<s>
such	O
as	O
an	O
artificial	B-Architecture
neural	I-Architecture
network	I-Architecture
,	O
statistical	B-General_Concept
classification	I-General_Concept
algorithm	O
,	O
</s>
<s>
kernel	O
estimator	O
or	O
a	O
linear	B-Algorithm
least	I-Algorithm
squares	I-Algorithm
.	O
</s>
<s>
In	O
principle	O
,	O
most	O
collocation	B-General_Concept
problems	O
can	O
be	O
solved	O
by	O
a	O
nearest	B-Algorithm
neighbor	I-Algorithm
search	I-Algorithm
,	O
</s>
<s>
There	O
are	O
at	O
least	O
two	O
main	O
considerations	O
when	O
performing	O
collocations	B-General_Concept
.	O
</s>
<s>
scanning	O
satellite	B-Application
instrument	O
.	O
</s>
<s>
kilometers	O
in	O
diameter	O
such	O
as	O
that	O
of	O
a	O
satellite-mounted	O
,	O
</s>
<s>
ease	O
the	O
collocation	B-General_Concept
process	O
.	O
</s>
<s>
binned	B-General_Concept
in	O
some	O
way	O
.	O
</s>
<s>
so	O
that	O
we	O
need	O
not	O
consider	O
collocations	B-General_Concept
with	O
a	O
time	O
error	O
of	O
more	O
than	O
an	O
hour	O
or	O
so	O
.	O
</s>
<s>
The	O
satellites	B-Application
that	O
most	O
concern	O
us	O
are	O
those	O
with	O
a	O
low-Earth	O
,	O
polar	O
orbit	B-Application
since	O
geostationary	O
satellites	B-Application
view	O
the	O
same	O
point	O
throughout	O
their	O
lifetime	O
.	O
</s>
<s>
instruments	O
mounted	O
on	O
three	O
satellites	B-Application
over	O
a	O
period	O
of	O
12	O
hours	O
.	O
</s>
<s>
This	O
illustrates	O
both	O
the	O
orbit	B-Application
path	O
and	O
the	O
scan	O
pattern	O
which	O
runs	O
crosswise	O
.	O
</s>
<s>
Since	O
the	O
orbit	B-Application
of	O
a	O
satellite	B-Application
is	O
deterministic	O
,	O
</s>
<s>
it	O
is	O
common	O
to	O
simply	O
feed	O
these	O
coordinates	O
to	O
a	O
nearest	B-Algorithm
neighbor	I-Algorithm
search	I-Algorithm
.	O
</s>
<s>
directly	O
than	O
satellite	B-Application
or	O
other	O
remote-sensing	B-Application
instruments	O
.	O
</s>
<s>
interpolation	B-Algorithm
is	O
likely	O
the	O
most	O
appropriate	O
method	O
for	O
performing	O
any	O
type	O
of	O
comparison	O
.	O
</s>
<s>
within	O
the	O
grid	O
and	O
interpolation	B-Algorithm
performed	O
between	O
the	O
nearest	O
neighbors	O
.	O
</s>
<s>
Linear	O
interpolation	B-Algorithm
(	O
bilinear	O
,	O
trilinear	O
etc	O
.	O
)	O
</s>
<s>
then	O
interpolation	B-Algorithm
can	O
eliminate	O
much	O
of	O
the	O
error	O
associated	O
with	O
collocation	B-General_Concept
.	O
</s>
<s>
Interpolation	B-Algorithm
may	O
also	O
be	O
appropriate	O
for	O
many	O
types	O
of	O
satellite	B-Application
instruments	O
,	O
</s>
<s>
of	O
trilinear	O
interpolation	B-Algorithm
.	O
</s>
<s>
grid	O
,	O
bilinear	O
interpolation	B-Algorithm
can	O
be	O
performed	O
.	O
</s>
<s>
we	O
need	O
a	O
more	O
general	O
form	O
of	O
interpolation	B-Algorithm
such	O
as	O
kernel	O
estimation	O
.	O
</s>
<s>
To	O
collocate	O
measurements	O
of	O
a	O
medium	O
-	O
to	O
long-lived	O
atmospheric	O
tracer	O
with	O
a	O
second	O
instrument	O
,	O
running	O
trajectories	B-Application
can	O
considerably	O
improve	O
the	O
accuracy	O
.	O
</s>
<s>
It	O
also	O
simplifies	O
the	O
analysis	O
somewhat	O
:	O
a	O
trajectory	B-Application
is	O
run	O
both	O
forwards	O
and	O
backwards	O
from	O
the	O
measurement	O
location	O
and	O
between	O
the	O
desired	O
time	O
window	O
.	O
</s>
<s>
Since	O
the	O
trajectories	B-Application
provide	O
a	O
location	O
for	O
every	O
point	O
in	O
time	O
within	O
the	O
time	O
window	O
,	O
there	O
is	O
no	O
need	O
to	O
check	O
multiple	O
measurements	O
from	O
the	O
second	O
instrument	O
.	O
</s>
<s>
Every	O
time	O
within	O
the	O
trajectory	B-Application
is	O
checked	O
for	O
the	O
distance	O
criterion	O
but	O
within	O
a	O
very	O
narrow	O
window	O
.	O
</s>
<s>
Alternatively	O
,	O
the	O
exact	O
times	O
of	O
the	O
measurements	O
for	O
the	O
second	O
instrument	O
are	O
interpolated	O
within	O
the	O
trajectory	B-Application
.	O
</s>
<s>
The	O
Pol-Ice	O
campaign	O
was	O
conducted	O
in	O
the	O
N	O
.	O
Baltic	O
in	O
March	O
2007	O
as	O
part	O
of	O
the	O
SMOS-Ice	O
project	O
in	O
preparation	O
for	O
the	O
launch	O
of	O
the	O
Soil	O
Moisture	O
and	O
Ocean	O
Salinity	O
satellite	B-Application
.	O
</s>
<s>
Since	O
the	O
flight	O
paths	O
of	O
both	O
aircraft	O
were	O
approximately	O
linear	O
,	O
the	O
first	O
step	O
in	O
the	O
collocation	B-General_Concept
process	O
was	O
to	O
convert	O
all	O
the	O
coincident	O
flights	O
to	O
Cartesian	O
coordinates	O
with	O
the	O
x-axis	O
being	O
lateral	O
distance	O
and	O
the	O
y-axis	O
transverse	O
distance	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
collocations	B-General_Concept
can	O
be	O
performed	O
in	O
two	O
ways	O
:	O
crudely	O
,	O
by	O
matching	O
only	O
the	O
x	O
distances	O
,	O
and	O
more	O
precisely	O
by	O
matching	O
both	O
coordinates	O
.	O
</s>
<s>
the	O
antenna	B-Device
response	I-Device
function	I-Device
for	O
the	O
radiometer	O
.	O
</s>
