104 lines
4.4 KiB
Mathematica
104 lines
4.4 KiB
Mathematica
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function status = navPartyChk(ndat)
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% This function is called to compute and status the parity bits on GPS word.
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% Based on the flowchart in Figure 2-10 in the 2nd Edition of the GPS-SPS
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% Signal Spec.
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%
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%status = navPartyChk(ndat)
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%
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% Inputs:
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% ndat - an array (1x32) of 32 bits represent a GPS navigation
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% word which is 30 bits plus two previous bits used in
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% the parity calculation (-2 -1 0 1 2 ... 28 29)
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%
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% Outputs:
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% status - the test value which equals EITHER +1 or -1 if parity
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% PASSED or 0 if parity fails. The +1 means bits #1-24
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% of the current word have the correct polarity, while -1
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% means the bits #1-24 of the current word must be
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% inverted.
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%--------------------------------------------------------------------------
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% SoftGNSS v3.0
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%
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% Written by Darius Plausinaitis, Kristin Larson
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%--------------------------------------------------------------------------
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%This program is free software; you can redistribute it and/or
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%modify it under the terms of the GNU General Public License
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%as published by the Free Software Foundation; either version 2
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%of the License, or (at your option) any later version.
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%
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%This program is distributed in the hope that it will be useful,
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%but WITHOUT ANY WARRANTY; without even the implied warranty of
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%MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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%GNU General Public License for more details.
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%
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%You should have received a copy of the GNU General Public License
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%along with this program; if not, write to the Free Software
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%Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
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%USA.
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%--------------------------------------------------------------------------
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% CVS record:
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% $Id: navPartyChk.m,v 1.1.2.5 2006/08/14 11:38:22 dpl Exp $
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% In order to accomplish the exclusive or operation using multiplication
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% this program represents a '0' with a '-1' and a '1' with a '1' so that
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% the exclusive or table holds true for common data operations
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%
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% a b xor a b product
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% -------------- -----------------
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% 0 0 1 -1 -1 1
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% 0 1 0 -1 1 -1
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% 1 0 0 1 -1 -1
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% 1 1 1 1 1 1
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%--- Check if the data bits must be inverted ------------------------------
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if (ndat(2) ~= 1)
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ndat(3:26)= -1 .* ndat(3:26); % Also could just negate
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end
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%--- Calculate 6 parity bits ----------------------------------------------
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% The elements of the ndat array correspond to the bits showed in the table
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% 20-XIV (ICD-200C document) in the following way:
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% The first element in the ndat is the D29* bit and the second - D30*.
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% The elements 3 - 26 are bits d1-d24 in the table.
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% The elements 27 - 32 in the ndat array are the received bits D25-D30.
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% The array "parity" contains the computed D25-D30 (parity) bits.
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parity(1) = ndat(1) * ndat(3) * ndat(4) * ndat(5) * ndat(7) * ...
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ndat(8) * ndat(12) * ndat(13) * ndat(14) * ndat(15) * ...
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ndat(16) * ndat(19) * ndat(20) * ndat(22) * ndat(25);
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parity(2) = ndat(2) * ndat(4) * ndat(5) * ndat(6) * ndat(8) * ...
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ndat(9) * ndat(13) * ndat(14) * ndat(15) * ndat(16) * ...
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ndat(17) * ndat(20) * ndat(21) * ndat(23) * ndat(26);
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parity(3) = ndat(1) * ndat(3) * ndat(5) * ndat(6) * ndat(7) * ...
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ndat(9) * ndat(10) * ndat(14) * ndat(15) * ndat(16) * ...
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ndat(17) * ndat(18) * ndat(21) * ndat(22) * ndat(24);
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parity(4) = ndat(2) * ndat(4) * ndat(6) * ndat(7) * ndat(8) * ...
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ndat(10) * ndat(11) * ndat(15) * ndat(16) * ndat(17) * ...
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ndat(18) * ndat(19) * ndat(22) * ndat(23) * ndat(25);
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parity(5) = ndat(2) * ndat(3) * ndat(5) * ndat(7) * ndat(8) * ...
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ndat(9) * ndat(11) * ndat(12) * ndat(16) * ndat(17) * ...
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ndat(18) * ndat(19) * ndat(20) * ndat(23) * ndat(24) * ...
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ndat(26);
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parity(6) = ndat(1) * ndat(5) * ndat(7) * ndat(8) * ndat(10) * ...
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ndat(11) * ndat(12) * ndat(13) * ndat(15) * ndat(17) * ...
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ndat(21) * ndat(24) * ndat(25) * ndat(26);
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%--- Compare if the received parity is equal the calculated parity --------
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if ((sum(parity == ndat(27:32))) == 6)
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% Parity is OK. Function output is -1 or 1 depending if the data bits
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% must be inverted or not. The "ndat(2)" is D30* bit - the last bit of
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% previous subframe.
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status = -1 * ndat(2);
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else
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% Parity failure
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status = 0;
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end
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