W. Liu et al. / Automatica 89 (2018) 274–289 277
Fig. 1. The group is in collaboration. (a) Four targets of a group move to a collective point from four positions; (b) Four targets of group forming a formation move to a point.
Throughout this paper, we assume that graph G
k
is a simple graph
and has no loops. According to whether each edge has an assigned
orientation, a graph can be classified into the undirected graph and
directed graph (digraph), respectively.
A digraph is a graph in which the edges have direction. This
is usually indicated with an arrow on the edge; more formally, if
v
k,i
and v
k,j
are vertices, an edge is an unordered pair {v
k,i
, v
k,j
},
while a directed edge, called an arc, is an ordered pair (v
k,i
, v
k,j
) or
(v
k,j
, v
k,i
). The arc (v
k,i
, v
k,j
) is drawn as an arrow from v
k,i
to v
k,j
(Guichard, 2014).
Definition 1. Two vertices v
k,i
and v
k,j
of a graph G
k
are said to be
connected if there is a path (v
k,i
, v
k,j
) in G
k
.
We further refer the following definition of a tree (Guichard,
2014):
Definition 2. A connected graph G
k
is a tree if it is a acyclic, that is,
it has no cycles. More generally, an acyclic graph is a forest.
In essential, from the viewpoint of graph theory, each group is
like a tree in the forest (all groups). Whether undirected graph or
digraph can be expressed by an adjacency matrix A(V
k
) defined by
A : V
k
× V
k
→ {0, 1}, where 0 denotes that the any two vertices
are not adjacent, otherwise adjacent. Therefore, the adjacency
matrix for undirected matrix is symmetric, while digraph often
asymmetric.
3. Dynamic models for group targets
3.1. Dependent relation for group targets
Different from common multi-target tracking, the group target
tracking focus on the group state, group dependent relation and
number of subgroups. A primary issue is to judge whether multiple
targets belong to group targets. That is to say, what are the group
targets indeed? In our idea, the group targets are the multiple
targets which move in certain mode of collaboration.
The term collaboration here has a very broad meaning. For
examples, sub-figure (a) of Fig. 1 shows that four targets take off
from several sites and fly to a default point at the same time.
Although the four targets have not a fixed formation and shape,
they are collaboration in the space and time and constitute a group.
Another scenario is shown in sub-figure (b), where four targets
keep a formation and thus are looked as group targets. In this paper,
we mainly focus on the second case of multiple formations which
remain fixed structure as sub-figure (b) of Fig. 1. Hence, we first
propose the following definition for group targets.
Definition 3. Group targets are a set of state points and the state
points move in state space in certain mode of collaboration.
In this definition, the key is the movement of collaboration.
Here, the collaboration denotes there are some kinds of dependent
relation between the targets. The group targets may involve multi-
ple subgroups and in each subgroup the targets are dependent and
form a connected graph. In special case, if the numbers of targets
in each subgroup reduces to one, the group targets will reduce to
the normal case of multiple targets.
Graph theory provides a powerful tool in modeling the de-
pendence between vertices and it has been successfully used in
formation control of flying vehicles (Anderson, Yu, & Fidan, 2006;
Yu, Hendrickx, Fidan, Anderson, & Blondel, 2007) and multi-agent
system control (Ferber, 1999). Inspired by the success, we combine
the graph theory with group targets and define the group targets
with graph structure G
k
as follows:
Definition 4. Given group targets with fixed number of targets
X
k
= {x
k,1
, . . . , x
k,N
k
} at time step k, the group targets with digraph
structure G
k
= (V
k
, E
k
), where vertex set V
k
is given by X
k
, i.e., V
k
≜
X
k
. The edge set E
k
is defined by the dependent relation between
the vertex set E
k
≜ X
k
× X
k
.
Remark 1. The structure of group targets can be seen as a kinds of
digraph. Specifically, according to their dependent relation, some
vertices (targets) are parents and some are children. Besides, the
group targets X
k
may involve multiple subgroups with different
structures. We analyze the structures of individual subgroups on
graph G
k
.
3.2. Group target dynamic models
Consider the following linear models for a target x
k,i
in group X
k
with digraph structure G
k
given by:
x
k+1,i
=
l∈P(i)
ω
k
(l, i)[F
k,l
x
k,l
+ b
k
(l, i)] + B
k,i
w
k,i
(17)
z
k+1,i
= H
k+1
x
k+1,i
+ v
k+1,i
(18)
x
k,i
∈ X
k
,
l∈P(i)
ω
k
(l, i) = 1, ω
k
(l, i) ∈ [0, 1] (19)
where w
k,i
∼ N (0, Q
k,i
), Q
k,i
is covariance matrix of process noise,
b
k
(l, i) is a standard (or required) displacement vector between
vertex i and its father vertex l, P(i) is a vertex set and denotes all the
father vertices for vertex i. If P(i) is a empty set ∅, let P (i), b
k
(l, i)
be i and zero, respectively. For simplicity, the weighted coefficients
{ω
k
(l, i), l ∈ P(i)} imply a convex combination of various vectors
{F
k,l
x
k,l
, l ∈ P(i)}. In short, the target i is dependent on the states of
its parents.
Besides, different from the usual multi-target dynamic models,
which is shown by Eq. (20), Eq. (17) has nothing to do with its