function [dphi, dlambda, h] = togeod(a, finv, X, Y, Z)
% TOGEOD   Subroutine to calculate geodetic coordinates latitude, longitude,
%         height given Cartesian coordinates X,Y,Z, and reference ellipsoid
%         values semi-major axis (a) and the inverse of flattening (finv).
%
% [dphi, dlambda, h] = togeod(a, finv, X, Y, Z);
%
%  The units of linear parameters X,Y,Z,a must all agree (m,km,mi,ft,..etc)
%  The output units of angular quantities will be in decimal degrees
%  (15.5 degrees not 15 deg 30 min). The output units of h will be the
%  same as the units of X,Y,Z,a.
%
%   Inputs:
%       a           - semi-major axis of the reference ellipsoid
%       finv        - inverse of flattening of the reference ellipsoid
%       X,Y,Z       - Cartesian coordinates
%
%   Outputs:
%       dphi        - latitude
%       dlambda     - longitude
%       h           - height above reference ellipsoid

% GNSS-SDR is a Global Navigation Satellite System software-defined receiver.
% This file is part of GNSS-SDR.
%
% SPDX-FileCopyrightText: 1987 C. Goad, 1996 Kai Borre
% SPDX-License-Identifier: GPL-3.0-or-later
%==========================================================================

h       = 0;
tolsq   = 1.e-10;
maxit   = 10;

% compute radians-to-degree factor
rtd     = 180/pi;

% compute square of eccentricity
if finv < 1.e-20
    esq = 0;
else
    esq = (2 - 1/finv) / finv;
end

oneesq  = 1 - esq;

% first guess
% P is distance from spin axis
P = sqrt(X^2+Y^2);
% direct calculation of longitude

if P > 1.e-20
    dlambda = atan2(Y,X) * rtd;
else
    dlambda = 0;
end

if (dlambda < 0)
    dlambda = dlambda + 360;
end

% r is distance from origin (0,0,0)
r = sqrt(P^2 + Z^2);

if r > 1.e-20
    sinphi = Z/r;
else
    sinphi = 0;
end

dphi = asin(sinphi);

% initial value of height  =  distance from origin minus
% approximate distance from origin to surface of ellipsoid
if r < 1.e-20
    h = 0;
    return
end

h = r - a*(1-sinphi*sinphi/finv);

% iterate
for i = 1:maxit
    sinphi  = sin(dphi);
    cosphi  = cos(dphi);

    % compute radius of curvature in prime vertical direction
    N_phi   = a/sqrt(1-esq*sinphi*sinphi);

    % compute residuals in P and Z
    dP      = P - (N_phi + h) * cosphi;
    dZ      = Z - (N_phi*oneesq + h) * sinphi;

    % update height and latitude
    h       = h + (sinphi*dZ + cosphi*dP);
    dphi    = dphi + (cosphi*dZ - sinphi*dP)/(N_phi + h);

    % test for convergence
    if (dP*dP + dZ*dZ < tolsq)
        break;
    end

    % Not Converged--Warn user
    if i == maxit
        fprintf([' Problem in TOGEOD, did not converge in %2.0f',...
            ' iterations\n'], i);
    end
end % for i = 1:maxit

dphi = dphi * rtd;
%%%%%%%% end togeod.m  %%%%%%%%%%%%%%%%%%%%%%
