Global Positioning Systems GPS

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Global Positioning SystemsGPS

• Steve Howard
• Central Virginia Governors School
• showard_at_cvgs.k12.va.us

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GPS Introduction

• GPS is a network of 24 NAVSTAR satellites
  orbiting the Earth at 11,000 miles
• Free to all users
• Developed by the US Dept of Defence
• These satellites launched from 1978-1994
• GPS provide specially coded satellite signals
  that can be processed by a receiver, enabling the
  receiver to compute position, velocity and time

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GPS Segments1 - satellite constellation

• 24 satellites transmitting signals (1575.42 MHz)
• 6 satellites can be seen at any time, anywhere
  on Earth
• Signal contains 3 different pieces of info
• Pseudo-random code which identifies each
  satellite along with timing data
• Ephemeris data
• Almanac data

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GPS Segments2 Operational Control Segment

• The OCS maintains the satellites in their proper
  positions and monitors satellite health and
  status
• Remote, unmanned monitor stations (Hawaii,
  Kwajalein-Pacific Ocean, Diego Garcia Indian
  Ocean, Ascention Island-Atlantic Ocean and
  Colorado Springs) constantly track and monitor
  signals sent from satellites and info to Master
  Control Station at Falcon Air Force Base in
  Colorado Springs
• MCS uploads navigation correction messages and
  other data to the satellites

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GPS Segments3 User receiving equipment

• GPS receivers process the signals from the
  satellites position, velocity and time
• Applications include
• Location
• Navigation
• Tracking
• Mapping
• Timing

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How GPS Works

• Trilateration
• Let's say that you are somewhere in the United
  States and you are TOTALLY lost -- you don't have
  a clue where you are. You find a friendly-looking
  person and ask, "Where am I?" and the person says
  to you, "You are 625 miles from Boise, Idaho."

You could be anywhere on a circle around Boise
that has a radius of 625 miles,
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How GPS Works

• Trilateration
• So you ask another person, and he says, "You are
  690 miles away from Minneapolis, Minnesota." This
  is helpful -- if you combine this information
  with the Boise information, you have two circles
  that intersect. You now know that you are at one
  of two points, but you don't know which one.

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How GPS Works

• Trilateration
• If a third person tells you that you are 615
  miles from Tucson, Arizona, you can figure out
  which of the two points you are at.
• GPS receivers work in a similar way by getting
  the distance from at least 3 satellites. A 4th
  satellite is needed for error correction and
  timing

Want more info? http//www.howstuffworks.com/gps.h
tm/
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How GPS Really Works

• In 3-Dimensions, if you know you are 11,000 miles
  from a satellite, you could be anywhere on a
  sphere
• 3 satellites leave you with 2 possible locations
  1 of which is usually impossible

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How to measure distance

• We assume both the satellite and the receiver are
  generating the same pseodo-random code at exactly
  the same time
• The receiver determines how out-of sync the 2
  codes are and then uses distrate x time, now
  knowing time and rate speed of light 186,000
  miles/sec

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How to measure distance

• BUT WAIT its not quite that easy. There is
  error involved.
• GPS signals pass through the ionosphere and
  troposphere, slowing them down and bending them
• The signals may also bounce off of local
  obstructions mountains, buildings, etc.
• The satellites orbit may also be off.

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What about that ERROR!

• Its important for receivers to fix the errors.
  At the speed of light, a timing error of 1/1000
  of a second results in a 200 mile location error!
• Readings from a 4th satellite can be used to
  approximate the timing error

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Another way to correct error WAAS (Wide Area
Augmentation System)

• Also called Differential GPS, WAAS uses
  monitoring stations that know where they are and
  where every satellite SHOULD be.
• The stations can then determine the timing
  differential in each satellite
• WAAS satellites then send this info out to GPS
  receivers

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Now lets try our hand at GPS

• Terms to know
• Location Fix
• Coordinate
• Waypoint
• Bearing
• GOTO