 +_____________________________________________________________________
  AUTHOR:                                          DR:              
  NORMAN PARKER          PASS PROGRAM NOTE              110220      
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  ORG/PHONE: USA/PASS OPS NOTE (X)  USER NOTE ( )  DATE:            
  (281)282-2850            RELEASE NOTE( )                  01/26/99
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  OI/FLIGHT EFFECTIVITY:          USER AFFECTED:                     
  OI-26 THROUGH OI-28             CREW (X)  GROUND (X)  DUMP ONLY ( )
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         MAJOR SOFTWARE AREA:        OPS MODES AFFECTED:             
 SSW ( )  GNC (X)  VU ( )  SM/PL ( ) G2, G3                          
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  SOFTWARE SUB-AREA:  IMU, IPL, NAV                                   
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  PRINCIPAL FUNCTIONS:  4.38, 4.72, 4.126                             
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  TITLE:  POSSIBLE COMPUTATION OF ERRONEOUS IMU ACCELERATION          
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  DESCRIPTION:                                                        
      WHEN A FRESH MAJOR FUNCTION BASE IS RETRIEVED FROM THE MASS     
  MEMORY (VIA AN ON-ORBIT IPL, FOR INSTANCE), THE POTENTIAL EXISTS    
  FOR A CORRUPTION OF THE NAVIGATION STATE TO OCCUR FOLLOWING THE     
  CALL UP OF THE IMU ALIGN DISPLAY (SPEC 21).  THIS ANOMALY RESULTS   
  FROM THE INITIALIZATION OF THE IMU "IN-OPERATE" DISCRETES IN THE    
  MFB TO OFF.  THE ACTUAL STATUS OF THE IMU'S IS NOT EVALUATED UNTIL  
  SPEC 21 BECOMES ACTIVE.  FOR ANY IMU FOUND TO BE IN OPERATE MODE,   
  THE DIFFERENCE BETWEEN THAT IMU'S VELOCITY PULSE COUNT AND THE      
  INITIALIZED ZERO COUNT CAN RESULT IN THE COMPUTATION OF A LARGE,    
  ERRONEOUS ACCELERATION.  SUCH AN ACCELERATION, WHEN USED IN THE     
  PROPAGATION ALGORITHMS, WILL ALTER THE NAVIGATION STATE TO A        
  VARYING DEGREE, DEPENDING UPON THE MAGNITUDE OF THE ACCELEROMETER   
  PULSE COUNT, AND MAY RESULT IN AN UNREALISTIC COMPUTED STATE. USE OF
  THIS ALTERED STATE VECTOR IN SUBSEQUENT TARGETING CALCULATIONS HAS  
  BEEN OBSERVED TO CAUSE ONE OR MORE OF THE FOLLOWING GPC ERRORS:     
                                                                      
    - EXPONENT ARGUMENT > 174.673 (GROUP 4, CODE 6)                   
    - SIN/COS ARGUMENT > PI*2.621E5 (GROUP 4, CODE 8)                 
    - EXPONENT OVERFLOW (GROUP 3, CODE 11)                            
    - SQRT ARG < 0 (GROUP 4, CODE 5)                                  
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 USER RESPONSE:                                                       
    THIS CONDITION IS UNAVOIDABLE FOLLOWING THE RETRIEVAL OF THE MFB  
  DIRECTLY FROM THE MASS MEMORY IN G2 OR G3.  A STATE VECTOR UPLINK   
  WILL CORRECT ANY CORRUPTED STATE WHICH MAY RESULT.                  
  FORCING A GOOD STATE VECTOR FROM GPS WILL ALSO CORRECT THE PROBLEM. 
  TO AVOID GPC ERRORS, DO NOT ATTEMPT TARGETING CALCULATIONS BEFORE   
  THE STATE IS CORRECTED.                                             
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 CONCURRENCE:                           APPROVAL:                    
 USA/PASS:  JOHN MAGLEY    DATE 03/16/99                             
         ----------------      ---------                             
 CHAIR:     KENNETH HILL   DATE 03/16/99                             
         ----------------      ---------                             
 PF MGR:    J. HOFF        DATE 03/09/99 DARRELL E. STAMPER 03/19/99 
         ----------------      ---------------------------- -------- 
            J. IMPASTATO        03/09/99 CHAIRMAN, SASCB      DATE   
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