WGS point to WGS line segment (great circle) distance

Question

How would I go about resolving this in C#?

I have a line segment (great circle distance) defined by two Lon-Lat pairs (call them points A and B). A third point somewhere on the Earth sphere.

The line segment closes the shortest distance between A and B. Third Lon-Lat pair is the point C.

How do I calculate the distance between the point C, and the closest point on the AB segment?

EDIT:
In fact, I am looking for an algorithm to find the spherical distance between an arbitrary point C and a geodesic segment AB.

What i have for now:

public static float DistanceInKilometres(PointF A, PointF B, PointF C)
{
    var d13 = Haversine_KM_F(A, C);
    var R = 3961.3;
    var brng12 = Bearing(A, B);
    var brng13 = Bearing(A, C);
    var dXt = Math.Asin(Math.Sin(d13 / R) * Math.Sin(brng13 - brng12)) * R;
    return (float)dXt;
}

public static Double Bearing(PointF coordinate1, PointF coordinate2)
{
    var latitude1 = coordinate1.Y.ToRadian();
    var latitude2 = coordinate2.Y.ToRadian();
    var longitudeDifference = (coordinate2.X - coordinate1.X).ToRadian();
    var y = Math.Sin(longitudeDifference) * Math.Cos(latitude2);
    var x = Math.Cos(latitude1) * Math.Sin(latitude2) -
    Math.Sin(latitude1) * Math.Cos(latitude2) * Math.Cos(longitudeDifference);
    return (Math.Atan2(y, x).ToDegree() + 360) % 360;
}

private static float ToRadian(this float angle)
{
    return (float)(Math.PI * angle / 180.0);
}

private static float ToDegree(this float angle)
{
  return (float)(Math.PI * angle / 180.0);
}

Answer 1

The more general problem, posed for an ellipsoid of revolution, is considered in Section 8 of

http://arxiv.org/abs/1109.4448v2

This gives solutions of the interception problem (the problem at hand) and the intersection problem using the ellipsoidal gnomonic projection. The same technique will apply to a sphere, of course.