Patent Publication Number: US-2020280944-A1

Title: Trusted Dissemination of a Reference Time Scale to User Terminals

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Italian Patent Application No. 102017000102599 filed on Sep. 13, 2017, the disclosure of which is incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to trusted dissemination of a Reference Time Scale to User Terminals. 
     The present invention finds advantageous but not limitative application to User Terminals (UTs) equipped with Global Navigation Satellite System (GNSS) radio receivers designed to receive radio Signals in Space (SIS) broadcasted by a GNSS satellite constellation, and to locally compute a GNSS Time Scale based on received GNSS SIS, and to which the following description will refer without, however, losing generality. 
     Nevertheless, the present invention may also find application to User Terminals equipped with other types of radio receivers designed to receive radio time signals based on which a Time Scale may be locally computed, such as radio receivers designed to receive radio, longwave, standard-frequency time signals broadcasted by a DCF77 radio station. 
     STATE OF THE ART 
     As is known, nowadays GNSS receivers are the user interfaces to any GNSS and allow Signals in Space (SIS) broadcasted by a GNSS satellite constellation to be received, and Position, Velocity, and Time (PVT) to be computed based thereon. 
     Civil aviation community imposes stringent requirements on precision, integrity, continuity and availability of GNSS services. One of the most essential aspects relies on integrity and its impact on safety, the major driver in civil aviation. GNSS integrity is the capability of providing timely warnings to users when a GNSS service should not be used. These drivers have pushed GNSS community to look for solutions that could guarantee integrity in the civil aviation domain, such as Receiver Autonomous Integrity Monitoring (RAIM). 
     OBJECT AND SUMMARY OF THE INVENTION 
     The Applicant has noticed that GNSS receivers compute integrity information directly based on the SIS, while accuracy of the computed PVT is estimated based the redundancy of the received SIS. Therefore, no security check is available to determine SIS corruption or spoofing and, hence, current GNSS receivers fail to ensure that the PVT are reliable and trustable. 
     The aim of the present invention is therefore to provide a technology that allows the aforementioned drawbacks to be overcome, in particular a technology that allows a reliable Reference Time Scale to be trustily disseminated to User Terminals. 
     This aim is achieved by the present invention, which relates to a Reference Time Scale Dissemination System as claimed in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a high-level block diagram of a Multi-Country implementation of a Reference Time Scale Dissemination System according to the present invention. 
         FIG. 2  shows a functional block diagram of a Reference Time Scale Dissemination Data Provider in the Reference Time Scale Dissemination System. 
         FIG. 3  shows a graphical timeline representation of a chronological sequence of events occurring during operation of the Reference Time Scale Dissemination System. 
         FIG. 4  shows Reference Time Scale Dissemination Data disseminated by the Reference Time Scale Dissemination System. 
         FIG. 5  shows a functional block diagram of a Reference Time Scale Dissemination Data Validation Service Provider in the Reference Time Scale Dissemination System. 
         FIG. 6  shows a functional block diagram of a User Terminal to which a Reference Time Scale is disseminated. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     The following description is provided to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the claimed invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein and defined in the appended claims. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments disclosed belongs. In the case of conflict, the present specification, including definitions, will control. In addition, the examples are illustrative only not intended to be limiting. In particular, the block diagrams depicted in the Figures and described are not to be construed to represent structural features, namely constructive limitations, but are to be construed to represent functional features, namely intrinsic properties of devices defined by the achieved effects or functional limitations and that may be implemented with different structures, so protecting the functionalities thereof (possibility to function). 
     For the purposes of promoting understanding of the embodiments described herein, reference will be made to certain embodiments and specific language will be used to describe the same. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present disclosure. 
     The basic idea underlying the present invention is to trustily disseminate a Reference Time Scale to User Terminals equipped with GNSS Receivers, so as to cause User Terminal Time Scales provided by User Terminal Clock Devices to reliably replicate the disseminated Reference Time Scale. 
     This goal is achieved by providing User Terminals with Reference Time Scale Dissemination Data indicative of the extent to which the GNSS Time Scale computed by the User Terminals differs from the Reference Time Scale, and based on which the User Terminal Local Time Scales may be corrected to reliably replicate the disseminated Reference Time Scale. 
     A high-level block diagram of a Reference Time Scale (RTS) Dissemination System (RTS-DS) according to the present invention is shown in  FIG. 1 , where a multi-country application is exemplarily considered, without losing generality. 
     The RTS Dissemination System comprises: 
     a Reference Time Scale (RTS) Dissemination Data Provider (RTS-DDP) designed to receive a Reference Time Scale to be disseminated to UTs and to compute RTS Dissemination Data (RTS-DD) to be disseminated to UTs so as to cause the UTs to reliably replicate the disseminated Reference Time Scale, 
     a Reference Time Scale (RTS) Dissemination Data (RTS-DD) Validation Service Provider (RTS-DD-VSP) designed to provide a RTS-DD Validation Service and to receive from the RTS Dissemination Data Provider and validate received RTS Dissemination Data, and transmit validated RTS Dissemination Data to the UTs, and 
     User Terminals (UTs) subscribed to the RTS-DD Validation Service and designed to receive from the RTS-DD Validation Service Provider and process validated RTS Dissemination Data to reliably replicate the disseminated Reference Time Scale. 
     The RTS Dissemination Data Provider, the RTS-DD Validation Service Provider and the UTs are each equipped with GNSS Receivers designed to receive GNSS SIS broadcasted by a GNSS satellite constellation, and to compute a GNSS Time Scale based on received GNSS SIS. 
     The RTS-DD Validation Service Provider is further equipped with an Internal Clock Device locked to a RTS-DD-VSP GNSS Time Scale computed by the RTS-DD Validation Service Provider based on received GNSS SIS and providing a RTS-DD-VSP Local Time Scale resultingly locked to the a RTS-DD-VSP GNSS Time Scale. 
     Similarly, each UT is further equipped with an Internal Clock Device locked to a UT GNSS Time Scale computed by the UT based on received GNSS SIS and providing a UT Local Time Scale resultingly locked to the UT GNSS Time Scale. 
     A functional block diagram of the RTS Dissemination Data Provider is shown in  FIG. 2 . 
     The RTS Dissemination Data Provider is designed to: 
     receive a Reference Time Scale from a Reference Time Scale Provider (not shown), such as the Reference Time Scale commonly known as UTC(k) Time Scale broadcasted by a national metrology institute and consisting of a national representation of an International Time Scale also known as Coordinated Universal Time (UTC), or the Reference Time Scale commonly known as TAI (from Temps Atomique International) Time Scale, or even a Reference Time Scale provided by any public/private company/institution/authority in order for it to be disseminated to UTs of its own employees/collaborators, etc., 
     compute a RTS-DDP GNSS Time Scale based on received GNSS SIS, and 
     at a (scheduled) RTS-DDP GNSS Time, hereinafter referenced by subscript i, compute Time Quantities indicative of the difference between the computed RTS-DDP GNSS Time Scale and the received Reference Time Scale, and comprising:
         a Time Scatter indicative of a difference between the RTS-DDP GNSS Time and a corresponding Reference Time, and   a Time Offset indicative of a mean value, computed over a timespan (preceding or across the RTS-DDP GNSS Time), of a number of differences between RTS-DDP GNSS Times and corresponding Reference Times, and       

     transmit to the RTS-DD Validation Service Provider RTS Dissemination Data containing the computed Time Quantities, the RTS-DDP GNSS Time when the Time Quantities are computed, and the RTS-DDP GNSS Time when the RTS Dissemination Data is transmitted to the RTS-DD Validation Service Provider. 
     RTS Dissemination Data may be scheduled to be computed at a frequency that depends on the performance that is to be guaranteed at the UTs and that depends on the subscribed Data Validation Service, as well as the computed RTS Dissemination Data may be scheduled to be transmitted individually, once computed, or in batches, at a frequency that depends on the performance that is to be guaranteed at the UTs and that depends on the subscribed Data Validation Service. 
     In particular, the Time Scatter and the Time Offset may be computed as follows: 
     
       
         
           
             
               
                 T 
                 Mi 
               
               = 
               
                 
                   T 
                   Si 
                 
                 - 
                 
                   T 
                   
                     R 
                      
                     i 
                   
                 
               
             
              
             
               
 
             
              
             
               
                 T 
                 Oi 
               
               = 
               
                 
                   1 
                   k 
                 
                  
                 
                   
                     ∑ 
                     
                       j 
                       = 
                       1 
                     
                     k 
                   
                    
                   
                     ( 
                     
                       
                         T 
                         
                           S 
                            
                           j 
                         
                       
                       - 
                       
                         T 
                         
                           R 
                            
                           j 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     where:
     T Si  is the RTS-DDP GNSS Time i   T Ri  is the Reference Time @ RTS-DDP GNSS Time i   T Mi  is the Time Scatter @ RTS-DDP GNSS Time i   T Oi  is the Time Offset @ RTS-DDP GNSS Time i   k is a cardinality of the set of differences between the RTS-DDP GNSS Times and the Reference Time based on which the Time Offset is computed.   

       FIG. 3  shows a timeline graphical representation of the chronological sequence of events occurring during operation of the RTS Dissemination System, while  FIG. 4  shows RTS Dissemination Data. 
     A functional block diagram of the RTS-DD Validation Service Provider is shown in  FIG. 5 . 
     The RTS-DD Validation Service Provider is designed to: 
     receive RTS Dissemination Data transmitted by the RTS Dissemination Data Provider, 
     compute a RTS-DD-VSP GNSS Time Scale based on received GNSS SIS, 
     check whether the RTS Dissemination Data is received by the RTS-DD Validation Service Provider within a given timespan (maximum acceptable transmission delay) from the transmission by the RTS Dissemination Data Provider, based on the RTS-DD-VSP GNSS Time when the RTS Dissemination Data is received by the RTS-DD Validation Service Provider and the RTS-DDP GNSS Time when the RTS Dissemination Data is transmitted by the RTS Dissemination Data Provider and contained in the received RTS Dissemination Data, 
     if the RTS Dissemination Data is determined to be received by the RTS-DD Validation Service Provider within the given timespan, compute at a RTS-DD-VSP GNSS Time, hereinafter referenced by subscript i, corresponding to the RTS-DDP GNSS Time i at which the Time Quantities contained in the received RTS Dissemination Data are computed:
         a first quantity indicative of a difference between the RTS-DD-VSP GNSS Time and the RTS-DD-VSP Local Time, and   a second quantity indicative of a difference between the Time Scatter and the Time Offset contained in the received RTS Dissemination Data,   check whether a validation criterion is met by the first and second computed quantities, and   if the validation criterion is determined to be met:
           validate the received RTS Dissemination Data, and   transmit validated RTS Dissemination Data to the UTs, wherein the validated RTS Dissemination Data contains the computed Time Quantities, the RTS-DDP GNSS Time when the RTS Dissemination Data are computed, and the RTS-DD-VSP GNSS Time when the validated RTS Dissemination Data is transmitted to the UTs.   
               

     Conveniently, the validation criterion may be defined by the first and second computed quantities being approximately equivalent or equal, as follows: 
     
       
      
       T 
       Si 
       −T 
       ˜Si 
       ≅T 
       Mi 
       −T 
       Oi  
      
     
     where:
     T Si  is the RTS-DD-VSP GNSS Time i   T ˜Si  is the RTS-DD-VSP Local Time @ RTS-DD-VSP GNSS Time i   T Mi  is the Time Scatter computed @ RTS-DDP GNSS Time i   T Oi  is the Time Offset computed @ RTS-DDP GNSS Time i.   

     It may be further appreciated that: 
     the first computed quantity quantifies the amount of variation or dispersion of the Local Time Scale with respect to the GNSS Time Scale in the RTS-DD Validation Service Provider at the same time as when the Time Quantities are computed, 
     the second computed quantity quantifies the amount of variation or dispersion of the GNSS Time Scale with respect to the Reference Time Scale in the RTS Dissemination Data Provider when the Time Quantities are computed, and 
     checking whether the first and second computed quantities are approximately equivalent amounts to checking whether at the time when the Time Quantities are computed, the variation or dispersion of the Local Time Scale with respect to the GNSS Time Scale in the RTS-DD Validation Service Provider is approximately equal or equivalent to the variation or dispersion of the GNSS Time Scale with respect to the Reference Time Scale in the RTS-DDP. 
     Different appropriate validation criteria may conveniently be adopted. 
     A functional block diagram of a UT is shown in  FIG. 6 . 
     The UT is designed to: 
     receive the validated RTS Dissemination Data transmitted by the RTS-DD Validation Service Provider, 
     compute a UT GNSS Time Scale based on received GNSS SIS, 
     check whether the received RTS Dissemination Data is received by the UT within a timespan (maximum acceptable transmission delay) from the transmission by the RTS-DD Validation Service Provider, based on the UT GNSS Time when the RTS Dissemination Data is received by the UT and the RTS-DD-VSP GNSS Time when the RTS Dissemination Data is transmitted by the RTS-DD Validation Service Provider and contained in the received validated RTS Dissemination Data, 
     if the RTS Dissemination Data is determined to be received by the UT within the given timespan, compute, at a UT GNSS Time, hereinafter referenced by subscript i, corresponding to the RTS-DDP GNSS Time i at which the Time Quantities contained in the received validated RTS Dissemination Data are computed:
         a first quantity indicative of a difference between the UT GNSS Time and the UT Local Time, and   a second quantity indicative of a difference between the Time Scatter and the Time Offset contained in the received validated RTS Dissemination Data,   check whether a validation criterion is met by the first and second computed quantities, and   if the validation criterion is determined to be met:
           compute a UT RTS-Aligned Time Scale in time alignment with the disseminated Reference Time Scale based on the Time Offset contained in the received validated RTS Dissemination Data, and   expose the UT RTS-Aligned Time Scale, along with an indication that the exposed UT RTS-Aligned Time Scale is trusted, namely is in time alignment with the disseminated Reference Time Scale,   
           if instead the validation criterion is determined not to be met,
           expose the UT Local Time Scale to the User, along with an indication that the exposed UT Local Time Scale may not be trusted, namely may not in time alignment with the disseminated Reference Time Scale.   
               

     Conveniently, the validation criterion may be defined by the first and second computed quantities being approximately equivalent or equal, as follows: 
     
       
      
       T 
       SRi 
       −T 
       ˜SRi 
       ≅T 
       Mi 
       −T 
       Oi  
      
     
     where:
     T SRi  is the UT GNSS Time i   T ˜SRi  is the UT Local Time @ UT GNSS Time i   T Mi  is the Time Scatter computed @ RTS-DDP GNSS Time i   T Oi  is the Time Offset computed @ RTS-DDP GNSS Time i.   

     Different appropriate validation criteria may conveniently be adopted. 
     The exposed UT RTS-Aligned Time Scale may be computed as follows: 
     
       
      
       T 
       U 
       =T 
       ˜SR 
       −T 
       Oi  
      
     
     where:
     T U  is the UT RTS-Aligned Time Scale   T ˜SR  is the UT Local Time Scale   T Oi  is the Time Offset @ RTS-DDP GNSS Time i.   

     The UT may be further designed to validate the exposed UT RTS-Aligned Time Scale by: 
     computing a UT Local Time Scale T ˜SR  based on the exposed UT RTS-Aligned Time Scale T U  and on the RTS-DDP Time Offset computed at a preceding RTS-DDP GNSS Time i−1 and contained in the previously received RTS Dissemination Data: 
     
       
      
       T 
       ˜SR 
       =T 
       U 
       +T 
       Oi−1  
      
     
     checking if the following validation criterion is met @ UT GNSS Time when the RTS Distribution Data is received by the UT: 
       ( T   ˜SR   −T   Sp )&lt;Δ R  
 
     where:
     T Sp  is the RTS-DD-VSP GNSS Time when the validated RTS Dissemination Data is transmitted to the UTs and contained in the received RTS Distribution Data   Δ R  is the timespan (maximum acceptable transmission delay) within which the validated RTS Dissemination Data are checked to be received by the UT from the transmission of the validated RTS Dissemination Data by the RTS-DD Validation Service Provider,   

     validating the exposed UT RTS-Aligned Time Scale if the validation criterion is determined to be met. 
     In view of the foregoing, it may be appreciated that the RTS Dissemination System according to the present invention allows a Reference Time Scale to be trustily disseminated to the UTs by transmitting, and validating the transmission of, Time Offsets and Time Scatters of the GNSS Time Scale with respect to the Reference Time Scale computed at the RTS Dissemination Data Provider, so as to cause the UT Local Time Scales to reliably replicate the disseminated Reference Time Scale. 
     Two main categories of UTs may benefit from the trusted Reference Time Scale dissemination according to the present invention. A first category comprises those UTs that need a reliable local time scale for billing or service availability/quality verification purposes, such as Power Utilities, Telephony, Motorways, Parking, Banking, etc., while a second category comprises those UTs that need a reliable local time scale to trace the development of an activity by timestamping the steps thereof for investigation purposes in case of disaster, such as Railways, Underground, Airport logistics (location supplement), Air Transportation, Logistics, Self-Driving Motor Vehicles (location supplement), etc. 
     It may be appreciated that various modifications may be made to the above-described embodiment. In a different embodiment, the RTD-DD Validation Service Provider may be omitted and the RTS Dissemination Data necessary to the UTs to replicate the Reference Time Scale may be broadcasted directly by the RTS Dissemination Data Provider.