include file: SWHBYA 3
      REAL HBYA,                       ! Typical height/earth's radius,    SWHBYA.2      
     &     HBYAP1,                     ! and constants based on it used    SWHBYA.3      
     &     HBYAX2                      ! to find DIRFAC                    SWHBYA.4      
      PARAMETER ( HBYA = .004, HBYAP1 = 1. + HBYA, HBYAX2 = 2. * HBYA )    SWHBYA.5