Time of arrival
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.
Compared to the TDOA technique, time of arrival 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 a position circle; data from a third base station is required to resolve the precise position to a single point. Many radiolocation systems, including GPS, use ToA.
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 frequency, hence different speed. Sound ranging to a lightning strike works 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,
Two way ranging is a cooperative method for determining the range between two radio transceiver units. When synchronisation of the oscillators of the involved transmitters is not viable, hence the clocks differ, then applying the measurement as a two ways travel to the receiver and mirrored back to the transmitter compensates for some of the phase differences between the oscillators involved. This concept is applied with the Real-time locating system (RTLS) concept as defined in the international standard ISO/IEC FCD 24730-5.
An introductory description of the concept is given with. In contrast to some faulty explanations, the concept may be applied as well with IEEE 802.15.4aCSS as with IEEE 802.15.4aUWB modulation.
- Information technology -- Real Time Locating Systems (RTLS) -- Part 5: Chirp Spread Spectrum (CSS) at 2,4 GHz air interface
- RTLS For Dummies by Ajay Malik, Wiley New York 2009
- Funknetzwerke daheim