The Npas mandate states that the aircraft will be at 126mph in the task direction 2 minutes after taskacceptance. This was how the deal was sold to Cambs police authority to lose their aircraft (google Cambs PA pdf).
I prefer direct clickable links and quotes
http://www.cambs-pa.gov.uk/user_file...ept%202011.pdf
The 20 minute flying time is calculated by including:
ground speed of the aircraft of 120 knots (2.3 miles per minute) and;
2 minutes to allow the aircraft to get airborne from the base.