capture.m

主要是关于GPS卫星C/A码信号的产生、捕获以及跟踪的S曲线图

%这是捕获伪码和多普勒的程序
%pn是伪码的相位偏移，capture_doppler是捕获到的多普勒偏移
function [pn,capture_doppler]=capture()
%产生本地码并采样
n=1023;
[code,ca] = generate(n);
for j = 1:n
    local_ca((j-1)*40 + 1:j*40) = repmat(ca(j),1,40);
end
%产生接收信号
receive = modulate(421,645);
%搜索伪码和doppler 
fc =10.23e+6;
fs = 40.92e+6;
change1 = 500;
t=0:1/fs:(0.001-1/fs);
correlation1 = zeros(1,21*2046);
hm = 0;
for j = -10:10
    local_wave = sin((fc+j*change1)*2*pi*t);
    for  i = 1:2046        
        k=20*i;
        local_ca_b = local_ca(k+1:1023*40);
        local_ca_a = local_ca(1:k);
        used_ca = [local_ca_b,local_ca_a];              
        local_signal = local_wave.* used_ca;
        hm = hm+1;
        correlation1(hm) = sum(local_signal.*receive);
    end
end
%找最大相关值  
[m,key] = max(abs(correlation1));
I = rem(key,2046)/2;
K = (ceil(key/2046)-11)*500;
figure;
plot(abs(correlation1));
title('搜索伪码和doppler的结果图');
%进一步搜索doppler
pn=I;
local_ca_b = local_ca(pn*40+1:1023*40);
local_ca_a = local_ca(1:pn*40);
used_ca = [local_ca_b,local_ca_a];
change2=10;
hn = 0;
for r=-25:25
    hn = hn+1;
    local_wave = sin((fc+K+r*change2)*2*pi*t);
    local_signal = local_wave.* used_ca;
    correlation2(hn) = sum(local_signal.*receive);
end
%找最大相关值并确定doppler
[x,index] = max(abs(correlation2));
index=index-26;
capture_doppler = K+index*10;