Determining whether an object has entered a certain space

An apparatus for determining whether a first object 203, which may be a drone or UAV, has entered a space 207, possibly restricted airspace. The appparatus includes object position measurer (101 fig.1) and a processor (111 fig.1) determines whether the first object has entered the space based on its position and information defining the space; if the object has encroached on the space a signal is transmitted to an immobiliser. The object position measurer may include a range finder (123 fig.1) and a direction sensor (125 fig.1). The apparatus may be included in a system which includes the immobilizer which comprises a second object, a projectile for carrying the second object and a launcher for launching the projectile towards the first object. The projectile deploys the second object, which may be a net, in the vicinity of the first object for capturing, immobilising or disabling the first object.