ToA (Time of Arrival) is a one-way radio transmission comprising transmitters, receivers and absolute time measurements. The latter measures the absolute time of when the signal is being sent and when the signal reaches the receiver. Stringent timing and synchronization requirements must be met among all transmitters and receivers. It requires three spatially distributed transmitters to calculate the position of a portable receiver, for example GPS relies on satellites as transmitters.
Systems based on TDoA (Time Difference of Arrival) measure the time differences between two receivers. To determine a location, the method requires three receivers for 2D positioning or four for 3D positioning. This method is also used in the RedFIR® technology.
The RTT (Round Trip Time) describes the time required for a signal to travel from one participant to another and back again. This way all transceivers know the distance to the other participants. A computer uses the data of the entire radio network to calculate individual positions.
Together with choosing the method appropriate for the individual case, propagation time measurement factors in environmental impacts. For example, real effects like multipath propagation caused by reflections off larger metal surfaces, like car bodies. They have to be addressed adequately. To ensure this the system is designed appropriately to distinguish distortion from the direct signal (LOS Line Of Sight).
Fraunhofer IIS finds the best balance between precision, range, degree of miniaturization and system cost suited to the individual application.