Just read Spacetrack Report No. 3 , p.69: "The function subroutine ACTAN is passed the values of sine and cosine in that order and it returns the angle in radians within the range of 0 to 2pi." That is in the range of 0 to 360 deg, the same range as the RAAN has. Quite obviously, the ACTAN function is not implemented in this way in SGP_MATH.PAS of TS Kelso's Pascal Library: Function AcTan(sinx,cosx : double) : double; begin if cosx = 0 then if sinx > 0 then Actan := pi/2 else Actan := 3*pi/2 else if cosx > 0 then Actan := ArcTan(sinx/cosx) else Actan := pi + ArcTan(sinx/cosx); end; {Function Actan} This function will not work as intended with sinx < 0 and cosx > 0, that is for angles in the range of 270 to 360 deg. These angles are returned in the range of -90 to 0 deg. I have changed Function AcTan to: Function AcTan(sinx,cosx : double) : double; begin if cosx = 0 then if sinx > 0 then Actan := pi/2 else Actan := 3*pi/2; if cosx > 0 then if sinx > 0 then Actan := ArcTan(sinx/cosx) else Actan := 2*pi + ArcTan(sinx/cosx); if cosx < 0 then Actan := pi + ArcTan(sinx/cosx); end; {Function Actan} This function returns all angles in the range of 0 to 360 deg. With this new function in use, it is no longer necessary to modify DEEP.FOR or SGP4SDP4.PAS. Rainer