function [a,b]=bounce(x,y,d,xym)

a=real(xym);
b=imag(xym);

      index = find(d>500); %500 meters is the boundary

            for p = 1:length(index)
                dummy=index(p);
                a(dummy)=x(dummy).*(2.*500-d(dummy))./d(dummy);
                %real(temp(dummy))=a(dummy);
            end

            for p = 1:length(index)
                dummy=index(p);
                b(dummy)=y(dummy).*(2.*500-d(dummy))./d(dummy);
                %imag(xym(dummy))=b(dummy);
            end