3
measurement
(| 1)
t
kkz
. Second, the valid measurements
are selected using the gating technique and the probability of
the target measurement being selected is
,Gt
P
[20]. Then, the
data association hypotheses are formulated and the
conditional densities of all targets as well as the probabilities
of all hypotheses are calculated. Finally, the marginalized
posterior densities of all targets are approximated using
Gaussian densities.
A hypothesis is a target-measurement association
event. In each hypothesis: 1) each target can be assigned
zero measurement or one of the measurements which falls in
the selection window of the target, 2) each measurement can
be allocated to zero or one of the target. Let
h
and
N
denote the hypothesis h and the total number of hypotheses,
respectively. The probability for
h
is computed as
()
1
() 1 ()
1
1!
( ) ( ); ( | 1), (k)
'
() (1 )
k
jh
th th
m
f
tt
htjj
j
n
tt
dd
t
PNkkk
c
V
PP
zz S
(8)
where
( ); ( | 1), (k)
tt
tj j
Nkkkzz S
is a Gaussian PDF with
mean
(| 1)
t
kkz
and covariance matrix
(k)
t
S
evaluated at
()
j
kz
,
'
() ( | 1)
tt
kkkSHP HR
is the residual error
covariance,
is the number of false measurements, V is the
volume of the valid space,
()
h
f
is the measurement
indicator (if measurement
()
j
kz
is associated with a target
in
h
,
()1
jh
f
; otherwise,
()0
jh
f
),
()
th
is the
target indicator (if target t is associated with a measurement
in
h
,
()1
th
; otherwise,
()0
th
), and
'c
is the
normalization factor.
Using the probabilities of hypotheses, the updated state
of target t is described as
1
() ( ) ()
N
tt
hh
h
kP kxx
(9)
where
()
t
h
kx
is the conditionally updated state of target t in
hypothesis
h
11
() ( | 1)1 ( ) () ( )
kk
mm
tt h th
h jth j jth
jj
kkk w kwxx x
(10)
with
1if
()
0other
h
th
h
jt h
w
(11)
denoting that
()
j
kz
can be associated with target t in
h
.
For the updated covariance of target t
''
1
() ( ) () () () () ()
N
tttttt
hh h h
h
kP kkkkkPPxxxx
(12)
Here,
()
t
h
kP
is the conditionally updated covariance of
target t in hypothesis
h
1
'
1
() ( | 1)1 ( )
( | 1) () () () ( )
k
k
m
tt h
hjth
j
m
tttth
th
j
kkk w
kk k k k w
PP
PKSK
(13)
and
'1
() ( | 1) ( ())
tt t
kkk kKP HS
is the filter gain.
From (9) and (12), one can see that the posterior PDF
is approximated by a single Gaussian PDF at each time step.
So, the performance of JPDA relies heavily on Gaussian
approximations. Nevertheless, JPDA is often described in an
alternative fashion, where each track is updated by a
weighted average of the measurements.
2.3. The RFS family
In this section, we show the relation between the
PDF of ordered (labelled) targets and the PDF of unordered
set of targets [18]. This relation provides the possibility to
switch the posterior PDF of JPDA, when target identity is
irrelevant. To formulate the problem, the joint state vector is
expressed as
T
TT T
12
,,,
n
Xx x x
(14)
where
t
x
is the state vector of target t and n is the number of
targets. In this paper, the targets are unordered and the RFS
is used to describe them. Since an RFS is without ordering,
the targets in (14) can be reordered without affecting the
RFS. The relation between the ordered PDF
()p X
and the
unordered RFS PDF
()R X
is as follows [18]
()
n
RpXX
(15)
where
n
is the set of all possible permutations of the joint
target state and the notation
X
describes the joint state
vector in permutation of
. That is, to go from an ordered
PDF to an RFS PDF, we sum the ordered PDF over all
possible permutations of the joint state vector.
There are several ordered PDFs corresponding to the
same RFS PDF. An ordered PDF
'
()p X
belonging to the
same RFS family of
()p X
can be obtained by multiplying
each
p X
with a constant weight
()
q
'
() ()
n
pqpXX
(16)
and the weight
()
q
must satisfy
() 1,0 () 1
n
qq
(17)
In this way, the ordered PDF
()p X
is switched into
'
()p X
.
For the JPDA, it is possible to switch between the ordered
PDFs in the same RFS family to obtain an PDF which can
provide a more accurate Gaussian approximation than others.
3. Proposed method
In this paper, a novel and efficient approach is
proposed to improve the tracking accuracy of the JPDA.
First, the posterior PDF of the JPDA is described as a GMM.
Then, the posterior PDF is switched into another ordered
PDF in the same RFS family. To obtain a good performance,
the switching criterion is important. Therefore, we first
develop the cost function in Section 3.1. In Section 3.2, the
iterative optimization of the posterior PDF is conducted
based on the cost function. Then, an indicative example is
shown in Section 3.3. In Section 3.4, the computational
complexity of the proposed method is analyzed.