UWB works like this: most of the time, the UWB chip ('U1') is 'off' because it draws a lot more energy than BLE does. BLE's signal strength gives a measure of

UWB works like this: most of the time, the UWB chip ('U1') is 'off' because it draws a lot more energy than BLE does. BLE's signal strength gives a measure of distance -- but generally BLE uses 1 antenna, so what you get is a signal strength for some unit within a radius of a sphere (again, approximately). When the BLE signal strength is considered 'strong enough', the iPhone commands the BLE to send a 'turn on the UWB in the tag' command. Now, the tag sends a UWB burst. Like this: https://media.nxp.com/static-files/34261e70-6908-4e13-8eec-8361dff859a3 https://media.nxp.com/static-files/34261e70-6908-4e13-8eec-8... The time-of-flight is measured, and the precise distance is now known. More detail is in this presentation https://media.nxp.com/static-files/8cf4341d-f16d-4971-9b92-c003c39bfa22 https://media.nxp.com/static-files/8cf4341d-f16d-4971-9b92-c... Further, UWB signal strength helps determine the direction. Above is the FiRa Consortium's literature. NXP makes a similar chip to Apple's "U1" UWB chip -- however, sigh, the standards are NOT compatible. Yes, they are both UWB and use the FCC rules for UWB RF requirements (frequencies, power, etc) -- but for reasons I do not understand, they are different elsewhere (Protocols? I don't know).