Series 500 does not require fixed readers
All nodes communicate with one another
Position calculation can be distributed
Mobile and fixed nodes are the same
System can be dynamically reconfigured
Fully meshed, including all nodes
Entire system is wireless
Relative positioning, also called collaborative positioning or cluster positioning is a novel technique in which all nodes in a wireless sensor network are capable of communicating with any neighbours within radio range.
Each device in the network periodically broadcasts a positioning message to all neighbours within range, and also receives and measures messages transmitted by neighbours. By using payload information contained in neighbour broadcasts along with accurately measured TOA (time-of-arrival) for the signal the device computes its position relative to its neighbours in the network.
The S500 system can be operated in a completely relative mode, in which case the output positions are not absolute. However, it is more usual to add context of the environment in which the system is operating by associating some of the nodes with features of the environment itself. For example, some of the nodes might be attached to the physical buildings making up the region into which the system is deployed.
These “fixed” nodes are no different from normal mobile ones, the difference is achieved by assigning each node a “personality” which in the case of fixed ones might be a known position.
Further flexibility is achieved by nodes adapting when and how often they transmit or listen to signals from neighbours. In this was they can fulfil different roles in the network, optimised to the individual requirements of the moment.
Conventional systems required fixed anchors
Tags only communicate with readers
Position calculation in central server
Readers and tag are physically different
System based on fixed deployment, inflexible
Limited or no meshing of radio network
Usually requires wired connection for readers
When setting up a system there are a number of factors to take into account:
● Number of visible neighbours: at least 4 for 3D positioning, but benefit in having 6 up to 16.
● Geometry of neighbours: good geometry gives better results. Use more "fixed" nodes to ensure good geometry at all times
● Every radio transmission consumes power which impacts battery life, but more measurements mean better accuracy.
● Larger networks with more devices require more messages which may affect the fastest update rate achievable.
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