function tm = ADtimestep(yitap, yitas, omega, dp, ds)
dp = abs(dp);
ds = abs(ds);
tm1 = (1 + omega) * yitap ./ (dp + omega * yitap);
tm2 = (1 + omega) * yitas ./ (ds + omega * yitas);
tm = min(min(tm1),min(tm2));