Yes, they do need to be carefully aimed, but the mobile systems use gyros to know “which end is up” as well as where the satellite is. The units adam yax linked to can move the antenna 45 degrees in under a second, so as long as you make a 90 degree turn in anything slower than two seconds, the system can keep up. Normally, airplanes change direction much slower than that so as to not scare the passengers.