DW1000 User Manual
© Decawave Ltd 2017
Version 2.12
Page 212 of 242
9.7 Location schemes
This part of the discussion on operational design choices relates to RTLS location schemes. Some of the ideas
and points discussed may be more generally applicable.
In general to locate a mobile node measurements are needed to be referenced to a number of fixed known
location “anchor” nodes. Typically a minimum of three anchor nodes are needed to locate a mobile node in
two dimensions, while a minimum four non-coplanar anchors are needed to locate a mobile node in three
dimensions. The spacing of anchors nodes in an installation has to be such that four anchors are always in
communication range of the mobile tag no matter where it is within the operating space. The
communication range is dependent on data rate and preamble length, the choice of which is influenced by
the node density requirements and perhaps also power consumption.
There are two general methods of doing location. These are time difference of arrival (TDOA) based location
and time of flight (TOF) based location, the main operational points of each are outlined below. In either
case the calculation of location, combining measurements from multiple anchors, is typically done by a
software functionality called the
central location engine
.
Time of flight location requires two-way communication from the mobile node (tag) to each of the anchor
nodes in its vicinity. Periodic message exchanges are used to measure the round trip delay and hence
calculate the one way flight time between tag and anchor. The TOF times multiplied by the speed of light
(and radio waves) gives the distance between the tag and the anchor. Each distance estimate defines a
spherical surface, centred on the anchor, on which the tag must lie. The tag’s 3D location is yielded by the
intersection of the spheres resulting from TOF measurements to the four anchors.
In time difference of arrival (TDOA) location the mobile tag blinks periodically and the blink message is
received by the anchor nodes in its vicinity. When the anchor nodes have synchronised clocks so that the
arrival time of the blink message at all nodes can be compared, then for each pair of anchors the time
difference in the arrival of the blink message defines a hyperbolic surface on which the sending tag must lie.
The tag’s 3D location is yielded by the intersection of the hyperbolic surfaces defined by the TDOA of the
blink at four pairs of anchors.
For low power RTLS deployments the TDOA scheme has benefits in that the tag needs to only send a single
message in order for it to be located. In contrast in the TOF scheme the tag has to send and receive multiple
messages with multiple anchors, and it needs to know what anchors are in the vicinity so it can address each
of them in turn correctly. TOF does not need synchronised anchors, and may suit the case where a hand
held device calculates its own location as part of a navigation system. The TDOA is a lower power solution as
there are fewer messages involved, and this also suits higher density deployments. The TDOA anchor clock
synchronisation may be achieved via a wired clock distribution. Alternatively there are wireless techniques
for clock synchronisation. Wired synchronisation may suit higher tag densities as it allows anchors to listen
all the time so no tag blinks are missed or collide with the wireless clock sync messages (potentially
disrupting synchronisation). Anchors should be wired for power and also with Ethernet to communicate the
arrival times to the central location engine.
Two-way ranging (TOF) is good for proximity detection and separation alarms, especially when both parties
in the exchange are mobile nodes.
