gps_scan.m

function [gprmc, gpgga, igps] = gps_scan(str)

% Usage:  [gprmc, gpgga, igps] = gps_scan(str)
%
% Scans a GPS string for messages $GPRMC and $GPGGA.
% Examples of such messages:
% $GPRMC,015409,A,4555.9652,N,12333.2382,W,000.0,000.0,190807,017.8,E*66
% $GPGGA,015409,4555.9652,N,12333.2382,W,1,03,3.1,,M,-20.5,M,,*5E
%
%    str           -  GPS character string
%    gprmc, gpgga  -  character arrays containing the information
%                     stored in the respective GPS messages
%    igps          -  indices in the input GPS string of all
%                     occurences of all GPS messages
%
% Uses function strsplit, which is stored in Toolbox/system_tools
% We could also check the checksums at the ends of the GPS messages
% but we are ignoring them for now.
%
% Author: Anna Kelbert, 21 Aug 2007
% 
% Last edited in 2020 by Brady Fry and Anna Kelbert. The following edits 
% to the GNSS NMEA sentence contained in the NGF NIMS binary data file
% format have been implemented. Backward compatibility has been maintained.
%
% NMEA Total Length:
% Because of the limitations inherent in the NIMS system, the NMEA sentences total
% length is limited to 140 bytes.  This causes the end of the GPGGA NMEA sentence
% to be truncated.
%
% NMEA Sentence Differences:
% RMC
% -Added '.00' to the data format
% -Latitude and Longitude increased to 5 decimal places
% -Declination is not provided (Currently calculated in SS/SSM)
% -Two new fields at the end
% 
% GGA
% -Added '.00' to the data format
% -Latitude and Longitude increased to 5 decimal places
% -Sentence truncated
% 
% Old NMEA Sentence Example:
% $GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A
% $GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47
% 
% New NMEA Sentence Example:
% $GPRMC,211510.00,A,4435.51438,N,12317.96682,W,0.073,,170420,,,A,V*12
% $GPGGA,211510.00,4435.51438,N,12317.96682,W,1,06,2.09,105.9,M,-22.6,M
%
% Finally, modified to use the native Matlab R2013a strsplit function
% (functionality was broken with default configuration so edits needed)
% and modified Toolbox/system_tools strsplit for compatibility.

gprmc = {};
nGPRMC = 12;
gpgga = {};
nGPGGA = 12;
igps = {};
good = [];

gpsStart = strfind(str,'$GPRMC');

if isempty(gpsStart)
    disp('The string contains no GPS messages.');
    return;
elseif length(gpsStart) == 1
    % The string contains exactly one GPS message
    gpsstr{1} = str;
else
    gpsstr = {};
    for k = 1:length(gpsStart) - 1
        gpsstr{k} = str(gpsStart(k):gpsStart(k+1)-1);
    end
    gpsstr{k+1} = str(gpsStart(k+1):end);
end


i = 1;
for k = 1:length(gpsstr)
    if length(gpsstr{k}) < 139
        continue
    end
    temp = gpsstr{k}(1:139);
    s = strfind(temp,'$'); 
    if length(s) <= 1
        continue % skip the GPS message: it is not complete
    end
    % GPRMC
    values = strsplit(temp(s(1):s(2)), ',', 'CollapseDelimiters', false);
    if length(values) >= nGPRMC % i.e. no transient errors
        gprmc(i, :) = values(1:nGPRMC);
    else
        continue
    end
    % GPGGA (if exists)
    values = strsplit(temp(s(2):end), ',', 'CollapseDelimiters', false);
    if length(values) >= nGPGGA % i.e. no transient errors
        gpgga(i,:) = values(1:nGPGGA);
    else
        gpgga(i,1:nGPGGA) = {''};
    end
    % go to next iteration
    good = [good; k];
    i = i + 1;
end

% compute the indices of all GPS messages in the GPS string
igps = gpsStart(good);

% compute the indices of all fixed (locked) GPS messages in the GPS string
%ifixed = igps(strcmp(gprmc(:,3),'A'));