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ACCURACY
You
may
have heard tales of
extraordinary accuracy
from GPS receivers.
The DOD
requires accuracy
of 10 to 15 meters from the satellite
system.
However,
only
the
military gets
this
precision.
The
way
the
military keeps
us and other unauthorized
people
from
using
the more
precise system
is
coding.
In other
words,
the data
coming
from the satellites is
encrypted.
Civilian GPS receivers use what's known as "C/A Code." It's
accuracy
is
intentionally
worsethan the
military's
'P Code." Inthis
manner,
civilian users
worldwide can benefit from excellent
position
fixes.
Meanwhile,
the
military
keeps
the most accurate
system away
from
potential
enemies. Theoreti-
cally,
C/Acodecan
give
accurate
position
fixes
upto
15 meters. This is more
than
adequate
for most
people.
However
(as
of this
writing),
the
military
isn't satisfied with C/A's
potential
accuracy
in the hands of the world.
So,
it's
degrading
it further with what's
called "Selective
Availability"
or SA. This is
small,
random errors intention-
ally
added tothe
system
so
your accuracy
will
typically
be within 100 meters.
Of
course, accuracy
also
depends
on the
angle
of the satellite above the
horizon, signal-to-noise ratio,
the number of satellites tracked
at one time
(the
more the
belier),
and other factors. The smallest
ranges
on the
plotter
may
not be usable if the SA
is
high,
because the
position
will move off the
screen even
while
you're sifting
still.
TRANSDUCER CONE ANGLES
The sound waves from the transducer
spread
out into the water in a
cone
shaped
beam. This looks much like the beam from a
flashlight.
The
angle
between the outside
edges
of the cone is the cone
angle.
Eagle
offers
a choice of transducers with either an 8 or 20
degree
cone
angle.
The transducer
supplied
with the UltraNavGPS has a 20
degree
cone
angle. Typically,
wide cone
angle
transducers
(20 degrees)
are
ideal for
operating
in shallow to medium water
depths.
The 20
degree
cone
angle
allows
you
to see more
of the underwater world. In 15 feet
of water the 20
degree
cone
covers an area about six feet across. The
8
degree
transducer covers
only
about a two foot circle.
The 20
degree
transducer is almost
always
the best to use in fresh
water,
the 8
degree mostly
in salt water. In a
deep
water
environment,
(300
feet
-
fresh
water,
100 feet
-
salt
water)
the narrow cone
angle
is
more desirable. Since the sound
energy
is concentrated in a smaller
area,
it can
penetrate
to much
deeper depths.
Don't let this
discourage you,
however. GPS
by
nature has much faster
updates
than other
systems (such
as
Loran),
and
typically
is much easier to
use.
Accuracy,
even with SA on is still belier than most other
navigation
systems.
You've
purchased
one of the finest
navigation
instruments on the
market
today.
We
hope you'll enjoy
it for
many years
to come.
The EAGLE GPS Module
This GPS receiver
(manufactured by Rockwell) currently
is used
in
all
Eagle
GPS receivers. It's
small,
rugged,
and fast. The five
channel
design
lets it
acquire
and track
up
to five satellites at one time. It transmits
position
information to the UltraNavGPS once
every
second.
By
incorporating
Rockwell's GPS receiver
technology
with
Eagle's
state-of-the-art
design
and
manufacturing capabilities, Eagle brings
to the consumer the most
advanced line of OPS marine
navigation systems
available in the world.
I
50'
100'
150'
200'
8° at 3 db
42
31
20° at 3 db
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