WANG AND TIAN: DISTRIBUTED NETWORK LOCALIZATION: ACCURATE ESTIMATION WITH NOISY MEASUREMENT 5929
Fig. 1. The relative position measurement under the case that all nodes share
a common reference frame.
network is described by digraph G = {V, E, A}, and its bearing-
sensing topology is described by
¯
G, the undirected graph induced
from G.
We denote by p
i
∈ C,i ∈Vthe accurate position of node i
in a global or called common coordinate system denoted by Σ
g
.
Denote by V
0
= {1,...,s} the set of anchor nodes whose po-
sitions p
i
,i∈V
0
, are known. Denote by V
1
= {s +1,...,N}
the set of sensor nodes whose positions p
i
,i∈V
1
aretobe
determined. Each node i ∈V
1
makes and records its all mea-
surements with respect to its own local coordinate system Σ
i
,
whose origin is assumed to be just at the position of node i, and
whose orientation with respect to Σ
g
is a constant denoted by
θ
i
∈ (−π, π]. θ
i
is assumed to be unknown for each sensor node
i ∈V
1
. We call all nodes share a common reference frame, if
directions of the coordinate axis of each node’s local coordinate
system are the same as ones of the common coordinate system
(i.e., θ
i
=0, ∀i ∈V). As for the anchor, since its position in Σ
g
is known, we set its orientation θ
i
=0, ∀i ∈V
0
, without loss of
generality.
Firstly, we assume all nodes share a common reference frame.
Then, as shown by Fig. 1,
b
ji
= p
j
− p
i
(1)
is the relative position of j measured by node i, which can be ex-
pressed as b
ji
= |b
ji
|e
ıδ
ji
with δ
ji
∈ (−π, π] denoting the bear-
ing of node j in Σ
i
. In practice, the relative position information
can be obtained by the node through the distance measurement
and bearing measurement, where an alternative distance mea-
surement method is the time of arrival (TOA) method (see, e.g.,
[2]) and the bearing can be measured by a bearing measurement
sensor, such as a camera (see, e.g., [21]) or an antenna array (see,
e.g., [22]). Let b
i
be the weighted sum of the relative positions
of all the node i’s neighbors measured by node i, i.e.,
b
i
=
j∈N
i
a
ij
b
ji
. (2)
For all anchor nodes, we let b
i
=0, ∀i ∈V
0
. Then, we can
aggregate (1) and (2) for all nodes into the following form
−Lp = b, (3)
Fig. 2. The bearing measurement in local coordinate systems and orientations.
where L is the Laplacian matrix of G(A) with partition L =
[
00
L
1
L
2
], p =[p
1
,...p
N
]
T
, b =[b
1
,...,b
N
]
T
.
Obviously, to ensure the localizability of the network with
relative position measurements, L
2
should be i nvertible. By
Lemma 3.1 given in [23], we know that the non-singularity of
L
2
is guaranteed if and only if the relative position sensing
topology G is 1-rooted at the anchor node set. A distributed
iterative localization algorithm can be constructed as
ˆp
i
(t +1)= ˆp
i
(t)+c
j∈N
i
a
ij
(ˆp
j
(t) − ˆp
i
(t) − b
ji
),i∈V
1
,
(4)
where t ∈ N, c is a small positive constant to be designed,
ˆp
i
∈ C,i ∈V
1
denotes estimate of the position p
i
made by
node i at the tth iteration. Let ¯p
T
(t)=[¯p
1
(t),...,¯p
N
(t)] with
¯p
i
(t)=ˆp
i
(t) − p
i
, ∀i ∈V. By (1), Eq. (4) together with ¯p
i
(t) ≡
0, ∀i ∈V
0
, can be aggregated into
¯p(t +1)=(I − cL)¯p(t).
(5)
Now, we can see that the relative-position based node-
localization problem with multiple anchor nodes can be trans-
formed into a consensus control problem with multiple leaders
at the same positions [23], [24].
Then, we consider the case that all nodes do not share a
common reference frame, which implies that there are some
nodes i ∈V
1
such that θ
i
s are unknown and θ
i
=0in general
(see Fig. 2). In this case, p
j
− p
i
is no longer equal to b
ji
, and
instead, we have
p
j
− p
i
= e
ıθ
i
b
ji
. (6)
In order to achieve the node-localization, each sensor node i ∈
V
1
needs to determine its own orientation θ
i
(or the orientation
vector e
ıθ
i
) first and then determine its position in the common
coordinate system.
Denote the relative bearing μ
ji
= δ
ji
− δ
ij
and define the
relative orientation
θ
ji
Δ
= PV(θ
j
− θ
i
), (7)
with PV(θ
j
− θ
i
)
Δ
=[(θ
j
− θ
i
+ π)mod2π] − π. The follow-
ing lemma, which is a restatement of the result of [16], shows the
relation between the relative orientation and the relative bearing.