CC2431
5.2 Nodes
The CC2431 Location Engine uses the RSSI value combined with the physical location of the
Reference Nodes to calculate its own position.
Any number of Reference Nodes can be used in the system, but a node can only calculate its
position if it is within range of at least three Reference Nodes
1
.
Experiments have shown that one Reference Node for each 100 m
2
gives good location
estimates. The nodes should be placed in a grid with one node for each 10 meters in both
directions.
5.2.1 Reference
Node
A node which has static location is called a Reference Node. This node must be configured
with an X and a Y value that correspond to the physical location.
The main task for a Reference Node is to provide “reference” packets to the Blind Node.
Reference packets contain the X and Y coordinates of the Reference Node.
5.2.2 Blind
Node
A Blind Node will communicate with its nearest Reference Nodes, collecting X, Y and RSSI
values for each of these nodes. Then it uses the location engine hardware to calculate its
position based on the collected parameters from several Reference Nodes.
5.2.3 Dongle
The Dongle will communicate with the entire network; it can request or configure the X,Y
values of all Reference Nodes and the A and N values of the Blind Nodes via the Z-location
Engine PC Application. The Z-location Engine can also configure any Blind Node to
automatically make a periodic position calculation and report (by default the Blind Node is
waiting for a command to perform a position calculation.)
5.3 RSSI
The RSSI value is typically in the range -40 dBm to -90 dBm, where -40 dBm is the highest
value. -40 dBm is approximately the measured signal strength on distance of one meter. Input
to the location engine hardware is the absolute value of the RSSI in dBm, so the range will
typically be from 40 to 90, where 40 is the highest signal strength.
1
Three is a lower theoretical limit, for a good estimate three Reference Nodes may be too
few.
SWRU076D
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