<s>
Focal-plane	B-Algorithm
arrays	I-Algorithm
(	O
FPAs	O
)	O
are	O
widely	O
used	O
in	O
radio	O
astronomy	O
.	O
</s>
<s>
Traditional	O
radio-telescopes	O
have	O
only	O
one	O
receiver	O
at	O
the	O
focus	O
of	O
the	O
telescope	O
,	O
but	O
radio-telescopes	O
are	O
now	O
starting	O
to	O
be	O
equipped	O
with	O
focal	B-Algorithm
plane	I-Algorithm
arrays	I-Algorithm
,	O
which	O
are	O
of	O
three	O
different	O
types	O
:	O
multi-beam	O
feed	O
arrays	O
,	O
bolometer	B-Application
arrays	O
,	O
and	O
the	O
experimental	O
phased-array	O
feeds	O
.	O
</s>
<s>
Examples	O
include	O
the	O
multi-beam	O
arrays	O
on	O
the	O
Parkes	O
Observatory	O
,	O
and	O
the	O
ALFA	O
array	O
at	O
Arecibo	O
Observatory	O
,	O
both	O
of	O
which	O
have	O
been	O
used	O
for	O
major	O
pulsar	B-Application
and	O
Hydrogen	O
line	O
studies	O
,	O
such	O
as	O
HIPASS	O
.	O
</s>
<s>
Bolometer	B-Application
arrays	O
are	O
arrays	O
of	O
bolometer	B-Application
receivers	O
which	O
measure	O
the	O
energy	O
of	O
incoming	O
radio	O
photons	O
.	O
</s>
<s>
Phased	O
Array	O
Feeds	O
are	O
an	O
experimental	O
type	O
of	O
focal	B-Algorithm
plane	I-Algorithm
array	I-Algorithm
using	O
phased	O
array	O
technology	O
in	O
which	O
antenna	O
elements	O
are	O
closely	O
spaced	O
so	O
that	O
they	O
do	O
not	O
act	O
independently	O
,	O
but	O
instead	O
act	O
as	O
sensors	O
of	O
the	O
electromagnetic	O
field	O
across	O
the	O
focal	O
plane	O
of	O
the	O
telescope	O
.	O
</s>
<s>
The	O
outputs	O
of	O
the	O
receivers	O
are	O
then	O
coherently	O
combined	O
in	O
a	O
beamformer	B-Algorithm
with	O
appropriate	O
weights	O
to	O
synthesise	O
several	O
discrete	O
beams	O
.	O
</s>
<s>
a	O
meteor	O
in	O
meteor	O
studies	O
)	O
can	O
be	O
estimated	O
using	O
amplitude	B-Algorithm
monopulse	I-Algorithm
.	O
</s>
