Sounds like a solution of what am i missing?
Ultrasound signals do not reflect off soft surfaces, like people, very well.
They do not reflect well off of curved surfaces, like people, very well.
They do not work well with moving things, like soccer players.
They do not travel long distances, like all the way across a soccer field and back.
You wouldn't know what the ultrasound signal reflected off of. All that you would know, if the previous things weren't problems, was that something was some distance away.
Aside from these trivial issues, it sounds like a great solution.