NAVSTAR GPS

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NAVSTAR GPS
GPS BASICS
Mike Mickelson KD8DZ 08 Dec. 2009
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GPS Global Positioning System Satellites are used
by the NAVSTAR Global Positioning System

• The first NAVSTAR GPS Satellite (NAVSTAR 1) was
  launched on Ferbruary 22nd 1978. The oldest
  functioning GPS Satellite dates from 1990.
• The GPS satellite constellation is operated by
  the 50th Space Wing of the USAF.

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Constellation of 24 to 32 GPS Satellites. Which
are monitored and controlled by ground stations.
These Orbit the earth once every 12 hours,
Which allows satellites to be updated twice a
day. In addition there is the Wide Area
Augmentation System of Geostationary Satellites,
WAAS which are updated every 5 seconds from
ground stations.
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• Satellite numbers
• Block Launch Period Satellites
  launched Currently in service
• I 19781985 10 1 1 0
• II 19851990 9 0
• IIA 19901997 19 11
• IIR 19972004 12 1 1 12
• IIR-M 20052009 8 7
• IIF 20092011 0 10 2 0
• IIIA 2014? 0 12 3 0
• IIIB 0 8 3 0
• IIIC 16 3
• Total 58 2 110 236 3 30
• 1 Failed
• 2 In preparation
• 3 Planned
• Last update 24 November 2009

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Satellite Complement, Software, and Hardware

• The basic GPS system consists of a constellation
  of between 24 to 32 satellites in medium Earth
  orbit. Medium Earth orbit lies between 1243
  miles and 22,236 miles.
• GPS Orbits at around 12,552 miles (20,200 km.
• Geostationary orbit at 22,236 miles (35,786 km.
• Each GPS satellite contains several cesium atomic
  clocks (usually 4) having a precision on the
  order of a few nanoseconds. One nanosecond is
  the time it takes light in a vacuum to travel
  30cm or about one foot.)
• Each satellite also contains its ephemeris and an
  almanac. The ephemeris contains the coordinate
  information for the satellite. The almanac
  contains other information about all the
  satellites.
• The satellite is also equipped with transmitters
  and receivers to allow for making corrections to
  the satellites clock, ephemeris, and almanac.
  These corrections are received from ground
  stations and rebroadcast to the GPS Receivers.

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GPS RADIO FREQUENCIES

• L1 (1575.42 MHz) Mix of Navigation Message,
  coarse-acquisition (C/A) code and encrypted
  precision P(Y) code.
• L2 (1227.60 MHz) Military access only.
• L3 (1381.05 MHz) Used by the Nuclear Detonation
  Detection System Payload for signal detection of
  nuclear detonations and other high-energy
  infrared events. Used to enforce nuclear test ban
  treaties.
• L4 (1379.913 MHz) Being studied for additional
  ionospheric correction.
• L5 (1176.45 MHz) Proposed for use as a civilian
  safety-of-life (SoL) signal. This frequency
  falls into an internationally protected range for
  aeronautical navigation, promising little or no
  interference under all circumstances.

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How Does NAVSTAR GPS Work?
Five Concepts

• Triangulation for satellites is the basis of the
  system.
• To triangulate, GPS measures distance using the
  travel time of a radio signal at the speed of
  light as modified by the Ionosphere and the
  Earths atmosphere.
• 3. To measure the travel time, GPS needs very
  accurate clocks.
• 4. Once you know the distance to a satellite, you
  then need to know where the satellite is in space
  using the on board ephemeris and almanac.
• As the GPS signal travels through the Ionosphere
  and the Earths atmosphere, corrections need to
  be made.

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Triangulation Steps
X
Surface of a Sphere of Radius R.
R
R c ?t
X GPS Satellite R distance from GPS
Satellite to GPS receiver. c speed of
light (radio signal) ?t time difference between
GPS Satellite clock and GPS Receiver clock.
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SIGNALS GENERATED BY THE SATELLITE
Pseudo Random Code
Navigational Message
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Clock from GPS Satellite
?t
GPS Receiver Clock
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ERROR PROPAGATION

• For example, an timing error of 0.001 seconds
  (one ms) results in a distance error of
  approximately 186 miles.
• Code Phase error. Up to 1 microsecond results in
  about 1000 feet. GPS receivers easily correct to
  1, or about 10 to 20 feet.

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INTERSECTION SPHERES FROM 2 SATELLITES
Sphere of Radius R1
Sphere of Radius R2
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3D Diagram of the intersection of two spheres of
radii R1 and R2. The GPS Receiver is anywhere on
the Circle of intersection of the two spheres.
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• The addition of a third sphere reduces the
  uncertainty to two points. At least one of which
  is on the surface of the Earth.
• The addition of a fourth sphere pinpoints the
  location of the GPS Receiver uniquely within the
  inherent errors of the system.
• The fourth satellite allows clock
  synchronization. Which allows the GPS unit to
  have Atomic Clock Accuracy.

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CORRECTIONS

• Most GPS Receivers track at least 12 GPS
  Satellites (12 channel receiver) if visible above
  the horizon. (For example the DeLorme LT-40 has
  a 16 channel receiver.)
• DGPS Differential GPS comes in two flavors.
• Fixed station at a known geographic location with
  several Rovers. Corrections are real time or
  post processed.
• SBAS Satellite Based Augmentation Systems such
  as WAAS (Wide Area Augmentation System) augmented
  GPS Receivers include real time corrections from
  the Geostationary WAAS Satellites above the
  continental US and Hawaii or EGNOS in Europe.
  These systems provide improved accuracy and are
  another form of Real Time Corrections.

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Programs such as the Delorme Street Atlas and GPS
units with NMEA 0183 Serial Protocol can be used
to provide real time geographical location.
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GPS Accuracy

• Older GPS units with Selective Availability, (SA)
  ON. 30 ft. 10m
• Older GPS unit with SA OFF, as of 2000, 15 feet
  (3-5m)
• Differential GPS Uses post processed data from a
  GPS Ground Station of known precise location.
  1 ft. (0.3m) Data is usually post processed.
• GPS-WAAS. Typical 3 to 6 ft. (1 to 2 meter
  accuracy).
• Carrier Phase wave GPS. Combines data from
  precisely located base station. Sub inch
  accuracy (few millimeters) used these days by
  surveyors.

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SOME USEFUL WEB SITES

• http//www.trimble.com/gps/sub_phases.shtml
  Trimble gps Tutorial.
• http//www.gps.gov/ This site describes some of
  the 21st Century GPS Augmentation on the horizon