Global Positioning System (GPS)
GPS BasicsGPS stands for Global Positioning System which measures 3-D locations on Earth surface using satellites
GPS operates using radio signals sent from satellites orbiting the earth
Created and Maintained by the US Dept. of Defense
System as a whole consists of three segmentsSatellites (space segment)Receivers (user segment)Ground stations (control segment)
GPS HistoryDevelopment began in 1973First satellite became operational in 1978Declared completely functional in 1995A total of 52 satellites have been launched in 4 phases30 satellites are currently functionalManaged by the U.S. Department of Defense
Originally developed for submarinesNow part of modern “smart bombs” and highly accurate missiles
SatellitesAt least 4 satellites are above the horizon anytime anywhere
GPS satellites are also known as “NAVSTAR satellites”
The satellites transmit time according to very accurate atomic clocks onboard each one
The precise positions of satellites are known to the GPS receivers from a GPS almanac
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.
Satellites cont.
The satellites are in motion around the earth
Like the sun and moon satellites rise and set as they cross the sky
Locations on earth are determined from available satellites (i.e., those above the horizon) at the time the GPS data are collected
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.
ReceiversGround-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once
Geographic position is determined using the time it takes signals from the satellites to reach the GPS receiver
Calculations result in varying degrees of accuracy that depend on:
Quality of the receiver User operation of the receiver (e.g., skill of user and receiver settings)Atmospheric conditionsLocal conditions (i.e., objects that block or reflect the signals)Current status of system
Ground Stations
Control stationsMaster station at Falcon (Schriever) AFB, Colorado4 additional monitoring stations distributed around the world
ResponsibilitiesMonitor satellite orbits & clocksBroadcast orbital data and clock corrections to satellites
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.
How GPS Works: Overview
Satellites have accurate atomic clocks onboard and all GPS satellites transmit the same time signal at the same time
Think “synchronize your watches”
The satellite signals contain information that includes
Satellite numberTime of transmission
How GPS Works: OverviewReceivers use an almanac that includes
The position of all satellites every secondThis is updated monthly from control stations
The satellite signal is received, compared with the receiver’s internal clock, and used to calculate the distance from that satellite
Trilateration (similar to triangulation) is used to determine location from multiple satellite signals
How GPS Works: Signal Processing
Distances between satellites and receivers is determined by the time is takes the signal to travel from satellite to receiver
Radio signals travel at speed of light (186,000 miles/second)All satellites send the identical time, which is also generated by the receiversSignal travel time = offset between the satellite signal and the receiver signal
Distance from each satellite to receiver = signal travel time * 186,000 miles/second
1sec
Receiver signal
Satellite signal
How GPS Works: Trilateration
Start by determining distance between a GPS satellite and your position
How GPS Works: Trilateration
Adding more distance measurements to satellites narrows down your possible positions
How GPS Works: Trilateration
How GPS Works: Trilateration
The 4th satellite in trilateration is to resolve any signal timing error
Unlike GPS satellites, GPS receivers do not contain an atomic clockTo make sure the internal clock in the receiver is set correctly we use the signal from the 4th satellite
GPS Error SourcesSatellite errors
Satellite position error (i.e., satellite not exactly where it’s supposed to be)Atomic clocks, though very accurate, are not perfect
AtmosphericElectro-magnetic waves travel at light speed only in a vacuumAtmospheric molecules, particularly those in the ionosphere, change the signal speed
Multi-path distortionThe signal may "bounce" off structures before reaching the GPS receiver – the reflected signal arrives a little later
Receiver error: Due to the receiver clock or internal noise
Selective AvailabilityNo longer an issue
Sources of Error
Satellite Clock & Satellite PositionAtomic clock errors+/- 2 meters of error
Satellite is not in precise orbit+/- 2.5 meters of error
Sources of Error
Atmospheric Delays/Bending+/- 5 meters or error
Sources of ErrorMulti Path Interference (signal bouncing off of buildings, trees, etc.)
+/- 1 meter of error
Sources of ErrorReceiver Timing/Rounding Errors
+/- 1 meter of error (depends on the quality of the GPS receiver)
Quadruple Redundant Atomic ClocksAccurate to Nanoseconds
$800,000 in clocks on each satellite2:02:01.23456789012
Powered by 4 AA Batteries~$2.99
2:02:01.2345
GPS - Selective AvailabilityA former significant source of error
Error intentionally introduced into the satellite signal by the U.S. Dept. of Defense for national security reasonsSelective Availability turned off early May 2, 2000
GPS Error: Position Dilution of PercisionSatellite Coverage: Position Dilution of Precision (PDOP)
Remember that satellites are moving, causing the satellite constellation to change
Some configurations of satellites are better than others
PDOP values range from 1 to 50, with values < 6 considered “good”
Poor PDOP Good PDOP
GPS - Error Budget
Typical Observed errors (meters)satellite clocks 0.6orbit (position error) 0.6receiver errors 1.2atmosphere 3.7
Total 6.1
Multiplied by PDOP (1-6)
Total error ~ 6.1 - 36.6 metersMeters
Atmosphere
Receivers
Orbit Error
SatelliteClocks
0 6 12 18 24 30
Example of typically observed error from a consumer GPS receiver:
GPS - Error Correction
2 Methods:Point AveragingDifferential Correction
Point AveragingPoint Averaging is one of the simplest ways to correct GPS point locations
Collect many GPS measurements at the same location and then average them to get one pointThe averaged point should have greater accuracy than a single point measurementAccuracy varies with this method but you should have a position that is within 5 meters of its true location 95% of the time
GPS - Point Averaging
AveragedLocation
This figure shows a successive series of 3-D positions taken using a receiver kept at the same location, and then averaged
GPS - Differential Correction
Differential correction collects points using a receiver at a known location (known as a base station) while you collect points in the field at the same time (known as a rover receiver)Any errors in a GPS signal are likely to be almost the same among all receivers within ~ 300 miles of each other
~ 300 miles (~ 480 km) or less
Base station (known location) Rover receiver
GPS - Differential Correction
The base station knows its own locationIt compares this location with its location at that moment obtained using GPS satellites, and computes errorThis known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same moment
Time GPS Lat GPS Long Lat. error Long. error3:12.53:13.03:13.53:14.03:14.53:15.0
35.5035.0534.9536.0035.3535.20
79.0578.6579.5580.4579.3079.35
.5
.05-.051.0.35.20
.5-.35.551.45.30.35
Example: Base Station File
GPS - Differential Correction
GPS error when using differential correction: 1 – 3 metersThere are two ways that differential correction can be applied:
Post-processing differential correction• Does the error calculations after the
rover has collected the points• Requires downloading a base-station file
Real-time differential correction• Done in real time by receiving a
broadcasted correction signal • May require additional hardware
• Generating mapped data for GIS databases • Collecting field data - travel to the field and
capture location & attribute information
• Other uses (many in real time):• 911/firefighter/police/ambulance dispatch• Car & boat navigation• Roadside assistance• Business vehicle/fleet management• Mineral/resource exploration• Wildlife tracking• Recreational (fishing, hunting, hiking, etc.• Ski patrol/medical staff location monitoring
GPS Applications
Strengths of GPS
Easy To Incorporate into ProjectOnce trained, just about anyone can use itCheapWidely Available
Weaknesses
Does require a training componentAccuracy Issues Differential Correction may not be an option in many parts of the world
Planning a GPS Project
GPS point collection can be an easy way to build your database but planning is essential
Planning a GPS ProjectIdentify Your Accuracy NeedsIdentify Error Correction Methodology
Point Averaging• How long will points be collected?
Differential Correction• Find a base station
Identify Point Collection MethodologyWhere will points be collected?Contingency plansData backups
Bottom Line
Cost depends on ProjectHardware
• GPS -- $200 to $100,000 or more• Differential Correction (yes? no? real-time?)• Additional hardware
– Computers, cables, batteries, antennas, etc.
People• Field Teams -- depends on length of field work• Simple projects in a day or two• More Complex projects can last months or years
Garmin GPS Introduction
Garmin GPS
Display Screen
Control Buttons
Antenna
Garmin GPS
Step 1: Turn on Unit
After a couple of seconds, the GPS unit will start looking for satellites.
Off/On Button
Search for SatellitesStep 2: Wait for Satellites
Battery Gauge
Signal Strengthfor satellite
Satellite Located,but not locked in
Satellite Located,and locked in
North Indicator
Outer circle representshorizon, inner circlerepresents 45 degreesabove horizon
Satellite Acquisition Page
Acquire PositionStep 3: Once enough satellites have been located, the GPS unit will provide you with a position
Position
Position Page
Time
Direction Indicator
Speed
Altitude
Collect a PointStep 4: Press the Mark button and begin collecting data.
MarkButton
Waypoint Page
To collect a single point, highlight Save and press ENTER.
To collect an averaged point, highlight Average, press the Enter button and wait for a few minutes, then highlight Save and stop point collection
Record PointStep 5: Go to Menu Princ. Page and view waypoint list and record coordinate
Main Menu Page
Waypoint ID
AveragedPosition
Waypoint Page
Other Pages
Compass PageMap Page
On Friday
Meet outside at the Old WellDon’t be late!Be prepared to walk short distanceYou will need a pencil or penRead your worksheet BEFORE class and remember to bring it on FridayRain Date – Monday, October 29