C:/Users/Andreas Schiffler/Desktop/SVN/NewGamePhysics/NewGamePhysics/Physics/GravityEarthGrsModel.cs

// --------------------------------------------------------
// GravityEarthGrsModel.cs
// (A. Schiffler, 2009)
// --------------------------------------------------------

namespace NewGamePhysics.Physics
{
    using System;

    public class GravityEarthGrsModel
    {
        public static double a = 6378137.0;

        public static double GM = 3986005.0e8;

        public static double J2 = 108263.0e-8;

        public static double omega = 7292115.0e-11;

        public static double b = 6356752.3141;

        public static double E = 521854.0097;

        public static double c = 6399593.6259;

        public static double e2 = 0.00669438002290;

        public static double ep2 = 0.00673949677548;

        public static double f = 1.0/298.257222101;

        public static double U0 = 62636860.850;

        public static double J4 = -0.00000237091222;

        public static double J6 = 0.00000000608347;

        public static double J8 = -0.00000000001427;

        public static double m = 0.00344978600308;

        public static double ya = 9.7803267715;

        public static double yb = 9.8321863685;

        public double Calculate(double latitude, double height)
        {
            if ((latitude < -90.0) || (latitude > 90.0))
            {
                throw new ArgumentOutOfRangeException("latitude");
            }

            if ((height < 0.0) || (height > 1000000.0))
            {
                throw new ArgumentOutOfRangeException("height");
            }

            // precalculations
            double phi = Math.PI * latitude / 180.0;
            double sl = Math.Sin(phi);
            double s2l = Math.Sin(2.0 * phi);
            double sl2 = sl * sl;
            double s2l2 = s2l * s2l;

            // calculate gravity flattening
            double fs = (yb - ya) / ya;
            double f4 = -0.5 * f * f + 2.5 * f * m;

            // calculate latitude dependent gravity
            double y = ya * (1.0 + fs * sl2 - 0.25 * f4 * s2l2);

            // calculate the height dependent gravity
            double yh = y * (1.0 - 2.0 / a * (1.0 + f + m - 2.0 * f * sl2) * height + 3.0 / (a * a) * (height * height));

            // result
            return yh;
        }
    }
}

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