The lower half of the screen
shows statistics for each of the
receiver's five channels. This
includes the satellite number
(PRN), its status (STAT), the
User Differential Range Error
(UDRE), Time status (TIME), _______
and if SA (Selective Availablity) CLEAR _________
is on or off. The UDRE is the _____
range error
from
your position
to the satellite. If there is an "OK" in this
field,
then
the
range error to that
satellite is 8 meters or less 68
percent of the time.
GPS
- HOW IT WORKS
The Global Positioning System (GPS) is the
best
approach to navigation
that has ever been devised. Conceived by the
Department of Defense
(DOD)
and the United States military,
the G PS
system
is an answerto their
needs of 24 hour
global positioning, 365
days
a
year.
Basically, the system works by using a constellation of satellites orbiting
Earth 11,000
miles in space. There will be 21 satellites in orbit when the
system
is
fully operational. Three more satellites will act as spares, for a
total of 24. When all satellites are in place, at least
four of them will be in
view
nearly anywhere on
Earthtwenty-four hours a
day.
The OPS receiver
requires at least three satellites to
give a "2D"
fix. (A
2D fix is
your position
in latitude/longitude.
A 3D fix is
your
latitude/longitude plus altitude.)
When
it locks
on to at least four satellites, it displays a 3D fix.
For more information, read
your
beacon receiver's manual.
SELECT UNITS OF
MEASURE
(See the Select
Units of Measure in the Plotter section of this manual.)
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 our
compasses point. It lies several hundred miles to the south of
true north, at a location in Canada.
Charts are usually
laid out
according
to a Mercator projectionthat usestrue
north. If
you plot
a course on a chart using
the Mercator projection, you'll
either have to convert magnetic readings to true or use true
readings.
The AccuNav II can display navigation information in magnetic or true.
When it's turned on for the first time magnetic is used. To switch to
true,
press the MENU key
while
a GPS screen is
displayed. Next, press the
key
next to the "CHANGE GPS SETTINGS" label. Now
press the
key
next to
the "SELECT UNITS OF
MEASURE" label. Now
simply press
the
key next
to the "BEARING" label. This
moves the black box
from "MAO" to "TRU."
Press the key next to the "EXIT' label when
you're
finished.
As the
receiver locks on to each satellite, it calculates
the distance from the
satellite by measuring the length
of time
it takes the radio
signal to reach
it. Each satellite has an extremely accurate clock that tells the receiver
when the radio transmission started. The receiver
compares that time
against its own clock, thus it knows how long it took the radio signal
(travelling
atthe
speed of
lightl)
to reach it. If
you
knowtime and
speed,
then
you can
calculate distance. Once
you have this from three
satellites, then
the receiver
can determine position.
11
15 OK 01< OK OFF
13 OK OK OK OFF
02 OK OK OK ON
ë7 OK OK OK ON
26 BAD OK BAD ON
It takes three satellites to
determine position.
38 7
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