Vaughan Wesson, December 1998

See the US Coast Guard GPS web pages for more details.

The (Global Positioning System) GPS is a space based radionavigation system comprising of three main components. The first is known as the space segment which consists of a constellation of 24 operational satellites. Each satellite generates a navigation message based on data periodically uploaded from the Control Segment and adds the message to a 1.023MHz Pseudo Random Noise Coarse/Acquisition (C/A) code sequence. The satellite modulates the resulting signal on to a 1575.42 MHz L-band carrier to create a spread spectrum ranging signal which it then transmits to the user community. Each C/A code is unique which allows each satellite to be identified.

The Control Segment consists of a Master Control Station (MCS), ground antennas and monitor stations. The MCS is located at Falcon Air Force Base, Colorado and is the central control node for the GPS satellite constellation. It is responsible for all aspects of command and control such as:

• Routine satellite bus and payload status monitoring
• Satellite maintenance and anomaly resolution
• Monitoring and management of position service performance
• Navigation data upload operations
• Prompt detection and response to service failures

The third segment is the receivers which can be used to determine their position and the current time. These are what is in your Kelunji recorder.

Each satellite provides data required to support the position determination process. The data includes information required to determine:

• satellite time of transmission
• satellite position
• satellite health
• satellite clock correction
• propagation delay effects
• time transfer to UTC
• constellation status

It does this using five subframes of information.

• Subframe 1 contains GPS week number, satellite accuracy and health, and satellite clock correction terms.
• Subframe 2 contains ephemeris parameters.
• Subframe 3 contains ephemeris parameters.
• Subframe 4 contains almanac and health data for satellites 25-32, special messages, satellite configuration flags and ionospheric and UTC data.
• Subframe 5 contains almanac and health data for satellites 1-24, almanac reference time and week number.

GPS time is established by the Control Segment and is referenced to a UTC (as maintained by the U.S.Naval Observatory) zero time point defined as midnight on the night of January 5, 1980. It is maintained to be within one microsecond of UTC (modulo one second). The largest unit used in stating GPS time is one week.