreaies and correct at finite word length effect eThe summation of the interior error that produces is preserved this sum of the deviations then with fixed point format;
Step 4: the frequency control word of the fixed point format that obtains with step 3, with signals sampling time interval T sBe the time beat, the phase place of signal is added up, judge then whether the time of cumulative process reaches the time interval T that finite word length effect is proofreaied and correct eIf, reached this interval, then add the sum of the deviations that step 3 obtains with this phase-accumulated value earlier, obtain the phase error correction value, this phase error correction value is right
Figure BSA00000627448800031
Preserve behind the delivery and output, get into step 5 then, if it is do not reach, then directly that phase-accumulated value is right Preserve behind the delivery and export this result, get into step 5 then;
Step 5: judge whether to reach parameter update time interval T uIf, reach, then return step 2; If do not reach, then return step 4.
Parameter update time interval T uBe greater than or equal to the time interval T that finite word length effect is proofreaied and correct e
Parameter update time interval T uBe the time interval T that finite word length effect is proofreaied and correct eIntegral multiple.
Said frequency control word is in IF carrier frequency control word, C/A code frequency control word and the navigation message frequency control word.
The time interval T that finite word length effect is proofreaied and correct eAdopt following method to confirm:
1) for each described frequency control word; Calculate its needed minimal accumulated number of times Min_N=Max_Phase_Err/Max_ Δ FCW according to the described maximum phase error Max_Phase_Err that can tolerate respectively; Wherein, Max_ Δ FCW is the absolute value of the maximum error that possibly cause when converting the frequency control word of floating-point format the frequency control word of fixed point format in the said step 3, for adopting the round up conversion method of mode of decimal place, Max_ Δ FCW=0.5; For adopting the conversion method of directly removing decimal place, Max_ Δ FCW=1;
2) for each described frequency control word, calculate T respectively e=T s* Min_N, wherein T sBe the described signals sampling time interval;
3) get 2) three T obtaining of step eIn minimum value, be the time interval T that finite word length effect is proofreaied and correct e
It is L that said step 3 converts the frequency control word of floating-point format to word length WThe step of fixed point format comprise:
4) frequency control word with said floating-point format multiply by
Figure BSA00000627448800041
5) with 1) value that obtains of step carry out round numbers computing that decimal place rounds up or with 1) value that obtains of step directly removes the rounding operation of decimal place.
The described time interval T that proofreaies and correct at finite word length effect of step 3 eError * T that the summation of the interior error that produces=said single conversion produces e/ T s
The method that the phase place of said step 4 pair signal adds up is the formula below circulation is carried out: the frequency control word of phase-accumulated value=phase-accumulated value+fixed point format that said step 3 obtains; Calculate for the first time its Doppler frequency parameter if be for this gps satellite of current simulation; Then the initial value of phase-accumulated value is the prima facies place value of signal; Otherwise the initial value of phase-accumulated value is the phase-accumulated value that the preservation in the said step 4 is got off.
Technical scheme of the present invention has been carried out good compromise on floating-point is realized and fixed a point to realize; In conjunction with floating-point accuracy that realizes and the simplicity of fixing a point to realize; Considered simultaneously that floating-point format is changeed in the fixed point format process because the error that finite word length effect is introduced, and it has been proofreaied and correct.Before the step 3 of present technique scheme, the CALCULATION OF PARAMETERS frequency is lower (to equal the parameter update interval T u), use floating-point to realize, to come the value of calculating horizontal rate control word more accurately according to the Doppler frequency of current time satellite-signal; After the step 3, calculated rate is higher (mainly to be by SI T sCarry out phase-accumulated calculating), use fixed point to realize, can produce satellite-signal more simply fast.Simultaneously, also calculated the accumulation of phase error that finite word length effect causes, and in the parameter update interval T of implementation part of fixing a point eProofread and correct at the place, has guaranteed the accuracy that fixed point realizes.
Description of drawings
The overview flow chart of the method for a kind of phase error that is used to proofread and correct the gps signal source that accompanying drawing 1 proposes for the present invention.
Embodiment
Specify the embodiment of technical scheme of the present invention below.
Step 1: initialization specifically comprises: setup parameter comprises gps signal carrier frequency f T=1575.42MHz, C/A code check f C=1.023MHz, navigation message code check f D=50Hz, SF f s=16.384MHz, SI T s=1/16.384us, intermediate frequency f I=5.12MHz, the word length L of digital platform W=32bit,, the maximum phase error tolerance value that three frequency control word errors cause is equal 8192, adopts the method that rounds up to realize fixed point, i.e. Max_ Δ FCW=0.5, the finite word length effect that obtains is thus proofreaied and correct interval T e=1ms proofreaies and correct interior at interval number of data points N e=T e/ T s=16384, in addition, setting parameter upgrades interval T u=2ms, N u=T u/ T e=2.
Step 2: the frequency control word that calculates floating-point format according to the Doppler frequency of current time
(a) the frequency control word FCW_Carrier_Float of calculating IF carrier, promptly
FCW _ Carrier _ Float = 5.12 MHz + f d 16.384 MHz
(b) the frequency control word FCW_Code_Float of calculating C/A sign indicating number, promptly
FCW _ Code _ Float = 1.023 MHz + f d × ( 1.023 / 1575.42 ) 16.384 MHz
(c) the frequency control word FCW_Data_Float of calculating navigation message, promptly
FCW _ Data _ Float = 50 Hz + f d × ( 50 Hz / 1575.42 MHz ) 16.384 MHz
F wherein dBe Doppler frequency.
Step 3: frequency control word is carried out fixed point handle, and write down the error that finite word length effect causes, specifically comprise the steps:
Step 3-1: the IF-FRE control word FCW_Carrier_Float to floating-point format carries out rounding up to decimal place after multiply by
Figure BSA00000627448800064
; Obtain the IF-FRE control word FCW_Carrier of fixed point format, promptly
FCW_Carrier=round{FCW_Carrier_Float×2 32}
Calculating is proofreaied and correct interval T at finite word length effect eIn, the IF carrier accumulation of phase error delta Phase_Carrier by finite word length effect causes still representes with fixed point format, promptly
ΔPhase_Carrier=round{(FCW_Carrier_Float×2 32-FCW_Carrier)×16384}
Step 3-2: carry out rounding up after the C/A code frequency control word FCW_Code_Float of floating-point format multiply by to decimal place; Obtain the C/A code frequency control word FCW_Code of fixed point format, promptly
FCW_Code=round{FCW_Code_Float×2 32}
Calculate finite word length effect and proofread and correct interval T eIn, the C/A code phase cumulative errors Δ Phase_Code by finite word length effect causes still representes with fixed point format, promptly
ΔPhase_Code=round{(FCW_Code_Float×2 32-FCW_Code)×16384}
Step 3-3: the navigation message frequency control word FCW_Data_Float to floating-point format carries out rounding up to decimal place after multiply by
Figure BSA00000627448800066
; Obtain the navigation message frequency control word FCW_Data of fixed point format, promptly
FCW_Data=round{FCW_Data_Float×2 32}
Calculate finite word length effect and proofread and correct interval T eIn, the navigation message accumulation of phase error delta Phase_Data by finite word length effect causes still representes with fixed point format, promptly
ΔPhase_Data=round{(FCW_Data_Float×2 32-FCW_Data)×16384}
Round{} computing in the above-mentioned formula, expression are accomplished floating-point format to the operation that rounds up of decimal place to the conversion of fixed point format except adopting the mode that decimal place is rounded up; Can also directly decimal place be given up; The way that promptly only keeps integer-bit converts the frequency control word of fixed point format to, if adopt this method, and Max_ Δ FCW=1; Other parameters relevant with Max_ Δ FCW also should be made corresponding change.
Step 4: it is phase-accumulated to utilize the frequency control word of fixed point format to carry out, and specifically comprises the steps:
Step 4-1: it is phase-accumulated to utilize the frequency control word of IF carrier to carry out, promptly
Phase_Carrier=Phase_Carrier+FCW_Carrier
Step 4-2: it is phase-accumulated to utilize the frequency control word of C/A sign indicating number to carry out, promptly
Phase_Code=Phase_Code+FCW_Code
Step 4-3: it is phase-accumulated to utilize the frequency control word of navigation message to carry out, promptly
Phase_Data=Phase_Data+FCW_Data
Before the beginning that phase place added up with frequency control word, need to judge whether the Doppler frequency that is adopted calculates this satellite for the first time, if the initial value of the phase place in then top three formula that add up is the first phase of this satellite-signal; If not, the initial value of the phase place in then top three formula that add up is the phase-accumulated value of preserving after last the adding up.
Step 5: proofread and correct interval T if arrive finite word length effect e, then utilize the cumulative errors amount that finite word length effect is proofreaied and correct, and will proofread and correct later accumulation result and preserve, specifically comprise the steps:
Step 5-1: proofread and correct the IF carrier phase place, promptly
Phase_Carrier=Phase_Carrier+ΔPhase_Carrier
Step 5-2: proofread and correct the C/A code phase, promptly
Phase_Code=Phase_Code+ΔPhase_Code
Step 5-3: proofread and correct the navigation message phase place, promptly
Phase_Data=Phase_Data+ΔPhase_Data
Do not proofread and correct interval T if reach finite word length effect e, then directly accumulation result is preserved.
Step 6: three phase-accumulated values to step 5 is preserved press 2 32Carry out delivery and output, specifically comprise the steps:
Step 6-1: the intermediate frequency carrier phase is carried out delivery, promptly
Phase_Carrier=mod{Phase_Carrier,2 32}
Step 6-2: the C/A code phase is carried out delivery, promptly
Phase_Code=mod{Phase_Code,2 32}
Step 6-3: the navigation message phase place is carried out delivery, promptly
Phase_Data=mod{Phase_Data,2 32}
In the fixed point implementation procedure, delivery is exactly simply to get low level, promptly to 2 32Delivery is exactly to take out low 32 of phase-accumulated value simply to get final product.
Step 7: judge whether to have reached the parameter update interval T uIf, reach, then return step 2, otherwise return step 4.
The step 2 of this specific embodiment adopts floating system to realize, step 3,4,5,6 adopts the fixed point mode to realize, describes and can find out from top step, has only reached the parameter update interval T uThe time, just can require the floating-point platform to recomputate each frequency control word, and the parameter update interval T uValue generally all proofread and correct interval T than finite word length effect eGrow, this has just well taken into account precision and system cost, makes that the floating-point operation amount of expensive and high power consumption is fewer, and a large amount of computings is placed on the fixed point Platform Implementation.Therefore, the present technique scheme is when concrete the realization, and suggestion is with the floating-point implementation part and fix a point implementation part separately; Place on the different platforms, as adopting the pattern of " DSP+FPGA ", DSP is responsible for the relevant calculating of floating-point; FPGA is responsible for phase-accumulated and tables look-up, and accomplishes the task of simulating GPS signal jointly.

Claims (8)

1. method that is used to proofread and correct the phase error in gps signal source is characterized in that in turn including the following steps:
Step 1: the word length of setting digital platform is L W, IF carrier, C/A sign indicating number and the navigation message signals phase error of largest tolerable separately are Max_Phase_Err, parameter update time interval T u, the time interval T that finite word length effect is proofreaied and correct e, signals sampling time interval T s
Step 2: utilize the Doppler frequency parameter of current time, calculate the frequency control word of floating-point format;
Step 3: it is L that the frequency control word of the floating-point format that step 2 is obtained converts word length to WThe frequency control word of fixed point format, the error that the record single conversion produces, and calculate the time interval T that proofreaies and correct at finite word length effect eThe summation of the interior error that produces is preserved this sum of the deviations then with fixed point format;
Step 4: the frequency control word of the fixed point format that obtains with step 3, with signals sampling time interval T sBe the time beat, the phase place of signal is added up, judge then whether the time of cumulative process reaches the time interval T that finite word length effect is proofreaied and correct eIf, reached this interval, then add the sum of the deviations that step 3 obtains with this phase-accumulated value earlier, obtain the phase error correction value, this phase error correction value is right
Figure FSA00000627448700011
Preserve behind the delivery and output, get into step 5 then, if it is do not reach, then directly that phase-accumulated value is right
Figure FSA00000627448700012
Preserve behind the delivery and export this result, get into step 5 then;
Step 5: judge whether to reach parameter update time interval T uIf, reach, then return step 2; If do not reach, then return step 4.
2. a kind of method that is used to proofread and correct the phase error in gps signal source according to claim 1 is characterized in that: parameter update time interval T uBe greater than or equal to the time interval T that finite word length effect is proofreaied and correct e
3. a kind of method that is used to proofread and correct the phase error in gps signal source according to claim 1 is characterized in that: parameter update time interval T uBe the time interval T that finite word length effect is proofreaied and correct eIntegral multiple.
4. a kind of method that is used to proofread and correct the phase error in gps signal source according to claim 1, it is characterized in that: said frequency control word is in IF carrier frequency control word, C/A code frequency control word and the navigation message frequency control word.
5. a kind of method that is used to proofread and correct the phase error in gps signal source according to claim 4 is characterized in that: the time interval T that finite word length effect is proofreaied and correct eAdopt following method to confirm:
1) for each described frequency control word; Calculate its needed minimal accumulated number of times Min_N=Max_Phase_Err/Max_ Δ FCW according to the described maximum phase error Max_Phase_Err that can tolerate respectively; Wherein, Max_ Δ FCW is the absolute value of the maximum error that possibly cause when converting the frequency control word of floating-point format the frequency control word of fixed point format in the said step 3, for adopting the round up conversion method of mode of decimal place, Max_ Δ FCW=0.5; For adopting the conversion method of directly removing decimal place, Max_ Δ FCW=1;
2) for each described frequency control word, calculate T respectively e=T s* Min_N, wherein T sBe the described signals sampling time interval;
3) get 2) three T obtaining of step eIn minimum value, be the time interval T that finite word length effect is proofreaied and correct e
6. a kind of method that is used to proofread and correct the phase error in gps signal source according to claim 1 is characterized in that: it is L that said step 3 converts the frequency control word of floating-point format to word length WThe step of fixed point format comprise:
1) frequency control word with said floating-point format multiply by
Figure FSA00000627448700021
2) with 1) value that obtains of step carry out round numbers computing that decimal place rounds up or with 1) value that obtains of step directly removes the rounding operation of decimal place.
7. a kind of method that is used to proofread and correct the phase error in gps signal source according to claim 1 is characterized in that: the described time interval T that proofreaies and correct at finite word length effect of step 3 eError * T that the summation of the interior error that produces=said single conversion produces e/ T s
8. a kind of method that is used to proofread and correct the phase error in gps signal source according to claim 1; It is characterized in that: the method that the phase place of said step 4 pair signal adds up is the formula below circulation is carried out: the frequency control word of phase-accumulated value=phase-accumulated value+fixed point format that said step 3 obtains; Calculate for the first time its Doppler frequency parameter if be for this gps satellite of current simulation; Then the initial value of phase-accumulated value is the prima facies place value of signal; Otherwise the initial value of phase-accumulated value is the phase-accumulated value that the preservation in the said step 4 is got off.
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