Abstract:
Methods and apparatus for implementing a receiver autonomous integrity monitoring (RAIM) algorithm are provided. The RAIM algorithm is for determining an integrity risk in a global navigation satellite system (GNSS) by processing several ranging signals received from satellites of the GNSS. The algorithm involves determining several integrity risks at an alert limit for different fault conditions of the ranging signals, and determining an overall integrity risk at the alert limit from the determined several integrity risks.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority under 35 U.S.C. §119 to European Patent Application No. 10 006 518.4, filed Jun. 23, 2010, the entire disclosure of which afore-mentioned document is herein expressly incorporated by reference 
       TECHNICAL FIELD 
       [0002]    The invention relates to a RAIM (Receiver Autonomous Integrity Monitoring) algorithm. 
       BACKGROUND 
       [0003]    RAIM (Receiver Autonomous Integrity Monitoring) provides integrity monitoring of a GNSS (Global Navigation Satellite System) to GNSS receivers. RAIM algorithms process GNSS signals and are based on statistical methods, with which faulty GNSS signals can be detected. Enhanced RAIM algorithms allow not only fault detection (FD) of any received GNSS signal, but also exclusion of a GNSS signal being detected as faulty from positioning, thus allowing a continuous operation of a GNSS receiver. This is known as fault detection and exclusion (FDE). 
         [0004]    RAIM is important for safety-critical applications such as in aviation navigation. GNSS based navigation in aviation is until now not standardized by the ICAO (International Civil Aviation Organization). However, the maximum allowable offsets, bends, and errors during precision landing of an airplane are categorized in three different categories CAT-I, CAT-II, and CAT-III, wherein CAT-III contains the smallest allowable offsets, bends, and errors. 
       SUMMARY OF INVENTION 
       [0005]    Exemplary embodiments of the invention provide an improved RAIM algorithm employable in a GNSS receiver, wherein the algorithm is particularly suitable to fulfill the requirements as outlined by the by ICAO for CAT-I. 
         [0006]    A basic idea underlying the present invention is to determine integrity risks at an alert limit. The determined integrity risks can then be transferred to a protection level concept, as employed by many RAIM algorithms. The transfer from Integrity at the alert limit to protection levels, which are commonly used in aviation, is for example described in “Combined Integrity of GPS and GALILEO”, F. Kneissl, C. Stöber, University of FAF Munich, January/February 2010, http://www.insidegnss.com. With the inventive RAIM algorithm it is possible to fulfill CAT-I requirements for precision landing. 
         [0007]    An embodiment of the invention relates to a RAIM algorithm for determining an integrity risk in a GNSS by processing several ranging signals received from satellites of the GNSS, wherein the algorithm comprises the following acts:
       determining several integrity risks at an alert limit for different fault conditions of the ranging signals, and   determining an overall integrity risk at the alert limit from the determined several integrity risks.       
 
         [0010]    The act of determining several integrity risks may comprise one or more of the following acts:
       determining the integrity risk at an alert limit for the fault condition that all ranging signals are fault free;   determining the integrity risk at an alert limit for the fault condition that only one ranging signal is faulty.       
 
         [0013]    The act of determining several integrity risks may further comprise the act of
       determining the integrity risk at an alert limit for the fault condition that more than one ranging signal is faulty.       
 
         [0015]    The act of determining an overall integrity risk may comprise the act of
       determining the overall integrity risk by bounding it by a maximum of all determined integrity risks.       
 
         [0017]    The act of determining an overall integrity risk may alternatively or additionally comprise the act of
       determining the overall integrity risk by it by on average over all intervals.       
 
         [0019]    The algorithm may further comprise the acts of
       defining a detection threshold for raising an alert and   determining a bounding of the probability to raise an alert under fault free conditions using the defined detection threshold,   defining an availability as the sum of the bounding of the probability to raise an alert and a fraction of instances where the overall integrity risk is above or equal to a tolerable integrity risk, and   tuning the detection threshold for raising an alert such that the availability is minimized.       
 
         [0024]    The act of defining an availability may comprise
       selecting for a fraction of instance a set of times at a location, where the sum is maximal, or a set of times at representative locations.       
 
         [0026]    According to a further embodiment of the invention, a computer program may be provided, which implements an algorithm according to the invention and as described above and enabling the determining of an integrity risk in a GNSS when executed by a computer. The computer program may be for example installed on a computing device with a receiver for ranging signals from a GNSS, for example ranging signal from NAVSTAR-GPS or the upcoming European GNSS GALILEO. 
         [0027]    According to a further embodiment of the invention, a non-transitory record carrier storing a computer program according to the invention may be provided, for example a CD-ROM, a DVD, a memory card, a diskette, or a similar data carrier suitable to store the computer program for electronic access. 
         [0028]    A yet further embodiment of the invention provides a receiver for ranging signals from the satellite of a GNSS, comprising
       a memory storing a computer program of the invention and as described before and   a processor being configured by the stored computer program to process received ranging signals with the algorithm implemented by the computer program.       
 
         [0031]    The receiver may be for example integrated in a mobile device such as a mobile navigation device, a smartphone, a tablet computer, or a laptop. 
         [0032]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
         [0033]    The invention will be described in more detail hereinafter with reference to exemplary embodiments. However, the invention is not limited to these exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0034]      FIG. 1  shows a flowchart of an embodiment of the RAIM algorithm according to the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    In the following, an embodiment of the inventive RAIM algorithm is explained. The RAIM algorithm processes ranging signals received at a certain position from satellites of a GNSS. A received ranging signal is also called a measurement. Another common term for a received ranging signal is pseudorange measurement. 
         [0036]    The RAIM algorithm processes the information contained in the received ranging signals in order to provide integrity monitoring for applications such as aviation. If the RAIM algorithm detects a faulty ranging signal, it may issue warning to a user for example a pilot by providing an alert. 
         [0037]    For CAT-I, the ICAO has outlined several requirements for GPS, which should be fulfilled by RAIM algorithms. The requirements refer to availability, accuracy, integrity, and continuity of a GNSS based positioning service. 
         [0038]    According to ICAO, integrity is defined as a measure of the trust that can be placed in the correctness of the information supplied by the total system. Integrity includes the ability of a system to provide timely and valid warnings to the user (alerts) when the system must not be used for the intended operation (or phase of flight). 
         [0039]    To describe the integrity performance the following parameters are used:
       Alert Limit: For a given parameter measurement, the error tolerance not to be exceeded without issuing an alert   Time-to-Alert: The maximum allowable time elapsed from the onset of the navigation system being out of tolerance until the equipment enunciates the alert.   Integrity Risk: Probability that a warning is not provided within the Time-to-Alert after the Alert Limit has been exceeded.       
 
         [0043]    The definitions for the terms Alert Limit and Time-to-Alert can be found on “International Standards and Recommended Practices Aeronautical Telecommunications Annex 10 to the Convention on International Civil Aviation Volume I (Radio Navigation Aids) (Amendments until amendment 84 have been considered)”, Sixth Edition, July 2006. The definition for the term Integrity Risk is, for example, specified in the GALILEO system requirements. 
       Position Error Model 
       [0044]    For the basic understanding of the inventive RAIM algorithm the Position Error 15 Model is explained in the following. 
         [0045]    It is assumed that the position solution is derived by a weighted least square algorithm. It is further assumed that the linearized relation between the position-time error Δx and the range errors Δr given by 
         [0000]      Δ x   0   =s   0   Δr   (0.1)
 
         [0000]      where 
         [0000]        S   0 =( G   T   W   ξ   G ) −1   G   T   W   ξ   (0.2)
 
         [0000]    where G is the design matrix and W ξ  is a diagonal matrix whose n-th diagonal element is a function of the satellite range error model ξ and the airborne error model assumed for the n-th satellite. N is the number of ranges in the position solution associated with S 0 . 
         [0046]    For GPS ξ=URA (User Range Accuracy) and it is 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         W 
                         URA 
                       
                       ) 
                     
                     
                       n 
                       , 
                       n 
                     
                   
                   = 
                   
                     1 
                     
                       
                         
                           ( 
                           
                             URA 
                             n 
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             σ 
                             
                               n 
                               , 
                               user 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             σ 
                             
                               n 
                               , 
                               tropo 
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0.3 
                   ) 
                 
               
             
           
         
       
     
         [0047]    For GALILEO ξ=SISMA (Signal In Space Monitoring Accuracy) and it is 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         W 
                         SISMA 
                       
                       ) 
                     
                     
                       n 
                       , 
                       n 
                     
                   
                   = 
                   
                     1 
                     
                       
                         
                           ( 
                           
                             SISMA 
                             n 
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             σ 
                             
                               n 
                               , 
                               user 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             σ 
                             
                               n 
                               , 
                               tropo 
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0.4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein
 
σ n,user  accounts for multipath and user receiver noise.
 
σ n,tropo  accounts for mismodelling of the troposphere.
 
         [0048]    The position error model for the position solution, where n-th satellite has been removed is given by 
         [0000]      Δ x   n   =S   n   Δr   (0.5)
 
         [0000]      where 
         [0000]        S   n =( G   T   M   n   W   ξ   G ) −1   G   T   M   n   W   ξ   (0.6)
 
         [0000]    with M n  being the identity matrix where the element (n,n) is set to zero. 
       Position Separation RAIM Algorithm 
       [0049]    In the following, the position separation as applied by the inventive RAIM algorithm is explained. 
         [0050]    The test statistics for the detection of a fault on satellite n is 
         [0000]        d   n   =Δx   n   −Δx   0   (0.7)
 
         [0000]    or more precisely, as the test statistics is in every spatial direction independent, 
         [0000]        d   n,i   =e   i   T (Δ x   n   −Δx   0 )≡(Δ x   n   −Δx   0 ) i   (0.8)
 
         [0000]    where e i  is the unit vector in direction i. i can take the values east, north or up, or any other value depending on the selected axis. 
         [0051]    With the definition of ΔS n  as follows 
         [0000]      Δ x   n   −Δx   0 =( S   0   −S   n )Δ r≡ΔS   n   Δr   (0.9)
 
         [0000]    it can be written 
         [0000]        d   n,i   =e   i   T   ΔS   n   Δr≡ΔS   n,i   Δr   (0.10)
 
       Detection Threshold and Alarm Probability 
       [0052]    In the following, the detection threshold and alarm probability according to the inventive RAIM algorithm are described. 
         [0053]    If the error distributions p r,m  m of the individual ranges Δr m  are paired bounded by 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    the following holds true for the probability distribution p d     n,i    of d n,i  for all L&gt;0 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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                    
                   
                       
                   
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                   with 
                 
               
               
                 
                   ( 
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                           Δ 
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                      
                     
                         
                     
                      
                     and 
                   
                 
               
               
                 
                   ( 
                   0.16 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
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                         Δ 
                          
                         
                             
                         
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                             , 
                             i 
                           
                         
                       
                     
                     = 
                     
                       
                         ∑ 
                         
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                         N 
                       
                        
                       
                         
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                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0.17 
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         [0054]    From (0.15) the inventive RAIM algorithm can determine that the probability p fa     n,i    to raise an alert under fault free conditions if a detection threshold of D n,i  is used is bounded by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       p 
                       
                         fa 
                         
                           n 
                           , 
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                      
                     
                       ( 
                       
                         D 
                         
                           n 
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                           i 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       [ 
                       
                         
                           ∏ 
                           
                             m 
                             = 
                             1 
                           
                           N 
                         
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                           m 
                         
                       
                       ] 
                     
                      
                     
                       [ 
                       
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                         - 
                         
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                             ] 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   0.18 
                   ) 
                 
               
             
           
         
       
     
         [0055]    Furthermore, the inventive RAIM algorithm can determine that the probability to raise an alert under fault free conditions is bounded by 
         [0000]    
       
         
           
             
               
                 
                   
                     p 
                     fa 
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
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                        
                       
                         
                           ∑ 
                           
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                             = 
                             1 
                           
                           N 
                         
                          
                         
                           
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                                    
                                   
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                     + 
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                         ∏ 
                         
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                           = 
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                        
                       
                         ( 
                         
                           
                             p 
                             
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                             p 
                             
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                               n 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0.19 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where p μ     m   ·p σ     n    is the probability that (0.11) to (0.14) hold true for pseudo range n under fault free conditions and I is the number of dimensions which are to be considered in the detection algorithm via alert limits. a(•) maps the enumeration of the dimensions to the actual dimensions. 
       Integrity Risk 
       [0056]    Next, the integrity risk determination according to the inventive RAIM algorithm is explained. 
         [0057]    Only one signal is considered as faulty. Therefore one of the Δ x     n,i    is the difference between the fault free solution with N−1 signals and the (faulty or fault free solution) with N signals. The inventive RAIM algorithm can determine that the integrity risk at the alert limit (p HMI ) is therefore bounded by the maximum of the maximum of all fault free integrity risk at the (alert limit minus d n,i ) (p HMI,fm ) and the fault free integrity risk (p HNI,ff ). 
         [0000]    
       
         
           
             
               
                 
                   
                       
                   
                    
                   
                     
                       p 
                       HMI 
                     
                     = 
                     
                       
                         max 
                          
                         
                           ( 
                           
                             
                               p 
                               
                                 HMI 
                                 , 
                                 ff 
                               
                             
                             , 
                             
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                                 HMI 
                                 , 
                                 fm 
                               
                             
                           
                           ) 
                         
                       
                       + 
                       
                         p 
                         mf 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0.20 
                   ) 
                 
               
             
             
               
                 
                   
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                       HMI 
                       , 
                       ff 
                     
                   
                   = 
                   
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                                 = 
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                                           ′ 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                       + 
                       1 
                       - 
                       
                         
                           ∏ 
                           
                             n 
                             = 
                             1 
                           
                           N 
                         
                          
                         
                           ( 
                           
                             
                               p 
                               
                                 μ 
                                 n 
                                 ′ 
                               
                             
                             · 
                             
                               p 
                               
                                 σ 
                                 n 
                                 ′ 
                               
                             
                           
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   0.21 
                   ) 
                 
               
             
             
               
                 
                   
                     p 
                     
                       HMI 
                       , 
                       fm 
                       , 
                       n 
                     
                   
                   = 
                   
                     
 
                   
                    
                   
                     
                       
                         [ 
                         
                           
                             ∏ 
                             
                               
                                 m 
                                 - 
                                 1 
                               
                               
                                 n 
                                 ≠ 
                                 m 
                               
                             
                             N 
                           
                            
                           
                             K 
                             m 
                             ′ 
                           
                         
                         ] 
                       
                       [ 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           I 
                         
                          
                         
                           [ 
                           
                             1 
                             - 
                             
                               erf 
                                
                               
                                 [ 
                                 
                                   
                                     
                                       AL 
                                       
                                         α 
                                          
                                         
                                           ( 
                                           i 
                                           ) 
                                         
                                       
                                     
                                     - 
                                     
                                       d 
                                       
                                         n 
                                         , 
                                         
                                           α 
                                            
                                           
                                             ( 
                                             i 
                                             ) 
                                           
                                         
                                       
                                     
                                     - 
                                     
                                       μ 
                                       
                                         S 
                                         
                                           n 
                                           , 
                                           
                                             α 
                                              
                                             
                                               ( 
                                               i 
                                               ) 
                                             
                                           
                                         
                                       
                                       ′ 
                                     
                                   
                                   
                                     
                                       2 
                                     
                                      
                                     
                                       σ 
                                       
                                         S 
                                         
                                           n 
                                           , 
                                           
                                             α 
                                              
                                             
                                               ( 
                                               i 
                                               ) 
                                             
                                           
                                         
                                       
                                       ′ 
                                     
                                   
                                 
                                 ] 
                               
                             
                           
                           ] 
                         
                       
                       ] 
                     
                     + 
                     1 
                     - 
                     
                       
                         ∏ 
                         
                           
                             m 
                             = 
                             1 
                           
                           
                             n 
                             ≠ 
                             m 
                           
                         
                         N 
                       
                        
                       
                         ( 
                         
                           
                             p 
                             
                               μ 
                               m 
                               ′ 
                             
                           
                           · 
                           
                             p 
                             
                               σ 
                               m 
                               ′ 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0.22 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       
                         p 
                         
                           HMI 
                           , 
                           fm 
                         
                       
                       = 
                       
                         
                           max 
                           
                             n 
                             = 
                             
                               1 
                                
                               
                                   
                               
                                
                               … 
                                
                               
                                   
                               
                                
                               N 
                             
                           
                         
                          
                         
                           ( 
                           
                             p 
                             
                               HMI 
                               , 
                               fm 
                               , 
                               n 
                             
                           
                           ) 
                         
                       
                     
                      
                     
                       
 
                     
                      
                     
                         
                     
                      
                     with 
                   
                 
               
               
                 
                   ( 
                   0.23 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       
                         μ 
                         
                           S 
                           
                             n 
                             , 
                             i 
                           
                         
                         ′ 
                       
                       = 
                       
                         
                           ∑ 
                           
                             m 
                             = 
                             1 
                           
                           N 
                         
                          
                         
                           
                             
                                
                               
                                 
                                   ( 
                                   
                                     S 
                                     
                                       n 
                                       , 
                                       i 
                                     
                                   
                                   ) 
                                 
                                 m 
                               
                                
                             
                             2 
                           
                            
                           
                             μ 
                             m 
                             ′ 
                           
                         
                       
                     
                      
                     
                       
 
                     
                      
                     
                         
                     
                      
                     and 
                   
                 
               
               
                 
                   ( 
                   0.24 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       
                         σ 
                         
                           S 
                           
                             n 
                             , 
                             i 
                           
                         
                         ′ 
                       
                       = 
                       
                         
                           ∑ 
                           
                             m 
                             = 
                             1 
                           
                           N 
                         
                          
                         
                           
                             ( 
                             
                               
                                 
                                   ( 
                                   
                                     S 
                                     
                                       n 
                                       , 
                                       i 
                                     
                                   
                                   ) 
                                 
                                 m 
                               
                                
                               
                                 σ 
                                 m 
                                 ′ 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                      
                     
                       
 
                     
                      
                     
                         
                     
                      
                     and 
                   
                 
               
               
                 
                   ( 
                   0.25 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       
                         μ 
                         
                           S 
                           
                             0 
                             , 
                             i 
                           
                         
                         ′ 
                       
                       = 
                       
                         
                           ∑ 
                           
                             m 
                             = 
                             1 
                           
                           N 
                         
                          
                         
                           
                             
                                
                               
                                 
                                   ( 
                                   
                                     S 
                                     
                                       0 
                                       , 
                                       i 
                                     
                                   
                                   ) 
                                 
                                 m 
                               
                                
                             
                             2 
                           
                            
                           
                             μ 
                             m 
                             ′ 
                           
                         
                       
                     
                      
                     
                       
 
                     
                      
                     
                         
                     
                      
                     and 
                   
                 
               
               
                 
                   ( 
                   0.26 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       σ 
                       
                         S 
                         
                           0 
                           , 
                           i 
                         
                       
                       ′ 
                     
                     = 
                     
                       
                         ∑ 
                         
                           m 
                           = 
                           1 
                         
                         N 
                       
                        
                       
                         
                           ( 
                           
                             
                               
                                 ( 
                                 
                                   S 
                                   
                                     0 
                                     , 
                                     i 
                                   
                                 
                                 ) 
                               
                               m 
                             
                              
                             
                               σ 
                               m 
                               ′ 
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0.27 
                   ) 
                 
               
             
           
         
       
     
         [0058]    The primes indicate that the overbounding might be different from the overbounding used for the computation of the alert probabilities. As the integrity risk is smaller than the alert probability, the probabilities p μ     n     ′ ·p σ     n     ′  most likely have to be smaller, which in turn result in possible larger μ m ′,σ′ and K′. 
         [0059]    p ff  is the probability that all ranging signals are fault free. 
         [0060]    p fm  is the probability that any one and only one of the signals is faulty. 
         [0061]    p mf  is the probability that more than one ranging signal is faulty. 
         [0062]    If one is not interested in bounding the integrity risk for a specific interval but on average over all intervals, a bound for the average integrity risk can be computed by the inventive RAIM algorithm with 
         [0000]    
       
         
           
             
               
                 
                   
                     p 
                     avHMI 
                   
                   = 
                   
                     
                       
                         p 
                         ff 
                       
                        
                       
                         p 
                         
                           HMI 
                           , 
                           ff 
                         
                       
                     
                     + 
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         N 
                       
                        
                       
                         
                           p 
                           
                             fm 
                             , 
                             n 
                           
                         
                          
                         
                           p 
                           
                             HMI 
                             , 
                             fm 
                             , 
                             n 
                           
                         
                       
                     
                     + 
                     
                       p 
                       mf 
                     
                   
                 
               
               
                 
                   ( 
                   0.28 
                   ) 
                 
               
             
           
         
       
     
         [0063]    The difference between (0.20) and (0.28) is that the first bounds the integrity risk for any interval whereas the second bounds the integrity risk on average over all intervals. 
       Tuning of the Detection Threshold 
       [0064]    Finally, the tuning of the detection threshold according to the inventive RAIM algorithm is explained. 
         [0065]    The detection thresholds in (0.19) have to be tuned such that the availability that is basically the sum of the false alert probability p fa  and the fraction of instances where the integrity risk at the alert limit computed with either (0.20) or (0.28) is above or equal to the tolerable integrity risk, where the are replaced by D n,i  for the evaluation of (0.20) respectively (0.28), is minimized. Depending on the definition of availability, different sets of instances have to be used. The set of instances can be a set of times at the location where the above sum is maximal. Or the set of instances can be sets of sets of times at representative locations. The set to be used depends on the definition of availability. 
         [0066]      FIG. 1  shows a flowchart of the inventive RAIM algorithm. In step S 10 , the integrity risk p HMI,ff  at an alert limit for the fault condition that all ranging signals are fault free is determined. In the next step S 12 , the integrity risk p HMI,fm  at an alert limit for the fault condition that only one ranging signal is faulty is determined. In a following step S 14 , the integrity risk p mf  at an alert limit for the fault condition that more than one ranging signal is faulty is determined. 
         [0067]    In step S 16 , the overall integrity risk by a maximum of all determined integrity risks is bounded, refer to equations (0.20) and (0.28) above. In a further step S 18 , a detection threshold D n,i  for raising an alert is defined. 
         [0068]    In step S 20 , a bounding of the probability to raise an alert under fault free conditions using the defined detection threshold is determined, refer to equations (0.18) and (0.19) above. 
         [0069]    In step S 22 , an availability as the sum of the bounding of the probability to raise an alert and a fraction of instances where the overall integrity risk is above or equal to a tolerable integrity risk is defined, as described above. 
         [0070]    Then in step S 24 , the detection threshold for raising an alert is tuned such that the availability is minimized, as described above. 
         [0071]    The present invention particularly allows a better balancing between different contributions to the integrity risk in a GNSS as well as to the false alarm probability. 
         [0072]    The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.