The code you show is almost exactly what you describe (except for the <0 instead of <1): if the user doesn't provide an input, it will set t to 50, otherwise it will set t to 50+time.
You could have found this error using the debugger to step through your code.
Note that this is a more common way to do this:
function satposit(time)
if nargin<1
time=0; % define the default value of time
end
t = 50 + time;
s = vt;
end
