A method for resolving half-cycle ambiguities in GNSS carrier phase

Abstract

According to the present invention, the four signals L transmitted by the GPS satellites are 1CA , L 1P , L 2P , and L 2C is received and, in a manner known in the art, 1CA and L 2C to the carrier phase φ LCA and φ L2C In addition, the down-converted signal L 1P and L 2P The cross-correlation P between the signals is determined taking into account the delay between said signals. According to the invention, the sign of P is 1CA Signal half-cycle ambiguity A L1CA According to a first embodiment, this is used in a novel method that allows the direct determination of L 2P Instead of determining the phase of the replica, φ L2C By replicating L with a phase derived from 2P This is done by down-converting the signal. This new method converts the P code directly into A L1CA According to a second embodiment, L 2P Replica phase φ L2P is determined by a tracking loop as in the prior art, and the half-cycle ambiguity A L1CA is based on the sign of P, and the carrier phase φ L2P and φ L2C The invention also relates to a GPS receiver comprising at least one channel adapted to carry out the method according to the first or second embodiment.

Description

[Technical field]

[0001] The present invention relates to Global Navigation Satellite Systems (GNSS), and in particular to the problem of half-cycle ambiguity in the carrier phase of satellite signals modulated with a navigation sequence. [Background technology]

[0002] Global Navigation Satellite Systems (GNSS) are widely used to provide position, velocity, and time to any user on or near the Earth's surface. GNSS comprises, among other things, a constellation of multiple navigation satellites (GNSS satellites) that transmit dedicated navigation signals known as "ranging signals." The best known is the Global Positioning System (GPS), developed by the United States. Other systems include the European Galileo system, the Russian GLONASS, and the Chinese BeiDou system.

[0003] All GNSS systems transmit multiple ranging signals in various frequency bands. For example, modern GPS satellites transmit at least two signals (L1, L2, L3) modulated on the L1 carrier at 1575.42 MHz. 1CA and L 1P ) and two signals modulated by the L2 carrier at 1227.6MHz (L 2P and L 2C ) and each signal consists of a pseudorandom code (PRN code) that is unique to each satellite. Some signals, known as "data signals", are further modulated by bits of the navigation message. Signals that are not modulated by navigation bits are called "pilot signals". For example, in GPS, 2C The signal is the pilot signal (or, more precisely, the L 2C Signal L 2CL The component is the pilot signal, and L 1CA , L 1P , and L 2P The signal is a data signal. For most signals, the PRN code is public and known to the receiver. However, for GPS L1P and L 2P For some signals such as , the PRN code is classified and unknown.

[0004] In a GNSS receiver, first, the different frequency bands are amplified and down-converted to baseband in an analog front-end circuit, and the signals from different satellites are acquired and tracked in a dedicated tracking channel. For each tracked signal of each satellite, the receiver measures the transmission delay from the satellite to the receiver and the phase of the received carrier. The measured delay multiplied by the speed of light is called the pseudorange. Although the measurement of the carrier phase is much more accurate than the pseudorange, it is subject to an ambiguity of an integer number of carrier wavelengths.

[0005] The phenomenon of integer ambiguity and techniques to overcome it are well known in the art. The present invention is directed to the problem of half cycle ambiguity affecting carrier phase measurements of data signals, as described in the following paragraphs.

[0006] In data signals, the navigation bit modulation follows the binary phase shift keying (BPSK) scheme, in which the phase of the carrier is offset by 0° or 180° (half the carrier period) depending on the value of the bit to be transmitted (1 or 0). In such a modulation scheme, a global shift of 180° of the unmodulated carrier is indistinguishable from an inversion of all navigation bits. For example, upon receiving the phase sequence (0°, 180°, 180°, 0°, 0°), the receiver cannot distinguish between the hypothesis that the unmodulated carrier phase is 0° and the navigation bits are (0, 1, 1, 0, 0) and the hypothesis that the carrier phase is 180° and the navigation bits are (1, 0, 0, 1, 1). Until the receiver can determine the correct bit sequence, a 180° ambiguity exists. This ambiguity is called "half cycle ambiguity".

[0007] Resolving half-cycle ambiguities typically involves waiting for the occurrence of a preamble (a short bit sequence transmitted at regular intervals whose contents are known in advance) and comparing it to the decoded sequence. In case of a sign reversal (if the bits of the preamble are decoded with their polarity reversed), the receiver adjusts its carrier phase by 180°. If there is a match, the carrier phase is confirmed and does not need to be changed. This process must be restarted after each loss of tracking (i.e., after each signal interruption).

[0008] Even if the half cycle ambiguity is resolved, it may be lost again during tracking, especially if the signal reception is weak. Such an event is known in the art as a "half cycle slip." A half cycle slip is difficult to detect until the next preamble is received. During that interval, the carrier phase measurement is inaccurate by a half cycle.

[0009] Because the availability of carrier phase with integer ambiguities is key to high precision positioning, there is a need for techniques to speed up the resolution of half-cycle ambiguities that affect data signals.

[0010] Many cycle slip detection and repair algorithms have been proposed that involve detecting the presence of discontinuities in the phase measurements. These methods typically compare the detected carrier phase to a modeled value obtained from past data, for example by extrapolation. Since extrapolation diverges quickly, these methods require the availability of recent data. For this reason, they are not suitable for resolving initial half-cycle ambiguities, in this case since the last time a given satellite was tracked may have been several hours ago. A technique is needed that can be applied regardless of the time a given satellite was last observed.

[0011] U.S. Patent Publication No. 6,150,978 describes a GPS L 1CAThis paper discloses a technique for rapid half-cycle ambiguity resolution applicable to satellite signals. The technique involves detecting a match or inverse match between an internally stored expected data message and the received data. The technique requires a memory buffer to store past data and relies on the assumption that the navigation messages transmitted by the satellites do not change frequently. Periodic refreshes of the memory buffer are required to keep track of navigation message updates, and the technique cannot be applied during those refreshes.

[0012] Son et al., "A new half cycle ambiguity resolution method for GPS attitude determination system," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, Utah, September 2000, pp. 2227-2232, propose a method to resolve half cycle ambiguities by comparing data sequences from multiple antennas. The method takes advantage of the fact that the demodulated data sequences from multiple antennas are identical for the same satellite. The technique is not applicable to single antenna configurations, and in multi-antenna systems it is ineffective in the most common case where a signal interruption concerns all antennas.

[0013] U.S. Patent Publication No. 6,061,390 describes how half-cycle ambiguity is related to GPS L 1CA Provided that it is resolved in carrier phase, GPS L 1P and L 2P It discloses a method to resolve half-cycle ambiguities in carrier phase measurements. 1CA Resolution of half-cycle ambiguities is still based on testing the sign of the preamble, as explained above. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] U.S. Patent Publication No. 6,150,978 [Patent Document 2] U.S. Patent Publication No. 6,061,390 [Patent Document 3] U.S. Patent No. 5,134,407 [Non-patent literature]

[0015] [Non-Patent Document 1] Son et al., "A new half cycle ambiguity resolution method for GPS attitude determination system," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, Utah, September 2000, pp. 2227-2232. [Non-Patent Document 2] Conference Paper "Woo, KT, "Optimum Semi-Codeless Carrier Phase Tracking of L2", Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, Tennessee, September 1999, pp. 289-306" Summary of the Invention [Means for solving the problem]

[0016] The present invention aims to provide a solution to the problems described above. This aim is achieved by a method and a GPS receiver as disclosed in the appended claims. According to the invention, the four signals L transmitted by the GPS satellites are 1CA , L 1P , L 2P , and L 2C is received, e.g., in a manner known in the art, 1CA and L 2C to the carrier phase φ LCA and φ L2C In addition, the down-converted signal L 1P and L 2P The cross-correlation P between the signals is determined taking into account the delay between said signals. According to the invention, the sign of P is 1CA Signal half-cycle ambiguity A L1CA According to a first embodiment, this is used in a novel method that allows the direct determination of L 2P Instead of determining the phase of the replica, φ L2C By replicating L with a phase derived from 2P This is done by down-converting the signal. This new method converts the P code directly into A L1CA According to a second embodiment, L 2P Replica phase φ L2P is determined by a tracking loop as in the prior art, and the half-cycle ambiguity A L1CA is based on the sign of P, and the carrier phase φ L2P and φ L2C The invention also relates to a GPS receiver comprising at least one channel adapted to carry out the method according to the invention.

[0017] In particular, the present invention relates to a method for resolving half-cycle ambiguities in a Global Positioning System (GPS) carrier phase, the method comprising the steps of: The following signals are received from GPS satellites: First carrier frequency f L1 The first signal L transmitted at 1CA and, First carrier frequency f L1 A second signal L transmitted at 1P wherein the second signal has a known phase offset δφ with respect to the first signal. L1P,CA A second signal L 1P and, Second carrier frequency f L2 A third signal L transmitted at 2C and, Second carrier frequency f L2 A fourth signal L transmitted at 2P wherein the fourth signal has a known phase offset δφ with respect to the third signal. L2P,C A fourth signal L 2P and receiving the Tracking a first signal and a third signal, thereby determining a carrier phase φ of the first and third signals. L1CA and φ L2C determining an estimate of each of the carrier phases φ of the first signal; L1CA The estimate of the term A L1CA .π, A L1CA is equal to 0 or 1, and A L1CA However, from then on, L 1CA step, called the half-cycle ambiguity of the carrier phase, Second and fourth signals L 1P and L 2P to baseband by multiplication with carrier replicas of the second and fourth signals, determining a delay between the downconverted second and fourth signals, and determining a cross-correlation P between the downconverted second and fourth signals by integrating a product of the downconverted second and fourth signals over an integration time T, taking into account the delay. Including, The sign of the cross-correlation P, as well as the known phase offset δφ L2P,C and δφ L1P,CA But half cycle ambiguity AL1CA The present invention relates to a method for determining

[0018] According to one embodiment, the first signal L 1CA Half-cycle ambiguity A L1CA teeth, ω L2 =2πf L2 Phase ω L2 t + φ L2C + δφ L2P,C A fourth signal L with a carrier replica having 2P The fourth signal L is generated by multiplying 2P performing said step of down-converting The following verification steps: If P is positive, then A L1CA = 0; and If P is negative, A L1CA = 1 and a verification step According to the above, L can be obtained directly from the sign of the cross-correlation P. 1CA Signal half-cycle ambiguity A L1CA determining is determined by.

[0019] In the previous embodiment, the delay applied between the downconverted second and fourth signals was L 1P and L 2P This can be determined by a delay locked loop (DLL) circuit (15) that tracks the delay between

[0020] Alternatively, the steps of tracking the first signal and the third signal include obtaining respective estimates of satellite distances R L1CA and R L2C , the delay applied between the downconverted second and fourth signals being the delay between the third and first signals L divided by the speed of light. 2C and L 1CA The difference between the respective estimates of satellite distances obtained from (PR L2C -PR L1CA ) / c.

[0021] According to another embodiment, the first signal L 1CA Half-cycle ambiguity A L1CA teeth, The carrier phase φ of the fourth signal that maximizes the amplitude of the cross-correlation P L2P is tracked by a tracking loop to obtain the fourth signal and phase ω L2 t + φ L2P down-converting the fourth signal by multiplication with a carrier replica having From the sign of the cross-correlation P, and φ L2P and φ L2C From the difference between the following steps, The parameter D is

[0022]

number

[0023] and determining If the integer closest to D is even, then P>0, then A L1CA If P<0, then A L1CA = 1, and If the integer closest to D is odd, then P>0, then A L1CA If P<0, then A L1CA = 0 and According to the first signal L 1CA Half-cycle ambiguity A L1CA The step of determining

[0024] The method according to the previous embodiment is such that the absolute value of the difference between D and the nearest integer is less than or equal to a predefined threshold D th and determining whether L is greater than or less than 1CA Signal half-cycle ambiguity A L1CA The step of determining is performed by determining whether the absolute value of the difference between D and the nearest integer exceeds a predefined threshold D th It will be executed only if it is less than

[0025] The method according to the invention is such that the absolute value of P is a predefined threshold P th and determining whether L is greater than 1CA Signal half-cycle ambiguity A L1CA The step of determining is |P|>P th This is executed only if

[0026] The method according to the present invention is to calculate the half-cycle ambiguity A L1CA The method may be performed repeatedly to allow for substantially continuous monitoring of the

[0027] According to one embodiment, the half cycle ambiguity A L1CA is determined only if the same result is obtained a given number of times.

[0028] According to one embodiment, the method of the present invention comprises: determining a half cycle ambiguity A based on a preamble transmitted in a first signal; L1CA is iteratively performed in parallel with the method for determining the half cycle ambiguity, such that the time between two successive determinations of the half cycle ambiguity is shorter than the time between two successive preambles.

[0029] According to one embodiment, the time between two successive determinations of a half cycle ambiguity is less than 6 seconds.

[0030] The invention likewise relates to the use of the method of the invention for detecting half-cycle slips during acquisition and processing of GPS signals.

[0031] The invention also relates to a GPS receiver comprising one or more receiver channels, each channel receiving the following signals from the GPS satellites: First carrier frequency f L1 The first signal L transmitted at 1CA and, First carrier frequency f L1 A second signal L transmitted at 1Pwherein the second signal has a known phase offset δφ with respect to the first signal. L1P,CA A second signal L 1P and, Second carrier frequency f L2 A third signal L transmitted at 2C and, Second carrier frequency f L2 A fourth signal L transmitted at 2P wherein the fourth signal has a known phase offset δφ with respect to the third signal. L2P,C A fourth signal L 2P and and at least one of said channels is configured to track L according to the method according to the invention. 1CA Signal half-cycle ambiguity A L1CA The method includes the step of:

[0032] Aspects of the present invention will now be described in more detail with reference to the accompanying drawings, in which like reference numbers indicate like features. [Brief description of the drawings]

[0033] [Figure 1] FIG. 1 illustrates the high level architecture of a receiver channel of a GPS receiver currently known in the art; high level means that only the basic components and their interactions are illustrated, while details of the components themselves are not included in the drawing. [Diagram 2] FIG. 2 shows two possible high-level architectures of a receiver channel of a GPS receiver that can be used to implement the method according to the first embodiment. [Diagram 3] FIG. 2 shows two possible high-level architectures of a receiver channel of a GPS receiver that can be used to implement the method according to the first embodiment. [Figure 4] 4 is a flow chart illustrating the operation of an L1CA half-cycle ambiguity computation unit available in a receiver channel configured to implement the method according to the first embodiment. [Diagram 5] FIG. 2 illustrates a high-level architecture of a receiver channel of a GPS receiver that can be used to implement the method according to the second embodiment. [Figure 6] 11 is a flow chart illustrating the operation of an L1CA half-cycle ambiguity computation unit available in a receiver channel configured to implement the method according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] During reception of the kth navigation bit, the L received from the GPS satellite 1CA , L 1P , L 2P , and L 2C The signal can be expressed as follows:

[0035]

number

[0036] Here, D i,k is the code of the k-th bit transmitted in signal i (D i,k can take the value -1 or +1), PRN i (t) is the PRN code used by the satellite in signal i, which is a sequence of −1 and +1 “chips” at a high chipping rate, and ω Li is the angular frequency of the L1 and L2 carriers, i.e., ω L1 =2πf L1 and ω L2 =2πf L2 where f1=1575.42MHz and f2=1227.6MHz. i is the amplitude of signal i, taking into account the various gains in the transmit and receive chains. L1P,CA , L 1P Signal and L 1CA is the phase offset between the L2P,C , L 2P Signal and L 2Cis the phase offset between the signal. Typically, δφ L1P,CA and δφ L2P,C Both offsets are equal to π / 2, but other values ​​are possible. These offsets are intentionally applied to the signals generated by the satellites, and are therefore known and documented in the GPS Interface Specification (IS-GPS-200). Δ i is the delay of propagation of signal i from the satellite to the receiver. This delay is different for every signal due to the dispersive effect of the ionosphere and the inter-signal delays of the satellite and the receiver. Φ Li is the phase shift of the L1 and L2 carriers in their propagation from the satellite to the receiver.

[0037] L 2C It is known in the art that a signal includes a medium-length data modulation code (CM) and a long-length dataless code (CL). In equation (1) and further in the text, "L 2C ", but L 2C Signal L 2CL Only the components are considered. 2C The signal is a pilot signal, so there is no bit modulation. 1P and L 2P PRN code (PRN p ) and bit (D p,k ) are classified and considered unknown here. However, they are 1P and L 2P The same applies to.

[0038] Therefore, equation (1) represents the following four signals: First carrier frequency f L1 The first signal L transmitted at 1CA And, ω L1 =2πf L1 and a first signal L modulated by a first known PRN code and a first navigation bit sequence. 1CA and, A first carrier frequency f modulated by an unknown PRN code and an unknown navigation bit sequence L1 A second signal L transmitted at 1P wherein the second signal has a known phase offset δφ with respect to the first signal. L1P,CA A second signal L 1P and, Second carrier frequency f L2 A third signal L transmitted at 2C And ω L2 =2πf L2 and a third signal L modulated by a second known PRN code. 2C and, A second carrier frequency f modulated by the unknown PRN code and the unknown navigation sequence. L2 A fourth signal L transmitted at 2P wherein the fourth signal has a known phase offset δφ with respect to the third signal. L2P,C A fourth signal L 2P and Represents the mathematical formula.

[0039] According to the state of the art, the high-level architecture of a receiver channel tracking the four signals of equation (1) is shown in Figure 1. A receiver channel is dedicated to tracking one particular satellite. Thus, the receiver comprises multiple channels, one for each satellite of the Global Positioning System.

[0040] The four signals of equation (1) are brought to baseband by mixing with sine waves 1 to 4 in multiplier circuits 5 to 8. The phase of each of sine waves 1, 3, and 4 is obtained from the outputs of phase-locked loop (PLL) circuits 9, 10, and 11. These outputs are used to generate the respective signals L 1CA , L 2C , and L 2P Carrier phase measurement value φ for L1CA , φ L2P , and φ L2C It is. 1P For a signal, the phase is given by the known L 1P -L 1CA Phase offset δφL1P,CA L 1CA The PLLs 9 and 10 optimize the carrier phase measurement φ by maximizing the in-phase correlation between the downconverted signal and a locally generated replica of said signal. L1CA and φ L2C , said correlations being generated by the respective correlators 12 and 13. L 2P The PLL 11 outputs the downconverted signal L determined in the P1-P2 cross-correlator circuit 14. 1P and L 2P By maximizing the in-phase cross-correlation P between 2p Carrier phase of the signal φ L2P Determine L 1P Signal and L 2P Delay between the signal and 2P -Δ 1P After compensation, the delay is determined by a delay locked loop (DLL) 15. The measured delay is applied to the input signal in a delay circuit 16.

[0041] The DLL circuits 17 and 18 receive the signal L 1CA and L 2C By tracking the delays of the known PRN codes transmitted by the L1CA and PR L2C The DLL circuits 17 and 18 determine the delay by locking a local replica of the PRN code to the input signal and determining the time delay between the transmitted signal and the locally generated replica. As is known in the art, this may be accomplished by successively generating early, late and punctual replicas of the PRN code and comparing their correlation with the input signal.

[0042] Assuming perfect locking of the tracking loop to the input signal, the L given at the beginning of Eq. 1CA A receiver channel tracking the signal produces the following carrier phase measurements in radians:

[0043]

number

[0044] A L1CA By L 1CA The half-cycle ambiguity of the carrier phase is either 0 or 1, N L1CA is an integer ambiguity.

[0045] L 1CA As a by-product of signal tracking, the receiver channel also receives navigation bits (D L1CA,k As is well known in the art, the sign of A L1CA If A is 0, the navigation bit is decoded with the correct code. L1CA If n is 1, the navigation bits are decoded with their sign inverted. Thus, a well-known method for resolving half-cycle ambiguities involves waiting for the occurrence of a known bit pattern and comparing it to the decoded sequence. For example, GPS L 1CA The navigation bit stream contains a known 8-bit pattern, the so-called preamble, that is repeated every 6 seconds. In the case of sign reversal (when the preamble is decoded by reversing the polarity), A L1CA is evaluated to be 1, otherwise, A L1CA is evaluated to be 0.

[0046] This process must be restarted after each loss of tracking (i.e., after each signal interruption). For example, if the receiver is used in a car, the process of resolving half-cycle ambiguities is repeated every time the car passes under a bridge that blocks the GPS signal. It takes several seconds to resolve the half-cycle ambiguities after each signal interruption.

[0047] The present invention is L1CA This is related to the fast resolution of ambiguities. 2C Carrier phase φ measured from the signal L2C But, L 1P and L 2PUsing the signal as a link between L1 and L2, 1CA Half-cycle ambiguity A L1CA In particular, the sign (+ or -) of the P1-P2 cross-correlation P, determined by integration over a short time period, e.g., 1 second ("short" meaning shorter than the time span between subsequent transmissions of the preamble), can be used to speed up the determination of the measured carrier phase φ L2C If used wisely, A L1CA The inventors have found two different ways to achieve this, which are described below as first and second embodiments of the method of the invention.

[0048] First embodiment Assuming perfect locking of the tracking loop to the signal, L given by equation (1) 1CA and L 2C A receiver channel tracking the signal produces the following carrier phase measurements:

[0049]

number

[0050] As mentioned above, equation (3) assumes perfect locking of the tracking loop to the input signal. In practice, the carrier-phase measurements may differ slightly from the above equation, for example due to noise or multipath effects, but these deviations may be considered small compared to the terms cited in equation (3). In any case, φ L1CA The estimate of is the half-cycle ambiguity A L1CA A term A equal to 0 or π depending on the value of L1CA π. Another way to define half-cycle ambiguity is to define it as the cited term itself, i.e., as equal to 0 or π. This definition of half-cycle ambiguity is equivalent to the definition used in this description and the appended claims.

[0051] L 2C Since is a pilot signal, its carrier-phase measurement is, by design, free of half-cycle ambiguities (but like all carrier-phase measurements, free of integer ambiguities N L2C (It is still affected by L 1CA and L 2C There is obviously no link between the carrier phases of L1 and L2. However, both the L1 and L2 carriers are 1P Signal and L 2P The fact that the signal is also modulated can be used to create a link.

[0052] In the prior art receiver channel of FIG. 2P The signal is L 2P The phase φ, which is the output of the PLL L2P Instead, in the present invention, L 2P The signal is measured by the phase of the measured L 2C Phase φ L2C The signal is converted to baseband by mixing it with a sine wave derived from the sine wave σ. The principle is shown in Figure 2.

[0053] L 2P The signal is measured L 2C Carrier phase φ L2C and known L 2P -L 2C Phase offset δφ L2P,C As in the prior art receiver, the L 1P The signal is measured L 1CA Carrier phase φ L1CA and known L 1P -L 1CA Phase offset δφ L1P,CA The baseband signal is then multiplied and integrated over a time T (preferably, as shown in FIG. 1, this means integrating between 0 and T and dividing the integral by T), where T can be, for example, 1 second.

[0054] From the signal definition in equation (1) and the carrier phase equation (3), the so-called punctual in-phase cross-correlation P (neglecting the twice the carrier frequency term) can be written as

[0055]

number

[0056] It can be calculated as follows:

[0057] Correlator circuit 14 may also output early and late cross-correlation values ​​to DLL 15 and quadrature cross-correlation values ​​to PLL 11. In the context of resolving half-cycle ambiguities according to any embodiment of the present invention, only the punctual in-phase cross-correlation P is needed. In the remainder of this description and in the claims, the term "cross-correlation P" should always be interpreted as "punctual in-phase cross-correlation."

[0058] P is A L1CA If =0, it is positive, and A L1CA If L = 1, it is negative. In other words, 2P The signal is ω L2 t + φ L2P Instead of phase ω L2 t + φ L2C + δφ L2P,C When the input signal is downconverted to baseband by a sine wave of 1CA Thus, according to the first embodiment of the present invention, L 1CA The half cycle ambiguities are resolved as follows. If P is positive, then A L1CA = 0, If P is negative, A L1CA =1.

[0059] Therefore, the method according to the first embodiment of the present invention calculates A directly based on the sign of P. L1CAThe method includes the steps summarized in the appended claim 2, which end with the determination of half cycle ambiguities. The method is preferably carried out repeatedly as the four signals of equation (1) are acquired and processed in the manner described above, so as to enable substantially continuous monitoring of the half cycle ambiguities. Preferably, this means that the half cycle ambiguities are determined substantially every T seconds. T may for example be 1 second or less, and in any case T may be significantly shorter than the 6 second interval between two successive preamble sequences used in prior art receivers to determine half cycle ambiguities. The invention thus allows a much more rapid determination of half cycle ambiguities and thus a more rapid recovery of integer ambiguity carrier phase after an interruption or signal slip.

[0060] In practice, the correlation P is affected by noise. To take noise into account, the above determination is preferably subjected to a threshold check on P. If the value of P is less than a given threshold P in absolute value, Th If it is below T, no decision is made and the test is performed again at the end of the next correlation interval T.

[0061] In parallel with the above procedure, a receiver channel equipped to carry out the method according to the first embodiment of the invention preferably comprises: 1CA The preamble code is still checked to resolve the half cycle ambiguities in a well-known manner. 1CA A half-cycle ambiguity is corrected as soon as at least one of the following conditions is met: The preamble was received and its code could be determined. L 2C The signal is tracked and 1P -L 2P The cross-correlation P is available with sufficient amplitude.

[0062] In most cases, L 2CHalf-cycle based ambiguity correction is faster, which is a major advantage of the present invention.

[0063] In a preferred embodiment, L 1CA The above process of determining half cycle ambiguities is performed repeatedly to detect half cycle slips, e.g., the preamble code is checked every 6 seconds and the code of the cross-correlation P is checked at intervals shorter than 6 seconds, e.g., every 1 second, or even shorter, as allowed by the fact that the integration time T can be much shorter than the 6 second interval between preambles. If the half cycle ambiguities appear to change, a cycle slip flag is erected and the new ambiguities are applied immediately.

[0064] L 1P and L 2P The process of cross-correlating signals is well known in the art, and several improvements to the P1-P2 cross-correlator 14 have been proposed to improve the SNR of the cross-correlation, as summarized, for example, in the conference paper "Woo, KT, "Optimum Semi-Codeless Carrier Phase Tracking of L2", Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, Tennessee, September 1999, pp. 289-306. Such improvements are directly applicable to the methods and receivers according to various embodiments of the present invention.

[0065] It is known in the art that the cross-correlation SNR can be significantly improved by performing a first pre-integration over a small portion of the L1P and L2P PRN codes, for example as disclosed in U.S. Patent No. 5,134,407. This improvement is compatible with the present invention and is advantageous as it reduces the noise in the cross-correlation P.

[0066] An embodiment of a receiver channel in a receiver according to the invention, arranged to apply the first embodiment of the method described above, is shown in FIG. 2, where (the term ω L2 In addition to t, the sine wave 4 is the measured carrier phase φ L2C and the known phase difference δφ L2P,C and the receiver channel is L, which receives the cross-correlation P as input. 1CA It can be seen that the system comprises an additional component called half-cycle ambiguity calculation unit 20, which is configured to verify the sign of P and deliver as output the value of the half-cycle ambiguity. As mentioned above, the calculation unit 20 is configured to check whether the absolute value of P is greater than a predefined threshold P th The sign of the half-cycle ambiguity can then be determined as |P|>P th It is generated only if

[0067] In the receiver channel of FIG. 2P PLL circuit 11 and L 2P The DLL circuit 15 is still included. The DLL 15 again introduces a delay Δ 2P -Δ 1P Determine, but L 2P The output of the PLL circuit 11 is not further used in the receiver channel.

[0068] This is a possible embodiment of the receiver channel according to the invention, but a more preferred embodiment is shown in figure 3. This embodiment corresponds to the following variant of the method according to the first embodiment.

[0069] In the context of the present invention, the delay Δ 2P -Δ 1P does not need to be known precisely, the only requirement is to obtain a reasonable level of correlation, L 2P Since the task is not to precisely track a signal, an accuracy of a few tens of nanoseconds is sufficient. At that level of accuracy, 1P Signal and L 2P The delay between the signal and1CA Signal and L 2C The delay between the receiver channel and the L 1P -L 2P Delay (Δ 2P -Δ 1P ), use L instead. 1CA and L 2C The delay between 2C -Δ 1CA ) is sufficient. A receiver channel implementing the present invention has at least L 1CA Signal and L 2C L 1CA and L 2C The delay between is simply L 2C The pseudorange and L 1CA The difference between the pseudoranges of the first embodiment and the pseudoranges of the second embodiment is divided by the speed of light c. 2P -Δ 1P Instead of delay Δ 2C -Δ 1CA may be used.

[0070] The receiver channel shown in FIG. 3 is according to the variant described above, i.e. with delay Δ 2P -Δ 1P Instead of delay Δ 2C -Δ 1CA This receiver channel is suitable for carrying out the method of the first embodiment using L 2P DLL circuit and L 2P It does not have a PLL circuit. 1P The delay applied to the signal is given by the formula: Δ 2C -Δ 1CA =(PR L2C -PR L1CA ) / c according to the pseudorange PR L1CA and PR L2C is obtained from

[0071] A flow chart of the method steps applied by the half-cycle ambiguity calculation unit 20, including checking the absolute value of P, is shown in FIG.

[0072] Compared to the prior art of FIG. 1, an important aspect of this embodiment of the present invention is that 1P Signal and L 2P The signal does not need to be tracked by the receiver channel. 1P and L 2P All the phase and delay information needed to compute the cross-correlation between 1CA and L 2C The phase and pseudorange measurements of L are available. The receiver according to the invention does not have to perform the steps necessary to track the P code (PLL and DLL) and only has to perform the cross-correlation. 1P and L 2P There is no need to calculate the code and carrier measurements. However, 2P In receivers where measurements are available, different options are possible as explained below.

[0073] Second embodiment Following the latest technology 1CA Signal and L 2P and a receiver that tracks the L signal and generates corresponding phase measurements. 1CA There is another way to quickly resolve half-cycle ambiguities. According to the state of the art (as shown in Figure 1), 1CA Signal and L 2P In the receiver channel that tracks the signal, L 2P The phase of the carrier replica (ω L2 t + φ L2P ) is L 2P Based on the output of the phase-locked loop (PLL) 11. φ L2P , L 2P This is the carrier phase measurement. The PLL 11 operates with φ L2P But half cycle ambiguity A L2P (0 or 1) and an integer ambiguity N L2P and s in formula (1) L2Pφ L1CA and φ L2P is given by the following equation, again assuming perfect lock of the tracking loop to the signal:

[0074]

number

[0075] The downconverted signal s taking into account the delay between the two signals L1P and s L2P The cross-correlation P in this case (neglecting the twice the carrier frequency term), obtained by integration over time T (defined above) of the product of

[0076]

number

[0077] As shown by equation (5), L 1CA Carrier Phase and L 2P Both carrier phases are half ambiguous A L1CA and A L2P However, A L1CA and A L2P are linked by the sign of the cross-correlation P. From equation (6), if P is positive, then A L2P =A L1CA while P is negative, A L2P =1-A L1CA This means that L 2P This allows the carrier phase to be rewritten as:

[0078]

number

[0079] The principle behind equation (7) has long been known, and L 1CA Once the half cycle ambiguity is resolved, L2P It is routinely applied to resolve half-cycle ambiguities (see, for example, U.S. Pat. No. 6,061,390). However, when the receiver 1CA and L 2P In parallel with L 2C If we also want to track signals, we can change equation (7) to A L1CA It has not heretofore been recognised that a cellular signal can be used for the rapid determination of . This is the basis of a second embodiment of the present invention.

[0080] The receiver is L 2C When tracking L, the carrier phase is given by (3) 2P Carrier phase measurement and L 2C The difference between the carrier phase measurements is

[0081]

number

[0082] is given by:

[0083] From equation (8), L 1CA The carrier phase ambiguity is calculated using the following quantities: 2C and L 2P It can be easily derived from the carrier phase measurements.

[0084]

number

[0085] N L2P , N L2C Considering that is an integer value, equation (10) is, when P is positive, A L1CA If is 0, D is expected to be even, and A L1CA indicates that D is expected to be odd when P is 1, and vice versa when P is negative. In practice, due to noise in the carrier-phase measurements, D will not be exactly an integer, but should be close to an integer under good signal-to-noise conditions.

[0086] Therefore, A according to this second embodiment of the invention L1CA The decision includes: Following the latest technology 2P Signal and L 2C and the corresponding carrier phase measurement φ L2P and φ L2C Generate. L 2C and L 2P The known phase difference δφ between L2P,C to calculate D according to equation (9). The following verification steps, i.e. If the integer closest to D is even, then P>0, then A L1CA =0, or if P<0, then A L1CA = 1; and If the integer closest to D is odd, then P>0, then A L1CA =1, or P<0, then A L1CA = 0 and a verification step From D to A by L1CA A receiver channel configured to carry out the method according to this second embodiment is shown in Fig. 5. This receiver channel is similar to the prior art shown in Fig. 1 in that it determines φ L2P 1, this embodiment requires a PLL circuit 10 to determine L 2C and L 2P L, which takes the carrier phase and the punctual cross-correlation value P as input. 1CA 6. Add a half-cycle ambiguity calculation unit 20. According to an alternative embodiment (not shown), the delay applied in circuit 16 of FIG. 6 is Δ 2C -Δ 1CA =(PR L2C -PR L1CA ) / c. In this case, the DLL circuit 15 in FIG.

[0087] According to a preferred embodiment, the verification step comprises: L1CAThis step is preceded by the determination of the threshold D Th , for example, by checking whether they differ by more than 0.1. L1CA The determination of D is made by dividing D by the nearest integer. Th If D is less than the nearest integer, the method described above is used. th If the differences are greater than , the noise is too large to make a decision and A L1CA is not determined until the next integration time T.

[0088] As in the first embodiment, to avoid decisions based on noise, a test on the P amplitude (|P|>P Th ) is added. The flow chart shown in FIG. 6 illustrates the preferred operation of the half-cycle ambiguity computation unit 20 when tests for both D and P are included. A L1CA If a change in is observed, a "cycle slip flag" can be raised.

[0089] As with the first embodiment, the method according to the second embodiment is preferably carried out repeatedly as the signal of equation (1) is tracked, so as to allow a substantially continuous monitoring of half-cycle ambiguities. The determination of the cross-correlation P is preferably carried out during successive integration times T that are shorter than the time between successive preambles, i.e. less than 6 seconds, for example 1 second or less. The method may be carried out each time P is so determined. Every determination of P is then preferably carried out over a respective threshold P th and D. th After evaluation of both P and D for the half-cycle ambiguity A L1CA φ is used together with the determination of the parameter D as input for the above-described method for determining φ. However, other practical ways of realizing the iterative implementation of the method according to the second embodiment are also possible. For example, the parameter D may be determined by the carrier phase measurement φ L2P and φ L2Ccan be determined at the rate at which P is updated, which may be, for example, 1 Hz or 10 Hz, or other suitable rate. The method can be performed at this same rate by using each carrier phase update to determine D in combination with the most recent determination of P.

[0090] In the method according to the second embodiment, a test of the carrier-to-noise density ratio (C / N0) can be added, L 2C Signal and / or L 2P If the C / N0 of the signal is below a given threshold, it may be decided not to make a decision.

[0091] In addition, it can be decided to make a decision only after a certain number of iterations have not provided consistent results. This can also be done when applying the method according to the first embodiment (FIGS. 3-5). The number of iterations required can be continuous, e.g., A L1CA A L1CA The number of iterations required may be non-sequential, e.g., A L1CA A L1CA for N out of M successive decisions.

[0092] Similar to what has been described above with respect to the method according to the first embodiment, the above method according to the second embodiment is preferably used in parallel with a conventional method of determining half-cycle ambiguities from the code of the preamble.

[0093] L 2P It is known in the art that the PLL (11) can be designed to ensure that the sign of P is always positive. This option is compatible with the second embodiment of the present invention. In that case, the test of the sign of P in Figure 6 can be bypassed since P is known to be positive by design.

[0094] While the present invention has been illustrated and described in detail in the drawings and the above description, such illustration and description should be considered as exemplary or explanatory and not restrictive. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure, and the appended claims, in carrying out the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the definite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be interpreted as limiting the scope. [Explanation of symbols]

[0095] 1 sine wave 2 sine wave 3 sine wave 4 sine wave 5 Multiplication Circuit 6 Multiplier Circuit 7 Multiplication Circuit 8 Multiplier Circuit 9 Phase Locked Loop (PLL), PLL 10 Phase Locked Loop (PLL), PLL, PLL circuit 11 Phase-locked loop (PLL), L 2P PLL, PLL, L2 P PLL circuit, L 2P Phase Locked Loop (PLL) 11 12 Correlator 13 Correlator 14 Cross-correlator circuit, correlator circuit, P1-P2 cross-correlator 15 Delay Lock Loop (DLL), DLL, L2 P DLL circuit 16 Delay circuit, circuit 17 DLL circuit 18 DLL circuit 20 L 1CA Half-cycle ambiguity calculation unit, calculation unit

Claims

1. 1. A method for resolving half-cycle ambiguities in a Global Positioning System (GPS) carrier phase, comprising: The following signals are received from GPS satellites: First carrier frequency f L1 The first signal L transmitted at 1CA and, The first carrier frequency f L1 A second signal L transmitted at 1P wherein the second signal has a known phase offset δφ with respect to the first signal. L1P,CA A second signal L 1P and, Second carrier frequency f L2 A third signal L transmitted at 2C and, The second carrier frequency f L2 A fourth signal L transmitted at 2P wherein the fourth signal has a known phase offset δφ with respect to the third signal. L2P,C A fourth signal L 2P and receiving the Tracking the first signal and the third signal, thereby determining a carrier phase φ of the first and third signals. L1CA and φ L2C determining an estimate of each of the carrier phases φ of the first signal; L1CA The estimate of the term A L1CA .π, A L1CA is equal to 0 or 1, and A L1CA However, from now on, 1CA step, called the half-cycle ambiguity of the carrier phase, The second and fourth signals L 1P and L 2P to baseband by multiplication with carrier replicas of the second and fourth signals, determining a delay between the downconverted second and fourth signals, and determining a cross-correlation P between the downconverted second and fourth signals by integrating a product of the downconverted second and fourth signals over an integration time T, taking into account the delay; Including, The sign of the cross-correlation P, as well as the known phase offset δφ L2P,C and δφ L1P,CA However, the half cycle ambiguity A L1CA The method of claim 1, wherein the method is used to determine

2. The first signal L 1CA The half cycle ambiguity A L1CA but, ω L2 =2πf L2 Phase ω L2 t +φ L2C + δφ L2P,C and the fourth signal L 2P The fourth signal L is obtained by multiplying 2P performing the step of down-converting The following verification steps: If P is positive, then A L1CA = 0; and If P is negative, A L1CA = 1 and a verification step According to the above, the L 1CA The half cycle ambiguity of the signal A L1CA determining The method of claim 1, wherein the determined

3. The delay applied between the downconverted second and fourth signals is L 1P and L 2P 3. The method of claim 2, wherein the delay between the first and second inputs is determined by a delay-locked loop (DLL) circuit (15) that tracks the delay between the first and second inputs.

4. The step of tracking the first signal and the third signal includes obtaining respective estimates R of satellite ranges. L1CA and R L2C determining a delay between the downconverted second and fourth signals, the delay being the sum of the delays of the third and first signals L divided by the speed of light; 2C and L 1CA The difference between the respective estimates of the satellite distances obtained from (PR L2C -PR L1CA 3. The method of claim 2, wherein

5. The first signal L 1CA The half cycle ambiguity A L1CA but, The carrier phase φ of the fourth signal that maximizes the amplitude of the cross-correlation P L2P is tracked by a tracking loop, and the fourth signal and phase ω L2 t + φ L2P performing the step of down-converting the fourth signal by multiplication with a carrier replica having From the sign of the cross-correlation P, and φ L2P and φ L2C From the difference between the following steps, The parameter D is [0010] and determining If the integer closest to D is even, then P>0, then A L1CA If P<0, then A L1CA = 1, and If the integer closest to D is odd, then P>0, then A L1CA If P<0, then A L1CA = 0 and According to the first signal L 1CA The half cycle ambiguity A L1CA The method of claim 1 , wherein the step of determining

6. The absolute value of the difference between D and the nearest integer is a predefined threshold D th and determining whether the L 1CA The half cycle ambiguity of the signal A L1CA wherein the step of determining is performed such that the absolute value of the difference between D and a nearest integer is less than the predefined threshold D th The method of claim 5 , wherein the method is executed only if

7. The absolute value of P is a predefined threshold P th and determining whether the L 1CA The half cycle ambiguity of the signal A L1CA The step of determining |P|>P th The method of claim 1, wherein the method is performed only if

8. The method further comprises: L1CA 13. The method of claim 1 , wherein the method is performed repeatedly to enable substantially continuous monitoring of the

9. The half cycle ambiguity A L1CA 9. The method of claim 8, wherein is determined only if the same result is obtained a given number of times.

10. The method is performed repeatedly, and the method includes: determining the half cycle ambiguity A based on a preamble transmitted in the first signal; L1CA and wherein a time between two successive determinations of the half cycle ambiguity is shorter than a time between two successive preambles.

11. 2. The method of claim 1, wherein a time between two successive determinations of the half-cycle ambiguity is less than 6 seconds.

12. Use of the method according to any one of claims 1 to 11 for detecting half cycle slips during acquisition and processing of GPS signals.

13. A GPS receiver with one or more receiver channels, each channel receiving the following four signals from GPS satellites: First carrier frequency f L1 The first signal L transmitted at 1CA and, The first carrier frequency f L1 A second signal L transmitted at 1P wherein the second signal has a known phase offset δφ with respect to the first signal. L1P,CA A second signal L 1P and, Second carrier frequency f L2 A third signal L transmitted at 2C and, The second carrier frequency f L2 A fourth signal L transmitted at 2P wherein the fourth signal has a known phase offset δφ with respect to the third signal. L2P,C A fourth signal L 2P and configured to track At least one of the channels is subjected to the L according to the method of any one of claims 1 to 11. 1CA Signal half-cycle ambiguity A L1CA a half-cycle ambiguity calculation unit (20) configured to calculate GPS receiver.

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