Coastal Navigation using GPS by dandanhuanghuang


									Session 7
Outline of Presentation
 GPS, how it works & what you
  need to check.
 Basic receiver functions –
  what is it telling you?
 Selecting a GPS receiver.
      How does GPS work?
 GPS is a satellite navigation system
  developed by the US Dept. of Defense
 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
5 ft wide
17.5 ft long
1,860 lbs
Delta II
Confused yet?
How GPS works
 in a few logical
The basis of GPS operation is „triangulation”
from the overhead satellites in view.
   In order to
    your GPS
 measures the
distance from a
 satellite using
   the time of
  travel of the
 radio signals
    from the
  In order to
measure time
  of travel, a
GPS set needs
very accurate
time which is
achieved with
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

 GPS   10 meters(33 ft) 95% of time

 DGPS   < 10 meters (33 ft)

 Availability--99.85%   of time

 Coverage--world   wide
 GPS   positions are not error free
 – User and/or Satellite clock bias.
 – Satellite signal can be blocked.
 – Atmospheric interference is
 – Poor satellite geometry is possible.
   You can monitor these errors
     with 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
             Dilution of
              Precision (DOP)
              is a measure of
 Good         satellite fix
Geometry      geometry quality
             Figure at far left
              has lower (better)
              DOP Poor
             GPS receivers
              measure and
              display DOP
Estimated Position Error
 Many  GPS units display EPE
 – It uses satellite geometry plus
   other calculations to improve
   accuracy of the estimated position
 – EPE is expressed in feet or meters
   – user choice
                    CEP = Circular Error Probable
                    SEP = Spherical Error Probable

SA – Selective Availability Error
         24 hours                   24 hours
         5/1/2000                   5/2/2000


                95 % of the time
within 45 meters          within 6.3 meters
   or 148 Feet                 or 21 Feet
  (operated by USCG)
enables local corrections
        for errors
 DGPS                        Series of
                              coastal stations
                              corrections to
                              GPS signal.
                             Former RDF
                              stations used
                              for this purpose.
                             Accuracy
                              improved to
Note: Stations on Great       better than 10
Lakes not shown on this       meters.
WAAS – Wide Area Augmentation

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 you purchase has
            Less than 10
  WAAS capability.
             feet of error

       Typical DGPS
Other things to
consider when
buying a GPS
         Intended Use

How are you going
Imagine your worst case scenario:
 to adjust
 – Stormy conditions. or read
 – Waves beating on the hull.
 – your GPS set in
   Boat bouncing back and forth.
 – Windshield covered with salt spray.
 – You can‟t conditions?
these let go of the helm without
    losing control of your boat.
 Mounting Capability
 Think about the
Mount near the helm.
You can‟t just lay the GPS
Swivel capability so that it can be
magnetic influence
          on the dash.
used or read by others.
Adjustable for sun glare.
that your GPS has
              your weather.
 Think offrom theworst case
on your compass!
Ability to view satellites.
           Screen Size
 Can  you read the display at a
 Avoid sets with tiny numbers and
 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.
RayNav 300
     Waterproof versus
Waterproof means that the device will
 survive a little dampness.
Water Resistant means it won‟t last
 long in a moist, damp marine
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
Key and Keyboard Size

Keys  too small for your
Lack of keys.
The argument for pre-
 planning your trips.
GPS Age and Processor Size
The faster a GPS calculates, the
 Smaller computing
more accurate the data that it
 Slower to calculate.
Most GPS projections are averages
of past activity.
 Less options.
Should be regenerating position at
 No DGPS or WAAS.
least once per second.
         Chart Plotters
 It is not a safe practice to
 Nice  to have but not really
be gazing at a chart plotter
 Colored charts are a definite
monitor when you should
  be focusing on the other
 Paper charts are easier to read in
   boats and obstructions
  bright sunlight.
           around you.
RayChart 420
         Display Clutter

 Too  much data being presented on a
What you eliminate
    small screen.
 Have to adjust your screen area
in the close-up to remove
 some data.
views, you also lose
Such as, depth readings, aids to
   in the wide-area
 navigation, etc.

     display views.
    Screen Capabilities

  How easy is it to switch on a GPS
Minimize your dependencyfrom one
screen. to another?
  Focus handling your boat.
Focus onon a few navigation screens
   for everyday use.
Pre-plot your trip.
  Avoid having to jump a multiple
Use a nautical chart and totrip log.
   screens for basic navigation data.
Use your GPS as a quality control
check for this process.
 Remote Antenna Need

GPS   need a clear view
 of the satellites.
May need a remote
 antenna on your boat.
   Multiplexing vs Parallel

    Only buy a
 Multiplexing   receivers often have
  only one channel to tune into
   parallel GPS
 Parallel receivers can acquire several
  satellites at a time. Come in 3 – 5 – 8

  and 12 channel versions.
  – More accurate processing
  – Quicker start ups.
   GPS Sensor Design

 NMEA   – National Marine
  Manufacturers Association
 Standard sensor capability to
  RADAR, Chart Plotters, Depth
  Sounders, etc.
 Not all sensors are compatible to
  other manufacturer‟s equipment.
 Check to be certain.
     Used Sets

The price is so low.
 buy new!
The best price for a
 used set is FREE.
        GPS Tips
GPS looks like a compass
GPS acts like a compass
Therefore it must be a

Any questions about?

AV Task 6-1 Differences between
GPS, DGPS and WAAS – page 139.

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