GPS Ground Segment

GPS
Title	GPS Ground Segment
Author(s)	GMV
Level	Basic
Year of Publication	2011

The GPS Ground Segment (also referred to as Control Segment or Operational Control System) is the responsible for the proper operation of the GPS system.

The GPS Control Segment is composed by a network of Monitor Stations (MS), a Master Control Station (MCS), a backup of the MCS and the Ground Antennas (GA).

The Master Control Station (MCS) processes the measurements received by the Monitor Stations (MS) to estimate satellite orbits (ephemerides) and clock errors, among other parameters, and to generate the navigation message. These corrections and the navigation message are uploaded to the satellites through the Ground Antennas, which are co-located in four of the Monitor stations (Ascension Island, Cape Canaveral, Diego Garcia, and Kwajalein).

Introduction

GPS Operational Control System

The Control Segment (also referred to as Ground Segment) is the responsible for the proper operation of the GPS system. The main tasks performed by the CS are the following:

• Maintenance of the satellites in their proper orbital positions;
• Monitoring of satellite subsystem health and status (solar arrays, battery power levels and propellant levels used for maneuvers);
• Activation of spare satellites;
• Update of parameters in the navigation message (ephemeris, almanac and clock corrections);
• Resolving satellite anomalies;
• Controlling Selective Availability (SA) and Anti-Spoofing (AS)
• Passive tracking of the satellites.

The Operational Control System (OCS) is comprised of four major subsystems:

• Master Control Station (MCS, soon to be replaced by a New Master Control Station [NMCS])
• Backup Master Control Station (BMCS, soon to be replaced by an Alternate Master Control Station [AMCS])
• Network of four ground antennas (GAs),
• Network of globally-distributed monitor stations (MSs).

The Master Control Station

Located at Colorado Springs, is the core of the Control segment. It is responsible for operating the system, providing command, control and maintenance services to the space segment.

The Monitor Stations

They are distributed around the world and equipped with atomic clocks standards and GPS receivers to continuously collect GPS data for all the satellites in view from their locations. The collected data is sent to the Master Control Station where it is processed to estimate satellite orbits (ephemerides) and clock errors, among other parameters, and to generate the Navigation Message.

Prior to the modernization program, the Monitor Stations network comprised five sites located in:

• Hawaii,
• Colorado Springs (Colorado, US),
• Ascension Island (South Atlantic),
• Diego Garcia (Indian Ocean),
• Kwajalein (North Pacific).

In order to increase performance and accuracy, new stations were incorporated into the ground segment providing greater visibility of the constellation: Cape Canaveral (Florida, US) was incorporated in 2001 and six new stations in 2005:

• Adelaide (Australia),
• Buenos Aires (Argentina),
• Hermitage (UK),
• Manama (Bahrain),
• Quito (Ecuador)
• Washington DC (USA).

Five more stations were added afterwards in 2006:

• Fairbanks (Alaska),
• Osan (South Korea),
• Papeete (Tahiti),
• Pretoria (South Africa)
• Wellington (New Zealand).

With this configuration, each satellite is seen from at least three monitor stations, which allows computing more precise orbits and ephemeris data, therefore improving system accuracy.

The Ground Antennas

GPS infrastructure in the world

The Ground Antennas uplink data to the satellites via S-band radio signals. These data includes ephemerides and clock correction information transmitted within the Navigation Message, as well as command telemetry from the MCS.

Every satellite can be uploaded three times per day, i.e. every 8 hours; nevertheless, it is usually updated just once a day.

The ground antennas are co-located in four of the Monitor stations:

• Ascension Island
• Cape Canaveral
• Diego Garcia
• Kwajalein