Part 1: GPS – Global Positioning System

GPS – Global Positioning System is space based radio navigation system used to determine precise position anywhere in the world. GPS structure is broadly divided into three segments (Space, Control and User Segments). GPS is maintained and monitored by Department of defense(DOD). Position determined by GPS is based on cartesian earth-centered, earth fixed coordinates as specified in World Geodetic System 1984 (WGS-84).
GPS Structure


  • Consist of 24 satellites equally distributed in 6 orbital planes surrounding the Earth
  • Orbital altitude of these satellites is 20,200 km from Earth surface
  • Minimum of 4 satellites in necessary for receivers to establish an accurate three-dimensional position
  • Receivers uses data transmitted(Ephemeris Data and Almanac Data) from satellite to establish position

Ephemeris and Almanac Data

Each satellite broadcasts two types of data – Ephemeris and Almanac. Ephemeris data is more specific in nature while almanac is general in nature. Ephemeris data is very precise orbital and clock correction for each satellite and is necessary for precision positioning. Almanac data describes orbital course of satellite and can be used to calculate its approximate location in orbit.

Ephemeris Data Almanac Data
  • each satellite broadcast ephemeris data for itself only
  • it is specific to each satellite
  • short validity period (approx 4 hours)
  • transmitted every 30s and takes less time to download
  • each satellite broadcasts almanac data for all satellites
  • almanac data is common for all satellites
  • long validity period (approx. 90 days)
  • takes more time to download

When GPS is initially turned on after being off for long time it searches satellites based on almanac and current time. This helps in expediting initial search. In next step ephemeris data is downloaded to define the precise position of aircraft. In case GPS was off for short period(less than 30 min), short start up is requires as ephemeris data need not to be downloaded again. This is also called warm start.

First start of the GPS system or if the device has been moved few hundred miles or accurate time is lost, start up takes significant time. Almanac data becomes invalid in this case and GPS will have to do “sky search”.


GPS control segment is global network of ground facilities to track GPS satellites and monitor their transmission. Control segment also perform analyses and send commands and data to these satellites constellation.

GPS control segment


User segment of GPS consist of receiver equipment which calculate user’s three dimensional position and time.

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