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GPS Global Positioning System Diana Cooksey, Montana State University, LRES Department
GPSGlobal Positioning System
Diana Cooksey, Montana State University, LRES Department
Overview
• What is GPS & how does it work?– Satellites
– Radio signals
– Almanacs
– Timing
What is GPS?
• Satellites orbiting the earth
• Positioning, navigation and timing
• Operates 24 hrs/day
• Used for any application requiring location information
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)
GPS Segments
UserControl
Space
Space Segment: GPS Satellites
• Power– Sun-seeking solar panels– Nicad batteries
• Timing– 4 atomic clocks
Satellite Orbits
• Orbit the earth at approx. 20,200 km (11,000 nautical miles)
• Satellites complete an orbit in approximately 12 hours
Satellite Signals
• Radio signals, 2 frequencies
• Two levels of service– Standard Positioning Service
(SPS)– Precise Positioning Service
(PPS)
Satellite Signals
• Radio signals contain– Unique pseudorandom code– Ephemeris– Clock behavior and clock
corrections– System time– Status messages– Almanac
Satellite Signals
• Require a direct line to GPS receivers
• Cannot penetrate water, soil, walls or other obstacles
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
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
User Segment
How Does GPS Work? Calculating a Position
• GPS receiver calculates its position by measuring the distance to satellites (satellite ranging)
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)
Measuring Satellite Signal Travel Time
• How do we find the exact time the signal left the satellite?– Synchronized codes
Timedifference
One measurement narrows down our position to the surface of a sphere
12,000 mileradius
A second measurement narrows down our position to the intersection of two spheres
11,000 mileradius
12,000 mileradius
A third measurement narrows down our position to just two points
12,000 mileradius
11,500 mileradius
11,000 mileradius
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
6 seconds4 seconds
AB
6 seconds4 seconds
5 seconds(wrong time)
7 seconds(wrong time)
AB
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
Overview
• How accurate is GPS?
– Error sources
– Differential correction
– Accuracy levels
GPS Error
• Atmospheric effects
• Multipath
• Satellite geometry
• Measurement noise (receiver error)
• Ephemeris data
• Satellite clock drift
• Selective availability (SA)
Ionospheric & Tropospheric Refraction
Multipath
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
Dilution of Precision (DOP)
Ephemeris Data
• A satellite’s positions as a function of time
– Each satellite broadcasts its individual ephemeris
– Can contain orbital position errors
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
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
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)
Roving receiver(unknownposition)
Base receiver(knownposition)
Radio link for real-time DGPS
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
Six Main Sources of GPS Error
• Atmospheric effects
• Multipath effects
• Satellite geometry
• Measurement noise
• Ephemeris data
• Satellite clock drift
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
