Andrew McLea

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GPS in (and out of)the Physics Classroom

• Andrew McLean
• Kelvin Barraclough
• Gisborne Secondary College

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What is GPS?

• 24 satellites

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What is GPS?

• 24 satellites
• Several ground stations

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What is GPS?

• 24 satellites
• Several ground stations
• GPS receiver

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Satellites

• Altitude 20200 km

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Satellites

• Altitude 20200 km
• Period 12 hours

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Satellites

• Altitude 20200 km
• Period 12 hours
• At least 4 visible everywhere

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Satellites

• Altitude 20200 km
• Period 12 hours
• At least 4 visible everywhere
• Solar powered, 50 W transmitter

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Satellites

• Altitude 20200 km
• Period 12 hours
• At least 4 visible everywhere
• Solar powered, 50 W transmitter
• Transmit on 1575.42 MHz

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Satellites

• Altitude 20200 km
• Period 12 hours
• At least 4 visible everywhere
• Solar powered, 50 W transmitter
• Transmit on 1575.42 MHz
• Fitted with atomic clocks

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Satellites

• Altitude 20200 km
• Period 12 hours
• At least 4 visible everywhere
• Solar powered, 50 W transmitter
• Transmit on 1575.42 MHz
• Fitted with atomic clocks
• Lifespan 10 years

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Ground Stations

• I master station, four remote

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Ground Stations

• I master station, four remote
• Monitor satellites and send back accurate orbital
  and time data

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The Signal

• The almanac. Approximate position of all
  satellites, transmitted and stored in the GPS.

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The Signal

• The almanac. Approximate position of all
  satellites, transmitted and stored in the GPS.
• The ephemeris. Accurate data of individual
  satellite position.

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The Signal

• The almanac. Approximate position of all
  satellites, transmitted and stored in the GPS.
• The ephemeris. Accurate data of individual
  satellite position.
• The pseudo random code. Effectively, an
  accurate time signal

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The Signal

• The almanac. Approximate position of all
  satellites, transmitted and stored in the GPS.
• The ephemeris. Accurate data of individual
  satellite position.
• The pseudo random code. Effectively, an
  accurate time signal
• Turned off while satellite orbit is adjusted

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Satellite positions
27400 km
20200 km
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Signal times
0.091 s
0.067 s
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Establishing position


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Establishing position


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Establishing position

• If receiver had a good enough clock
• Two satellites somewhere on a circle

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Establishing position

• If receiver had a good enough clock
• Two satellites somewhere on a circle
• 3rd satellite cuts this circle twice, but
  usually one solution is obvious

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Establishing position

• If receiver had a good enough clock
• Two satellites somewhere on a circle
• 3rd satellite cuts this circle twice, but
  usually one solution is obvious
• 4th satellite removes ambiguity

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Establishing position

• With a small, cheap clock -

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Establishing position

• With a small, cheap clock
• 3 satellites give 2D fix

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Establishing position

• With a small, cheap clock
• 3 satellites give 2D fix
• 4 satellites give 3D fix

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Accuracy

• Given clear view of sky typically 5 10 m

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Receivers

• Arent equal

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Receivers

• Arent equal
• Need good view of sky

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Possible information

• Position

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Possible information

• Position
• Distance travelled

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Possible information

• Position
• Distance travelled
• Speed

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Possible information

• Position
• Distance travelled
• Speed
• Direction

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Possible information

• Position
• Distance travelled
• Speed
• Direction
• Altitude

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Gathering data low tech
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Gathering data hi tech

• Almost all GPS receivers can be connected to a
  computer to transfer data

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Suitable software (1)

• OziExplorer

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Suitable software (1)

• OziExplorer
• 130

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Suitable software (1)

• OziExplorer
• 130
• Uses raster mapping

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Suitable software (1)

• OziExplorer
• 130
• Uses raster mapping
• Suitable for all brands of GPS

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Suitable software (1)

• OziExplorer
• 130
• Uses raster mapping
• Suitable for all brands of GPS
• Transferring data from Ozi is awkward

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Suitable software (2)

• Garmin Mapsource

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Suitable software (2)

• Garmin Mapsource
• Free with Garmin GPS

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Suitable software (2)

• Garmin Mapsource
• Free with Garmin GPS
• Uses Vector mapping

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Suitable software (2)

• Garmin Mapsource
• Free with Garmin GPS
• Uses Vector mapping
• Allows easy transfer of data to other programs
  (eg Excel)

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Some possibilities

• Real aircraft - loop the loop
• Model aircraft
• Racing cars, dragsters, motorbikes
• Skydivers, bungee jumpers
• Yacht racing
• Theme park rides
• Relative motion on moving ship

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Cost?

• Cheap GPS 100
• Garmin GPS Map 60 CSx 470
• OziExplorer 130
• Oztopo maps for Garmin (all of Australia) 229