Time is a scarce and valuable resource in our modern world. We are all used to synchronising our clocks with the radio time signal, or the clock on TV. However, this is not very accurate: we are always late because of the delay taken to send the signal across the radio or TV channel. Time Transfer is the art and science of synchronising time between two different places so that the clocks are identical even though the signal used to communicate time takes time to travel between them.

There are a surprising number of applications that rely on having accurate time:

● Mobile cellular telephone networks accurately synchronise time in the base stations

● Financial transactions are timed to extremely high precision

● Sport - although most sporting events take place in a local area, so true time transfer may not be required

● Defence and space - many defence and space applications rely on extremely high levels of time synchronisation

● Astronomical observations

● Navigation

Amongst techniques for accurate time transfer and synchronization, GPS is one of the most widely used.

 

The Omnisense S500 System includes the capability of accurate time transfer between nodes in the network. This can be achieved to very high accuracy and, in the S534, node a physical clock pulse can be generated.

Each S500 node operates from its own asynchronous clock. Nodes measure the time of arrival of signals from neighbours using their own local clocks as a reference.

When a node broadcasts its navigation signal (chirp), the data message contains information about itself including its estimated position, the time of the transmission based on its own clock and a subset of its neighbour observations.

Within the Omnisense JTLE (Joint Timing and Location Engine) sophisticated algorithms are used to track clock offsets between nodes and to compute the positions of the nodes.

In this way a highly accurate absolute time synchronised soft clock is established in each node.

Each node, therefore, has a free running unsynchronised local clock based on its internal low cost crystal oscillator, and a very accurate synchronised soft clock.

The soft clock may be used to generate a physical clock pulse (typically a pulse-per-second output similar to GPS). A low resolution pulse is available on the present S534 product. A very high precision pulse may be available in the future if applications demand.

● Internal Soft Clock accurate to 5 ns

● Low resolution pulse on S534 to 100 μs

● Future high resolution pulse to 10 ns

The S500 system can be slaved to GPS time using a 1PPS time pulse from a GPS time transfer receiver.

 

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