Ephemeris: Current satellite position and timing information transmitted as part of the satellite data message.

Horizon: The imaginary line where the sky appears to meet the Earth. The horizon can be divided into 360 degrees of azimuth.

Large-Scale Map: A map which shows a small area in large detail. Large-scale maps are characterized by fractional scales with small denominators, e.g., 1/10,000 (in contrast with a small-scale map large-denominator fractional scale such as 1/10,000,000).

Radius: One half of the diameter of a circle.

Waypoint: A permanently stored and named position in the GPS receiver's memory.

Zenith: The imaginary point in the sky directly overhead an observer. Everyone has their own personal zenith, and it follows you everywhere you go!