Andrew McLean Kelvin Barraclough

Gisborne Secondary College

What is GPS?

• 24 + satellites

What is GPS?

• 24 + satellites• Several ground stations

What is GPS?

• 24 + satellites• Several ground stations• GPS receiver

Satellites

• Altitude ≈ 20200 km

Satellites

• Altitude ≈ 20200 km• Period 12 hours

Satellites

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

Satellites

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

Satellites

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

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

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

Ground Stations

• I master station, four remote

Ground Stations

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

orbital and time data

The Signal

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

The Signal

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

• The “ephemeris”. Accurate data of individual satellite position.

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

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

Satellite positions

27400 km

20200 km

Signal times

0.091 s

0.067 s

Establishing position

Establishing position

Establishing position

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

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

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

Establishing position

• With a small, cheap clock -

Establishing position

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

Establishing position

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

Accuracy

• Given clear view of sky typically 5 – 10 m

Receivers

• Aren’t equal

Receivers

• Aren’t equal• Need “good view” of sky

Possible information

• Position

Possible information

• Position• Distance travelled

Possible information

• Position• Distance travelled• Speed

Possible information

• Position• Distance travelled• Speed• Direction

Possible information

• Position• Distance travelled• Speed• Direction• Altitude

Gathering data – “low tech”

Gathering data – “hi tech”

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

Suitable software (1)

• “OziExplorer”

Suitable software (1)

• “OziExplorer”• $130

Suitable software (1)

• “OziExplorer”• $130• Uses raster mapping

Suitable software (1)

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

Suitable software (1)

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

Suitable software (2)

• Garmin “Mapsource”

Suitable software (2)

• Garmin “Mapsource”• Free with Garmin GPS

Suitable software (2)

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

Suitable software (2)

• Garmin “Mapsource”• Free with Garmin GPS• Uses Vector mapping• Allows easy transfer of data to other programs

(eg Excel)

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

Cost?

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