Understanding GPS

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Understanding GPS
2
Overview

• What is GPS?
• How does it work?
• Different types of GPS
• Understanding coordinate systems
• Practical exercise

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What is GPS?

• The Global Positioning System (GPS) is a
  worldwide radio-navigation system formed from a
  constellation of 24 satellites and their ground
  stations.
• GPS uses these "man-made stars" as reference
  points to calculate positions accurate to a
  matter of meters. In fact, with advanced forms of
  GPS you can make measurements to better than a
  centimeter!
• In a sense it's like giving every square meter on
  the planet a unique address.
• GPS receivers have been miniaturized to just a
  few integrated circuits and so are becoming very
  economical. And that makes the technology
  accessible to virtually everyone.
• These days GPS is finding its way into cars,
  boats, planes, construction equipment, movie
  making gear, farm machinery, even laptop
  computers.
• Soon GPS will become almost as basic as the
  telephone. Indeed, many telephones now have GPS
  build in!

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History of GPS

• GPS was invented by the Ratheon corporation in
  the 1950s under the guidance of Dr. Ivan Getting
• Ratheon was responding to a solicitation from the
  US Air Force to create a guidance system for
  ICBMs.
• The original system launched during the 1960s
  used eighteen satellites, six in each of three
  orbital planes spaced 120º apart, and their
  ground stations.
• The system cost 12 BILLION!!!

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1960

• April 13 The first navigation satellite TRANSIT
  IB is launched for use by the U.S. Navy to
  accurately locate ballistic missile submarines
  and ships.
• May 15 Drs. Ivan Getting and Shep Arikin of
  Raytheon propose a radio-navigation system called
  MOSAIC (Mobile System for Accurate ICBM Control)
  to the U.S. Air Force.
• June 3 The Aerospace Corporation is established
  "to aid the United States Air Force in applying
  the full resources of modern science and
  technology to the problem of achieving those
  continuing advances in ballistic missiles and
  military space systems which are basic to
  national security." Dr. Ivan Getting becomes the
  company's first president.

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1963

• Project 57 begins at The Aerospace Corporation.
  The study seeks to clarify areas where space
  systems could be used for military applications.
  According to Dr. Ivan Getting, it was "in this
  study that the concept for GPS was born."
• Under the direction of the Air Force, the Project
  57 study becomes Project 621B, and Aerospace is
  asked to continue its work on determining
  navigation coordinates from satellite signals.
  Dr. Brad Parkinson notes that Project 621B "had
  many of the attributes that you now see in GPS.
  It has probably never been given its due credit."

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1964-66

• Aerospace scientists and engineers conduct a
  series of satellite navigation studies within the
  companys Systems Planning Division. These
  studies arrive at the operational concept for GPS
  as we know it today.

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• 1972
• November Air Force Col. Dr. Brad Parkinson is
  assigned by Gen. Ken Schultz to manage the 621B
  program. Parkinson's recognition that a synthesis
  of three competing satellite navigation proposals
  was needed marked the beginning of the first real
  progress toward the eventual approval of GPS by
  the Defense Department.
• 1973
• April U.S. Navy TIMATION system and the Air Force
  System 621B 3d navigation system combine in an
  effort to develop a Defense Navigation Satellite
  System, which would later become NAVSTAR or GPS.
• 1974
• August 17 The deputy secretary of defense
  suggests a program based on the GPS concept be
  established, marking the start of the
  conception-validation phase of the program.
• 1978
• February 22 After an initial launch failure, the
  first the GPS Block I satellites is launched.
  Block I comprised 10 developmental satellites
  launched from 1978 through 1989.

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• 1983
• May 20 The Air Force signs a 1.2 billion
  contract for the production of 28 GPS Block II
  satellites with Rockwell Space Systems.
• September A Korean civilian airliner is shot down
  by Russian fighters after accidentally intruding
  into Soviet air space. To prevent any such
  tragedy from happening again, President Ronald
  Reagan declassifies NAVSTAR GPS becomes
  available to civilians.
• 1985
• October 9 The last of the Block 1 satellites is
  launched.
• 1989
• February 14 The first of the GPS Block II
  production satellites is launched. From 1989 to
  1997, 28 production satellites are launched the
  last 19 satellites in the series are updated
  versions, called Block IIA.
• 1990
• December NAVSTAR GPS becomes operational.

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• 1991
• The Persian Gulf War enables American military
  forces to validate the usefulness of GPS in
  combat situations. Although not fully
  operational, GPS allows the military to obtain
  accurate coordinates in the featureless Iraqi
  desert and to achieve a quick victory.
• 1992
• The Aerospace Corporation, as part of the GPS
  team, receives the Collier Trophy, the nation's
  most prestigious aeronautical award for the work
  it has done developing GPS.
• 1994
• January 17 The last of the Block IIA satellites
  is launched, completing the GPS constellation.
• February 17 The Federal Aviation Administration
  announces that GPS is operational an integrated
  as a part of the U.S. air traffic control system.
• March 9 The Air Force announces the completion of
  the 24 Block II GPS satellite constellation.
• 1995
• April 27 Air Force Space Command declares the
  Block II NAVSTAR GPS constellation fully
  operational.
• 1996
• March 29 The National Security Councils Office
  of Science and Technology Policy details a
  comprehensive national policy for the use and
  management of GPS

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• 1997
• January 17 The Delta rocket carrying the first of
  the GPS Block IIR satellites explodes after
  liftoff.
• 2001-2003
• Military battles in Afghanistan following the
  September 11 attacks and during Operation Iraqi
  Freedom demonstrate the precision of GPS in
  military conflict.
• 2003
• October 11 Dr. Ivan Getting dies at the age of 91
  at his home in Coronado, California.
• 2004
• March Drs. Ivan Getting and Brad Parkinson are
  awarded the Charles Stark Draper Prize by the
  National Academy of Engineering.
• March 18 GPS satellite 2R-11 is dedicated to the
  late Dr. Ivan A. Getting, who envisioned these
  lighthouses in the sky serving all mankind.A
  plaque inscribed with his words is attached to
  the satellite.
• May
• Drs. Ivan Getting and Brad Parkinson are inducted
  into the National Inventors Hall of Fame.
• 2005
• The first of GPS Block IIF satellites are
  scheduled to launch.

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Elements of GPS

• GPS has three parts
• The space segment. The space segment consists of
  a constellation of 24 satellites (and about six
  "spares"), each in its own orbit 11,000 nautical
  miles above Earth.
• The user segment. The user segment consists of
  receivers, which you can hold in your hand or
  mount in a vehicle, like your car.
• The control segment. The control segment consists
  of ground stations (six of them, located around
  the world) that make sure the satellites are
  working properly. The master control station at
  Schriever Air Force Base, near Colorado Springs,
  Colorado, runs the system.

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A Constellation of Satellites

• An orbit is one trip in space around Earth.
• GPS satellites each take 12 hours to orbit Earth.
  
• Each satellite is equipped with an atomic clock
  so accurate that it keeps time to within three
  nanosecondsthats 0.000000003, or
  three-billionths, of a secondto let it broadcast
  signals that are synchronized with those from
  other satellites.

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• The signal travels to the ground at the speed of
  light. Even at this speed, the signal takes a
  measurable amount of time to reach the receiver.
  The difference between the time when the signal
  is received and the time when it was sent,
  multiplied by the speed of light, enables the
  receiver to calculate the distance to the
  satellite.
• To make this measurement as accurate as possible,
  the GPS navigation signals are specially designed
  to make it easy for GPS receivers to measure the
  time of arrival and to allow all the satellites
  to operate on the same frequency without
  interfering with each other.
• To calculate its precise latitude, longitude, and
  altitude, the receiver measures the distance to
  four separate GPS satellites. By using four
  satellites, the receiver calculates both its
  position and the time and doesn't need an
  expensive atomic clock like those on the
  satellites.

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Receivers

• GPS receivers can be carried in your hand or be
  installed on aircraft, ships, tanks, submarines,
  cars, and trucks.
• These receivers detect, decode, and process GPS
  satellite signals.
• More than 100 different receiver models are
  already in use.
• The typical hand-held receiver is about the size
  of a cellular telephone, and the newer models are
  even smaller and fit in a wristwatch or a
  Personal Data Assistant.

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• The commercial hand-held units distributed to
  U.S. armed forces personnel during the Persian
  Gulf War weighed only 28 ounces (less than two
  pounds).
• Since then, basic receiver functions have been
  miniaturized onto integrated circuits that weigh
  about one ounce.

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Ground Stations

• The GPS control segment consists of several
  ground stations located around the world.
• A master control station is located at Schriever
  Air Force Base in Colorado
• Six unsafe monitor stations Hawaii and Kwajalein
  in the Pacific Ocean Diego Garcia in the Indian
  Ocean Ascension Island in the Atlantic Ocean
  Cape Canaveral, Florida and Colorado Springs,
  Colorado
• Four large ground-antenna stations that send
  commands and data up to the satellites and
  collect telemetry back from them.

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• The monitor stations track the navigation signals
  and send their data back to the master control
  station.
• There, the controllers determine any adjustments
  or updates to the navigation signals needed to
  maintain precise navigation and update the
  satellites via the ground antennas.
• To further improve system accuracy, in 2005, the
  master control station added data from six
  monitor stations operated by the National
  Geospatial-Intelligence Agency to the six GPS
  monitor stations.

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How it works

• The principle behind GPS is the measurement of
  distance (or range) between the satellites and
  the receiver.
• The satellites tell us exactly where they are in
  their orbits by broadcasting data the receiver
  uses to compute their positions. It works
  something like this
• If we know our exact distance from a satellite in
  space, we know we are somewhere on the surface of
  an imaginary sphere with a radius equal to the
  distance to the satellite radius.
• If we know our exact distance from two
  satellites, we know that we are located somewhere
  on the line where the two spheres intersect.
• And, if we take a third and a fourth measurement
  from two more satellites, we can find our
  location. The GPS receiver processes the
  satellite range measurements and produces its
  position.

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Types of GPS

• Recreational
• Survey grade
• Mapping grade

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Recreational GPS

• As it exists today, the accuracy of an
  off-the-shelf, lower end GPS unit is about 10
  meters on a good day
• These systems only rely upon satellite timing
  signals to calculate position
• This degree of accuracy works just fine for most
  uses

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Survey-grade GPS

• Surveying requires increased accuracy
• Would you want the borders of your property to be
  /- 32 ft?
• Surveying practice uses the addition of a known
  surface point to refine the satellite positioning
  information.
• This process is called Real-Time Kinematic GPS
  (RTK) and uses a Differential GPS methodology

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DGPS

• Differential GPS is a process for corecting raw
  GPS signals
• It is sometimes is confused with other in-place
  systems such as those maintained by the US Coast
  Guard

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Survey-grade uses two GPS units

• One unit is called a Base
• The Base occupies a known point that has been
  surveyed and certified by the US Geodetic Survey
• The Base compares the satellite signals it is
  receiving with the location entered as the known
  point can creates a correction factor that
  reconciles the satellite position with the known
  position
• The other unit is called a Rover and is used to
  make the actual position measurement
• The Base send the Rover the correction
  information to refine the measurement to better
  than 1 cm accuracy!

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Survey grade GPS

• A typical survey grade GPS system costs 30,000
  55,000!!!
• Is there a middle ground?

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Mapping grade GPS

• As with DGPS, the WAAS system uses base stations
  at known reference points to calculate the
  accuracy of the GPS signal.
• This is accomplished at each of the 25 ground
  reference stations (Currently only in the US)
  receiving a standard GPS signal.
• A set of correction data deetermined from the
  difference between the GPS calculated position
  and the known position is transferred to one of
  two ground control stations that then uplink the
  data to the WAAS satellite.
• The WAAS (InMarSat) satellites then transmit this
  information back down to the GPS user using a
  GPS-like signal complete with the correction
  information.
• The GPS receiver then decodes this information
  and applies it to its calculated position to
  significantly improve the accuracy.
• WAAS is maintained by the US Coast Guard and is
  accurate to 3 meters!

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• Currently the system is only accurate in North
  America and primarily in the United States.
  However, the signal can be received over half of
  the world on the Inmarsat AOR-W and POR
  satellites.
• This means that in parts of the world not covered
  by the base station corrections, you will get a
  WAAS signal, but the corrections will place you
  well off your mark in Australia, South America
  and Europe, for example. This is why it is best
  to turn off the WAAS reception outside North
  America.

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2 Problems with GPS

• The Earth isnt round
• The surface of the Earth isnt flat
• Not such a big deal for horizontal measurements,
  but a nightmare for elevation measurements!

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Ellipsoids

• Ellipsoidal earth models are required for
  accurate range and bearing calculations over long
  distances. Loran-C, and GPS navigation receivers
  use ellipsoidal earth models to compute position
  and waypoint information. Ellipsoidal models
  define an ellipsoid with an equatorial radius and
  a polar radius. The best of these models can
  represent the shape of the earth over the
  smoothed, averaged sea-surface to within about
  one-hundred meters.
• Reference ellipsoids are defined by semi-major
  (equatorial radius) and semi-minor (polar radius)
  axes.
• Other reference ellipsoid parameters such as
  flattening, and eccentricity are computed from
  these two terms.

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Geoid Models

• The topographical surface of the earth is the
  actual surface of the land and sea at some moment
  in time. Aircraft navigators have a special
  interest in maintaining a positive height vector
  above this surface.
• Sea level is the average (methods and temporal
  spans vary) surface of the oceans. Tidal forces
  and gravity differences from location to location
  cause even this smoothed surface to vary over the
  globe by hundreds of meters.
• Gravity models attempt to describe in detail the
  variations in the gravity field. The importance
  of this effort is related to the idea of
  leveling. Plane and geodetic surveying uses the
  idea of a plane perpendicular to the gravity
  surface of the earth, the direction perpendicular
  to a plumb bob pointing toward the center of mass
  of the earth. Local variations in gravity, caused
  by variations in the earth's core and surface
  materials, cause this gravity surface to be
  irregular.

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Geoid Models, Contd

• Geoid models attempt to represent the surface of
  the entire earth over both land and ocean as
  though the surface resulted from gravity alone.
  Bomford described this surface as the surface
  that would exist if the sea was admitted under
  the land portion of the earth by small
  frictionless channels.
• The WGS-84 Geoid defines geoid heights for the
  entire earth.
• The U. S. National Imagery and Mapping Agency
  (formerly the Defense Mapping Agency) publishes a
  ten by ten degree grid of geoid heights for the
  WGS-84 geoid.
• By using a four point linear interpolation
  algorithm at the four closest grid points, the
  geoid height for any location can be determined.

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http//www.pbs.org/wgbh/nova/longitude/gps/