gps global positioning system diana cooksey, montana state university, lres department
TRANSCRIPT
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GPSGlobal Positioning System
Diana Cooksey, Montana State University, LRES Department
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Overview
• What is GPS & how does it work?– Satellites
– Radio signals
– Almanacs
– Timing
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What is GPS?
• Satellites orbiting the earth
• Positioning, navigation and timing
• Operates 24 hrs/day
• Used for any application requiring location information
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GPS Constellations
• United States– NAVSTAR GPS (Navigation Satellite Timing &
Ranging system); 28 satellites
• European Union– Galileo; 30 satellites
• Russia– Global Navigation Satellite System (GLONASS);
24 satellites (10 healthy)
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GPS Segments
UserControl
Space
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Space Segment: GPS Satellites
• Power– Sun-seeking solar panels– Nicad batteries
• Timing– 4 atomic clocks
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Satellite Orbits
• Orbit the earth at approx. 20,200 km (11,000 nautical miles)
• Satellites complete an orbit in approximately 12 hours
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Satellite Signals
• Radio signals, 2 frequencies
• Two levels of service– Standard Positioning Service
(SPS)– Precise Positioning Service
(PPS)
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Satellite Signals
• Radio signals contain– Unique pseudorandom code– Ephemeris– Clock behavior and clock
corrections– System time– Status messages– Almanac
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Satellite Signals
• Require a direct line to GPS receivers
• Cannot penetrate water, soil, walls or other obstacles
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Satellite Almanac
• Sent along with position and timing messages
• Prediction of all satellite orbits
• Needed to run satellite availability software
• Valid for about 30 days
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Control Segment: US DoD Monitoring
Colorado Springs
Hawaii
AscensionDiego Garcia
Kwajalein
Orbits precisely measured
Discrepancies between predicted orbits (almanac) and actual orbits transmitted back to the satellites
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User Segment
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How Does GPS Work? Calculating a Position
• GPS receiver calculates its position by measuring the distance to satellites (satellite ranging)
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Measuring Distance to Satellites
• 1. Measure time for signal to travel from satellite to receiver
• 2. Speed of light x travel time = distance
• Distance measurements to 4 satellites are required to compute a 3-D position (latitude, longitude and altitude)
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Measuring Satellite Signal Travel Time
• How do we find the exact time the signal left the satellite?– Synchronized codes
Timedifference
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One measurement narrows down our position to the surface of a sphere
12,000 mileradius
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A second measurement narrows down our position to the intersection of two spheres
11,000 mileradius
12,000 mileradius
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A third measurement narrows down our position to just two points
12,000 mileradius
11,500 mileradius
11,000 mileradius
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Correcting for Timing Offset
• The first three measurements narrow down our position
• A fourth measurement is needed to correct for timing offset (difference in synchronization between satellite and receiver clocks)– Satellites use highly accurate atomic clocks– Receivers use accurate quartz clocks
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6 seconds4 seconds
AB
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6 seconds4 seconds
5 seconds(wrong time)
7 seconds(wrong time)
AB
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5 Things to Take Away
1. 3 GPS segments
2. Satellites transmit radio signals containing– Unique pseudorandom code
– Ephemeris
– Clock behavior and clock corrections
– System time
– Status messages
– Almanac
3. Formula for satellite ranging (D = t ∙ v)
4. 4 satellites to compute an accurate 3-D position (the 4th measurement is needed to correct for timing offset)
5. We are not the only country with a GPS system
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Overview
• How accurate is GPS?
– Error sources
– Differential correction
– Accuracy levels
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GPS Error
• Atmospheric effects
• Multipath
• Satellite geometry
• Measurement noise (receiver error)
• Ephemeris data
• Satellite clock drift
• Selective availability (SA)
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Ionospheric & Tropospheric Refraction
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Multipath
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Satellite GeometryGeometric Dilution of Precision (GDOP)
• GDOP can magnify or lessen other GPS errors
• Wider angles better measurements
• Components of GDOP– HDOP; H=horizontal lat/long– VDOP; V=vertical altitude– TDOP; T=time clock offset
PDOP values
<=4 excellent
5-8 acceptable
>=9 poor
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Dilution of Precision (DOP)
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Ephemeris Data
• A satellite’s positions as a function of time
– Each satellite broadcasts its individual ephemeris
– Can contain orbital position errors
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Selective Availability (SA)
• The accuracy of GPS signals was intentionally degraded by the DoD
•
• SA was the largest component of GPS error
• SA was turned off on May 1, 2000
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GPS Error Budget
• Ionosphere..................................5.0 meters (0.4)• Troposphere................................0.5 meters (0.2)• Ephemeris data..............................2.5 meters (0)• Satellite clock drift........................1.5 meters (0)• Multipath....................................0.6 meters (0.6)• Measurement noise.......... ..........0.3 meters (0.3)• Selective availability.....................30-100 meters
• Total.................................................~ 10 meters
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Differential Correction
• GPS receiver on the ground in a known location (base station)
• Acts as a static reference point1. Transmits error correction messages to other
GPS receivers in the local area (real-time)
2. Differential correction can be done on computer after GPS data are collected (post-processed)
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Roving receiver(unknownposition)
Base receiver(knownposition)
Radio link for real-time DGPS
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How accurate is GPS?• Recreational and mapping grade.........................10-15 m
– C/A code– Autonomous
• Recreational and mapping grade.............................1-5 m– C/A code– With differential correction
• Submeter mapping grade.............................10 cm to 1 m– C/A code & carrier– With differential correction
• Survey grade.............................................................1 cm– Dual frequency– Advanced survey methods
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Six Main Sources of GPS Error
• Atmospheric effects
• Multipath effects
• Satellite geometry
• Measurement noise
• Ephemeris data
• Satellite clock drift
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Things to Take Away
• 6 major sources of error affect the accuracy of GPS positions
– Atmospheric error largest source
– Previously SA
• Almanac and ephemeris data are different
• Differential correction increases accuracy