Code example of MATLAB reading GNSS observation value o file

One 、 preparation
Observation data reading is the premise of data processing , Usually , The data formats of observations are rtcm、ubx、rinex, Manufacturers also have custom formats . Data reading is the simplest job , Mastering the data organization format means mastering the data reading strategy , in short , We do a series of operations on strings . I think that's right. o The following preparations should be made for file reading , Namely ：
（1） install Matlab Of PC;
（2) Study rinex.pdf Information ;
（3）o Document preparation ;
（4） Pseudocoding , Clear the data flow ;
（5） Code testing
We are using location methods SPP/PPP/RTK when , It is to read the observation value of the satellite according to the epoch （PR/CP）,Doppler,SNR Etc , The subsequent processing is to cycle through all the satellites , Do different things .
Two 、 Problems to be solved
The problem is ： To achieve the use of Matlab Draw the pseudo range observation value sequence diagram of satellites with different satellite systems in the observation arc .
In the above problem analysis , Easy to know satellite PRN The number is the only one , Can be used as key; The satellite corresponds to the observed value （PC/CP/Doppler）, Can be used as value. In other words ,key and value have access to map As a data structure , among ,value You can use a structure to save . The most important link has been determined , That is to use map and struct.
Next , Let's take the pseudo range value in the observation arc of a satellite , There are several possible situations ：
（1） In the observation arc , The satellite has been captured , namely 100% There is a pseudo range value ;
（2） In the observation arc , The satellite was not captured at the beginning of the epoch , Then they were captured ;
（3） stay （2） There are still uncapped , Capture , Not captured , Capture, etc ;
The observed value is 0, meanwhile , We should also pay attention to , If there is a pseudo range, there is not necessarily a carrier , There must be no pseudo range without carrier . Our pseudo code mainly judges the above three situations .
3、 ... and 、 Code implementation
Specific analysis of specific problems , Solve the problems mentioned above , The specific code is as follows ：

% function: read the body of rinex obs file
clear all
close all
clc

fid=fopen(' example.obs');
if fid==-1
    disp('fail to open obs file!');
end

%  Create structure obs, Use Map mapping ,prn As the key (char),sv Value (table)
obs=containers.Map;
record_counter=0;

%  Read body Record 
while feof(fid)==0
    line=fgets(fid);
    if strcmp(line(1),'>')
        sv_sum=str2double(line(34:35));
        time=line(3:29);
        if sv_sum>0
            for i=1:sv_sum
                line=fgets(fid);
                %  key  -> prn
                key=line(1:3);
                %  value  -> sv(pr cp dop)
                sv.pr=str2double(line(6:17));
                cp=line(21:33);
                if isnan(cp)
                    sv.cp=0;
                else
                    sv.cp=str2double(line(21:33));
                end
                sv.dop=str2double(line(41:49));
                %  Value logic judgment 
                if isKey(obs,key)
                    jump_sec=record_counter-length(obs(key));
                    if jump_sec~=0 %  Solve the problem of relocking after losing the star 
                        temp.pr=0;
                        temp.cp=0;
                        temp.dop=0;
                        tmp_strcut=repmat(temp,1,jump_sec);
                        value=[obs(key) tmp_strcut sv];
                    else %  The satellite has been locked , Add directly 
                        if strcmp(key,'C18')==1
                            if length(obs(key))>5713
                                disp(time);
                            end
                        end
                        value=[repmat(obs(key),1) repmat(sv,1)]; % mark
                    end
                else %  The satellite is locked for the first time 
                    if record_counter~=0 %  The first second is unlocked 
                        temp.pr=0;
                        temp.cp=0;
                        temp.dop=0;
                        tmp_strcut=repmat(temp,1,record_counter);
                        value=[tmp_strcut sv];
                    else %  First second lock 
                        value=sv;
                    end
                end
                obs(key)=value;
            end
        end
        record_counter=record_counter+1;
    end
end
disp('create struct obs(body) successfully!');

%  mapping 
sv_sum=obs.Count; %  The number of visual satellites 
keySet=keys(obs); %  Visual satellite name 
disp(keySet);

v=299792458.0;
bf1=1.561098E9; % bds L1  frequency 
gf1=1.57542E9; % gps L1  frequency 
gps_sum=0;bds_sum=0;

for i=1:sv_sum
    prn=char(keySet(i));
    if strcmp(prn(1),'G')==1 % GPS
        gps_sum=gps_sum+1;
    else if strcmp(prn(1),'C')==1 % BDS
            bds_sum=bds_sum+1;
        end
    end
end
subfig1=ceil(sqrt(bds_sum));
subfig2=ceil(sqrt(gps_sum));

x=1:record_counter;
for i=1:sv_sum
    prn=char(keySet(i));
    info=obs(prn);
    pr=zeros(1,record_counter);cp=pr;
    for j=1:length(info)
        pr(j)=info(j).pr;
        cp(j)=info(j).cp;
    end
    if i==1 || i==bds_sum+1
        figure;
    end
    subplot(n,n,pos);
    plot(x,pr,'r',x,cp*f,'g'); 
    title(char(prn));
end

Four 、 Sample results
I only focus on the problems encountered in my work rinex Version data processing ,gps/bds Dual system , Single frequency data , The results are as follows ：

The sample code is simple , There are also corresponding notes .