;+ ; NAME: ; ECI2GEO ; ; PURPOSE: ; Convert Earth-centered inertial coordinates to geographic spherical coords ; EXPLANATION: ; Converts from ECI (Earth-Centered Inertial) (X,Y,Z) rectangular ; coordinates to geographic spherical coordinates (latitude, longitude, ; altitude). JD time is also needed as input. ; ; ECI coordinates are in km from Earth center at the supplied time (True of ; Date). Geographic coordinates are in degrees/degrees/km ; Geographic coordinates assume the Earth is a perfect sphere, with radius ; equal to its equatorial radius. ; ; CALLING SEQUENCE: ; gcoord=eci2geo(ECI_XYZ,JDtime) ; ; INPUT: ; ECI_XYZ : the ECI [X,Y,Z] coordinates (in km), can be an array [3,n] ; of n such coordinates. These should be at the supplied ; Julian Date (TOD - true of date). ; JDtime: the Julian Day time, double precision. Can be a 1-D array of n ; such times. ; ; KEYWORD INPUTS: ; None ; ; OUTPUT: ; a 3-element array of geographic [latitude,longitude,altitude], or an ; array [3,n] of n such coordinates, double precision ; ; COMMON BLOCKS: ; None ; ; PROCEDURES USED: ; CT2LST - Convert Local Civil Time to Local Mean Sidereal Time ; ; EXAMPLE: ; IDL> gcoord=eci2geo([6378.137+600,0,0], 2452343.38982663D) ; IDL> print,gcoord ; 0.0000000 232.27096 600.00000 ; ; (The above is the geographic direction of the vernal point on ; 2002/03/09 21:21:21.021, in geographic coordinates. The chosen ; altitude was 600 km.) ; ; gcoord can be further transformed into geodetic coordinates (using ; geo2geodetic.pro) or into geomagnetic coordinates (using geo2mag.pro) ; ; MODIFICATION HISTORY: ; Written by Pascal Saint-Hilaire (Saint-Hilaire@astro.phys.ethz.ch) on ; 2001/05/13 ; Modified on 2002/05/13, PSH : vectorization + use of JD times ; Document use of TOD epoch R. Redmon April 2014 NOAA/NGDC ;- ;============================================================================= FUNCTION eci2geo,ECI_XYZ,JDtim Re=6378.137 ; Earth's equatorial radius, in km coord=DOUBLE(ECI_XYZ) JDtime= DOUBLE(JDtim) theta=atan(coord[1,*],coord[0,*]) ; azimuth ct2lst,gst,0,0,JDtime angle_sid=gst*2.*!DPI/24. ; sidereal angle lon= (theta - angle_sid ) MOD (2* !DPI) ;longitude r=sqrt(coord[0,*]^2+coord[1,*]^2) lat=atan(coord[2,*],r) ; latitude alt=r/cos(lat) - Re ; altitude lat=lat*180./(!DPI) ; to convert from radians into degrees... lon=lon*180./(!DPI) ss=WHERE(lon LT 0.) IF ss[0] NE -1 THEN lon[ss]=lon[ss]+360. RETURN,[lat,lon,alt] END ;====================================================================================