GPS 计算卫星坐标

得到的结果是卫星发射信号时刻坐标，计算的也是卫星发送信号时刻的接收机坐标。
即当前时刻得到的是70ns之前的接收机坐标。当然这70ns也没有什么影响。

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
%Copyright (c) by Kai Borre
%Updated by Darius Plausinaitis, Peter Rinder and Nicolaj Bertelsen
%
% CVS record:
% $Id: satpos.m,v 1.1.2.17 2007/01/30 09:45:12 dpl Exp $

%% 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);
%GPS时间（TOW（transmitTime））在每周六午夜零时重新置零。
%TOW-t_oc>or<half_week = 302400时，需要调整。
    %--- 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);
%规划时间
    %Initial mean motion
    n0  = sqrt(GM / a^3);
%卫星平均角速度
    %Mean motion
    n   = n0 + eph(prn).deltan;
%校正后的卫星平均角速度
    %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);
%M平近点角
    %Initial guess of eccentric anomaly
    E   = M;
%迭代法求偏近点角；初始值为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);
%相对论时钟校正量
    %Calculate the true anomaly
    nu   = atan2(sqrt(1 - eph(prn).e^2) * sin(E), cos(E)-eph(prn).e);
%nu真近点角
    %Compute angle phi
    phi = nu + eph(prn).omega;
    %Reduce phi to between 0 and 360 deg
    phi = rem(phi, 2*gpsPi);
%phi升交点角距
    %Correct argument of latitude
    u = phi + ...
        eph(prn).C_uc * cos(2*phi) + ...
        eph(prn).C_us * sin(2*phi);%二次谐波摄动校正后的升交点角距
    %Correct radius
    r = a * (1 - eph(prn).e*cos(E)) + ...
        eph(prn).C_rc * cos(2*phi) + ...
        eph(prn).C_rs * sin(2*phi);%二次谐波摄动校正后的卫星矢量长度
    %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);%二次谐波摄动校正后的轨道倾角

    %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);
%升交点赤经
    %--- 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);
%卫星在WGS-84坐标系下的坐标

%% 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