GPS
Orientation
THE OFFICIAL
NAME IS
NAVISTAR GPS
NAVISTAR GPS
NAVIgatonal
System using
Timing And
Ranging
Global
Positioning
System
GPS is a satellite
navigation system
developed by the US
Dept. of Defense
(DOD)
GPS provides
continuous,
all-weather, threedimensional positioning
information
(position, speed and time)
How does GPS work?
GPS is a satellite navigation system
developed by the US Dept. of Defense
(DOD).
 GPS provides continuous, all-weather,
three-dimensional positioning information
(position, velocity and time).


GPS is used by aviators, mariners,
surveyors, truck and auto operators, and
hikers.
5 ft wide
17.5 ft long
1,860 lbs
Delta II
Confused yet?
How GPS works
in a few logical
steps
The basis of GPS operation is ‘triangulation”
from the overhead satellites in view.
In order to
triangulate,
your GPS
receiver
measures the
distance from a
satellite using
the time of
travel of the
radio signals
from the
satellite.
In order to
measure time
of travel, a
GPS set needs
very accurate
time which is
achieved with
some
electronic
tricks.
Along with the distance, the
satellite must know exactly
where it is positioned
in space at all times.
High orbits and careful
monitoring are required.
All delays that the
GPS signal
experiences as it
travels through
the atmosphere
must be
corrected.
ANIMATION
Specifications
Accuracy
 GPS
10 meters(33 ft) 95% of time
 DGPS
< 10 meters (33 ft)
 Availability--99.85%
 Coverage--world
of time
wide
SUMMARY
 GPS
Fixes are not error free
– User and/or Satellite clock bias.
– Satellite signal can be blocked.
– Atmospheric interference is
possible.
– Poor satellite geometry is possible.
You must monitor these errors
on your GPS receiver.
Signal Quality and Accuracy
The height of the bars
in display shows
signal quality from
each satellite
The number of items
show the positions of
satellites currently in
view to your GPS.
Dilution of Precision
Good
Geometry
Poor
Geometry
Estimated Position Error
“EPE”
Many GPS units display “EPE”
It uses satellite geometry plus other
calculations to improve the
accuracy of the estimated position
error.
“EPE” can be expressed in feet or
meters. It is a user choice.
CEP = Circular Error Probable
SEP = Spherical Error Probable
SA – Selective Availability Error
DGPS
(operated by USCG)
enables local corrections
for errors
DGPS
Note: Stations on Great
Lakes not shown on this
illustration

Series of
coastal stations
broadcast
corrections to
GPS signal.

Former RDF
stations used
for this purpose.

Accuracy
improved to
better than 10
meters.
24 hours
5/1/2000
24 hours
5/2/2000
With
95 % of the time
within 45 meters
within 6.3 meters
or 148 Feet
or 21 Feet
WAAS – Wide Area Augmentation
System
System of ground-based reference stations
and two stationary satellites that broadcast
precision differential GPS corrections.
Typical GPS with SA OFF)
Be sure that the GPS
that
WAASyou purchase has
Less than 10
WAAS capability.
feet of error
Typical DGPS
Other things to
consider when
buying a GPS
set.
Intended Use
How are you going
Imagine your worst case scenario:
adjust
–to
Stormy
conditions. or read
– Waves beating on the hull.
GPS
– your
Boat bouncing
back andset
forth. in
– Windshield covered with salt spray.
these
conditions?
– You can’t let
go of the helm without

losing control of your boat.
Mounting Capability
Think about the
magnetic influence
that your GPS will
have on your
compass!
Mount near the helm.
You can’t
just so
lay
the
GPS
Swivel
capability
that
it can
be
used or read
others.
on by
the
dash.
Adjustable for sun glare.
Think
of
your
worst
case
Protected from the weather.
scenario.
Ability to view
satellites.
Screen Size
 Can
you read the display at a
distance?
 Avoid sets with tiny numbers and
letters.
 Some sets sacrifice readability for
more chart area on their screens.
 If you can’t clearly read the screen at
an arm’s length, you won’t be happy
with the set in stormy conditions.
Waterproof versus
Submergible
Waterproof means that the device will
survive a little dampness.
Water Resistant means it won’t last
long in a moist, damp marine
environment.
Submergible means that the device
will survive a dunking in the ocean.
Display Lighting
Buy a back-lighted display so you can
view it at night,
Color displays are usually easier to
read than black and white.
Viewable in bright sunlight.
Glare resistant screen.
12v Attachment
Be
sure your set comes
with a power cord that
will attach to your boat’s
12V system.
Carry spare batteries for
backup.
Chart Plotters
It is not
a safe
practice
to
to have
but not
really
benecessary.
gazing at a chart plotter
 Colored charts are a definite
monitor when you should
plus.
be
focusing
on
the
other
 Paper charts are easier to read in
boatssunlight.
and obstructions
bright
around you.
 Nice
Remote Antenna Need
GPS need a clear view of
the satellites.
May need a remote
antenna on your boat.
Remote
Used Sets
The price is so low.
buy new!
The best price for a
used set is FREE.