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What is GPS?The Global Positioning System (GPS) is a satellite based navigational aid. Essentially it is a radio positioning navigation and time transfer system. It provides accurate information on position, velocity and time of an object or a platform at any moment, anywhere in the globe.
A Constellation of Earth-Orbiting Satellites Maintained by the United States Government for the Purpose of Defining Geographic Positions On and Above the Surface of the Earth. It consists of Three Segments:
Control Segment
Space Segment
User Segment
Position and coordinates.
The distance and direction between any two waypoints, or a position and a waypoint.
Travel progress reports.
Accurate time measurement.
Four Basic Functions of GPS
•Control Segment
•Space Segment
•User Segment
Three Segments of the GPSThree Segments of the GPS
•Monitor Stations
•GroundAntennas
•Master Station
•Kwajalein Atoll
•US Space Command
Control SegmentControl Segment
•Hawaii
•Ascension Is. •Diego Garcia
•Cape Canaveral
•Ground Antenna•Master Control Station •Monitor Station
•Falcon AFB, Colorado springs
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Control Segment: Maintaining the System
(5) Monitor Stations
• Correct Orbitand clockerrors• Create new navigation message
• Observeephemerisand clock
Falcon AFBUpload Station
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Space Segment 24+ satellites
– 6 planes with 55° inclination
– Each plane has 4-5 satellites
– Broadcasting position and time info on 2 frequencies
– Constellation has spares Very high orbit
– 20,200 km– 1 revolution in approx. 12 hrs– Travel approx. 7,000 mph
Considerations– Accuracy – Survivability– Coverage
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Military. Search and rescue. Disaster relief. Surveying. Marine, aeronautical and terrestrial navigation. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). Recreation.
User Segment
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How the system works
Space Segment24+ Satellites
The Current Ephemeris is Transmitted to UsersMonitor
Stations• Diego Garcia• Ascension Island• Kwajalein • Hawaii• Colorado Springs GPS Control
Colorado Springs
End User
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Triangulation
Satellite 1 Satellite 2
Satellite 3 Satellite 4
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Position is Based on TimePosition is Based on Time
•T + 3
•Distance between satellite and receiver = “3 times the speed of light”
•T
•Signal leaves satellite at time “T”
•Signal is picked up by the receiver at time “T + 3”
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Distance Measuring
Rate = 186,000 miles per second (Speed of Light)
Time = time it takes signal to travel from the SV to GPS receiver
Distance = Rate x Time
Each satellite carries around four atomic clocks
Uses the oscillation of cesium and rubidium atoms to measure time
Accuracy?
plus/minus a second over more than 30,000 years!!
The whole system revolves around
time!!!
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SV and Receiver Clocks
• SV Clocks– 2 Cesium & 2 Rubidium in each SV– $100,000-$500,000 each
• Receiver Clocks– Clocks similar to quartz watch– Always an error between satellite
and receiver clocks ( t)
• 4 satellites required to solve for x, y, z, and t
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• PROBLEM– Can’t use atomic
clocks in receiver
• SOLUTION– Receiver clocks
accurate over short periods of time
– Reset often– 4th SV used to
recalibrate receiver clock
Cesium Clock = $$$$$$$!!!
Size of PC
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PRN code
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•GPS Satellite Signals
•• The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers.•• Three binary codes shift the L1 and/or L2 carrier phase. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidth.•The C/A code repeats every 1023 bits (one millisecond). There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS.•• The P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P (Y)-Code is the basis for the PPS. The Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters.
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Breaking the Code
Transmission Time
Receiver
The Carrier Signal...
combined with…
The PRN code...
produces the
Modulated carrier signal which is transmitted... demodulated...
And detected by receiver, Locked-on, but With a time delay...
Time delay
Satellite
Pseudo Random Noise CodePseudo Random Noise Code
•Receiver PRN
•Satellite PRN
•Time Difference
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Accuracy and Precision in GPS
• Accuracy– The nearness of a measurement to the
standard or true value
• Precision– The degree to which several
measurements provide answers very close to each other.
What affects accuracy and precision in GPS?
Sources of GPS Error• Standard Positioning Service (SPS ): Civilian Users
• Source Amount of Error Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters Selective Availability (see notes) User error: Up to a kilometer or more
• Errors are cumulative and increased by PDOP.
Receiver Errors are Cumulative!Receiver Errors are Cumulative!
•User error = +- 1 km
•System and other flaws = < 9 meters
Sources of Signal InterferenceSources of Signal Interference
•Earth’s Atmosphere
•Solid Structures
•Metal •Electro-magnetic Fields
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Sources of Error• Clock Error
– Differences between satellite clock and receiver clock
• Ionosphere Delays– Delay of GPS signals as they pass through the layer of charged ions and free electrons known as the ionosphere.
• Multipath Error–Caused by local reflections of the
GPS signal that mix with the desired signal
Direct
Sig
nal
Ref
lect
ed S
igna
l
GPSAntenna
Reflected Signal
Hard Surface
Satellite
GPS Satellite Geometry
Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration.
Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver.
There are five distinct kinds of DOP. Position Dilution of Precision (PDOP) is the DOP value used
most commonly in GPS to determine the quality of a receiver’s position.
It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation.
Some GPS receivers allow DOP to be manipulated by the user.
Ideal Satellite GeometryIdeal Satellite Geometry•N
•S
•W •E
Good Satellite GeometryGood Satellite Geometry
Good Satellite GeometryGood Satellite Geometry
Poor Satellite GeometryPoor Satellite Geometry•N
•S
•W •E
Poor Satellite GeometryPoor Satellite Geometry
Poor Satellite GeometryPoor Satellite Geometry
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Sources of Error• Geometric Dilution of
Precision (GDOP) – Describes sensitivity of receiver to changes in the geometric
positioning of the SVs
• The higher the DOP value, the poorer the measurement
QUALITY DOP
Very Good 1-3Good 4-5Fair 6Suspect >6
•DGPS Site
•x+30, y+60
•x+5, y-3
•True coordinates = x+0, y+0
•Correction = x-5, y+3
•DGPS correction = x+(30-5) and y+(60+3)
•True coordinates = x+25, y+63
•x-5, y+3
Real Time Differential GPSReal Time Differential GPS
•DGPS Receiver•Receiver
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Differential GPS • Method of removing errors that affect GPS
measurements
• A base station receiver is set up on a location where the coordinates are known
• Signal time at reference location is compared to time at remote location
• Time difference represents error in satellite’s signal
• Real-time corrections transmitted to remote receiver– Single frequency (1-5 m)– Dual frequency (sub-meter)
• Post-Processing DGPS involves correcting at a later timeReference location
Remote location
= Error
www.ngs.noaa.gov/OPUS
Online post-processing
•Wide Area Augmentation System•Wide Area Augmentation System•Geostationary WAAS satellites
•GPS Constellation
•WAAS Control Station (West Coast)
•Local Area System (LAAS)
•WAAS Control Station (East Coast)
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Wide Area Augmentation System (WAAS)
• System of satellites and ground stations that provide GPS signal corrections
• 25 ground reference stations across US
• Master stations create GPS correction message
• Corrected differential message broadcast through geostationary satellites to receiver
• 5 Times the accuracy (3m) 95% of time
• Only requires WAAS enabled GPS
•How good is WAAS?•How good is WAAS?
•+ -3
meters
•+-15 meters
•With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.*
•Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.*
•* Precision depends on good satellite geometry, open sky view, and no user induced errors.
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Differential GPS
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