Image Processing Reference
In-Depth Information
The RTT method requires a deterministic and constant reaction time to guarantee the accuracy.
Therefore in most systems, the ACK is automatically generated by a specialized Media Access Control
(MAC)-layer without passing higher communication layers [Sta].
6.2.4 Symmetric Double-Sided Two-Way Ranging
The unavoidable clock drift between sender and receiver limits the accuracy of the distance es-
timation noticeable. To increase the accuracy, Nanotron proposed an improved method called
symmetric double-sided two-way ranging (SDS-TWR) [Tec]. Likewise in RTT, a message is sent
from
S
to
S
and is acknowledged immediately. Based on this round trip,
S
determines a distance
d
.hen,node
S
initiates a round trip by transmitting a second message to
S
.
After reception of
S
's ACK,
S
computes
d
and sends it to
S
.Finally,node
S
calculates the arith-
metic average of both distances
d
and
d
and, therefore, reduces clock skew effects compared to
the RTT method.
=
.
c
⋅(
∆
t
−
∆
t
R
)
6.2.5 Hop Count
In some sensor networks, the required accuracy of the determined position is very low. In these cases,
also imprecise distance estimations are sufficient to approximate a position. In a multihop network
with many uniformly distributed nodes, a distance can be estimated by the number of hops (hop
count) a message is traveling through the network as shown in Figure .. The beacons
B
and
B
are in line of sight of each other. herefore, a message between both nodes must be forwarded by
S
.
Thus, the estimated hop distance between
B
and
B
is
h
=
incontrasttotheabsolutedistance
d
e
,
and has another
unit than the coordinates. If there exist several alternative routes between two nodes, the minimal
hop distance is chosen.
Usually, distributed nodes are not in line with each other. Therefore, hop distances and real dis-
tances do not correlate linearly. Thus, the scaling error differs as exemplarily visualized for the
connections
B
=
m. Hence, this distance estimation has a scaling error of
e
S
,
=
/
=
−
B
(
h
=
,
d
e
,
=
m,
e
S
,
=
) and
B
−
B
(
h
=
,
d
e
,
=
m,
.). To determine a distance with correct units, Langendoen et al. proposed the Sum_Dist
method [LR,BWHF]. In this algorithm, all single distances between beacons are simply accu-
mulated. But besides the unhandled scaling errors, even all measurement errors are accumulated too,
andinallyreducetheaccuracy.
e
S
,
=
Sensor node
S
i
B
2
Beacon
B
j
S
4
S
3
Possible connection
Connection between
B
1
and
B
2
S
2
S
1
B
3
Connection between
B
1
and
B
3
S
6
S
5
B
1
FIGURE .
Hop count between several nodes.
