Geographic Information Geographic Information SystemsSystems

Global Positioning Systems Global Positioning Systems (GPS)(GPS)

1. GPS1. GPS► A method used in surveying. It uses a A method used in surveying. It uses a constellation of satellites orbiting constellation of satellites orbiting the earth at very high altitudesthe earth at very high altitudes

GPS ..GPS ..

► The GPS technology allows accurate The GPS technology allows accurate geodetic surveys by using specially geodetic surveys by using specially designed designed receiversreceivers that, when that, when positioned at a point on the earth, positioned at a point on the earth, measure the distance from that point measure the distance from that point to to three or more orbiting satellitesthree or more orbiting satellites

► Through the geometric calculations of Through the geometric calculations of triangulationtriangulation, the , the coordinates of the coordinates of the pointpoint on the surface of the earth are on the surface of the earth are determineddetermined

X, Y, Z (position) and time from 4 satellites to calculate position

Pamela E. Jansma, University of Arkansas

2. NAVSTAR GPS2. NAVSTAR GPS ► NAVigation Satellite Timing And Ranging NAVigation Satellite Timing And Ranging ((NAVSTARNAVSTAR) Global Positioning System ) Global Positioning System

► Developed by U.S. Department of DefenseDeveloped by U.S. Department of Defense► A constellation system of 28-30 A constellation system of 28-30 satellites orbiting the earth at a high satellites orbiting the earth at a high altitude twice a dayaltitude twice a day

► Transmitting precise time and position Transmitting precise time and position information information

► For world-wide and all weather For world-wide and all weather navigationsnavigations

NAVSTAR ..NAVSTAR ..

► 24 satellites in 6 orbital planes24 satellites in 6 orbital planes► Orbit the earth at approx. 20,200 km Orbit the earth at approx. 20,200 km ► 555500 inclination inclination ► Satellites complete an orbit in Satellites complete an orbit in approximately 12 hoursapproximately 12 hours

http://www.montana.edu/places/gps/lres357/assignments.html

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

3. Operation Principles3. Operation Principles ► The basis of GPS is triangulationThe basis of GPS is triangulation

► SatelliteSatellite► GPS receiver GPS receiver ► Determination of LocationDetermination of Location► Coordinate systemsCoordinate systems

3. Operation - Distance to 3. Operation - Distance to One SatelliteOne Satellite► SatelliteSatellite

- continuously broadcasts time and its - continuously broadcasts time and its location location

► GPS receiver GPS receiver

- receives the signal and calculates the - receives the signal and calculates the distance distance between the satellite and the between the satellite and the receiverreceiver::

Distance = Velocity x TimeDistance = Velocity x Time

where time = the time a signal takes to where time = the time a signal takes to travel from the satellite to the receiver, travel from the satellite to the receiver, velocity = ?velocity = ?

3. Operation - Determine 3. Operation - Determine LocationLocation

► Determination of locationDetermination of location

- - 3 or more3 or more simultaneous distance simultaneous distance measurements are needed measurements are needed

to determine the to determine the location of the location of the receiverreceiver

► Coordinate systems Coordinate systems

- A GPS provides its position in - A GPS provides its position in geographicalgeographical//UTMUTM/ / State State

Plane Plane coordinates coordinates and and altitudealtitude

One measurement narrows down our One measurement narrows down our position to the surface of a sphereposition to the surface of a sphere

12,000 mileradius

http://www.montana.edu/places/gps/lres357/assignments.html

A second measurement narrows A second measurement narrows down our position to the down our position to the intersection of two spheres, intersection of two spheres, which is a circlewhich is a circle

11,000 mileradius

12,000 mileradius

http://www.montana.edu/places/gps/lres357/assignments.html

A third measurement narrows down our position A third measurement narrows down our position to just two points, because the intersection to just two points, because the intersection of a circle and a sphere is two pointsof a circle and a sphere is two points

12,000 mileradius

11,500 mileradius

11,000 mileradius

http://www.montana.edu/places/gps/lres357/assignments.html

A four measurement narrows A four measurement narrows our position down to one our position down to one pointpoint

http://www.bbk.ac.uk/geog/study/courses/pgis/GPSlecture10.pdf

3. Operation- Determine 3. Operation- Determine location ..location ..

Pamela E. Jansma, University of Arkansas

4. GPS Receiver4. GPS Receiver

► GGPS receivers PS receivers can be hand can be hand carried out or carried out or installed on installed on aircraft, ships, aircraft, ships, tanks, tanks, submarines, cars submarines, cars and trucksand trucks

GPS Receiver ..GPS Receiver .. ► Radio channelsRadio channels► Internal clock Internal clock ► ComputerComputer

GPS Receiver ..GPS Receiver .. ► Radio channels Radio channels

It needs one or more channels to receive It needs one or more channels to receive signals of signals of

        (1) the time a signal is sent, and (1) the time a signal is sent, and                

(2) a pseudorandom code to identify the (2) a pseudorandom code to identify the signalsignal

► Internal clockInternal clock  - Synchronized with the satellite in order to - Synchronized with the satellite in order to

calculate the signal travel time preciselycalculate the signal travel time precisely

GPS Receiver ..GPS Receiver .. ► Computer Computer

- Calculates distance and location- Calculates distance and location

- Stores location readings- Stores location readings

- Calculates the satellite - Calculates the satellite availabilityavailability

5. Autonomous vs. 5. Autonomous vs. Differential GPSDifferential GPS

► Autonomous GPS: one receiver unit Autonomous GPS: one receiver unit

► Selective availability interference Selective availability interference ► Differential GPS Differential GPS ► WAAS (Wide Area Augmentation System)WAAS (Wide Area Augmentation System)

Differential GPS ..Differential GPS ..

► Selective availability interferenceSelective availability interference

-- The Us government used to insert The Us government used to insert random errors in the signals in order random errors in the signals in order to maintain optimum military to maintain optimum military effectiveness of the system. It was effectiveness of the system. It was turned off in 2000turned off in 2000

► Differential GPSDifferential GPS

Differential GPSDifferential GPS

► Selective availability interferenceSelective availability interference

► Differential GPSDifferential GPS

- Uses two receiver units, with one - Uses two receiver units, with one placed at a known location, such as a placed at a known location, such as a surveyed control point, to detect the surveyed control point, to detect the inserted errors and calculate the inserted errors and calculate the needed corrections. The correction needed corrections. The correction information is used by another information is used by another receiver to overcome its selective receiver to overcome its selective availability interferenceavailability interference

Differential GPS Differential GPS (DGPS) ..(DGPS) ..

► The DGPS corrections can be applied The DGPS corrections can be applied to the GPS data in real-time (radio to the GPS data in real-time (radio modems). Or, can be done later on a modems). Or, can be done later on a PCPC

► There are a series of radio beacons There are a series of radio beacons to transmit the DGPS corrections for to transmit the DGPS corrections for accurate navigationaccurate navigation

5. Differential Correction5. Differential Correction

http://www.bbk.ac.uk/geog/study/courses/pgis/GPSlecture10.pdf

5. DGPS - WAAS Satellite 5. DGPS - WAAS Satellite DGPSDGPS

► Wide Area Augmentation System Wide Area Augmentation System ((WAASWAAS))

  - a real-time differential correction service

- is based on a network of approximately 25 ground reference stations that cover a very large service area

http://www.montana.edu/places/gps/lres357/assignments.html

WAAS Satellite DGPS ..WAAS Satellite DGPS ..

- Signals from GPS satellites are received by wide area ground reference stations (WRSs)

- Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist

- These WRSs are linked to form the - These WRSs are linked to form the U.S. WAAS network. This service is U.S. WAAS network. This service is primarily for aviation purposesprimarily for aviation purposes

WAAS Satellite DGPS ..WAAS Satellite DGPS ..

► Improves the accuracy, integrity Improves the accuracy, integrity and availability of the basic GPS and availability of the basic GPS signalssignals

► Safety - critical navigation system Safety - critical navigation system for aviationfor aviation

► Owned and operated by Federal Owned and operated by Federal Aviation Administration (FAA)Aviation Administration (FAA)

► Type: real-timeType: real-time► Cost: freeCost: free

WAAS Stations

http://www.montana.edu/places/gps/lres357/assignments.html

DGPS ..DGPS ..

► OmnistarOmnistar► Type: Real-timeType: Real-time

- Network of reference stations- Network of reference stations

70 reference stations70 reference stations

3 network control centers3 network control centers

- Worldwide coverage- Worldwide coverage► Cost: ~ $800/yearCost: ~ $800/year

http://www.montana.edu/places/gps/lres357/assignments.html

6. Static vs. Kinematic GPS6. Static vs. Kinematic GPS ► Static: two or more receivers receive Static: two or more receivers receive data for a lengthy period of time at data for a lengthy period of time at a fixed locationa fixed location

► Kinematic: one receiver is placed at Kinematic: one receiver is placed at a known location, while one or more a known location, while one or more receivers move from point to pointreceivers move from point to point

► Real Time Kinematic (RTK)Real Time Kinematic (RTK)

7. Advantages and 7. Advantages and LimitationsLimitations

► Advantages: fast, more accurate, lower Advantages: fast, more accurate, lower cost than manual approachescost than manual approaches

► Disadvantages: obstructed locationsDisadvantages: obstructed locations

8. Readings8. Readings ► Chapter 2Chapter 2

Omnistar – How it WorksOmnistar – How it Works

http://www.montana.edu/places/gps/lres357/assignments.html

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

5. DGPS - Omnistar 5. DGPS - Omnistar CoverageCoverage

North AmericaNorth America

http://www.montana.edu/places/gps/lres357/assignments.html