Radar guns have what's known as "Instant-On" radar. This keeps the transmitter in "hot standby" mode, ready to be activated by an officer when the target is within 200-300 yards. Practically speaking, you can't really defend yourself against Instant-On radar; if it's been aimed at you, your speed has been measured by the time your detector gives an alert. However, if the radar was targeted on a car ahead of you, a detector with sensitive reception will alert you. High sensitivity is what allows manufacturers to promote a detector as giving Instant-On Protection.