I would like to measure the position of a soccer player
on a field with an as high as possible accurancy.
Will RF signal strength measuring work or is there another good
way to do it (not too expensive)?
Thanks.
Especially if you want the ball to continue to have the regulation qualities
What's the ball got to do with it?
To the OP: no, there is no simple, cheap way to do it.
Example: only this week, the owner of Hawk-Eye, used at Wimbledon and in test match cricket, put the company up for sale.
The owner's surname?
Getty. 
Will RF signal strength measuring work
As stated above no.
The best way is to have several TV cameras and image processing software but that is expensive.
The cheap way (if you have enough man power) is to assign one observer on each player and have him record the position of the player on a touch screen or graphics tablet.
Thanks for the answers so far.
I realize it is not that easy but I thought of a solution
that might work in my opinion.
What if there are 4 poles around the field containing RF transmitters
and UltraSound transmitters. The player wears a RF receiver and UltraSound receiver. The RF signal speed is almost endless so the travel time is almost equal to zero.
From the poles I send an RF signal and an US signal at the same time.
On the player I start measuring when the RF comes in and stop the timer if the US comes in.
The speed of sound is 343 m/s. The clockrate of the Arduino is 16mHz. Sounds like a solution of what am i missing?
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.
I would like to measure the position of a soccer player
Is it only one player? If so how about a variation on what Mike suggested.
Three people strategically placed around the field, each has a pointer of some kind that is calibrated so you know the angle they are pointing, then they all follow the player and you triangulate the readings.
Works for the whole team too, but you'd need a sh*t load of people and pointers 
Sounds like a solution of what am i missing?
On the player I start measuring when the RF comes in and stop the timer if the US comes in.
Is the problem, ultra sound receivers are not unidirectional, also ultra sound does not travel that far in air, well not without a large loss, especially over the large distance of a football field.
Translators note:- for football anyone in the U.S. please replace with soccer.
Incidentally Ultra Sound were a rock group that split up in about 2001, they have reformed and play there first gig in Leeds this Friday (10/09/10 UK format), I have just received tickets to see them. This has nothing to do with this thread.