GPS Guide

Born of the best minds from a concept so simple is an exceptional complex system, the GPS tracking system.  It is made up of three components satellites that orbit the earth, control and monitoring stations and the GPS receivers.  It is a radio navigation system providing reliable positioning, navigating, and timing services not just for the military but also to civilian users as well in any part of the world.  The GPS tracking system has drastically changed the world of navigation through land, sea and air, in all weather, 24/7.


The Global Positioning System (GPS) is a U.S. space-based radio navigation system that provides reliable positioning, navigation, and timing services to civilian users on a continuous worldwide basis -- freely available to all. For anyone with a GPS receiver, the system will provide location and time. GPS provides accurate location and time information for an unlimited number of people in all weather, day and night, anywhere in the world.


In the 1960s the U.S. Navy and Air Force worked together on a number of systems that would be able to provide a variety of navigation applications.  In 1973 the Department of Defense aimed to direct the services in unifying their systems.  The concept of the tracking system would be atomic clocks onboard satellites was tested successfully by an earlier Navy program called TIMATION.  The Air Force would operate the new system called the Navstar Global Positioning System.  From then on, it has since been known as simply GPS or Global Positioning System.


The GPS tracking System Satellites

Today's GPS Operational Constellation is funded and controlled by the U.S. Department of Defense and being operated by the U.S. Military.  It is comprised of three major components:  a "constellation" of satellites in Earth orbit, ground stations that controlled the system and receivers hand carried by the users.  It was so designed so that these receivers could be made small and inexpensive and would not require atomic clocks.


The GPS tracking system is an aerospace technology comprising of at least 24 operational satellites at all times.  Each satellite is on a circular orbit 20,000 kilometers above the Earth on a 12-hour period.  In order to make sure that these satellites are detected from anywhere on the surface of the Earth, the satellites are divided into six groups having four space vehicles each. The six orbital planes completely surround the Earth, equally spaced 60 degrees apart and at an angle of about 50 degrees with respect to the equatorial plane.  Each group is assigned a different path to follow.  The orbit altitude is such that the GPS satellites recapitulate the same ground track and configuration at approximately over any point each in 24 hours.


The GPS tracking system's navigation message consists of time-tagged data bits that mark the time of transmission of each subframe at the time they are being transmitted by the space vehicle.  A data bit frame consists of 1500 bits divided into five 300-bit subframes and a data frame is being transmitted at an interval of 30 seconds.  There are three six-second subframes containing orbital and clock data.  Precise clock corrections are sent in subframe one and orbital data sets or ephemeris data parameters for the transmitting space vehicle are sent in subframes two and three.  Various pages of the system data are being transmitted in subframes four and five.  So the GPS tracking system has an entire set of twenty-five frames or 125 subframes creating the complete navigation message sent over a 12.5-minute period.


The GPS Tracking System Equipment on the Ground

Ground stations are used so that each satellite orbit is accurately tracked.  The GPS tracking system has ground stations which is comprised of a receiver and antenna, as well as communication tools to transmit the data to the data center.  When the GPS satellites supply specially coded signals, the omni-directional antenna at each site picks up the signals and that is processed in a receiver.  The receiver then separates the signals in various channels for a particular satellite and frequency at a particular time.  Once the signals are isolated, the receiver now decodes them and splits them into individual frequencies to calculate position, velocity, direction and time anywhere on Earth.


At least four GPS satellite signals consisting of space segments are used in computing positions in three-dimensional locations - latitude, longitude and altitude - and the time offset in the receiver clock.  All GPS tracking system satellites have synchronized operations so that the signals are transmitted at the speed of light to the equipment on the ground. Each GPS satellite broadcasts data indicating its location and current time.


Since the GPS tracking system sites are constructed throughout the world and are being monitored by different institutions worldwide, there are many different data center locations.
The role of the data center is to both monitor and control the GPS tracking system stations. The data center uses an automated computer system in retrieving and analyzing data from the receivers.  Once processed, the data, including the original raw data, are sent to scientists around the glob for use in various applications.


From laboriously built landmarks by our ancestors to shooting the stars to chronometers to satellites, our technology has evolved faster than we could ever imagine.  The GPS tracking system is the most recent and significant advancement in navigation and positioning technology.  This new constellation of artificial stars serves this important need for both the military and civilian users and is now fast becoming important in everyday life.