function [week,sec,nav]=prnnav(prn,filename)
% -----------------------------------------------------------------------------
% Purpose: Read GPS RINEX Navigation File
%
% Input:   prn         - requested PRN number
%          filename    - GPS navigation file name
% Output:  week(i)     - array of GPS weeks
%          sec(i)      - array of GPS seconds
%          nav(i,1:31) - nav messages
%
%          i=1,...,n   - number of message blocks for prn
%
% Author:  Urs Hugentobler, FESG
% Date  :  19-10-2006
% Changes: 06-10-2007, PS: Check if PRN found
% -----------------------------------------------------------------------------

% read file
data = readnav (filename);

n=0;
time=[0,0];
for i=1:size(data,1)
    if data(i,1) == prn & any(time ~= data(i,2:3))
        n=n+1;
        time   =data(i,2:3);
        week(n)=data(i,2);
        sec(n) =data(i,3);
        nav(n,1:31)=data(i,4:34);
    end;
end;

% Check if PRN found
if exist('nav')==0
    error(['No navigation information found for PRN ',num2str(prn)]);
end