Introduction to the Global Positioning System (GPS)

Giovanni Zambotti GIS Specialist – CGA Harvard University [email protected] September 24, 2013

Outline • • • • •

Before GPS What is GPS GPS segments (Space, Control, User) How GPS works GPS uses

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Before GPS • • • •

Sextant (clear sky) LORAN (1940) (all-weather) Sputnik (1957) The Navy's TRANSIT navigation system (1965) – 6 satellites (90 minutes/200 m) • GPS (1973) – DoD – 1978 prototype – 1989 launch

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What is GPS?

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The Global Positioning System (GPS) • Aerospace technology that uses satellites and ground equipment to provide geographic locations at any time, anywhere ON or NEAR the Earth. • Free for anyone to use, with a GPS receiver • 24 satellites • elevation ~ 12500 Miles • 6 orbits, 4 satellite each • 3 segments

The Space segment

Solar panels – power Atomic clock – precisely records time Receiver – data from the Control segment Radio transmitter – broadcasts a continuous signal containing the time, and ephemeris (identification and positional information) 6

GPS satellite @ San Diego Aerospace Museum

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The Control segment • Network of ground stations • Control the shape and velocity of the satellites' orbits. • Monitor Stations:  very precise GPS receivers at known location  record discrepancies between known and calculated positions (variations in satellite orbits)

• Master Control Station (Colorado Springs):  produce data describing the orbits  upload to the satellites

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The User segment (GPS receiver) • Contain a clock, computer, radio wave receiver. • Dime sized to dinner plate sized • Receive signals from GPS satellites

GPS receiver (Survey, Mapping, Recreational) Survey grade – Very sophisticated receivers, able to receive many signals. Centimeter accuracy possible. Cost: several thousand $$. Mapping grade – ~1 meter accuracy, possible. Cost: $700 and up. Recreational grade – User friendly, 10 meter accuracy. Cost: $100 and up.

How GPS work • Each GPS satellites transmit signals to GPS receivers on the ground (current time and ephemeris - identification and positional information ). • The GPS receiver receive the signal passively: • computes distance to the satellite by comparing time (time signal sent vs. time signal received) [distance = time * speed] • computes position using the trilateration method

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Trilateration A GPS receiver "knows“: 1) The location of the satellites. 2) The distance from the satellite

It is located somewhere on the surface of an imaginary sphere centered at the satellite. The receiver is located where these spheres intersect. Watch the explanation by a NASA scientist: http://www.youtube.com/watch?v=3zRlbb oMvb0

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GPS uses – Navigation (land, air, water)

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GPS uses - Commercial Precision agriculture

Fleet management

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GPS uses: Military Troop deployment and navigation

Weapon targeting and guidance 15

GPS use at Harvard – mapping rural households for a public health study in Indonesia

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GPS use at Harvard – Quantifying air pollution Accra, Ghana

Boston, MA

GPS use at Harvard – Child physical activity study

GPS / GIS integration

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Recreational GPS use Geocaching Collaborative mapping opencyclemap.org

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GPS / GIS integration • Load Geographic Information System (GIS) maps and data onto a GPS. • Display basemap • Collect feature attributes

ID

Type

Diameter

Health

1 coniferous

14"

poor

2 deciduous

8"

good

3 coniferous

10"

poor

4 deciduous

10"

good

GPS data file • .gpx format – standard GPS data format

GPS data file • Text format

GPS Accuracy GPS accuracy depends on: 1) The number of signals received. More signals = better accuracy 2) The receiver’s ability to filter noise. Better filtering = better accuracy. More signals can be acquired by:  Accessing the L1 and L2 radio bands from GPS satellites.  Accessing other Global Navigation Satellite Systems:  GLOSNASS (Russia),  Galileo (European Union)  China (Compass/Beidou)  India (IRNSS – not yet operational)  Japan (QZSS – proposed)

GPS Sources for Error GPS Satellites - orbit and clock Atmosphere – Charged ionic particles and water vapor slow the signal. Ground – Deflects GPS signals. Buildings, tree canopy – Deflect and obstruct GPS signals.

Mission Planning to aid accuracy

GPS Device Vendors • Trimble: http://trimble.com/ (High-end surveying and mapping receivers) • Leica: http://leica.com/ (High-end surveying and mapping receivers) • Magellan: http://www.magellangps.com/ (Consumer GPS: in-car navigation systems, low-end handheld receivers) • Garmin: http://www.garmin.com/ (Consumer GPS: in-car navigation systems, low-end handheld receivers) • TomTom: http://www.tomtom.com/ (Consumer GPS: in-car navigation systems).