 +_____________________________________________________________________
 ] AUTHOR:            ]                             ] SMS: B27046      ]
 ] K. Ayers, G. Rook  ]    BFS PROGRAM NOTE         ] DR:  109694      ]
 ]____________________]                             ]__________________]
 ] ORG/PHONE:         ] OPS NOTE (X)  USER NOTE ( ) ] DATE: 04/30/97   ]
 ] BNA/(562) 922-3714 ]      RELEASE NOTE ( )       ] REV A: 09/11/97  ]
 ]____________________]_____________________________]__________________]
 ] OI/FLIGHT EFFECTIVITY:        ]  USER AFFECTED:                     ]
 ] OI26, OI26B                   ]  CREW (X)  GROUND (X)  DUMP ONLY ( )]
 ]_______________________________]_____________________________________]
 ]           MAJOR SOFTWARE AREA:            ] OPS MODES AFFECTED:     ]
 ]BSL( ) BSS( ) GNC(X) SEQ( ) SM/SP( ) IPL( )]          1              ]
 ]_____________________________________________________________________]
 ] SOFTWARE SUB-AREA:  NAV                                             ]
 ]_____________________________________________________________________]
 ] PRINCIPAL FUNCTIONS:  SOD 80-0366/BFS Coding Standard A1            ]
 ]_____________________________________________________________________]
 ] TITLE:  Divide By Zero Possible in AS_UPP_INIT                      ]
 ]                                                                     ]
 ]_____________________________________________________________________]
 ] DESCRIPTION:                                                        ]
 ] For an in plane, due-east, rendezvous launch, on the descending     ]
 ] intersection with the launch latitude, division by zero may occur   ]
 ] in the AS_UPP_INIT calculation for ALP_DP.  The divisor which may   ]
 ] be equal to zero under these conditions is the following:           ]
 ]                                                                     ]
 ]   TAN((THET - PHI_DP + AZ_DP) / 2)                                  ]
 ]                                                                     ]
 ] An infinite loop of divide-by-zero errors/software restarts will be ]
 ] entered from which BFS cannot recover. This infinite error/restart  ]
 ] loop will prevent the FSW from executing the code that would annun- ]
 ] ciate an alarm. The following will be the immediate visible effects:]
 ]                                                                     ]
 ] (1) BFS displays will display the "Big X" with "POLL FAIL" on the   ]
 ]     scratch pad line.           (continued on next page)            ]
 ]_____________________________________________________________________]
 ] USER RESPONSE:                                                      ]
 ] For due east rendezvous missions, launch must not occur when GMT is ]
 ] +/- 2 seconds to the time when the location of the launch pad is in ]
 ] plane on the descending arc of the orbit plane.  For the unique case]
 ] where the inclination is less than or equal to the launch latitude, ]
 ] this time is defined by T_GMTLO_REF.  Otherwise, it is a time after ]
 ] T_GMTLO_REF, fixed by the pad to orbit plane geometry of the day.   ]
 ] Therefore, if a due east rendezvous launch were planned, a non-     ]
 ] launch time period within the overall launch window would need to   ]
 ] be established to preclude this condition from occurring.           ]
 ]_____________________________________________________________________]
 ]CONCURRENCE:                           ]APPROVAL:                    ]
 ]BNA:<s> D. Corbin         DATE 09/24/97]                             ]
 ]    --------------------      ---------]                             ]
 ]ASD:  ERIC C. BINGHAM     DATE 10/23/97]                             ]
 ]    --------------------      ---------] DARRELL E. STAMPER 10/29/97 ]
 ]PF MGR:   M. N. MONTEZ    DATE 10/23/97]------------------- -------- ]
 ]       -----------------      ---------] CHAIRMAN, SASCB      DATE   ]
 ]_____________________________________________________________________]
 @




 +_____________________________________________________________________
 ]                                                                     ]
 ]                       SUPPLEMENTAL SHEET                            ]
 ]                                                                     ]
 ]_____________________________________________________________________]
 ] AUTHOR:            ]                             ] SMS: B27046      ]
 ] K. Ayers, G. Rook  ]    BFS PROGRAM NOTE         ] DR:  109694      ]
 ]____________________]                             ]__________________]
 ] ORG/PHONE:         ] OPS NOTE (X)  USER NOTE ( ) ] DATE: 04/30/97   ]
 ] BNA/(562) 922-3714 ]      RELEASE NOTE ( )       ] REV A: 09/11/97  ]
 ]____________________]_____________________________]__________________]
 ] TITLE:  Divide By Zero Possible in AS_UPP_INIT                      ]
 ]                                                                     ]
 ]                                                                     ]
 ]_____________________________________________________________________]
 ] DESCRIPTION:                                                        ]
 ]                                                                     ]
 ] (continued from previous page)                                      ]
 ]                                                                     ]
 ] (2) GPC Error Log will receive the following pair of GPC errors     ]
 ]     every pass through the infinite loop: 9C (INVALID FLOATING      ]
 ]     POINT DIVIDE) and B2 (RESTART SVC).                             ]
 ] (3) Downlist data on each minor cycle will contain either stale data]
 ]     (no downlink) or the GPC error log data, depending on whether   ]
 ]     the error/restart occurred prior to downlink on that minor cycle]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]_____________________________________________________________________]
 ] USER RESPONSE:                                                      ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]                                                                     ]
 ]_____________________________________________________________________]
 @
