Name

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    GEO2ECI

Purpose

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    Convert geographic spherical coordinates to Earth-centered inertial coords

Explanation

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    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.

Calling Sequence

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      ECIcoord=geo2eci(gcoord,JDtime)

Input

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      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

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      None

Output

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      a 3-element array of ECI [X,Y,Z] coordinates, 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

Examples

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      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

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      Written by Pascal Saint-Hilaire (shilaire@astro.phys.ethz.ch)
            on 2002/05/14