Where am I?

Each satellite constantly sends out the current time as a message contained within radio waves Speed of light = 3X10 8 meters/second

Each satellite constantly sends out the current time as a message contained within radio waves Speed of light = 3X10 8 meters/second 4 seconds for the radio wave to return 600,000,000 meters away 600,000 kilometers away 372,000 miles away (about twice the distance from the earth to the moon)

Each satellite constantly sends out the current time as a message contained within radio waves with one satellite, you could be anywhere on a sphere a set distance from the sensor

Each satellite constantly sends out the current time as a message contained within radio waves with two satellites, you could be anywhere along a circle (red line) a set distance from the two sensors

Each satellite constantly sends out the current time as a message contained within radio waves with three satellites, you could be at one of two points – red points

Each satellite constantly sends out the current time as a message contained within radio waves with four satellites, you can only be at one point – red point

Accuracy of Position Atmospheric errors (ionosphere, troposphere – 5.5m of error) Multipathing (bouncing off objects – 1m of error) Geometric Dilution of Precision (GDOP – highly variable) Imprecision in satellite orbits (2.5m of error) Slight timing errors in clock updates (2m of error) Relativistic effects (10km of error!) 7.2 microseconds slower due to high velocity 30.8 microseconds faster due to low gravity

Accuracy of Position Atmospheric errors (ionosphere, troposphere – 5.5m of error) Multipathing (bouncing off objects – 1m of error) Geometric Dilution of Precision (GDOP – highly variable) Imprecision in satellite orbits (2.5m of error) Slight timing errors in clock updates (2m of error) Relativistic effects (10km of error!) 7.2 microseconds slower due to high velocity 30.8 microseconds faster due to low gravity Differential GPS – Ground station corrections WAAS (wide area augmentation system)