Civil Engineering Reference
In-Depth Information
Figure 6.11 Sapphire UWB tracking system
arrives at its antenna. The extremely wide bandwidth of the UWB pulses
permits the receivers to measure these times-of-arrival (TOA) to sub nano-
second precision.
*
To determine the actual tag position from these measurements, the
Sapphire
DART
Hub/Processor, using calibration data from the
Sapphire DART
UWB
reference tag, determines the differential times-of-arrival (DTOA) between
receiver pairs from these individual receiver measurements and implements an
optimization algorithm to determine the location using a multilateration or
hyperbolic positioning technique (Figure 6.12). In general,
N
receivers provide
N
-1 halves of two-sheeted hyperboloids, which are approximated by hyper-
bolas. In this case, given four receivers, the tag is located along three DTOA
hyperbolas (i.e. DTOA
1-2
, DTOA
1-3,
and DTOA
1-4
) at their intersections. Since
the speed of light is approximately 0.98 feet per nanosecond, these DTOA are
readily converted into the appropriate measurement distances (Multispectral
Solutions, 2007) and the tag location is determined.
The outputs resulting from a UWB tracking system application are provided
from the hub to the client machine in the following format:
GData HeaderH, Gtag #H, GXH, GYH, GZH, GbatteryH,
GtimestampH, GunitHGLFH
“Data Header” represents the tag dimensional information. There are many
expected values for the data header but the one of interest in this research is
R
,
which reflects the 3D calculation for x, y, and z. “tag #” is the tag ID. “X, Y, Z
are
the calculated tag coordinates in feet or meters with respect to a user supplied
origin.
”
“
”
is the tag's low battery indicator (range value 0-15, where 15
represents a fully charged battery).
Battery
“
Timestamp
”
represents the hub system time.
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