As elandel says, there is no special hardware for working this out.
Basically - you are taught a way (by CP!!) that is really easy:
1. Find an a/c on the final approach at the 10-15 mile mark.
2. Then add up in 5's - as a 3 mile spaced aircraft will catch up one mile going downwind to base (and reducing speed to 180) and one mile on the closing hdg (and speed back to 160/170)
3. Obviously, if there are a lot of heavies then they need 4 miles so some add up in 6's then. But if there is a mix, it is easier to just add up in 5's!! (my 5x table is better than addin a few 4's, a couple of 3's, a 5 and a six!!!!)
Interstingly, a lot of the experienced controllers went to the Air Traffic Management Development Centre recently to conduct a trial with a new computer which works out miles and helps get the aircraft off the stacks at the right time. EVERYONE was proved to be UNDERESTIMATING this range from touchdown! Still at trial stage though!!!
Hope the range is given correctly, as CDA's are becoming increasingly important at LHR.
Ranges should be given when handed over from No 1 Director (119.72/134.97) which is an estimate as the No 2 will fine tune and will give a more accurate range on first contact. That is why they may be a bit different before you ask!!!
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B-L