Name

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    ECI2GEO

Purpose

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    Convert Earth-centered inertial coordinates to geographic spherical coords

Explanation

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    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.
    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

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    gcoord=eci2geo(ECI_XYZ,JDtime)

Input

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      ECI_XYZ : the ECI [X,Y,Z] coordinates (in km), can be an array [3,n]
                of n such coordinates.
      JDtime: the Julian Day time, double precision. Can be a 1-D array of n
                such times.

Keyword Inputs

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      None

Output

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      a 3-element array of geographic [latitude,longitude,altitude], or an
        array [3,n] of n such coordinates, double precision

Common Blocks

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      None

Procedures Used

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      CT2LST - Convert Local Civil Time to Local Mean Sidereal Time

Example

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      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

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      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