  Prediction of the shocks' arrival times (SATs) at the Earth is very important
for space weather forecast. There is a well-known SAT model, STOA, which is
widely used in the space weather forecast. However, the shock transit time from
STOA model usually has a relative large error compared to the real
measurements. In addition, STOA tends to yield too much `yes' prediction, which
causes a large number of false alarms. Therefore, in this work, we work on the
modification of STOA model. First, we give a new method to calculate the shock
transit time by modifying the way to use the solar wind speed in STOA model.
Second, we develop new criteria for deciding whether the shock will arrive at
the Earth with the help of the sunspot numbers and the angle distances of the
flare events. It is shown that our work can improve the SATs prediction
significantly, especially the prediction of flare events without shocks
arriving at the Earth.
