Abstract:
An apparatus and method for automatically adjusting for data protocol discrepancies. The apparatus includes a unit that receives a signal from GPS receiver. The signal was formatted according to a transmission protocol. The unit includes a first component that estimates User Equivalent Range Error (UERE) based on the received signal and adjusting at least one value of the received signal based on the estimated UERE and one or more predefined scale factor and a second component that uses the adjusted at least one value.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Global Positioning Systems (GPS) units transmit GPS data using the Aeronautical Radio, Inc (ARINC) 743 or 743A protocols. These protocols are nearly identical except for the scaling of the Figure of Merit (FOM) terms. Currently, avionics system require the GPS installation (unit) to specify which type of ARINC protocol is used. However, if the GPS installation has been upgraded to use the ARINC 743A protocol, but the GPS installation is still reporting itself as using the ARINC 743 protocol, the outputted GPS data is improperly scaled. Improperly scaled FOM data can adversely affect avionic devices, such as an Enhanced Ground Proximity Warning System (EGPWS). 
         [0002]    Therefore, there is a need to easily and cheaply determine the protocol type of GPS data. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    The present invention provides an apparatus and method for automatically adjusting for data protocol discrepancies. The apparatus includes a unit that receives a signal from GPS receiver. The signal is formatted according to a transmission protocol. The unit includes a first component that estimates User Equivalent Range Error (UERE) based on the received signal and adjusts at least one value of the received signal based on the estimated UERE and one or more predefined scale factor and a second component that uses the adjusted at least one value. 
         [0004]    In one aspect of the invention, the at least one value includes a Horizontal Figure of Merit (HFOM) and Vertical Figure of Merit (VFOM). 
         [0005]    In another aspect of the invention, the first component includes a component for computing the estimate of UERE using a HFOM value and a HDOP value and a component for filtering the estimated UERE. The first component also includes a component for determining if the filtered UERE estimate is greater than or less than a range of UERE, a component for multiplying at least one of HFOM or VFOM by one of two scale factors, if the filtered UERE estimate is determined less than the UERE range, and a component for dividing at least one of HFOM or VFOM by one of the two scale factors, if the filtered UERE estimate is determined greater than the UERE range. 
         [0006]    In still another aspect of the invention, the transmission protocols include ARINC 743 and ARINC 743A. The two scale factors include an HFOM scale factor between 28 and 29 and a VFOM scale factor between 9 and 10 and wherein the range of UERE is 4 to 33. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0007]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
           [0008]      FIG. 1  illustrates a block diagram of an example environment and aircraft formed in accordance with an embodiment of the present invention; and 
           [0009]      FIGS. 2 and 3  illustrate flow diagrams of an example process performed by the auto detector of  FIG. 1  in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]      FIG. 1  illustrates a block diagram of an example environment  18  formed in accordance with an embodiment of the present invention. The environment  18  includes a plurality of satellites  22  and an aircraft  20  having a Global Positioning System (GPS) receiver  28  and a Terrain Awareness Warning System, such as an Enhanced Ground Proximity Warning System (EGPWS)  26  or another system that uses GPS information received by the GPS receiver  28  from the plurality of satellites  22 . In one embodiment, the EGPWS  26  includes an automatic adjustor  30 . 
         [0011]    The automatic adjustor  30  analyzes the data sent from the GPS receiver  28  and scales certain GPS data if it determines there is a discrepancy between the expected protocol from the GPS receiver  28  and the actual protocol used to transmit the data. More specifically, the auto adjustor  30  uses Figure of Merit (FOM) information and Dilution of Precision (DOP) data for the Horizontal channel to back compute User Equivalent Range Error (UERE). A normal UERE should fall within the range of approximately 4 to 33 meters. If the EGPWS  26  is configured to receive GPS data according to Aeronautical Radio, Inc (ARINC) 743 and the GPS receiver  28  is sending according to ARINC 743A protocol or vice a versa, the computed UERE is sufficiently outside a normal UERE range. The automatic adjustor  30  then automatically adjusts the FOMs to the correct value (i.e. equivalent to both systems using the same protocol). 
         [0012]      FIGS. 2 and 3  illustrate a flow diagram of an example process  50  performed by the automatic adjustor  30  of  FIG. 1 , in accordance with an embodiment of the invention. As shown in  FIG. 2 , the process  50  begins at a block  52  whereby the GPS receiver  28  receives GPS signals from one or more of the plurality of GPS satellites  22 . At a block  54 , the GPS receiver  28  processes the received GPS signals and formats the processed information according to either the ARINC 743 or 743A protocols. Next, at a block  56  the adjustor  30  receives the GPS signals from the GPS receiver  28 . At a block  58 , the auto adjustor  30  analyzes the sent data and adjusts the sent data if it was determined during analysis that a protocol discrepancy exists. 
         [0013]      FIG. 3  shows a process  70  at block  58  from  FIG. 2  for analyzing and adjusting, if necessary, GPS data received from the GPS receiver  28 . First, at a decision block  74 , the process  70  determines whether Horizontal FOM (HFOM) and Horizontal DOP (HDOP) values are greater than 0. The process  70  is complete if the results of the decision block  74  indicate that either the HFOM or HDOP values are equal to or less than 0. However, if both the HFOM and HDOP values are greater than 0, the process  70  continues to block  76  where an estimated UERE is computed. The UERE estimate is computed with Equation (1): 
         [0000]    
       
         
           
             
               
                 
                   UERE 
                   = 
                   
                     HFOM 
                     
                       2 
                        
                       
                         ( 
                         HDOP 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0014]    At a block  78 , the result of the computation performed at block  76  is sent through a low pass filter. The filtered UERE is used to compare to the normal limits. 
         [0015]    The horizontal scale factor from 743 to 743A (K h ) is 16 nm/1024 meters or approximately 28.856. The vertical scale factor from 743 to 743A (K v ) is 32768 ft/1024 m or approximately 9.727. At a decision block  80 , if the filtered UERE is greater than an upper limit (e.g. 115 meters (i.e. 4 m*K h )), a Rescale flag is set to HIGH at a block  82  and the FOMs for vertical and horizontal are divided by their respective scale factor K (K h , K v ) at block  90 . In this situation, the FOMs are being transmitted via the ARINC 743 protocol, but the EGPWS  26  interprets the FOMs are being transmitted via the ARINC 743A protocol. 
         [0016]    If the filtered UERE is less than the upper limit and at decision block  86  is less than a lower limit (e.g. 1.5 meters (i.e. 33 m/K h )), the Rescale flag is set to LOW at block  88  and the vertical FOM (VFOM) and HFOM are multiplied by their respective scale factor K (K h , K v ) at block  92 . In this situation, the FOMs are being transmitted via the ARINC 743A protocol, but the EGPWS  26  interprets the FOMs are being transmitted via the ARINC 743 protocol. If the filtered UERE is within normal UERE limits, the Rescale flag is set to NORMAL and the FOMs remain unchanged—the protocol understood by the EGPWS  26  is the protocol that is being used to send the data from the GPS receiver  28 . The resultant adjusted or unadjusted FOMs are used by the EGPWS  26  or other avionics. 
         [0017]    The values used at the decision blocks  80  and  86  may vary depending upon desired results. Also, in one embodiment the low pass filter is pre-charged, which is a standard operation on filters. It involves setting the internal state to a specific value. 
         [0018]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.