Alexander,<br /><br />The liftoff time makes not a single difference to the TLE. The TLE is computated using SGP4 using X, Y, Z and X dot, Y dot, Z dot and epoch time of the state vector.<br /><br />Then the TLE is calculated with the spacecraft drag co-efficient. The launch time then comes into play simply calculating the REV number.<br /><br />Simply explained the state vector is simply a Earth Centered Inertia or Earth Fixed Greenwich 'snap shot' of absolute velocities and heights at a given instant with reference to the earth, there is NO reference to REV number.<br /><br />So even a minute inaccurancy in the launch time will not effect a object orbiting every 80 to 90 minutes.<br /><br />I guarantee our vectors from the cape used to generate TLEs are within 30 meters of the real position (depending on your clock accurancy and age of the TLE), infact quite often the cape send us differences which we may trim or increase the drag co-efficient. The differences can only be seen by using a tracking station and comparing the downlinked telemetry from the shuttle.<br /><br />Hope this helps you?