User Guide

SELECT NMEA
The GlobalNav II sends data out the white wire on the
power
cable
according
to standards set
by
the NMEA
(National
Marine Electronics
Association).
This allows the GlobalNav II to send
position, depth,
and
navigation
information to 'listener"
units,
such as
charting instruments,
autopilots,
and other marine instruments. The GlobalNav II uses the
following
NMEA data
protocols:
NMEA 0180 and 0183. NMEA 0180
sends
steering
information
only.
It's useful
mainly
for
autopilots.
NMEA
0183 sends
position, steering, speed,
and more. In order to use this
feature,thewhitewire
onthe
powercable
must beconnectedtothe NMEA
data
input
on the other
instrument.
See the installation section in the front
of this manual for
wiring
connection information.
Onceyou connectthewiring properly,
the GlobalNav II must betoldwhich
dataformatto use. Consuitthe owner's manual of the
'istener"equipment
to see which format it needs. Then set the GlobalNav II as follows:
GPS
-
HOW IT WORKS
The Global
Positioning System (GPS)
isthe
bestapproach
to
navigation
that
haseverbeen devised. Conceived
bythe Departmentof
Defense
(DOD)
andthe
United States
military,
the GPS
system
is an answertotheirneedsof 24 hour
global positioning,
365
days ayear.
Basically, thesystemworks by usingaconstellation
of
satellitesorbiting
Earth
11
,000miles
in
space.
Therewill be2l satellites in orbitwhenthe
system isfully
operational.
Three more satelliteswill actas
spares,
foratotal of 24. When all
satellitesareinplace,
atleastfourofthemwill
beinviewnearlyanywhereon
Earth
twenty-fourhoursa day.
The GPS
receiverrequiresat
leastthree satellitesto
give
a"2D"fix.
(A2Dfix isyourposition
in
latitude/longitude.
A3Dfix
isyour
latitude/longitude plus altitude.)
When it locks on to at
leastfoursatellites,
it
displays
a3D fix.
First, press
the MENU
key. Next, press
the
key
next to the
"Change
GPS
Settings"
label. Now
press
the
key
next to the "Select NMEA" label. The
screen shown below
appears.
The data format
currently
in use shows
at the
top
of the screen. Press the
key
next to the desired data
output.
The GlobalNav II will return to
the
last
used GPS screen and send
NMEA data out the white wire
on the
power
cable.
TRUE and MAGNETIC POSITION
True and
magnetic
north are not
always
the same. True north is the
top
of the world. It's where all lines of
longitude
converge. Magnetic
north is
the location
ourcompasses point.
Itliesseveral hundred milestothe
south
of true
north,
at a location in Canada.
Charts
are
usually
laid out
according
to a Mercator
projection
that uses
true north. If
you
plot
a course on a chart
using
the Mercator
projection,
Asthe receiver locks on to each
satellite,
it calculates the distance
from the
satellite
bymeasuringthe length
of time ittakesthe
radiosignalto
reach it. Each
satellite has an
extremely
accurateclockthattellsthe receiverwhen the radio
transmission started. The
receivercomparesthattimeagainst
its own
clock,
thus it knows how
long
ittookthe radio
signal (travelling
atthe
speed
of
light!)
to reach it. If
you
knowtimeand
speed,
then
you
can calculate distance. Once
you
havethisfromthree
satellites,
then the receivercan determine
position.
It takes three satellites to determine
position.
40 9
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