GPS Technology and Accuracy
By: Aaron Osicki
Overview
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What is the Global Positioning System (GPS)?
Who created the GPS?
When was the GPS created?
Why was the GPS created?
What does the GPS consist of?
How does the GPS work?
What errors are involved in GPS measurements?
What is the accuracy of GPS measurements?
What Is the Global Positioning System (GPS)?
GPS permits land, sea, and airborne users to
determine their three-dimensional position, velocity, and
time 24 hours a day, in all weather, anywhere in the world
with a precision and accuracy far better than other
radionavigation systems available today or in the
foreseeable future (http://gps.faa.gov/).
Who Created the GPS?
• The U.S. Department of Defense
– At an initial cost of around 12 billion dollars.
• It is currently being managed by the Interagency GPS
Executive Board (IGEB).
– Consisting of military and federal personnel.
When Was the GPS Created?
• The first GPS satellite was launched in 1978.
• 2nd generation (Block II) satellites started being launched
in 1989.
• The first full constellation of 24 satellites was achieved by
1994.
• Non-military accessibility to GPS technology permitted in
1983.
• Selective Availability (SA) switched off in May, 2000.
Why Was the GPS Created?
• Initially intended for military use for ICBM targeting.
– Intercontinental Ballistic and Cruise Missiles (ICBMs)
– During the arms race with the U.S.S.R.
– Need to know location information of launch source and
destination to accurately launch missiles.
– Most of the U.S. nuclear arsenal was in the ocean on submarines.
– The U.S. needed a way to allow the submarines to surface, fix their
exact position, launch the ICBMs, and submerge in a matter of
minutes.
– U.S.S.R. created a similar system GLONASS (Global Navigation
Satellite System).
What Does the GPS Consist of?
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Three main segments:
1.
Space Segment (Satellites)
2.
Control Segment (Ground Stations)
3.
User Segment (GPS Receiver)
Space Segment
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24 Satellites (4 Satellites visible at
any location on the Earth at any point in
time).
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Satellite orbit
– 20,200 Km
• Low orbital degradation
– 55o Angle to Equatorial Plane
• Full Earth Coverage
– 12 Hour Orbit
• Good temporal coverage
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Continually broadcasting
positional and time data.
GPS Satellite
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Name: NAVSTAR
Manufacturer: Rockwell International
Weight: 1900 lbs (in orbit)
Size:17 ft with solar panels extended
Planned Lifespan: 7.5 years
Current constellation: 24 Block II
production satellites
Future satellites: 21 Block IIrs
developed by Martin Marietta
General Equipment: Radio transmitter
Atomic Clock (nanosecond accuracy – one billionth
of a second), which use the oscillations of a particular
atom as their "metronome”.
Control Segment
• Ground Stations
– Monitor Satellites’ operational health and exact position in space.
• Master Control
– Transmits corrections of satellites’ ephemeris constant and clock
offsets (satellites incorporate this data into their signal).
How Does the GPS Work?
• Triangulation (Trilateration/Resection)
• Two Key Pieces of Information:
– The exact location of the satellite (provided)
– The satellite’s distance from the receiver (calculated)
1 Satellite – limits infinite possible locations to a sphere around the
satellite (at a given radius).
X Km
2 Satellites – defines the circumference of a 2-D circle as the possible
location.
3 Satellites – defines 2 points, one on the Earth’s surface and one far off in
space.
Either a 4th satellite or elevation data will identify the correct satellite.
Determining Distance
• Mathematical Formula:
Velocity * Time = Distance
Speed of Light (3*108 ms) * Average Time (0.06 s) = Average
Distance (18,000 Km)
GPS Method for Determining Distance
• Measure the lag in receiving a Pseudo Random Code
(PRC) from each satellite, which determines “time” in the
distance formula.
– Each satellite has two carrier frequencies
• L1 – 1575.42 MHz (civilian)
• L2 – 1227.6 MHZ (military)
– Coarse Acquisition (C/A) code modulates L1
» Repeats every 1023 bits
» Modulates at 1 MHz
– Precise (P) code (“Y” code when encrypted) Modulates both L1 and L2
» Repeats every 7 days
» Modulates at 10 MHz
GPS Measurement Errors
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Ionosphere & Troposphere Delay
Signal Multi-path
Receiver Clock Error
Orbital Error
Number of Visible Satellites
Satellite Geometry
Intentional Degradation of the Satellite Signal
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Differential GPS, Augmented GPS, Carrier-Phase GPS are various
alterations which attempt to eliminate much of the error associated
with GPS measurements.
GPS Measurement Accuracy
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100 meters
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15 meters
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Typical GPS position
accuracy without SA.
3-5 meters
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Accuracy of the original GPS system, which was subject to accuracy degradation
under the government-imposed Selective Availability (SA) program.
Typical differential
GPS (DGPS) position
accuracy.
< 3 meters
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Typical WAAS position
accuracy.
Useful Links
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National Air and Space Museum – GPS Page
http://www.nasm.edu/galleries/gps/intro.html
A Practical Guide to GPS - UTM
http://members.home.net/don.bartlett/gpsutm.htm#moreinfo
Global Positioning System (GPS) Resources
http://www.cnde.iastate.edu/staff/swormley/gps/gps.html
GPS World Online
http://www.gpsworld.com/
The Federal Aviation Administration - Global Positioning System Product Team
http://gps.faa.gov/
Garmin
http://www.garmin.com/
Trimble
http://www.trimble.com/index.html