That is why you must point the remote control toward the TV and make sure it does not get blocked by the book on the table. Also, under certain circumstances involving direct sunlight or reflections, infrared systems can be highly directional and even inoperative. A woman entering a parking garage could use the transmitter to activate the surveillance cameras and intercom. She could then request the security personnel watch her get to her vehicle safely. In this case, the action is not taken as an emergency, just a normal remote escort procedure.
Ultrasonic signals are merely rapid changes in air pressure.
Ultrasonic devices do not require government licenses as do some RF devices. The modulated signal used in the PPi PAS system nearly eliminates interference from all possible sources. Ultrasonic signals will not penetrate walls. All the transmitters and all the receivers work on the same frequencies. The monitoring center knows exactly where to direct the response.