Aston Martin DB 9 2010 Manual page 104

5.18
In heavy rain or similar adverse conditions, the rear parking
assist sensors may not always be able to accurately measure
distance to close objects. A fully laden vehicle or irregular
obstacles may also cause inaccurate measurements.
The Parking Assist system defaults to OFF when the ignition is set to
ON. The system comes ON when reverse gear is selected, or if the
Parking Assist button (A) is pressed at speeds below 9 mph (15 km/h).
The system will set to OFF when the vehicle moves forwards above 9
mph (15 km/h). The Parking Assist button LED will come ON when
the system is set to ON. The LED will flash if a fault is detected in the
system.
If an obstacle is detected at the front or rear of the vehicle, a series of
beeps will be heard from the front or rear speaker respectively, which
increases in rate as the vehicle nears the obstacle.
The beep becomes a continuous tone when an obstacle is detected
at or within approximately 12 in (300 mm) from the rear or 10 in (250
mm) from the front of the vehicle.