Abstract:
An apparatus and method for detecting the type of Selective Availability (SA) engine in a Global Positioning System (GPS) receiver. The apparatus reverse computes a User Equivalent Range Error (UERE) value and filters it. The filtered value is run through a hysteresis to determine the GPS engine type. The engine type determination is used to adjust values used by other systems.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     There are currently at least three types of Global Positioning System (GPS) engines used on GPS receivers for generating GPS data: Selective Availability (SA)-on, SA-on/off, and Dynamic SA-on/off. These engines attempt to compensate for the SA state of satellites and atmospheric or other satellite errors by inflating the actual Horizontal Figure of Merit (HFOM). A single aircraft may include a combination of these GPS receiver engines.  
         [0002]     SA was turned off in May of 2000 and will continue to be off for the foreseeable future. With SA off, the GPS accuracy is possible to around 10 meters. However, the three types of GPS engines report different values for HFOM because of their differing assumptions about the satellites. The SA-on engine will report numbers with accuracy of 55 to 100 meters. The SA-on/off engine will report HFOMs of 18 to 46 meters. The Dynamic SA-on/off engine will report HFOMs of 10 to 18 meters. Although the HFOM is generally acceptable when received from the SA-on/off and Dynamic SA-on/off engines, accommodating the SA-on engine requires expanding the threshold values.  
         [0003]     Therefore, there is a need to detect the GPS engine type of a receiver to allow receiving systems to optimally operate.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     In one embodiment, the invention determines the engine type by using Horizontal Figure of Merit (HFOM) and Horizontal Dilution of Precision (HDOP) values determined by a Global Positioning System (GPS) receiver to compute a User Equivalent Range Error (UERE) and filter it. The filtered UERE value is run through a hysteresis to determine the GPS receiver type. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0005]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0006]      FIG. 1  illustrates a block diagram of an example environment and aircraft formed in accordance with an embodiment of the present invention; and  
         [0007]      FIGS. 2 and 3  illustrate a flow diagram 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  
       [0008]      FIG. 1  illustrates a block diagram of an example environment and aircraft  20  formed in accordance with an embodiment of the present invention. The aircraft  20  includes a Global Positioning System (GPS) receiver  28  and an Enhanced Ground Proximity Warning System (EGPWS)  26  or another system that uses GPS information that is received by the GPS receiver  28  from a plurality of satellites  22 . In one embodiment, the EGPWS  26  includes an automatic GPS engine detector  30 . The automatic GPS engine detector  30  analyzes data sent from each of the satellites  22  and determines the type of GPS engine included in the receiver  28 . If the auto detector  30  determines that a GPS engine is of the Selective Availability (SA)-on type, limits for some of the data that is sent are reset to more applicable values.  
         [0009]      FIGS. 2 and 3  illustrate a flow diagram of an example process  50  performed by the detector  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 . Next, at a block  54  the detector  30  receives the GPS signals from the GPS receiver  28  and determines the engine type for the GPS receiver  28 . The process performed at the block  54  is described in more detail below with regard to  FIG. 3 .  
         [0010]     At a block  56 , Horizontal Figure of Merit (HFOM) limits for each position solution are set based on the determined engine type. In one embodiment, the HFOM limits are not changed if the engine type is determined to be an SA-on/off or Dynamic SA-on/off type. The HFOM limits are set to a pre-defined value if the step performed at the block  54  determines that the engine  70  type is an SA-on type. Thus, the process  50  can be performed at start-up of the receiver  28  and detector  30 .  
         [0011]      FIG. 3  shows a process at block  54  from  FIG. 2  for determining the GPS receiver engine type, in accordance with an embodiment of the invention. First, at a decision block  74 , the process  70  determines whether HFOM and a Horizontal Dilution of Precision (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 estimate User Equivalent Range Error (UERE) is computed. The UERE estimate is computed with the following equation:  
       HFOM     2   ⁢     (   HDOP   )             
         [0012]     At a block  78 , the result of the computation performed at block  76  is sent to a low pass filter. A first decision block  80  determines if the result of the low pass filtering performed at the block  78  is greater than 27 meters. If the output of the low pass filter is greater than 27 meters, then the GPS engine for the receiver is determined to be an SA-on type. If the output of the low pass filter is less than 27 meters, then a second decision block  86  determines whether the output of the low pass filter is less than 14 meters. If the output of the low pass filter is not less than 14 meters, then the process ends. If the output of the low pass filter is less than 14 meters, then the GPS engine for the receiver is determined to be an SA-off type. Thus, the process  70  described in the above embodiment determines whether a GPS engine is an SA-on or SA-off type. The values used at the decision blocks  80  and  86  may vary depending upon desired results. Also, the low pass filter is pre-charged to a central value of approximately 20.5 meters, but could be of other values depending upon the desired filtering results.  
         [0013]     In one particular embodiment, a Runway Awareness and Advisory System (RAAS) uses the determination of whether the engine is an SA-on or SA-off type to set appropriate thresholds. RAAS sets the threshold for operation to 0.05 nm for an SA-on receiver engine and to 0.02 nm for an SA-off receiver engine to reduce nuisances while improving availability. In another particular embodiment, EGPWS uses the determination of whether the engine is an SA-on or SA-off type to evaluate whether to re-compute the HFOM value. The HFOM value is not re-computed when the engine type is an SA-off; however, the HFOM value is recomputed when the engine type is an SA-on (e.g. HFOM=2 ×8 meters x HDOP).  
         [0014]     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.