Convert latitude and longitude to ECEF coordinates system

Question

I am studying pArk Apple sample code, and how it is works. anyone knows how convert the latitude and longitude to ECEF coordinates, and Covert ECEF to ENU coordinates centered at given lat, lon functions are work? I just want to understand what is going on in this function!

thanks.

void latLonToEcef(double lat, double lon, double alt, double *x, double *y, double *z)
{   
    double clat = cos(lat * DEGREES_TO_RADIANS);
    double slat = sin(lat * DEGREES_TO_RADIANS);
    double clon = cos(lon * DEGREES_TO_RADIANS);
    double slon = sin(lon * DEGREES_TO_RADIANS);

    double N = WGS84_A / sqrt(1.0 - WGS84_E * WGS84_E * slat * slat);

    *x = (N + alt) * clat * clon;
    *y = (N + alt) * clat * slon;
    *z = (N * (1.0 - WGS84_E * WGS84_E) + alt) * slat;
}
// Coverts ECEF to ENU coordinates centered at given lat, lon
void ecefToEnu(double lat, double lon, double x, double y, double z, double xr, double yr, double zr, double *e, double *n, double *u)
{
    double clat = cos(lat * DEGREES_TO_RADIANS);
    double slat = sin(lat * DEGREES_TO_RADIANS);
    double clon = cos(lon * DEGREES_TO_RADIANS);
    double slon = sin(lon * DEGREES_TO_RADIANS);
    double dx = x - xr;
    double dy = y - yr;
    double dz = z - zr;

    *e = -slon*dx  + clon*dy;
    *n = -slat*clon*dx - slat*slon*dy + clat*dz;
    *u = clat*clon*dx + clat*slon*dy + slat*dz;
}

Answer 1

The latLonToEcef method is an implementation of the algorithm outlined in the Geographic coordinate conversion - From geodetic to ECEF coordinates wikipedia page:

where

Φ is latitude, λ is longitude, and

Likewise the ecefToEnu method is an implementation of the ECEF to ENU algorithm:

If you need further references, they can be found at the bottom of that Wikipedia page. You might also refer to the World Geodetic System 1984 spec.