1. Satellite Network

2. Satellite Network 24 satellites in orbit, plus 3 backups 3000-4000 lbs, solar powered Orbit height is roughly 20,000 km (Earth radius is 6400 km) Complete orbit twice per day At any point on Earth, at least four should be visible at any time

3. Finding Location Based on principle of triangulation or trilateration E.g. – in two dimensions, three overlapping circles is enough to find a precise point In 3 dimensions, three spheres overlap in a single point Minimum – you need two satellites and the Earth’s surface

4. Distance calculation Satellites send out low-power, high frequency radio signals with time stamps Time stamps come from long sequences of pseudo-random numbers that restart each day at midnight Satellites have extremely accurate atomic clocks

5. Distance calculation Receivers have normal quartz clocks – not accurate enough for this Quartz clocks are calibrated by using four satellites for measurements Travel time can be calculated from delay, because speed is known ( 300,000 km per second)

6. Possible Problems Waves travel through atmosphere; this changes speed of light Orbits can change due to moon/sun gravity (DoD monitors and includes corrections in signal) Reflections off of ground stuff Bad position data from satellite foils almanac plan

7. Differential GPS Use a base station to improve accuracy Fixes some systematic inaccuracies, because the position of base station is known, and offsets can be calculated

8. Accuracy Hand-held – horizontal accuracy of 10- 20 meters, worse for elevations Differential – submeter accuracy, but calculation is difficult and not real-time In May 2000, military ended the practice of including errors in the signals, which used to limit accuracy to 100m for non-military GPS units.

9. Horizontal Error David Wilson’s GPS Accuracy Page (URL next slide)

10. Vertical Error http://users.erols.com/dlwilson/gpsvert.htm