<s>
The	O
SAO/NASA	B-Application
Astrophysics	I-Application
Data	I-Application
System	I-Application
(	O
ADS	O
)	O
is	O
an	O
online	O
database	O
of	O
over	O
16	O
million	O
astronomy	O
and	O
physics	O
papers	O
from	O
both	O
peer	O
reviewed	O
and	O
non-peer	O
reviewed	O
sources	O
.	O
</s>
<s>
Abstracts	O
are	O
available	O
free	O
online	O
for	O
almost	O
all	O
articles	O
,	O
and	O
full	O
scanned	O
articles	O
are	O
available	O
in	O
Graphics	O
Interchange	O
Format	O
(	O
GIF	O
)	O
and	O
Portable	B-Application
Document	I-Application
Format	I-Application
(	O
PDF	B-Application
)	O
for	O
older	O
articles	O
.	O
</s>
<s>
The	O
service	O
is	O
distributed	O
worldwide	O
,	O
with	O
twelve	O
mirror	O
sites	O
in	O
twelve	O
countries	O
on	O
five	O
continents	O
,	O
with	O
the	O
database	O
synchronized	O
by	O
means	O
of	O
weekly	O
updates	O
using	O
rsync	B-Application
,	O
a	O
mirroring	O
utility	O
which	O
allows	O
updates	O
to	O
only	O
the	O
portions	O
of	O
the	O
database	O
which	O
have	O
changed	O
.	O
</s>
<s>
Originally	O
this	O
data	O
was	O
stored	O
in	O
ASCII	B-Protocol
format	O
,	O
but	O
eventually	O
the	O
limitations	O
of	O
this	O
encouraged	O
the	O
database	O
maintainers	O
to	O
migrate	O
all	O
records	O
to	O
an	O
XML	B-Protocol
(	O
Extensible	B-Protocol
Markup	I-Protocol
Language	I-Protocol
)	O
format	O
in	O
2000	O
.	O
</s>
<s>
Bibliographic	O
records	O
are	O
now	O
stored	O
as	O
an	O
XML	B-Protocol
element	O
,	O
with	O
sub-elements	O
for	O
the	O
various	O
metadata	O
.	O
</s>
<s>
Older	O
articles	O
have	O
been	O
scanned	O
,	O
and	O
an	O
abstract	O
is	O
created	O
using	O
optical	B-Application
character	I-Application
recognition	I-Application
software	I-Application
.	O
</s>
<s>
Scanned	O
articles	O
are	O
stored	O
in	O
TIFF	O
format	O
,	O
at	O
both	O
medium	O
and	O
high	B-Algorithm
resolution	I-Algorithm
.	O
</s>
<s>
The	O
TIFF	O
files	O
are	O
converted	O
on	O
demand	O
into	O
GIF	O
files	O
for	O
on-screen	O
viewing	O
,	O
and	O
PDF	B-Application
or	O
PostScript	B-Language
files	O
for	O
printing	O
.	O
</s>
<s>
The	O
generated	O
files	O
are	O
then	O
cached	B-General_Concept
to	O
eliminate	O
needlessly	O
frequent	O
regenerations	O
for	O
popular	O
articles	O
.	O
</s>
<s>
The	O
separation	O
of	O
the	O
databases	O
allows	O
searching	O
in	O
each	O
discipline	O
to	O
be	O
tailored	O
,	O
so	O
that	O
words	O
can	O
automatically	O
be	O
given	O
different	O
weight	B-Algorithm
functions	I-Algorithm
in	O
different	O
database	O
searches	O
,	O
depending	O
on	O
how	O
common	O
they	O
are	O
in	O
the	O
relevant	O
field	O
.	O
</s>
<s>
ADS	O
's	O
database	O
links	O
preprints	O
with	O
subsequently	O
published	O
articles	O
wherever	O
possible	O
,	O
so	O
that	O
citation	O
and	O
reference	B-Algorithm
searches	O
will	O
return	O
links	O
to	O
the	O
journal	O
article	O
where	O
the	O
preprint	O
was	O
cited	O
.	O
</s>
<s>
The	O
scripts	O
are	O
designed	O
to	O
be	O
as	O
platform	B-Operating_System
independent	I-Operating_System
as	O
possible	O
,	O
given	O
the	O
need	O
to	O
facilitate	O
mirroring	O
on	O
different	O
systems	O
around	O
the	O
world	O
,	O
although	O
the	O
growing	O
use	O
of	O
Linux	B-Application
as	O
the	O
operating	B-General_Concept
system	I-General_Concept
of	O
choice	O
within	O
astronomy	O
has	O
led	O
to	O
increasing	O
optimization	O
of	O
the	O
scripts	O
for	O
installation	O
on	O
that	O
platform	O
.	O
</s>
<s>
The	O
main	O
ADS	O
server	O
is	O
located	O
at	O
the	O
Center	O
for	O
Astrophysics	O
Harvard	O
&	O
Smithsonian	O
in	O
Cambridge	O
,	O
Massachusetts	O
,	O
and	O
is	O
a	O
dual	O
64-bit	O
X86	O
Intel	O
server	O
with	O
two	O
quad-core	O
3.0	O
GHz	O
CPUs	O
and	O
32	O
GB	O
of	O
RAM	B-Architecture
,	O
running	O
the	O
CentOS	O
5.4	O
Linux	B-Application
distribution	O
.	O
</s>
<s>
The	O
service	O
may	O
receive	O
data	O
referring	O
to	O
the	O
same	O
article	O
from	O
multiple	O
sources	O
,	O
and	O
creates	O
one	O
bibliographic	O
reference	B-Algorithm
based	O
on	O
the	O
most	O
accurate	O
data	O
from	O
each	O
source	O
.	O
</s>
<s>
The	O
common	O
use	O
of	O
TeX	B-Application
and	O
LaTeX	B-Application
by	O
almost	O
all	O
scientific	O
journals	O
greatly	O
facilitates	O
the	O
incorporation	O
of	O
bibliographic	O
data	O
into	O
the	O
system	O
in	O
a	O
standardized	O
format	O
,	O
and	O
importing	O
HTML-coded	O
web-based	O
articles	O
is	O
also	O
simple	O
.	O
</s>
<s>
ADS	O
utilizes	O
Python	B-Language
and	O
Perl	B-Language
scripts	O
for	O
importing	O
,	O
processing	O
and	O
standardizing	O
bibliographic	O
data	O
.	O
</s>
<s>
For	O
scanned	O
articles	O
,	O
reference	B-Algorithm
extraction	O
relies	O
on	O
OCR	O
.	O
</s>
<s>
The	O
reference	B-Algorithm
database	O
can	O
then	O
be	O
"	O
inverted	O
"	O
to	O
list	O
the	O
citations	O
for	O
each	O
paper	O
in	O
the	O
database	O
.	O
</s>
<s>
References	O
in	O
and	O
citations	O
of	O
articles	O
in	O
the	O
major	O
journals	O
are	O
fairly	O
complete	O
,	O
but	O
references	O
such	O
as	O
"	O
private	O
communication	O
"	O
,	O
"	O
in	O
press	O
"	O
or	O
"	O
in	O
preparation	O
"	O
cannot	O
be	O
matched	O
,	O
and	O
author	O
errors	O
in	O
reference	B-Algorithm
listings	O
also	O
introduce	O
potential	O
errors	O
.	O
</s>
<s>
The	O
search	O
engine	O
is	O
tailor-made	O
for	O
searching	O
astronomical	O
abstracts	O
,	O
and	O
the	O
engine	O
and	O
its	O
user	B-Application
interface	I-Application
assume	O
that	O
the	O
user	O
is	O
well-versed	O
in	O
astronomy	O
and	O
able	O
to	O
interpret	O
search	O
results	O
which	O
are	O
designed	O
to	O
return	O
more	O
than	O
just	O
the	O
most	O
relevant	O
papers	O
.	O
</s>
<s>
It	O
works	O
by	O
first	O
gathering	O
synonyms	B-Application
and	O
simplifying	O
search	O
terms	O
as	O
described	O
above	O
,	O
and	O
then	O
generating	O
an	O
"	O
inverted	O
file	O
"	O
,	O
which	O
is	O
a	O
list	O
of	O
all	O
the	O
documents	O
matching	O
each	O
search	O
term	O
.	O
</s>
<s>
An	O
example	O
of	O
an	O
entry	O
in	O
the	O
author	O
synonym	B-Application
list	O
is	O
:	O
</s>
<s>
The	O
system	O
uses	O
data	O
from	O
the	O
SIMBAD	O
,	O
the	O
NASA/IPAC	B-Application
Extragalactic	I-Application
Database	I-Application
,	O
the	O
International	O
Astronomical	O
Union	O
Circulars	O
and	O
the	O
Lunar	O
and	O
Planetary	O
Institute	O
to	O
identify	O
papers	O
referring	O
to	O
a	O
given	O
object	O
,	O
and	O
can	O
also	O
search	O
by	O
object	O
position	O
,	O
listing	O
papers	O
which	O
concern	O
objects	O
within	O
a	O
10arcminute	O
radius	O
of	O
a	O
given	O
Right	O
Ascension	O
and	O
Declination	O
.	O
</s>
<s>
These	O
databases	O
combine	O
the	O
many	O
catalogue	O
designations	O
an	O
object	O
might	O
have	O
,	O
so	O
that	O
a	O
search	O
for	O
the	O
Pleiades	B-Device
will	O
also	O
find	O
papers	O
which	O
list	O
the	O
famous	O
open	O
cluster	O
in	O
Taurus	O
under	O
any	O
of	O
its	O
other	O
catalog	O
designations	O
or	O
popular	O
names	O
,	O
such	O
as	O
M45	O
,	O
the	O
Seven	O
Sisters	O
or	O
Melotte	O
22	O
.	O
</s>
<s>
Once	O
search	O
terms	O
have	O
been	O
pre-processed	O
,	O
the	O
database	O
is	O
queried	O
with	O
the	O
revised	O
search	O
term	O
,	O
as	O
well	O
as	O
synonyms	B-Application
for	O
it	O
.	O
</s>
<s>
As	O
well	O
as	O
simple	O
synonym	B-Application
replacement	O
such	O
as	O
searching	O
for	O
both	O
plural	O
and	O
singular	O
forms	O
,	O
ADS	O
also	O
searches	O
for	O
a	O
large	O
number	O
of	O
specifically	O
astronomical	O
synonyms	B-Application
.	O
</s>
<s>
ADS	O
's	O
synonym	B-Application
list	O
was	O
created	O
manually	O
,	O
by	O
grouping	O
the	O
list	O
of	O
words	O
in	O
the	O
database	O
according	O
to	O
similar	O
meanings	O
.	O
</s>
<s>
As	O
well	O
as	O
English	O
language	O
synonyms	B-Application
,	O
ADS	O
also	O
searches	O
for	O
English	O
translations	O
of	O
foreign	O
search	O
terms	O
and	O
vice	O
versa	O
,	O
so	O
that	O
a	O
search	O
for	O
the	O
French	O
word	O
soleil	O
retrieves	O
references	O
to	O
Sun	O
,	O
and	O
papers	O
in	O
languages	O
other	O
than	O
English	O
can	O
be	O
returned	O
by	O
English	O
search	O
terms	O
.	O
</s>
<s>
Synonym	B-Application
replacement	O
can	O
be	O
disabled	O
if	O
required	O
,	O
so	O
that	O
a	O
rare	O
term	O
which	O
is	O
a	O
synonym	B-Application
of	O
a	O
much	O
more	O
common	O
term	O
(	O
such	O
as	O
'	O
dateline	O
 '	O
rather	O
than	O
'	O
date	O
 '	O
)	O
can	O
be	O
searched	O
for	O
specifically	O
.	O
</s>
<s>
Also	O
returned	O
are	O
links	O
to	O
the	O
SIMBAD	O
and/or	O
NASA	B-Application
Extragalactic	I-Application
Database	I-Application
object	O
name	O
databases	O
,	O
via	O
which	O
a	O
user	O
can	O
quickly	O
find	O
out	O
basic	O
observational	O
data	O
about	O
the	O
objects	O
analyzed	O
in	O
a	O
paper	O
,	O
and	O
find	O
further	O
papers	O
on	O
those	O
objects	O
.	O
</s>
<s>
Most	O
users	O
access	O
the	O
system	O
from	O
institutes	O
of	O
higher	O
education	O
,	O
whose	O
IP	B-Protocol
address	I-Protocol
can	O
easily	O
be	O
used	O
to	O
determine	O
the	O
user	O
's	O
geographical	O
location	O
.	O
</s>
