ACCURACY
DGPS
(Differential
GPS)
BEACON RECEIVER SETUP
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 "C/A Code." It's
accuracy
is
intentionallyworse
than the
military's
"P Code." In
this
manner,
civilian users worldwide can
benefit from excellent
position
fixes.
Meanwhile,
the
military keeps
the
most accurate
system away
from
potential
enemies.
Theoretically,
C/A
code can
give
accurate
position
fixes
up
to 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
"Selective
Availability"
or SA. This is
small,
random
errors
intentionally
added to the
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
better),
and otherfactors. The smallest
ranges
on
the
plotter
may
not be usable if the SA is
high.
The
present position symbol
can move
off the
screen even while
you're sitting
still.
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 better than most
other
navigation
systems.
You've
purchased
one of thefinest
navigatiorrinstruments
on the
market
today.
We
hope you'll enjoy
it for
many years
to come.
The EAGLE EGP-1 GPS Module
This GPS receiver is
currently
used in all
Eagle
GPS receivers. It's
small,
rugged,
and fast. The five channel
design
lets it track all satellites in view
and
acquire up
to five satellites at one time. It sends
position
information
to the AccuNav II once
every
second.
By incorporating
Rockwell's GPS
receiver
technology
with
Eagle's
state-of-the-art
design
and manufactur-
ing capabilities,
Eagle brings
to the consumer the most advanced line of
GPS marine
navigation systems
available in the world.
1'
You'll have to tell the
AccuNav II which beacon receiver's data to
expect
and
set
up
the
parameters
for that data. To do
this,
first
press
the MENU
key,
then
pressthe keynexttothe
"CHANGE GPS SETTINGS" label. Now
press
the
key
next to the
"SELECT NMEA" label. The screen on the
previous page appears.
Press the
key
next to the
desired beacon receiver.
The beacon receiver is
selected,
now
you'll
need to set it
up
for the
frequency
and bit rate of the station
you'll
be
using
in
your
area.
Repeat
the above
steps
to
getthe
NMEA
screen as shown on the
previous
page.
Now
press
the
key
next to the
"SETUP BEACON" label. The screen
shown below
appears.
The station
frequency
and bit
rate are shown on the
right
side
of the screen.
Press the
key
adjacent
to the "INC FREQ" to
increase the station
frequency
orthe "DEC
FREQ"to decrease
it. Do the same
for the bit rate.
When the station
frequency
and
bit rate are
adjusted
to
their
proper settings, press
the
key
next to the "EXIT" label. The
AccuNav II returns to the last
used GPS screen. The letters "DGPS"
appear
next to the
"POSITION" on all screens that show
your present
position, showing
that the beacon receiver is
working.
To viewthe status of the beacon
signal, pressthe
MENU
key
until the fourth
menu
page appears.
Now
press
the
key
adjacent
to the
"DIFFERENTIAL
GPS STATUS" label. If the
beacon receiver is
receiving
data from the
transmitter and is connected
properly
to the AccuNav
II,
a
screen similar
to the one shown at
right ap-
pears.
This
page
shows the station ID
number,
its
frequency
and bit
rate,
the "health" of the station
(0
=
best,
5
=
worst), signal
strength (the higher
the num-
ber,
the
better),
and the
signal-
to-noise ratio
(SNR).
The
high-
erthe SNR
number,
the better.
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8
37
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OK OK OK
OFF
fl13
OK OK OK OFF
tJ02
OK OK OK ON
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OK
OK OK ON
26 BAD OK BAD
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