function [X,Y,Z]=get_k(num,k,xs,ys,zs,flag)

%*********************************
% ** %求出k顆可見星的坐標
% ** flag為系統標志 gps:flag = 1;galileo: flag = 2; beidou:flag = 3 
% **********************************

switch flag
    case 1
        starnum = 24;
    case 2
        starnum = 27;
    case 3
        starnum = 2;
end

Xs=reshape(xs',1,starnum);
Ys=reshape(ys',1,starnum);
Zs=reshape(zs',1,starnum);
X1=[];Y1=[];Z1=[];
for i=1:k
    X1=[X1,Xs(1,num(1,i))];
    Y1=[Y1,Ys(1,num(1,i))];
    Z1=[Z1,Zs(1,num(1,i))];
end
X=X1(1,:);
Y=Y1(1,:);
Z=Z1(1,:);