Abstract:
The invention relates to a method for tracking the carrier phase of a signal received from a satellite by a carrier using a carrier loop of the carrier phase, said signal being acquired by a navigation system of the carrier, said navigation system including a receiver for location by radio navigation, and a self-contained unit, wherein the receiver is suitable for acquiring and tracking the phase of the carrier of the signal from the satellite.

Description:
GENERAL TECHNICAL FIELD 
       [0001]    The invention relates to radio navigation with satellites, notably to satellite radio navigation of the GNSS (&lt;&lt;Global Navigation Satellite Systems&gt;&gt;) type, and more particularly of the GPS (&lt;&lt;Global Positioning System&gt;&gt;), Galileo, GLONASS (&lt;&lt;Global Navigation Satellite System&gt;&gt;). 
       STATE OF THE ART 
       [0002]    With satellite radio navigation, it is possible to obtain the position of a receiver by a method close to triangulation. The distances are measured from signals sent by satellites. 
         [0003]    More specifically, a receiver for positioning via satellites notably allows delivery of a set of integrated pseudo-velocities or dopplers, each representing the projection on the axis connecting the receiver to each satellite with view to obtaining the relative velocity vector between the receiver and said satellite. 
         [0004]    The signals transmitted by the satellites are formed by modulating the carrier of the signal with a spreading code. Thus, the satellite signals allow two types of measurements for localizing the receiver. Further, the modulation of the carrier with a spreading code extends the spectrum in the spectral band, which increases the resistance of the system to interference. And, additionally, this gives the possibility of disassociating the satellites (by using a different code for each satellite in the GPS case). 
         [0005]    The first type of measurement uses the code of the received signal. The measurements based on the code, unlike the ones based on the carrier (see below) are not ambiguous, since the receiver is capable of evaluating the code period integer between the satellite and the receiver. But the measurements based on the code are much less accurate than those based on the carrier. 
         [0006]    The second type of measurement is a measurement based on the carrier of the receiver. The measurements based on the carrier are accurate but ambiguous. Indeed, the receiver is only capable of evaluating the phase of the carrier, therefore the number of wavelengths between the satellite and the receiver remains unknown: there is therefore an ambiguity which has to be removed. 
         [0007]    In order to carry out both of these types of measurements, the receiver acquires and tracks the received signal. For this, it generates replicas of the code and of the carrier, so called local replicas, which it correlates with the received signal. As the code and the carrier are inconsistent pieces of information, the generations of the code and carrier replicas are subordinated to two distinct loops. 
         [0008]    The carrier loop is generally a phase-locked loop (PLL). The code loop as for it generally includes a double correlation allowing evaluation of the shift between the local code and the received code which corresponds to a difference of measurable energies. 
         [0009]    The receiver uses both of these loops in order to obtain unambiguous accurate measurements. 
         [0010]    In a first phase, a so-called acquisition phase, the receiver operates in an open loop for seeking the received signal by testing several position and velocity assumptions on the local code and on the local carrier. Each control loop is closed when the uncertainty on its input (position for the code and frequency for the carrier) becomes less than the field of application of the discriminant of the loop. 
         [0011]    Thus, both loops are complementary during the phase for tracking the received signal: the carrier loop provides accuracy while the code loop provides robustness. 
         [0012]    However, a source of a feedback error of the carrier loop is due to the crossing of the ionosphere by the waves from the satellites. 
         [0013]    Such inaccuracies cause cycle jumps on the carrier loop, and therefore measurement errors without necessarily causing unlocking of the carrier loop so that there is no re-locking step of the carrier loop: the navigation solution is therefore erroneous. 
       PRESENTATION OF THE INVENTION 
       [0014]    The invention allows the aforementioned drawbacks to be overcome. 
         [0015]    For this purpose, according to a first aspect, the invention relates to a method for tracking the carrier phase of a signal received from a satellite with a carrier by means of a carrier phase-locked loop, said signal being acquired by a navigation system of the carrier which comprises a localization receiver by radio navigation and a self-contained unit, the receiver being adapted so as to acquire and track the phase of the carrier of the signal stemming from the satellite. 
         [0016]    The method comprises the following steps:
       determining a closed-loop control error of the carrier phase loop, said closed-loop control error being determined between two sampling instants and corresponding to a first phase deviation;   determining a variation of acceleration of the carrier between both sampling instants by means of a self-contained unit;   projecting the variation of acceleration on a satellite-receiver view axis in order to obtain a second phase deviation;   comparing the first and second deviations in order to detect an error on the measurement of the carrier phase tracked by said carrier phase loop.       
 
         [0021]    The method according to the invention may further include either one of the following aspects:
       the variation of acceleration of the carrier is obtained by means of a numerical model implemented in a module of the navigation system;   the variation of acceleration of the carrier is determined by means of an inertial unit of the navigation system;   the comparison consists of determining inconsistency of the first deviation with the second deviation;   the inconsistency is integrated over a sliding period with a duration of the order of one to five times a time constant of a filter of the carrier phase loop and then compared with a threshold, typically λ/4, the method comprises a step for generating a replica signal of the received signal from the phase deviation {δγ n ·δt 2 } projected  stemming from the self-contained unit;   it comprises a step for determining a navigation solution from integrated dopplers stemming from the generated replica signal,   if the inconsistency is greater than the threshold, typically of the order of λ/4, the method comprise a step for correcting the integrated dopplers required for the navigation solution, the correction consists of adding a term k·λ/2 to said integrated dopplers wherein k is a relative integer such that the absolute value of the integrated inconsistency is less than a threshold, the threshold being typically of the order of λ/4;   it comprises a step for determining a navigation solution from the corrected integrated dopplers;   the comparison consists of calculating an inconsistency term defined in the following way:       
 
         [0000]    
       
         
           
             
               
                 λ 
                 
                   2 
                    
                   π 
                 
               
               · 
               
                 
                   { 
                   
                     δϕ 
                     n 
                   
                   } 
                 
                 rectified 
               
             
             - 
             
               
                 { 
                 
                   
                     
                       δγ 
                       n 
                     
                     · 
                     δ 
                   
                    
                   
                       
                   
                    
                   
                     t 
                     2 
                   
                 
                 } 
               
               projected 
             
           
         
       
     
         [0000]    wherein {δφ n } rectified  is the first phase deviation and {δγ n ·δt 2 } projected  is homogeneous to a distance corresponding to the second phase deviation and λ is the wavelength associated with the carrier frequency of the received signal. 
         [0030]    According to a second aspect, the invention relates to a navigation system comprising means for applying a method according to the first aspect of the invention. 
     
    
     
       PRESENTATION OF THE FIGURES 
         [0031]    Other features and advantages of the invention will become further apparent from the description which follows, which is purely illustrative and non-limiting and should be read with reference to the appended drawings wherein: 
           [0032]      FIG. 1  schematically illustrates a navigation system according to the invention; 
           [0033]      FIG. 2  schematically illustrates steps of the method according to the invention; 
           [0034]      FIG. 3  illustrates in a detailed way a navigation system according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    In  FIG. 1 , a navigation system on board a carrier, typically an aircraft, to be localized, is illustrated. 
         [0036]    Such a navigation system includes a receiver  10  for localization by radio navigation, preferably a GPS or GNSS receiver. The receiver may be a multi-channel receiver and in this case each channel corresponds to one satellite which transmits a signal received by the receiver  10 . 
         [0037]    The receiver  10  includes a receiving antenna  100  capable of receiving a signal stemming from one or several satellites (not shown). 
         [0038]    The case is considered when the signal received by the navigation system is a GPS signal. 
         [0039]    In a known way, the radio navigation signals transmitted by satellites appear as a carrier modulated by a spread waveform containing a pseudo-random binary code. The modulation of the carrier causes spreading of the spectrum around the frequency of the carrier, the radio navigation signals have a spread spectrum. 
         [0040]    The pseudo-random codes represent an identifier of the signal and therefore of the transmitter satellite. 
         [0041]    Further, certain signals for positioning by satellite may also convey useful data (for example the navigation message) as a binary sequence (at a significantly lower rate than the pseudo-random code) modulating the signal from the carrier modulated by the code. 
         [0042]    In the case of the GPS, the radio navigation signals are transmitted in the frequency bands L 1 , centred on 1575.42 MHz and L 2 , centred on 1227.6 MHz. 
         [0043]    Further, the navigation system of  FIG. 1  includes a self-contained unit  20  and a unit  30  for detecting and correcting possible loop control errors. It is specified here that the self-contained unit  20  does not receive any signal from one or more satellites and is consequently autonomous. The measurements which it provides are subsequently distinguished from the other ones by the descriptive term of autonomous. 
         [0044]    The receiver  10  operates in a known way in an acquisition or tracking mode. 
         [0045]    It is considered here that one is in a tracking mode, i.e. that the receiver gives the possibility of providing a navigation solution, from a set of pseudo-distances and of pseudo-velocities or integrated dopplers which allow localization of the carrier. 
         [0046]    It is from these measurements that the navigation solution of the carrier is determined. 
         [0047]    In particular, this is the resolution of a set of equations obtained from the pseudo-measurements. These processing operations will not be detailed subsequently since they are well known to one skilled in the art. 
         [0048]    More specifically, the reception of a radio navigation signal comprises a first demodulation by means of an internal replica of the carrier phase generated in the receiver by an oscillator driven by a carrier phase tracking loop and a second demodulation by means of an internal replica of the form of the spreading code produced by a code tracking loop. 
         [0049]    The control signals of the carrier phase and code tracking loops are used by the receiver for determining the pseudo-measurements, with which the navigation solution may be obtained. 
         [0050]    As this was already mentioned above, the navigation system includes a unit  30  for detecting and correcting closed-loop control errors. 
         [0051]    This unit  30  gives the possibility of applying a method for monitoring the loop for tracking the carrier phase of the signal received from a satellite. Such a method is of course implemented for each channel in the case when the receiver is a multi-channel receiver. 
         [0052]    The object of this method is to detect one or several cycle jumps in the phase of the carrier and to correct them. 
         [0053]    Steps of such a method are schematically illustrated in  FIG. 2 . 
         [0054]    In a first step E 1 , a closed-loop control error of the carrier phase loop is determined between two instants. Thus this closed-loop control error is a first phase deviation. This phase deviation allows the carrier phase tracking loop to get back in step. 
         [0055]    This first phase deviation is taken between consecutive samples measured at the output of the carrier phase loop. Such a phase deviation is expressed by δφ n =φ n −φ n-1  wherein n is an index corresponding to the calculation instant. 
         [0056]    In a second step E 2  (which may practically be applied before or in parallel with the first step E 1  described above) a variation of acceleration of the carrier is determined between both instants. 
         [0057]    This variation of acceleration is determined by the self-contained unit  20  of the navigation system. 
         [0058]    According to an embodiment, the self-contained unit  20  may include an inertial numerical model which allows determination of the acceleration variation of the carrier relatively to the satellite. 
         [0059]    Taking into account that the movement of the satellite (the carrier in this case) is mainly governed by Kepler&#39;s laws; accordingly, the position of the satellite is determined from the orbital Kepler parameters. The velocity of a satellite is determined by differentiation (preferentially an exact formula) or by differentiation of the position. The acceleration is obtained by differentiation of the velocity. 
         [0060]    According to another embodiment, the self-contained unit  20  is an inertial unit. 
         [0061]    As this is known, an inertial unit mainly consists of two groups of three sensors. The groups of sensors are gyrometers (rotation measurements) and accelerometers (acceleration measurements). The three sensors of each group are oriented in order to capture the movements in space (in three dimensions). Integration of the accelerometric measurements provides the velocity along the axis of each accelerometer, and integration of the velocities provides the position of each accelerometer along its axis. The integrations call for determination of the initialization constants: this is the subject of inertial alignment. 
         [0062]    As the movement of a mobile is arbitrary, the orientation of the axes of the accelerometers varies, therefore it is necessary to project the acceleration measurements in a reference coordinate system: this is the purpose of the gyrometers to determine the rotation of the measurement axes of the accelerometers. 
         [0063]    Thus, in a third step E 3 , the acceleration variation is projected on a satellite-receiver view axis so as to obtain a relevant measurement, this projected acceleration variation representing a second phase deviation. 
         [0064]    The projection of the acceleration variation is homogeneous to meters (m) and as a wavelength is equivalent to 2π radians, it is possible to simply switch from the projected acceleration variation to a carrier phase deviation. 
         [0065]    Next, in a fourth step E 4 , the first and second deviations are compared in order to detect a possible error on the carrier phase tracked by the carrier phase loop. 
         [0066]    The question here is in particular to identify whether the phase deviation allowing the carrier phase loop to be driven is erroneous or inconsistent or else if it is subject to cycle jumps. 
         [0067]    Finally, in a fifth step E 5 , the navigation solution is determined from the phase deviation measured by the carrier loop and optionally corrected from the projected acceleration variation stemming from the measurements carried out by the self-contained unit  20 . 
         [0068]    The radio navigation solution is determined from pseudo-measurements, for example by a least squares algorithm, and applies a phase deviation. 
         [0069]    The comparison E 4  notably consists of determining inconsistency of the first deviation with the second deviation. 
         [0070]    If the determined inconsistency is greater than the threshold, an inconsistency alert may be suppressed, the second phase deviation replaces the first and the determination E 6  of the navigation solution stems from the corrected closed-loop control. 
         [0071]    Alternatively, the determination E 6  of the navigation solution is carried out from corrected pseudo-measurements. 
         [0072]    A detailed scheme of a navigation system is also illustrated in  FIG. 3 . 
         [0073]    Like in the diagram of  FIG. 1 , a signal of the GPS type is received by a radio navigation antenna  100 . 
         [0074]    At the antenna  100 , the signal is pre-amplified and then filtered, undergoes lowering in frequency and finally undergoes analogue/digital conversion before being processed digitally. 
         [0075]    In a known way, in a tracking mode, a carrier phase loop tracks the carrier phase of the received signal. The carrier phase loop is driven by a local oscillator  18  with which during the tracking of the carrier phase a phase deviation between the local replica and the received signal may be corrected. Indeed, the navigation solution is calculated from the local signal which has to be a quasi-perfect replica of the received signal. 
         [0076]    At the local oscillator  18 , a local replica signal is generated. Such a replica signal is formed in a known way by a carrier (sine wave) modulated with a pseudo-random binary code; it is sampled at a frequency of the order of a few MHz to a few tens of MHz. The frequency of the replica of the carrier is equal to the transmission frequency of the carrier by the satellite (1.57542 GHz in the case of GPS L 1 ), reduced with the frequency lowering by the receiver, or increased by the relative satellite/carrier Doppler component. The purpose of the closed-loop control on the carrier phase is specifically to determine this relative Doppler component. The frequency of the replica of the code is equal to the transmission frequency of the code by the satellite (1.023 MHz in the case of the GPS C/A code), increased by the driving of the code by the carrier (i.e. by the estimated Doppler component by the carrier loop), and increased by the code-carrier inconsistency, object of the code control loop. 
         [0077]    From the received signal and the local signal generated by the local oscillator  18 , a phase channel I and a quadrature channel Q of the correlation product of both signals are determined, channels which will be used subsequently. 
         [0078]    Channel I is in particular the integral of the product of the received signal by the local signal, this integration being carried out at a frequency of the order of a few to a few tens of MHz (see the sampling frequency of the local oscillator  18 ) and on a horizon of 1 ms or an epoch of the C/A code, the channel I then being provided at the output and then reset to zero. 
         [0079]    Channel Q is determined in a similar way to channel I but the local signal has a carrier phase advance of π/2 relatively to the one used for calculating channel I. 
         [0080]    From these two channels I, Q, a carrier loop discriminant is determined  12 . Such a discriminant is obtained by calculating 
         [0000]    
       
         
           
             
               Arctan 
                
               
                 ( 
                 
                   Q 
                   I 
                 
                 ) 
               
             
             . 
           
         
       
     
         [0000]    From this discriminant, a value of the phase of the carrier φ n  is inferred. 
         [0081]    As the received signal includes navigation data coding a piece of information, the carrier phase is rectified  13 ,  14 ,  15  for removing these data. 
         [0082]    The question is to notably determine  13 ,  15  a phase deviation taken between two calculation instants (or else two samples): δφ n =φ n −φ n-1 . 
         [0083]    This phase deviation δφ n  will give the possibility of obtaining  14  a rectified phase deviation {δφ n } rectified . 
         [0084]    In particular, if δφ n &gt;π/2 then δφ n  is reduced by π while if δφ n &lt;−π/2 then δφ n  is increased by π. 
         [0085]    This is therefore a rectified phase deviation δφ n  which is used for determining whether there is an ambiguity on the carrier phase. 
         [0086]    It is this phase deviation {δφ n } rectified  which is integrated  16  during the whole duration of the servo-control and then filtered by a loop of the third order  17  and then sent to the input of the local oscillator  18  for generating a replica signal of the received signal. 
         [0087]    The error detection and correction module  30  may have to correct an error beforehand. 
         [0088]    To do this, the module  30  receives the phase deviation {δφ n } rectified  stemming from the carrier phase loop (dynamics measured by the carrier phase loop) on the one hand and the autonomous dynamics which in fact is an acceleration variation projected on a satellite-carrier view axis. 
         [0089]    The acceleration variation determined by the self-contained unit  20  is obtained either from an inertial model or from an inertial unit  23 . 
         [0090]    Such an acceleration variation is obtained by determining autonomous dynamics processed by a module for integrating the navigation  22  in order to obtain an autonomous measurement and therefore an absolute measurement of the acceleration γ n  of the position P n  and of the velocity V n . 
         [0091]    The acceleration variation is projected on a satellite-receiver view axis by a projection module  23 . 
         [0092]    From this projection, a therefore autonomous variation of the acceleration projected on a {δγ n ·δt 2 } projected  view axis is obtained. 
         [0093]    The acceleration variation is determined over the same calculation period as the one for determining the phase variation at the carrier phase loop. 
         [0094]    In order to detect a possible error, the correction module  30  determines an inconsistency term defined in the following way: 
         [0000]    
       
         
           
             
               
                 λ 
                 
                   2 
                    
                   
                       
                   
                    
                   π 
                 
               
               · 
               
                 
                   { 
                   
                     δϕ 
                     n 
                   
                   } 
                 
                 rectified 
               
             
             - 
             
               
                 { 
                 
                   
                     
                       δγ 
                       n 
                     
                     · 
                     δ 
                   
                    
                   
                       
                   
                    
                   
                     t 
                     2 
                   
                 
                 } 
               
               projected 
             
           
         
       
     
         [0000]    wherein {δφ n } rectified  is the first phase deviation, {δγ n ·δt 2 } projected  is homogeneous to a distance corresponding to the second projected phase deviation and λ is the wavelength associated with the carrier frequency of the received signal. 
         [0095]    If the term above is greater than the threshold, the measurement of the carrier phase {δφ n } rectified  is marred with an error. 
         [0096]    In this case, there are two solutions for suppressing the error. 
         [0097]    The first solution consists of using E 5  the phase deviation obtained from the self-contained unit  20 , {δγ n ·δt 2 } projected  for generating the replica signal. 
         [0098]    In that case, the threshold is of the order of 3σ φ     n    at λ/4 wherein σ φ     n    is a function of the signal-to-noise ratio. 
         [0099]    It is this replica signal which will be used for measuring the pseudo-velocities or integrated dopplers required for calculating the navigation solution. 
         [0100]    The second consists of correcting E 5 ′ the integrated dopplers by adding to them a term k·λ/2 wherein k is a relative integer such that |inconsistency| integrated &lt;threshold. In particular the inconsistency is integrated over a sliding period with a duration of the order of 1 to 5 times the constant of the filter of the control loop on the carrier. The threshold is of the order of λ/4. 
         [0101]    The first solution is only applicable when the estimation  25  of {δγ n ·δt 2 } projected  is available at the calculation instant of the filtering  17 . 
         [0102]    The second solution is applicable in all cases, but becomes mandatory when the first solution is not applicable. In this case, the carrier phase deviation {δφ n } rectified  has to be stored in memory so that the calculation of the inconsistency term deals with synchronized (i.e. stemming from the same time period) autonomous and radio navigation measurements.