;+ ; NAME: ; GEO2ECI ; ; PURPOSE: ; Convert geographic spherical coordinates to Earth-centered inertial coords ; ; EXPLANATION: ; Converts from geographic spherical coordinates [latitude, longitude, ; altitude] to ECI (Earth-Centered Inertial) [X,Y,Z] rectangular ; coordinates. JD time is also needed. ; ; Geographic coordinates are in degrees/degrees/km ; Geographic coordinates assume the Earth is a perfect sphere, with radius ; equal to its equatorial radius. ; ECI coordinates are in km from Earth center at epoch TOD (True of Date) ; ; CALLING SEQUENCE: ; ECIcoord=geo2eci(gcoord,JDtime) ; ; INPUT: ; gcoord: geographic [latitude,longitude,altitude], or a an array [3,n] ; of n such coordinates ; JDtime: Julian Day time, double precision. Can be a 1-D array of n ; such times. ; ; KEYWORD INPUTS: ; None ; ; OUTPUT: ; a 3-element array of ECI [X,Y,Z] coordinates, or an array [3,n] of ; n such coordinates, double precision. The TOD epoch is the ; supplied JDtime. ; ; COMMON BLOCKS: ; None ; ; PROCEDURES USED: ; CT2LST - Convert Local Civil Time to Local Mean Sidereal Time ; ; EXAMPLES: ; ; IDL> ECIcoord=geo2eci([0,0,0], 2452343.38982663D) ; IDL> print,ECIcoord ; -3902.9606 5044.5548 0.0000000 ; ; (The above is the ECI coordinates of the intersection of the equator and ; Greenwich's meridian on 2002/03/09 21:21:21.021) ; ; MODIFICATION HISTORY: ; Written by Pascal Saint-Hilaire (shilaire@astro.phys.ethz.ch) ; on 2002/05/14 ; Update documentation to specify epoch is TOD. ; R. Redmon NOAA/NGDC April 2014 ; ;- ;==================================================================================== FUNCTION geo2eci,incoord,JDtim Re=6378.137 ; Earth's equatorial radius, in km lat = DOUBLE(incoord[0,*])*!DPI/180. lon = DOUBLE(incoord[1,*])*!DPI/180. alt = DOUBLE(incoord[2,*]) JDtime= DOUBLE(JDtim) ct2lst,gst,0,0,JDtime angle_sid=gst*2.*!DPI/24. ; sidereal angle theta=lon+angle_sid ; azimuth r=(alt+Re)*cos(lat) X=r*cos(theta) Y=r*sin(theta) Z=(alt+Re)*sin(lat) RETURN,[X,Y,Z] END ;====================================================================================