You can also just constantly move it there, but IMO that sometimes looks slightly unprofessional(slightly) so what I like to do is have a location around the area the unit should be in, that is larger than it, and then if the unit isn't in the smaller location, but is in the bigger one, I deselect the unit by giving it to another player then back, then I order it back, I usually also add another "just in case" location that simply moves them back if they somehow manage to get past the second location.

You can use an inverted location exactly the size of the dropship to detect when it moves even a pixel