<s>
The	O
ALSE	B-Algorithm
(	O
Apollo	B-Algorithm
Lunar	I-Algorithm
Sounder	I-Algorithm
Experiment	I-Algorithm
)	O
(	O
also	O
known	O
as	O
Scientific	O
Experiment	O
S-209	O
,	O
according	O
to	O
NASA	O
designations	O
)	O
was	O
a	O
ground-penetrating	B-Algorithm
radar	I-Algorithm
(	O
subsurface	O
sounder	O
)	O
experiment	O
that	O
flew	O
on	O
the	O
Apollo	O
17	O
mission	O
.	O
</s>
<s>
This	O
experiment	O
used	O
radar	B-Application
to	O
study	O
the	O
Moon	O
's	O
surface	O
and	O
interior	O
.	O
</s>
<s>
Radar	B-Application
waves	O
with	O
wavelengths	O
between	O
2	O
and	O
60	O
meters	O
(	O
frequencies	O
of	O
5	O
,	O
15	O
,	O
and	O
150MHz	O
)	O
were	O
transmitted	O
through	O
a	O
series	O
of	O
antennas	O
near	O
the	O
back	O
of	O
the	O
Apollo	O
Service	O
Module	O
.	O
</s>
<s>
This	O
was	O
possible	O
because	O
very	O
long	O
radar	B-Application
wavelengths	O
were	O
used	O
and	O
because	O
the	O
Moon	O
is	O
very	O
dry	O
,	O
which	O
allowed	O
the	O
radar	B-Application
waves	O
to	O
penetrate	O
much	O
deeper	O
into	O
the	O
Moon	O
than	O
would	O
have	O
been	O
possible	O
if	O
water	O
were	O
present	O
in	O
lunar	O
rocks	O
.	O
</s>
<s>
(	O
A	O
radar	B-Application
experiment	O
on	O
the	O
Space	O
Shuttle	O
has	O
been	O
similarly	O
used	O
to	O
map	O
ancient	O
river	O
valleys	O
beneath	O
the	O
Sahara	O
Desert	O
.	O
)	O
</s>
<s>
In	O
addition	O
to	O
studying	O
the	O
Moon	O
,	O
the	O
experiment	O
also	O
measured	O
radio	O
emissions	O
from	O
the	O
Milky	O
Way	O
galaxy	B-Application
.	O
</s>
<s>
Based	O
on	O
the	O
properties	O
of	O
the	O
reflected	O
radar	B-Application
waves	O
,	O
the	O
structures	O
are	O
believed	O
to	O
be	O
layering	O
within	O
the	O
basalt	O
that	O
fills	O
both	O
of	O
these	O
mare	O
basins	O
.	O
</s>
<s>
The	O
ALSE	B-Algorithm
instrument	O
operated	O
in	O
two	O
HF	B-Algorithm
bands	O
(	O
5MHz	O
-	O
HF1	O
-	O
and	O
15MHz	O
-	O
HF2	O
)	O
center	O
frequencies	O
and	O
one	O
VHF	B-Algorithm
band	I-Algorithm
(	O
150MHz	O
)	O
,	O
each	O
with	O
a	O
bandwidth	O
of	O
10%	O
(	O
using	O
a	O
chirped	O
signal	O
)	O
.	O
</s>
<s>
The	O
two	O
HF	B-Algorithm
bands	O
shared	O
the	O
same	O
center-feed	O
dipole	O
antenna	O
,	O
while	O
a	O
7	O
elements	O
Yagi	O
antenna	O
was	O
used	O
for	O
the	O
VHF	B-Algorithm
channel	O
.	O
</s>
<s>
Two	O
different	O
transceiver	O
were	O
used	O
for	O
the	O
HF	B-Algorithm
(	O
alternating	O
operation	O
between	O
HF1	O
and	O
HF2	O
on	O
a	O
PRF-by-PRF	O
basis	O
)	O
and	O
VHF	B-Algorithm
,	O
sharing	O
a	O
common	O
optical	O
recorder	O
.	O
</s>
<s>
It	O
was	O
not	O
possible	O
to	O
operate	O
in	O
VHF	B-Algorithm
and	O
HF	B-Algorithm
simultaneously	O
.	O
</s>
<s>
The	O
two	O
halves	O
of	O
the	O
dipole	O
antenna	O
were	O
retractable	O
,	O
on	O
the	O
two	O
sides	O
of	O
the	O
service	O
module	O
itself	O
,	O
while	O
the	O
Yagi	O
used	O
for	O
VHF	B-Algorithm
was	O
stowed	O
close	O
to	O
the	O
main	O
engine	O
and	O
then	O
deployed	O
into	O
position	O
after	O
launch	O
.	O
</s>
<s>
the	O
range	O
sidelobes	O
of	O
the	O
compressed	O
chirp	B-Application
:	O
they	O
can	O
mask	O
weak	O
subsurface	O
echoes	O
if	O
not	O
properly	O
controlled	O
.	O
</s>
<s>
ALSE	B-Algorithm
was	O
designed	O
to	O
have	O
a	O
minimum	O
peak-to-sidelobes	O
ratio	O
of	O
45dB	O
after	O
the	O
3rd	O
lobe	O
.	O
</s>
<s>
In	O
both	O
the	O
HF	B-Algorithm
and	O
VHF	B-Algorithm
transceivers	O
,	O
the	O
chirp	B-Application
signal	O
was	O
generated	O
by	O
a	O
swept	O
oscillator	O
synchronized	O
with	O
a	O
STAble	O
Local	O
Oscillator	O
(	O
STALO	O
)	O
in	O
order	O
to	O
preserve	O
the	O
phase	O
coherency	O
for	O
the	O
SAR	B-Application
processing	O
.	O
</s>
<s>
The	O
received	O
signal	O
was	O
converted	O
at	O
IF	O
and	O
the	O
signal	O
amplitude	O
was	O
used	O
to	O
amplitude-modulate	O
a	O
CRT	B-Device
(	O
swept	O
at	O
PRF	O
rate	O
)	O
,	O
in	O
turn	O
impressing	O
a	O
70mm	O
film	O
for	O
optical	O
recording	O
of	O
the	O
data	O
.	O
</s>
<s>
Due	O
to	O
the	O
high	O
recording	O
speed	O
required	O
for	O
the	O
wider	O
bandwidth	O
VHF	B-Algorithm
channel	O
,	O
to	O
minimize	O
the	O
amount	O
of	O
recorded	O
data	O
,	O
this	O
channel	O
used	O
an	O
echo	O
tracking	O
system	O
to	O
acquire	O
and	O
record	O
only	O
the	O
main	O
surface	O
return	O
and	O
the	O
70μs	O
of	O
echoes	O
immediately	O
following	O
it	O
.	O
</s>
<s>
The	O
processing	O
facility	O
on	O
ground	O
allowed	O
for	O
both	O
full	O
optical	O
processing	O
(	O
at	O
that	O
time	O
,	O
the	O
standard	O
approach	O
for	O
SAR	B-Application
processing	O
)	O
performing	O
azimuth	O
and/or	O
range	O
compression	O
,	O
or	O
digitization	O
of	O
rough	O
or	O
azimuth-compressed	O
data	O
for	O
later	O
digital	O
processing	O
.	O
</s>
<s>
During	O
the	O
developments	O
phase	O
,	O
a	O
modified	O
ALSE	B-Algorithm
prototype	O
was	O
installed	O
on	O
board	O
a	O
KC-135	O
aircraft	O
to	O
perform	O
sounding	O
tests	O
over	O
the	O
southeast	O
US	O
and	O
over	O
Greenland	O
,	O
demonstrating	O
the	O
capabilities	O
of	O
the	O
system	O
.	O
</s>
<s>
The	O
main	O
parameters	O
of	O
the	O
ALSE	B-Algorithm
radar	B-Application
are	O
summarized	O
in	O
the	O
table	O
below	O
:	O
</s>
