A number of transmitting devices are also allowed to operate on the same frequencies used by police radar. These are not considered false signals. For example, the automatic door openers commonly used by business are low powered X band transmitters. They produce signals that appear legitimate to a radar detector and cause it to alarm. Other sources produce "harmonics" signals, ghosts or look alike K or Ka band radar signals which make a radar detector give a false alert. A better radar detector reduces these types of false alarms.