GLOBAL POSITIONING SYSTEMS

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GLOBAL POSITIONING SYSTEMS
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Introduction
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Introduction

• Geographic Positioning Systems have in many ways
  taken taken over the need to know where you are
  going.
• You can dial your destination in, and then be
  taken there, but once you get there, you may not
  have the slightest idea of where you are.

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Outline
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Outline

• Why GPS
• What is GPS
• How GPS works
• What you need to know about GPS
• What can you do with GPS
• Applications of GPS

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Why GPS at Sea?

• Location, and Information!

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Why GPS at Sea?

• There are no street signs or addresses at sea.
• There are an infinite number of courses, some of
  which will take you into dangerous waters, to get
  to a destination that has no address.

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What is GPS?
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What is GPS?

• Global Positioning System is that - a system -
  not a single unit!
• It was developed by the US Department of Defense
  at a cost of gt 12 billion.
• Funding was contingent on making the system
  available to the public.

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Global Positioning System

• There are three major components in this system
• Satellites
• Ground Control Stations
• GPS Receivers (or units)

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Satellites

• There are 24-32 satellites up at any given time
  orbiting the earth at 11,000 nautical miles.
• The EXACT location of each satellite is known,
  and they have VERY accurate clocks.
• The satellites continuously transmit radio
  signals towards earth.
• These radio signals are picked up by the
  individual GPS receivers.

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Control Stations

• There are five control stations that continuously
  track satellites, transmit information to and
  update the positions of each satellite.
• Without control stations, the accuracy of the
  system would degrade in a matter of days.

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GPS Receivers

• GPS units are referred to as receivers.
• They receive information (radio signals) from
  satellites.

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GPS Receivers

• The GPS receiver knows how long it takes the
  signal to travel from the satellite to the
  receiver.
• The Receiver is therefore able to calculate its
  distance from the satellite.
• Distance time x velocity
• Distance time x 186,355 mi/sec

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GPS Receivers

• The receiver can calculate the time that signal
  traveled from the satellite to the receiver.
• The receiver is therefore able to determine its
  exact distance from the satellite.

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How GPS Works

• GPS uses triangulation
• You can find any point if given distances from 3
  other points

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How GPS Works
If your satellite receiver picks up a signal from
the yellow satellite, you must be somewhere on
the yellow circle. If you're also picking up
signals from the blue and red satellites, you
must be at the black dot where the signals from
the three satellites meet.
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Simplified Triangulation

• One satellite tells you that you are 64 km from
  Nanaimo, BC

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Nanaimo
64km
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• If the GPS receiver obtains two satellites, it
  tells you that you are also 16 km from Sea Island.

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Nanaimo
Sea Island
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x
Nanaimo
Sea Island
x
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• A third satellite tells you that you are 49 km
  from Duncan, BC.

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Nanaimo
Sea Island
Duncan
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Nanaimo
Sea Island
x
Duncan
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• These three position circles will give you a
  position at the Tsawwassen Ferry Terminal (Gift
  shop)
• A fourth satellite is required to determine exact
  location and elevation.

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What you need to know about GPS
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What you need to know about GPS

• Signal Accuracy Issues
• Selective Availability
• Tricks of the Trade

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Signal Accuracy

• There are 2 types of GPS Signals
• P-code (Precise code)
• This is only available to the military and some
  selected public officials.
• Very precise, not degraded.
• C-code (Civilian Code).
• Less precise
• Signal can be degraded (by scrambling the signal)
  especially in times of conflict.

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Selective Availability (SA)

• For national security reasons, the military
  sometimes degrades the C-code signal. This is
  called selective availability.
• These errors are random
• Errors be as high as 300 feet

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Selective Availability

• SA errors can put you on the wrong side of a
  stream, or even a different city block or street!
• 300 feet is a lot of real estate!!!

The GPS tells you that you are located here But
your real location is here
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Selective Availability

• It is possible to correct for Selective
  Availability (as well as other inherent signal
  errors).
• However, SA has not been employed for many years.
  If it was, there would be media attention since
  it would affect car GPS systems and many others.
• This process is called Differential Correction

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Differential Correction

• There are already established base stations
  established around the U.S., whose position has
  been determined precisely.
• Each base station has a GPS receiver, which
  collects incoming (possibly slight incorrect)
  signals, and determines a position.
• The true (surveyed) position is then compared to
  the GPS coordinates.
• The correction values are then sent to other GPS
  receivers in the field.

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WAAS

• The Wide Area Augmentation System (WAAS) is a
  differential GPS system that is being constructed
  to support GPS accuracy in aircraft.
• WAAS also provides additional accuracy on the
  ground.

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WAAS
Note Not all GPS receivers are WAAS compatible.
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Averaging

• Averaging A GPS receiver can collect points
  continuously for 15-30 seconds.
• The receiver can then average all these locations
  together
• This only works when you are standing still!!

GPS Collected Points
GPS Averaged Position
True location
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Errors
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Positional Dilution of Precision

• It is better for your receiver to get a fix on 4
  satellites spread around the horizon, than 4
  clustered satellites.

Good Satellite Distribution
Poor Satellite Distribution
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Multipath Error

• Try and stay away from buildings and other
  structures when using a GPS receiver.
• Satellites may not be visible.
• This can introduce error.

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Loss of Satellite Signal

• GPS has worldwide coverage
• HOWEVER
• You can lose satellite coverage (or received
  degraded signals) in areas with dense foliage, in
  urban canyons, etc.
• You may also lose satellite coverage (or receive
  degraded signals) in deep inlets, valleys or
  gorges.

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How Accurate is a GPS?

• Thats the million dollar question

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How Accurate is a GPS?
SA OFF 15m
SA ON 100m
YOU ARE HERE
WAAS 3m
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What Can You Do With a GPS?
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Speed

• The GPS can calculate your vessels speed over
  the ground.

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Waypoints

• Collect and store points (latitude and longitude
  positions) called WAYPOINTS.
• Download the points onto your computer and
  integrate them with other mapping programs

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Waypoints
Lat 50 24.00N Long 125 12.00W
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Routes

• Collect and store ROUTES
• Routes are similar to TRACKS, but are created by
  joining Waypoints together to set up a passage
  plan.

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Routes
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Tracks

• Collect and store the path that you have sailed,
  - this is called TRACKS.
• Activate Tracks and see where you have been.
• Monitor tracks for an efficient coverage of an
  assigned area in a search pattern.

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Man Overboard

• The Manoverboard is a particular GoTo function,
  which marks the position on the plotter when the
  MOB Button is pressed, to allow you to
  expeditiously return to that spot, and pick up
  someone who had fallen overboard.

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GoTo

• Using the GOTO function, the GPS will guide you
  to a predefined Waypoint (you choose which one)
  using a compass and pointer
• The GOTO/FIND function is like using Autopilot
• You can program the GPS to beep when you are
  within a certain distance of a selected Waypoint

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Tide Tables

• Many of the marine GPSs have built in tide
  tables. They provide tidal information and
  ranges for any date and any place.

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Elevation

• A pocket or vehicle GPS can give you your
  elevation.

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The Future

• The US Department of Defense is upgrading the
  existing GPS satellite constellation
• A new WAAS satellite was launched in early 2007
• -better coverage availability (i.e. in forested
  areas)
• -anticipated greater accuracy (even for the
  recreational grade GPS receivers)

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GPS Accuracy Getting Better

• Starting in 2009 there will be 12 European
  satellite system which will greatly increase the
  number of satellites.
• These European satellites are also more accurate
  than many of the existing.
• Russia is also launching new satellites

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GPS Receivers

• Continue to get better and better with
• -better antennae,
• -more efficient power consumption,
• -smaller
• -increasingly more bells whistles (maps,
  hard drives, etc.)
• -inexpensive

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What its not

• Perfect