Time of arrival
Time of Arrival (ToA), also named Time of Flight (ToF), which both means the travel time of a radio signal from a single transmitter to a remote single receiver. By the relation between light speed in vacuum and the carrier frequency of a signal the time is a measure for the distance between transmitter and receiver. However, in some publications the fact is ignored, that this relation is well defined for vacuum, but is different for all other material when radio waves pass through.
Similar to the TDOA technique, this TOA or Time of arrival called technology only differs in the fact that it uses the absolute time of arrival at a certain base station rather than the measured time difference between departing from one and arriving at the other station. The distance can be directly calculated from the time of arrival as signals travel with a known velocity. Time of arrival data from two base stations will narrow a position to two circles and data from a third base station is required to resolve the precise position with the third circle when matching in a single point. There are many ToA-based localization systems, including GPS.
Ways of synchronization
As with TDOA, synchronization of the network base station with the locating reference stations is important. This synchronization can be done in different ways:
- With exact synchronous clock on both sides. Inaccuracy in the clock synchronization translates directly to an imprecise location.
- With two signals which have different frequencies and hence spreading speed. Distance to a lightning strike can be measured in this way (speed of light and sound velocity).
- Via measurement to or triggering from a common reference point.
- Without direct synchronisation, but with compensation of clock phase differences, see Two Way Ranging