167 lines
7.1 KiB
Mathematica
167 lines
7.1 KiB
Mathematica
|
function [firstSubFrame, activeChnList] =TrackResults2NavBits(trackResults, ...
|
||
|
|
settings)
|
||
|
|
% findPreambles finds the first preamble occurrence in the bit stream of
|
||
|
|
% each channel. The preamble is verified by check of the spacing between
|
||
|
|
% preambles (6sec) and parity checking of the first two words in a
|
||
|
|
% subframe. At the same time function returns list of channels, that are in
|
||
|
|
% tracking state and with valid preambles in the nav data stream.
|
||
|
|
%
|
||
|
|
%[firstSubFrame, activeChnList] = findPreambles(trackResults, settings)
|
||
|
|
%
|
||
|
|
% Inputs:
|
||
|
|
% trackResults - output from the tracking function
|
||
|
|
% settings - Receiver settings.
|
||
|
|
%
|
||
|
|
% Outputs:
|
||
|
|
% firstSubframe - the array contains positions of the first
|
||
|
|
% preamble in each channel. The position is ms count
|
||
|
|
% since start of tracking. Corresponding value will
|
||
|
|
% be set to 0 if no valid preambles were detected in
|
||
|
|
% the channel.
|
||
|
|
% activeChnList - list of channels containing valid preambles
|
||
|
|
|
||
|
|
%--------------------------------------------------------------------------
|
||
|
|
% SoftGNSS v3.0
|
||
|
|
%
|
||
|
|
% Copyright (C) Darius Plausinaitis, Peter Rinder and Nicolaj Bertelsen
|
||
|
|
% Written by Darius Plausinaitis, Peter Rinder and Nicolaj Bertelsen
|
||
|
|
%--------------------------------------------------------------------------
|
||
|
|
%
|
||
|
|
%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: findPreambles.m,v 1.1.2.10 2006/08/14 11:38:22 dpl Exp $
|
||
|
|
|
||
|
|
% Preamble search can be delayed to a later point in the tracking results
|
||
|
|
% to avoid noise due to tracking loop transients
|
||
|
|
searchStartOffset = 0;
|
||
|
|
|
||
|
|
%--- Initialize the firstSubFrame array -----------------------------------
|
||
|
|
firstSubFrame = zeros(1, settings.numberOfChannels);
|
||
|
|
|
||
|
|
%--- Generate the preamble pattern ----------------------------------------
|
||
|
|
preamble_bits = [1 -1 -1 -1 1 -1 1 1];
|
||
|
|
|
||
|
|
% "Upsample" the preamble - make 20 vales per one bit. The preamble must be
|
||
|
|
% found with precision of a sample.
|
||
|
|
preamble_ms = kron(preamble_bits, ones(1, 20));
|
||
|
|
|
||
|
|
%--- Make a list of channels excluding not tracking channels --------------
|
||
|
|
activeChnList = find([trackResults.status] ~= '-');
|
||
|
|
|
||
|
|
%=== For all tracking channels ...
|
||
|
|
for channelNr = activeChnList
|
||
|
|
|
||
|
|
%% Correlate tracking output with preamble ================================
|
||
|
|
% Read output from tracking. It contains the navigation bits. The start
|
||
|
|
% of record is skiped here to avoid tracking loop transients.
|
||
|
|
bits = trackResults(channelNr).I_P(1 + searchStartOffset : end);
|
||
|
|
|
||
|
|
% Now threshold the output and convert it to -1 and +1
|
||
|
|
bits(bits > 0) = 1;
|
||
|
|
bits(bits <= 0) = -1;
|
||
|
|
|
||
|
|
% Correlate tracking output with the preamble
|
||
|
|
tlmXcorrResult = xcorr(bits, preamble_ms);
|
||
|
|
|
||
|
|
%% Find all starting points off all preamble like patterns ================
|
||
|
|
clear index
|
||
|
|
clear index2
|
||
|
|
|
||
|
|
xcorrLength = (length(tlmXcorrResult) + 1) /2;
|
||
|
|
|
||
|
|
%--- Find at what index/ms the preambles start ------------------------
|
||
|
|
index = find(...
|
||
|
|
abs(tlmXcorrResult(xcorrLength : xcorrLength * 2 - 1)) > 153)' + ...
|
||
|
|
searchStartOffset;
|
||
|
|
|
||
|
|
%% Analyze detected preamble like patterns ================================
|
||
|
|
for i = 1:size(index) % For each occurrence
|
||
|
|
|
||
|
|
%--- Find distances in time between this occurrence and the rest of
|
||
|
|
%preambles like patterns. If the distance is 6000 milliseconds (one
|
||
|
|
%subframe), the do further verifications by validating the parities
|
||
|
|
%of two GPS words
|
||
|
|
|
||
|
|
index2 = index - index(i);
|
||
|
|
|
||
|
|
if (~isempty(find(index2 == 6000)))
|
||
|
|
|
||
|
|
%=== Re-read bit vales for preamble verification ==============
|
||
|
|
% Preamble occurrence is verified by checking the parity of
|
||
|
|
% the first two words in the subframe. Now it is assumed that
|
||
|
|
% bit boundaries a known. Therefore the bit values over 20ms are
|
||
|
|
% combined to increase receiver performance for noisy signals.
|
||
|
|
% in Total 62 bits mast be read :
|
||
|
|
% 2 bits from previous subframe are needed for parity checking;
|
||
|
|
% 60 bits for the first two 30bit words (TLM and HOW words).
|
||
|
|
% The index is pointing at the start of TLM word.
|
||
|
|
bits = trackResults(channelNr).I_P(index(i)-40 : ...
|
||
|
|
index(i) + 20 * 60 -1)';
|
||
|
|
|
||
|
|
%--- Combine the 20 values of each bit ------------------------
|
||
|
|
bits = reshape(bits, 20, (size(bits, 1) / 20));
|
||
|
|
bits = sum(bits);
|
||
|
|
|
||
|
|
% Now threshold and make it -1 and +1
|
||
|
|
bits(bits > 0) = 1;
|
||
|
|
bits(bits <= 0) = -1;
|
||
|
|
|
||
|
|
%--- Check the parity of the TLM and HOW words ----------------
|
||
|
|
if (navPartyChk(bits(1:32)) ~= 0) && ...
|
||
|
|
(navPartyChk(bits(31:62)) ~= 0)
|
||
|
|
% Parity was OK. Record the preamble start position. Skip
|
||
|
|
% the rest of preamble pattern checking for this channel
|
||
|
|
% and process next channel.
|
||
|
|
|
||
|
|
firstSubFrame(channelNr) = index(i);
|
||
|
|
|
||
|
|
%%
|
||
|
|
Nsamp=floor((length(trackResults(channelNr).I_P)-index(i)+40)/20)*20 ;
|
||
|
|
tmpbits = trackResults(channelNr).I_P(index(i)-40 : index(i)-40+Nsamp-1)';
|
||
|
|
|
||
|
|
%--- Combine the 20 values of each bit ------------------------
|
||
|
|
tmpbits = reshape(tmpbits, 20, (size(tmpbits, 1) / 20));
|
||
|
|
tmpbits = sum(tmpbits);
|
||
|
|
|
||
|
|
% Now threshold and make it -1 and +1
|
||
|
|
tmpbits(tmpbits > 0) = '1';
|
||
|
|
tmpbits(tmpbits <= 0) = '0';
|
||
|
|
|
||
|
|
fid=fopen(['NAV\navascii',num2str(trackResults(channelNr).PRN)],'wt');
|
||
|
|
fwrite(fid,tmpbits,'char');
|
||
|
|
fclose(fid);
|
||
|
|
%%
|
||
|
|
break;
|
||
|
|
end % if parity is OK ...
|
||
|
|
|
||
|
|
end % if (~isempty(find(index2 == 6000)))
|
||
|
|
end % for i = 1:size(index)
|
||
|
|
|
||
|
|
% Exclude channel from the active channel list if no valid preamble was
|
||
|
|
% detected
|
||
|
|
if firstSubFrame(channelNr) == 0
|
||
|
|
|
||
|
|
% Exclude channel from further processing. It does not contain any
|
||
|
|
% valid preamble and therefore nothing more can be done for it.
|
||
|
|
activeChnList = setdiff(activeChnList, channelNr);
|
||
|
|
|
||
|
|
disp(['Could not find valid preambles in channel ', ...
|
||
|
|
num2str(channelNr),'!']);
|
||
|
|
end
|
||
|
|
|
||
|
|
end % for channelNr = activeChnList
|