GPS导航电文 IODC
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function [eph, TOW] = ephemeris(bits, D30Star)
%Function decodes ephemerides and TOW from the given bit stream. The stream
%(array) in the parameter BITS must contain 1500 bits. The first element in
%the array must be the first bit of a subframe. The subframe ID of the
%first subframe in the array is not important.
%
%Function does not check parity!
%
%[eph, TOW] = ephemeris(bits, D30Star)
%
% Inputs:
% bits - bits of the navigation messages (5 subframes).
% Type is character array and it must contain only
% characters '0' or '1'.
% D30Star - The last bit of the previous nav-word. Refer to the
% GPS interface control document ICD (IS-GPS-200D) for
% more details on the parity checking. Parameter type is
% char. It must contain only characters '0' or '1'.
% Outputs:
% TOW - Time Of Week (TOW) of the first sub-frame in the bit
% stream (in seconds)
% eph - SV ephemeris
%--------------------------------------------------------------------------
% SoftGNSS v3.0
%
% Copyright (C) Darius Plausinaitis and Kristin Larson
% Written by Darius Plausinaitis and Kristin Larson
%--------------------------------------------------------------------------
%This program is free software; you can redistribute it and/or
%modify it under the terms of the GNU General Public License
%as published by the Free Software Foundation; either version 2
%of the License, or (at your option) any later version.
%
%This program is distributed in the hope that it will be useful,
%but WITHOUT ANY WARRANTY; without even the implied warranty of
%MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
%GNU General Public License for more details.
%
%You should have received a copy of the GNU General Public License
%along with this program; if not, write to the Free Software
%Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
%USA.
%--------------------------------------------------------------------------
%CVS record:
%$Id: ephemeris.m,v 1.1.2.7 2006/08/14 11:38:22 dpl Exp $
%% Check if there is enough data ==========================================
if length(bits) < 1500
error('The parameter BITS must contain 1500 bits!');
end
%% Check if the parameters are strings ====================================
if ~ischar(bits)
error('The parameter BITS must be a character array!');
end
if ~ischar(D30Star)
error('The parameter D30Star must be a char!');
end
% Pi used in the GPS coordinate system
gpsPi = 3.1415926535898;
%% Decode all 5 sub-frames ================================================
for i = 1:5%5个子帧，每个子帧300比特
%--- "Cut" one sub-frame's bits ---------------------------------------
subframe = bits(300*(i-1)+1 : 300*i);
%--- Correct polarity of the data bits in all 10 words ----------------
for j = 1:10%每个子帧10个字，每个字30比特
[subframe(30*(j-1)+1 : 30*j)] = ...
checkPhase(subframe(30*(j-1)+1 : 30*j), D30Star);
D30Star = subframe(30*j);
end
%收到的D,要恢复d,d为原始信息比特
%--- Decode the sub-frame id ------------------------------------------
% For more details on sub-frame contents please refer to GPS IS.
subframeID = bin2dec(subframe(50:52));%交接字HOW的第20-22比特是子帧识别标志
%其共有5个有效二进制值，001表示该子帧是第一子帧，以此类推。。。
%--- Decode sub-frame based on the sub-frames id ----------------------
% The task is to select the necessary bits and convert them to decimal
% numbers. For more details on sub-frame contents please refer to GPS
% ICD (IS-GPS-200D).
switch subframeID
case 1 %--- It is subframe 1 -------------------------------------
% It contains WN, SV clock corrections, health and accuracy
eph.weekNumber = bin2dec(subframe(61:70)) + 1024;
%每一子帧的第三个字的前10位，即Z计数器的高10位，指示当前GPS星期
%因为星期数最大为2……10-1=1023，其计时起点可能不是这次星期数循环
%的起点，所以就加1024了
eph.accuracy = bin2dec(subframe(73:76));
%用户测距精度，是对有GPS地面监控部分和空间星座部分引起的误差的一个
%统计值，值越大表示测距值精度越低。N=15时，此时用户要自己承担使用该卫星的风险
eph.health = bin2dec(subframe(77:82));
%卫星健康状况，最高位为0表示完全正确，为1表示出错了，低5位指示出错情况
eph.T_GD = twosComp2dec(subframe(197:204)) * 2^(-31);
%T_GD群波延时校正值
eph.IODC = bin2dec([subframe(83:84) subframe(197:204)]);
%IODC时钟数据期号。一个IODC值对应一套时钟校正参数
%IODC至少7天不会重复，是连续变化的。依据IODC值是否变化，判断卫星是否更新时钟校正参数；
%若没变化，则不必每30s去重复读解这一数据块中的时钟校正参数
eph.t_oc = bin2dec(subframe(219:234)) * 2^4;
%t_oc是第一数据块的参考时间，它在时钟校正模型中被用作时间参考点
eph.a_f2 = twosComp2dec(subframe(241:248)) * 2^(-55);
eph.a_f1 = twosComp2dec(subframe(249:264)) * 2^(-43);
eph.a_f0 = twosComp2dec(subframe(271:292)) * 2^(-31);
%a_f0f1f2是卫星时钟校正模型方程中的三个系数
case 2 %--- It is subframe 2 -------------------------------------
% It contains first part of ephemeris parameters
eph.IODE_sf2 = bin2dec(subframe(61:68));
%星历数据期号，6小时之内不会出现重复类似于上面IODC
eph.C_rs = twosComp2dec(subframe(69: 84)) * 2^(-5);
%轨道半径的正弦调和修正项的幅度
eph.deltan = ...
twosComp2dec(subframe(91:106)) * 2^(-43) * gpsPi;
%计算值的平均移动误差
eph.M_0 = ...
twosComp2dec([subframe(107:114) subframe(121:144)]) ...
* 2^(-31) * gpsPi;
%t_oe时的平近地点
eph.C_uc = twosComp2dec(subframe(151:166)) * 2^(-29);
%升交点角距余弦调和校正振幅
eph.e = ...
bin2dec([subframe(167:174) subframe(181:204)]) ...
* 2^(-33);
%轨道偏心率
eph.C_us = twosComp2dec(subframe(211:226)) * 2^(-29);
%升交点角距正弦调和校正振幅
eph.sqrtA = ...
bin2dec([subframe(227:234) subframe(241:264)]) ...
* 2^(-19);
%轨道长半轴的平方根
eph.t_oe = bin2dec(subframe(271:286)) * 2^4;
%星历参考时间
case 3 %--- It is subframe 3 -------------------------------------
% It contains second part of ephemeris parameters
eph.C_ic = twosComp2dec(subframe(61:76)) * 2^(-29);
%倾斜角的余弦调和修正项的幅度
eph.omega_0 = ...
twosComp2dec([subframe(77:84) subframe(91:114)]) ...
* 2^(-31) * gpsPi;
%在每星期历元轨道平面上的升点经度
eph.C_is = twosComp2dec(subframe(121:136)) * 2^(-29);
%倾斜角的正弦调和修正项的幅度
eph.i_0 = ...
twosComp2dec([subframe(137:144) subframe(151:174)]) ...
* 2^(-31) * gpsPi;
%参考时间的轨道倾角
eph.C_rc = twosComp2dec(subframe(181:196)) * 2^(-5);
%轨道半径的正弦调和修正项的幅度
eph.omega = ...
twosComp2dec([subframe(197:204) subframe(211:234)]) ...
* 2^(-31) * gpsPi;
%近地点的辐角
eph.omegaDot = twosComp2dec(subframe(241:264)) * 2^(-43) * gpsPi;
%赤经的速率
eph.IODE_sf3 = bin2dec(subframe(271:278));
%数据与星历发布号
eph.iDot = twosComp2dec(subframe(279:292)) * 2^(-43) * gpsPi;
%倾斜角的速率
case 4 %--- It is subframe 4 -------------------------------------
% Almanac, ionospheric model, UTC parameters.
% SV health (PRN: 25-32).
% Not decoded at the moment.
case 5 %--- It is subframe 5 -------------------------------------
% SV almanac and health (PRN: 1-24).
% Almanac reference week number and time.
% Not decoded at the moment.
end % switch subframeID ...
end % for all 5 sub-frames ...
%% Compute the time of week (TOW) of the first sub-frames in the array ====
% Also correct the TOW. The transmitted TOW is actual TOW of the next
% subframe and we need the TOW of the first subframe in this data block
% (the variable subframe at this point contains bits of the last subframe).
TOW = bin2dec(subframe(31:47)) * 6 - 30;
%乘以6得到下一子帧起始的GPS时间，减6是当前子帧的GPS时间，减30为啥阿阿
%我知道了。这里得到TOW用的子帧是第五个子帧，所以得到的下一子帧起始时间是第6子帧的
%起始时间，-30s,是得到接收到的第一子帧的开始时间。
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