An ambiguity-free tracking method for high-order BOC modulated navigation signals

Abstract

The present invention discloses a method for unambiguous tracking of a high-order BOC modulated navigation signal, including: counting the period of the subcarrier operation; adding a variable - the subcarrier half-cycle count accumulation value Acc_sc and initializing it; statistically calculating the count difference of the subcarrier half-cycle count Cnt2_sc of two FTF time-scale signals as the increment and accumulating Acc_sc; continuously comparing Acc_sc with the updated subcarrier half-cycle ambiguity value Cnt_sc; setting the locked state; if the number of consecutive equal times reaches the lock-in threshold value, entering the locked state, replacing the original subcarrier half-cycle ambiguity value Cnt_sc with Acc_sc, and at this time outputting the subcarrier observation value externally as the ranging observation value; setting the unlocked state; if the number of consecutive unequal times reaches the unlock threshold value, entering the unlocked state, re-initializing Acc_sc, and at this time outputting the code observation value externally as the ranging observation value. The present invention solves the traditional subcarrier tracking ambiguity problem, can obtain better anti-multipath effect and stable high-precision ranging observation quantity, thereby improving the positioning accuracy.

Description

Technical Field

[0001] The present invention relates to the field of satellite positioning and navigation, and particularly to a method for ambiguity-free tracking of high-order BOC modulated navigation signals. Background Art

[0002] Modern satellite navigation systems generally use BOC modulated signals. Compared with the traditional BPSK modulation, they can be transmitted on the existing carrier frequency without affecting the existing BPSK modulated signals. At the same time, BOC modulated signals can obtain better anti-multipath effects and higher ranging accuracies. The high-order BOC factor has a high carrier rate, with the advantage of obtaining a higher ranging accuracy, but the disadvantage is that there are multiple peaks in the correlation, resulting in unstable ranging accuracy. High-order BOC modulated signals have been applied in existing navigation systems. The Beidou navigation system has a BOC(14, 2) navigation signal at the B1 frequency point, and Galileo has a BOC(15, 2.5) navigation signal at the E6 frequency point.

[0003] BOC modulation multiplies a periodic square wave signal on the traditional PRN code, moving the original spectrum at the center frequency point to both sides of the center frequency point. The BOC(m, n) signal is defined as the pseudo-code rate f c = n×1.023e6, and the rate of the sub-carrier period is f s = m×1.023e6, and the spreading ratio is K = 2m / n. When the phase of the square wave signal relative to the PRN code is 0, that is, sine sub-carrier modulation, its signal expression is: S BOCs = c(t)×s(t). The signal modulation schematic diagram is as shown in Figure 1 where, T c = 1 / f c , T s = 1 / (2f s ).

[0004] In the data branch of the GPS L1 signal, BOC(1, 1) modulation is used, and the spreading ratio is 2. In Galileo, BOC(15, 2.5) modulation is used in the data branch, and the spreading ratio is 12.

[0005] Currently, for high-order BOC modulated signals, the mainstream tracking methods are: single-sideband tracking, peak-hopping tracking, double-sideband tracking, and double-estimation tracking. The ranging accuracies obtained by the first three are the accuracies of the pseudo-code, and only half of the signal energy is used in single-sideband tracking. The double-estimation tracking method can utilize all the signal energy and obtain a higher ranging accuracy by using the sub-carrier. The principle of the double-estimation tracking method is as shown in Figure 2 .

[0006] Due to relative motion between the satellite and the receiver, as well as factors such as receiver clock drift, the received signal will have Doppler shift, which is reflected in the carrier, code, and subcarrier. If the single-sideband tracking method is adopted, only the carrier and code loops need to be tracked. If the dual-estimation tracking BOC signal is adopted, the tracking loop consists of three loops: the carrier loop, the code loop, and the subcarrier loop. The carrier loop completes the tracking of the carrier frequency and phase of the received signal, and at the same time obtains the carrier observable. The code loop completes the tracking of the pseudo-code signal of the received signal, and at the same time obtains the pseudo-code observable. The subcarrier loop completes the tracking of the subcarrier signal of the received signal, and at the same time obtains the subcarrier observable.

[0007] To calculate the distance, the receiver needs the observables at the same epoch. By subtracting the observables from the local time, the distance can be obtained. Using the conventional dual-estimation tracking method, since the subcarrier is tracked in a way similar to the pseudo-code tracking, the accuracy of the code loop or subcarrier loop observables is often insufficient, resulting in the phenomenon that the subcarrier half-cycle ambiguity value Cnt_sc is 1 more or 1 less. For the BOC(15,2.5) signal, an error of about 9.7 meters will occur, ultimately leading to positioning errors. Summary of the Invention

[0008] The object of the present invention is that currently, Beidou uses high-order BOC modulated navigation signals at the B1 frequency point, and Galileo uses high-order BOC modulated navigation signals at the E6 frequency point. To achieve the tracking of high-order BOC, while obtaining better anti-multipath effects and higher ranging accuracy, on the basis of dual-estimation tracking, the statistical circuit of the subcarrier observable is improved to solve the subcarrier half-cycle ambiguity problem when the signal is weak or the signal is subject to short-term interference; thus, a non-ambiguous tracking method for high-order BOC modulated navigation signals is provided.

[0009] The object of the present invention is achieved through the following technical solutions:

[0010] A non-ambiguous tracking method for high-order BOC modulated navigation signals includes the following steps:

[0011] Step 1: Count the periods of the subcarrier through a counting circuit; then, add a variable - subcarrier half-cycle count accumulation value Acc_sc in the software program, and calculate the subcarrier half-cycle ambiguity value Cnt_sc at the current FTF time scale signal through the code observation value and initialize it;

[0012] Step 2: Statistically calculate the counting difference of the subcarrier half-cycle count Cnt2_sc between two FTF time scale signals as the increment Delta_Cnt_sc, and accumulate it to the subcarrier half-cycle count accumulation value Acc_sc; and calculate the Cnt_sc value through the code phase;

[0013] Step 3: Continuously compare Acc_sc with the updated Cnt_sc value;

[0014] Step 4: Set the locked state; preset a locked threshold. If the number of consecutive equal occurrences reaches the locked threshold, enter the locked state. At this time, use the subcarrier half-cycle count cumulative value Acc_sc to replace the original subcarrier half-cycle ambiguity value Cnt_sc as the integer part of the subcarrier observation value's half-cycle; use the phase Ph_sc within the subcarrier half-cycle as the fractional part of the subcarrier observation value's half-cycle. The sum of the two is the subcarrier observation value, and at this time, output the subcarrier observation value externally as the ranging observation value.

[0015] Step 5: Set the unlocked state; preset an unlocked threshold. If the number of consecutive unequal occurrences reaches the unlocked threshold, enter the unlocked state, and re-initialize the subcarrier half-cycle count cumulative value Acc_sc. At this time, output the code observation value externally as the ranging observation value.

[0016] Further, the initialization process is specifically as follows: Calculate the subcarrier half-cycle ambiguity value Cnt_sc under the current FTF time-scale signal through the code observation value as the initial subcarrier half-cycle count cumulative value Acc_sc.

[0017] Further, the locked threshold is 45 times.

[0018] Further, the unlocked threshold is 30 times.

[0019] Further, between two FTF time-scale signals, the distance difference corresponding to the subcarrier half-cycle count is equal to the distance difference corresponding to the code phase.

[0020] Further, in step 1, the counting of the subcarrier running period takes the arrival of each FTF time-scale signal as the counting opportunity.

[0021] Further, the integer part of the subcarrier observation value's half-cycle in step 4 is obtained by using the subcarrier half-cycle count cumulative value Acc_sc to replace the original subcarrier half-cycle ambiguity value Cnt_sc.

[0022] Further, in step 4, output the subcarrier observation value externally as the ranging observation value, and the ranging observation value is the distance calculation parameter required in navigation and positioning.

[0023] Advantages of the present invention: The present invention solves the problem of subcarrier tracking ambiguity existing in traditional high-order BOC navigation signal tracking circuits, can stably output a relatively high-precision ranging value. By adopting this processing method, without affecting the existing circuit, adding a small amount of logic circuits, making good use of the signal advantages of high-order BOC, a better anti-multipath effect and stable high-precision ranging observations can be obtained, thereby improving the positioning accuracy. Description of the Drawings

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0025] Figure 1 It is a schematic diagram of a BOC modulation signal.

[0026] Figure 2 It is a schematic diagram of the principle of the dual-estimation tracking method.

[0027] Figure 3 It is a flowchart of the method of the present invention.

[0028] Figure 4 It is a principle block diagram of the improved unambiguous tracking observable statistical circuit of the present invention.

[0029] Figure 5 It is a principle block diagram of the observable statistical circuit of the conventional dual-estimation tracking method. Detailed implementation manners

[0030] It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0032] In this embodiment, as Figure 3 and Figure 4 shown, an unambiguous tracking method for a high-order BOC modulated navigation signal includes the following steps:

[0033] Step 1: Count the period of the subcarrier operation through a counting circuit; then, add a variable - the subcarrier half-cycle count accumulation value Acc_sc in the software program, and calculate the subcarrier half-cycle ambiguity value Cnt_sc at the current FTF time scale signal through the code observation value and initialize it;

[0034] Step 2: Statistically calculate the counting difference of the subcarrier half-cycle count Cnt2_sc of two FTF time scale signals as the increment Delta_Cnt_sc, accumulate the subcarrier half-cycle count accumulation value Acc_sc; and calculate the Cnt_sc value through the code phase;

[0035] Step 3: Continuously compare Acc_sc with the updated Cnt_sc value;

[0036] Step 4: Set the locked state; the preset lock-in threshold is 45 times. If the number of consecutive equal times reaches the lock-in threshold of 45 times, enter the locked state. At this time, use the subcarrier half-cycle count cumulative value Acc_sc to replace the original subcarrier half-cycle ambiguity value Cnt_sc as the integer part of the half-cycle of the subcarrier observation value; use the phase Ph_sc within the subcarrier half-cycle as the fractional part of the half-cycle of the subcarrier observation value. The sum of the two is the subcarrier observation value. At this time, output the subcarrier observation value externally as the ranging observation value;

[0037] Step 5: Set the unlocked state; the preset unlock threshold is 30 times. If the number of consecutive unequal times reaches 30 times, enter the unlocked state, and re-initialize the subcarrier half-cycle count cumulative value Acc_sc. At this time, output the code observation value externally as the ranging observation value. Among them, the ranging observation value is the distance calculation parameter required in navigation and positioning.

[0038] Among them, the initialization process is specifically: calculate the subcarrier half-cycle ambiguity value Cnt_sc under the current FTF time-scale signal through the code observation value as the initialized subcarrier half-cycle count cumulative value Acc_sc.

[0039] In this embodiment, the lock-in threshold needs to be considered comprehensively. If the lock-in threshold is too small, although the lock-in time is short, it is easy to enter the false lock state, resulting in errors in subsequent subcarrier observation values; if the lock-in threshold is too large, the probability of false lock will become smaller, but the lock-in time is longer. In practical applications, this value can be adjusted according to specific applications.

[0040] In this embodiment, the unlock threshold also needs to be considered comprehensively. If the unlock threshold is too small, it is easy to unlock, and then it needs to be locked again. Multiple lock-in processes and unlock processes will result in less validity of the subcarrier observation value; if the unlock threshold is too large, the unlock time will be longer, and it cannot be unlocked in time, resulting in errors in the subcarrier observation value. In practical applications, this value can be adjusted according to specific applications, but this value should not be greater than the lock-in threshold.

[0041] Among them, between two FTF time-scale signals, the distance difference corresponding to the subcarrier half-cycle count is equal to the distance difference corresponding to the code phase.

[0042] Among them, the counting of the subcarrier running period takes the arrival of each FTF time-scale signal as the counting opportunity.

[0043] In the existing scheme, to calculate the distance, the receiver needs the observation value at the same epoch. By subtracting the observation value from the local time, the distance can be obtained. The principle block diagram of the observation value statistical circuit of the conventional double-estimation tracking method is as Figure 5As shown in the figure. Among them, the FTF time scale signal is the basic time frame of the timer circuit, which latches the code loop observation, carrier observation, and subcarrier observation. The code observation values include the code phase Ph_pn within a chip, the chip count value Cnt_pn within a millisecond, and the millisecond count value Cnt_ms; the carrier observation includes the carrier phase Ph_car and the carrier integer cycle count Cnt_car; the subcarrier observation includes the phase Ph_sc within a half cycle of the subcarrier.

[0044] The dual estimation tracking method tracks the subcarrier through a tracking method similar to that of the pseudo-code. By combining the subcarrier observation and the code observation, the subcarrier half-cycle ambiguity value Cnt_sc after deducting the phase within a half cycle of the subcarrier from the chip is:

[0045] Cnt_sc = (Ph_pn - Ph_sc) / T s

[0046] The subcarrier observation value Ph1_sc after considering the subcarrier phase is:

[0047] Ph1_sc = Cnt_sc × T s + Ph_sc

[0048] Theoretically, the subcarrier half-cycle ambiguity value Cnt_sc is an integer, but in the actual environment, it will be a floating-point number, and there is a rounding process. If the accuracy of the code loop or subcarrier loop observation is insufficient, this value will be 1 more or 1 less. For the BOC(15,2.5) signal, an error of about 9.7 meters will occur, ultimately leading to positioning errors.

[0049] After using the technology improved by this solution, by adding a subcarrier half-cycle count Cnt2_sc circuit to the circuit and adding a variable subcarrier half-cycle count accumulation value Acc_sc to the software, using the subcarrier half-cycle count accumulation value Acc_sc to replace the original subcarrier half-cycle ambiguity value Cnt_sc, the subcarrier half-cycle count accumulation value Acc_sc is the count difference of the subcarrier half-cycle count Cnt2_sc of two FTF time scale signals, used as the increment Delta_Cnt_sc, and the subcarrier half-cycle count accumulation value Acc_sc is accumulated. The calculation formula for the new subcarrier observation value Ph1_sc' is

[0050] Ph1_sc' = Acc_sc × T s + Ph_sc

[0051] The increment Delta_Cnt_sc is an integer, and the accumulated sub-carrier half-cycle count Acc_sc is also an integer. Therefore, there will be no data error problem caused by rounding of floating-point numbers for the sub-carrier half-cycle ambiguity value Cnt_sc. Using the sub-carrier observation value improved by this solution as the ranging observation value, there is no ambiguity tracking problem, and the positioning result is more stable and has higher accuracy.

[0052] It should be noted that, for the foregoing method embodiments, for the sake of simple description, they are all expressed as a series of action combinations. However, those skilled in the art should know that this application is not limited by the described action sequence, because according to this application, some steps can be performed in other sequences or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and units involved are not necessarily essential to this application.

[0053] In the above embodiments, the descriptions of the respective embodiments have their own emphases. For the parts not detailed in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0054] Those of ordinary skill in the art can understand that all or part of the processes of implementing the above method embodiments can be completed by instructing relevant hardware through a computer program. The program can be stored in a computer-readable storage medium. When the program is executed, it can include the processes of the above method embodiments. Among them, the storage medium can be a magnetic disk, an optical disk, a ROM, a RAM, etc.

[0055] The above-disclosed are only the preferred embodiments of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. Therefore, equivalent changes made according to the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. An ambiguity-free tracking method for high-order BOC modulated navigation signals, characterized in that, it includes the following steps: Step 1: Count the period of the subcarrier operation through a counting circuit; then, in the software program, add a variable - the subcarrier half-cycle count accumulation value Acc_sc, and calculate the subcarrier half-cycle ambiguity value Cnt_sc under the current FTF time scale signal through the code observation value and initialize it. The specific initialization process is: calculate the subcarrier half-cycle ambiguity value Cnt_sc under the current FTF time scale signal through the code observation value as the initialized subcarrier half-cycle count accumulation value Acc_sc; Step 2: Statistically calculate the count difference of the subcarrier half-cycle count Cnt2_sc of two FTF time scale signals as the increment Delta_Cnt_sc, and accumulate it to the subcarrier half-cycle count accumulation value Acc_sc; and calculate the Cnt_sc value through the code phase; Step 3: Continuously compare Acc_sc with the updated Cnt_sc value; Step 4: Set the locked state; preset a locked threshold. If the number of consecutive equal times reaches the locked threshold, enter the locked state. At this time, use the subcarrier half-cycle count accumulation value Acc_sc to replace the original subcarrier half-cycle ambiguity value Cnt_sc as the half-cycle integer part of the subcarrier observation value; use the phase Ph_sc within the subcarrier half-cycle as the half-cycle decimal part of the subcarrier observation value. The sum of the two is the subcarrier observation value. At this time, output the subcarrier observation value externally as the ranging observation value; Step 5: Set the unlocked state; preset an unlocked threshold. If the number of consecutive unequal times reaches the unlocked threshold, enter the unlocked state, and re-initialize the subcarrier half-cycle count accumulation value Acc_sc. At this time, output the code observation value externally as the ranging observation value.

2. The ambiguity-free tracking method for high-order BOC modulated navigation signals according to claim 1, characterized in that, the locked threshold is 45 times.

3. The ambiguity-free tracking method for high-order BOC modulated navigation signals according to claim 1, characterized in that, the unlocked threshold is 30 times.

4. The ambiguity-free tracking method for high-order BOC modulated navigation signals according to claim 1, characterized in that, between two FTF time scale signals, the distance difference corresponding to the subcarrier half-cycle count is equal to the distance difference corresponding to the code phase.

5. The ambiguity-free tracking method for high-order BOC modulated navigation signals according to claim 1, characterized in that, in Step 1, the counting of the subcarrier operation period takes the arrival of each FTF time scale signal as the counting opportunity.

6. The ambiguity-free tracking method for high-order BOC modulated navigation signals according to claim 1, characterized in that, the half-cycle integer part of the subcarrier observation value in Step 4 is obtained by using the subcarrier half-cycle count accumulation value Acc_sc to replace the original subcarrier half-cycle ambiguity value Cnt_sc.

7. The ambiguity-free tracking method for high-order BOC modulated navigation signals according to claim 1, characterized in that, In step 4, the externally output subcarrier observation values are used as ranging observation values, and the ranging observation values are distance calculation parameters required in navigation and positioning.

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