function [satPositions, satClkCorr] = satpos(transmitTime, prnList, ...
    eph, settings)
% SATPOS Computation of satellite coordinates X,Y,Z at TRANSMITTIME for
% given ephemeris EPH. Coordinates are computed for each satellite in the
% list PRNLIST.
%[ satPositions, satClkCorr] = satpos(transmitTime, prnList, eph, settings);
%
%   Inputs:
%       transmitTime  - transmission time
%       prnList       - list of PRN-s to be processed
%       eph           - ephemerides of satellites
%       settings      - receiver settings
%
%   Outputs:
%       satPositions  - position of satellites (in ECEF system [X; Y; Z;])
%       satClkCorr    - correction of satellite clocks

%--------------------------------------------------------------------------
%                           SoftGNSS v3.0
%--------------------------------------------------------------------------
% Based on Kai Borre 04-09-96
% Updated by Darius Plausinaitis, Peter Rinder and Nicolaj Bertelsen
%
% GNSS-SDR is a Global Navigation Satellite System software-defined receiver.
% This file is part of GNSS-SDR.
%
% SPDX-FileCopyrightText: Kai Borre
% SPDX-License-Identifier: GPL-3.0-or-later

%% Initialize constants ===================================================
numOfSatellites = size(prnList, 2);

% GPS constatns

gpsPi          = 3.1415926535898;  % Pi used in the GPS coordinate
% system

%--- Constants for satellite position calculation -------------------------
Omegae_dot     = 7.2921151467e-5;  % Earth rotation rate, [rad/s]
GM             = 3.986005e14;      % Universal gravitational constant times
% the mass of the Earth, [m^3/s^2]
F              = -4.442807633e-10; % Constant, [sec/(meter)^(1/2)]

%% Initialize results =====================================================
satClkCorr   = zeros(1, numOfSatellites);
satPositions = zeros(3, numOfSatellites);

%% Process each satellite =================================================

for satNr = 1 : numOfSatellites

    prn = prnList(satNr);

    %% Find initial satellite clock correction --------------------------------

    %--- Find time difference ---------------------------------------------
    dt = check_t(transmitTime - eph(prn).t_oc);

    %--- Calculate clock correction ---------------------------------------
    satClkCorr(satNr) = (eph(prn).a_f2 * dt + eph(prn).a_f1) * dt + ...
        eph(prn).a_f0 - ...
        eph(prn).T_GD;

    time = transmitTime - satClkCorr(satNr);

    %% Find satellite's position ----------------------------------------------

    %Restore semi-major axis
    a   = eph(prn).sqrtA * eph(prn).sqrtA;

    %Time correction
    tk  = check_t(time - eph(prn).t_oe);

    %Initial mean motion
    n0  = sqrt(GM / a^3);
    %Mean motion
    n   = n0 + eph(prn).deltan;

    %Mean anomaly
    M   = eph(prn).M_0 + n * tk;
    %Reduce mean anomaly to between 0 and 360 deg
    M   = rem(M + 2*gpsPi, 2*gpsPi);

    %Initial guess of eccentric anomaly
    E   = M;

    %--- Iteratively compute eccentric anomaly ----------------------------
    for ii = 1:10
        E_old   = E;
        E       = M + eph(prn).e * sin(E);
        dE      = rem(E - E_old, 2*gpsPi);

        if abs(dE) < 1.e-12
            % Necessary precision is reached, exit from the loop
            break;
        end
    end

    %Reduce eccentric anomaly to between 0 and 360 deg
    E   = rem(E + 2*gpsPi, 2*gpsPi);

    %Compute relativistic correction term
    dtr = F * eph(prn).e * eph(prn).sqrtA * sin(E);

    %Calculate the true anomaly
    nu   = atan2(sqrt(1 - eph(prn).e^2) * sin(E), cos(E)-eph(prn).e);

    %Compute angle phi
    phi = nu + eph(prn).omega;
    %Reduce phi to between 0 and 360 deg
    phi = rem(phi, 2*gpsPi);

    %Correct argument of latitude
    u = phi + ...
        eph(prn).C_uc * cos(2*phi) + ...
        eph(prn).C_us * sin(2*phi);
    %Correct radius
    r = a * (1 - eph(prn).e*cos(E)) + ...
        eph(prn).C_rc * cos(2*phi) + ...
        eph(prn).C_rs * sin(2*phi);
    %Correct inclination
    i = eph(prn).i_0 + eph(prn).iDot * tk + ...
        eph(prn).C_ic * cos(2*phi) + ...
        eph(prn).C_is * sin(2*phi);

    %Compute the angle between the ascending node and the Greenwich meridian
    Omega = eph(prn).omega_0 + (eph(prn).omegaDot - Omegae_dot)*tk - ...
        Omegae_dot * eph(prn).t_oe;
    %Reduce to between 0 and 360 deg
    Omega = rem(Omega + 2*gpsPi, 2*gpsPi);

    %--- Compute satellite coordinates ------------------------------------
    satPositions(1, satNr) = cos(u)*r * cos(Omega) - sin(u)*r * cos(i)*sin(Omega);
    satPositions(2, satNr) = cos(u)*r * sin(Omega) + sin(u)*r * cos(i)*cos(Omega);
    satPositions(3, satNr) = sin(u)*r * sin(i);


    %% Include relativistic correction in clock correction --------------------
    satClkCorr(satNr) = (eph(prn).a_f2 * dt + eph(prn).a_f1) * dt + ...
        eph(prn).a_f0 - ...
        eph(prn).T_GD + dtr;

end % for satNr = 1 : numOfSatellites
