Aerospace and Electronic Systems Magazine July 2017 Tutorial XI - 26

Introductory View of Anomalous Change Detection
class of EC distributions whose PDF is a monotonically decreasing
function of the Mahalanobis distance. In this case the PDFs of the
observation e(i, j) conditioned on the two hypotheses are [3]

(

)

(

)

f e ( i, j ) H 0 =
f e ( i, j ) H1 =

1

( 2π )

L
2

Γ0

1

( 2π )

L
2

)

(

)

hL d1 ( i, j )

1
2

Γ1

(

hL d 0 ( i, j )

1
2

(37)

APPENDIX II
In this Appendix we prove that the test statistics of the HACD and
the SACD are invariant to a linear transformation applied to one of
both the vectors y(i, j) and z(i, j) in e(i, j). Particularly, denoting as
HACD(y, z; i, j) the test statistic THACD(i, j) and as SACD(y, z; i, j)
the test statistic TSACD(i, j), we prove that
HACD ( y , z; i, j ) = HACD ( R ⋅ y + b, K ⋅ z + m; i, j )

SACD ( y , z; i, j ) = SACD ( R ⋅ y + b, K ⋅ z + m; i, j )

where Γ0 and Γ1 are the covariance matrices under the two hypotheses, hL(x) is a monotonically decreasing function of x and depends also on the data dimension (L), and d0(i, j) and d1(i, j) are the
squared Mahalanobis distances:
T

T

1

−1
0

0

−1
1

1

(38)
z T ( i , j ) = K ⋅ z ( i, j ) + m

1

In (38) μ0 and μ1 are the mean vectors of e(i, j)|H0 and e(i, j)|H1,
respectively.
Following the same reasoning as in Section II, we can express
the GLRT with respect to the parameter μ1 as
Λ G ( i, j ) =

Γ0
Γ1

1
2
1
2

))
( (
h ( d ( i, j ) )

max hL d1 ( i, j )
μ1

H1
>

Λ G ( i, j ) =

Γ1

1
2
1
2

hL ( 0 )

1

(

hL d 0 ( i, j )

Considering that the ratio hL ( 0 )

)

H1
>
<

λG

(40)

H0

Γ0

1/ 2

Γ1

1/ 2

is greater than zero and that

hL(d0(i, j)) is a monotonically decreasing function of d0(i, j), the
GLRT in (40) can be reformulated as

(

d 0 ( i, j ) = e ( i , j ) − μ 0

)

T

(

Γ 0−1 e ( i, j ) − μ 0

)

H1
>
<

λ*

(41)

H0

As in the Gaussian model case considered in Section II, being μ0
and Γ0 not known a priori, they are replaced by their ML estimate
Γˆ 0 and μˆ 0 from the secondary data. The resulting detector becomes

(

T ( i, j ) = e ( i, j ) − μˆ 0

)

T

(

Γˆ 0−1 e ( i, j ) − μˆ 0

)

H1
>
<

λ

(42)

H0

which is exactly the same expression as that reported in (9).

26

e T ( i, j ) = eT ( i, j ) − μˆ 0,T

μˆ 0,T = μˆ yT T , μˆ zT T 

Being hL(d1(i, j)) a monotonically decreasing function of d1(i, j) it
has its maximum value in d1(i, j) = 0, thus:
Γ0

T

T
T
eT ( i, j ) =  y T ( i, j ) , zT ( i, j ) 



(39)

λG

<
H0

0

L

where R and K are two generic L × L full rank matrices and b and
m are two generic L × 1 vectors.
We start by letting
y T ( i, j ) = R ⋅ y ( i, j ) + b

(
) Γ ( e ( i, j ) − μ )
d ( i , j ) = ( e ( i, j ) − μ ) Γ ( e ( i , j ) − μ )
d 0 ( i, j ) = e ( i, j ) − μ 0

(43)

T

(44)

with μˆ yT and μˆ zT obtained according to (14) from all the pixels yT(i,
j) and zT (i, j).
It can be easily proved that μˆ yT = R ⋅ μˆ y + b and μˆ zT = K ⋅ μˆ z + m.
So e T ( i, j ) can be rewritten as
R 0 
e T ( i, j ) = 
 ⋅ e ( i, j ) = D ⋅ e ( i, j )
 0 K
R 0 
D=

 0 K

(45)

where 0 represents the L × L matrix with all-zero entries and
e ( i, j ) = e ( i, j ) − μˆ 0. According to the expressions in (14) and the
notation in (44) and (45), it is straightforward to obtain the following relationships:
NS NL

Γˆ yT =

1
NS ⋅ NL

 y ( i, j ) ⋅ y ( i, j )

Γˆ zT =

1
NS ⋅ NL

 z ( i, j ) ⋅ z ( i, j )

Γˆ yT zT =

IEEE A&E SYSTEMS MAGAZINE

T

NS NL

i =1 j =1

1
NS ⋅ NL

− μˆ yT ⋅ μˆ TyT = R ⋅ Γˆ y ⋅ R T

T

T

i =1 j =1

T

T

T

NS NL

− μˆ zT ⋅ μˆ TzT = K ⋅ Γˆ z ⋅ K T (46)

 y ( i, j ) ⋅ z ( i, j )
i =1 j =1

T

T

T

− μˆ yT ⋅ μˆ TzT = R ⋅ Γˆ yz ⋅ K T

JULY 2017, Part II of II



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