Modern global navigation services use orbiting satellites to meet many demands.
AVAILABILITY
Positioning information must be available all the time, regardless of time of day, weather conditions, or other factors.
COVERAGE
Services must be available anywhere in the world, so satellites need to be visible anywhere on Earth and their orbits must be carefully tracked.
ACCURACY
Precise positions need to be determined to identify tiny land features, locate individual structures, and allow vehicles, ships, and aircraft to avoid hazards.
USER EQUIPMENT
Equipment used on Earth must be small and portable, requiring satellites to operate precise time and data systems in the harsh environment of space.
USABILITY
Positions must be determined quickly, without requiring users to receive extensive training or perform multiple steps.
