Global Positioning System

The Global Positioning System (GPS) is a satellite-based navigation system that was developed by the U.S. Department of Defense (DOD) in the early 1970s. Initially, GPS was developed as a military system to fulfill U.S. military needs. However, it was later made available to civilians. Today GPS is under dual-use system that can be accessed by both military and civilian users. GPS provides continuous positioning and timing information, anywhere in the world under any weather conditions. Because it serves an unlimited number of users as well as being used for security reasons, GPS is a one-way system, which means that users can only receive the satellite signals.

GPS normally consists of a constellation of 24 operational satellites. Such constellation, known as the initial operational capability (IOC), was completed by the U.S. Department of Defense (USDOD) in July 1993. The official IOC announcement, however, was made on December 8, 1993. It became fully operational in 1994.

Key Facts

• NAVSTAR is the official U.S. Department of Defense name for GPS
• The first GPS satellite was launched in 1978.
• A full constellation of 24 satellites was achieved in 1994.
• Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
• A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
• Transmitter power is only 50 watts or less.

GPS Constellation

To ensure continuous worldwide coverage, GPS satellites are arranged so that four satellites are placed in each of six orbital planes. With such a constellation geometry, four to ten GPS satellites are visible anywhere in the world.

GPS Orbits

GPS satellite orbits are nearly circular (an elliptical shape with a maximum eccentricity is about 0.01), with an inclination of about 55° to the equator. The semimajor axis of a GPS orbit is about 26,560 km (i.e., the satellite altitude of about 20,051 km above the Earth’s surface). The corresponding GPS orbital period is about 12 sidereal hours (11 hours, 58 minutes).

GPS Segments

GPS consists of three segments: the space segment, the control segment, and the user segment.  The space segment consists of the 24-satellite constellation. Each GPS satellite transmits a signal, which has a number of components: two sine waves, also known as carrier frequencies, two digital codes, and a navigational message. The codes and the navigation message are added to the carriers as binary biphase modulations. The carriers and the codes are used mainly to determine the distance from the user’s receiver to the GPS.

The 24 satellites are arranged in 6 orbital planes of 55- degree inclination, 20,051 kilometers above the Earth.

Each satellite completes one orbit in one-half of a sidereal day and, therefore, passes over the same location on earth once every sidereal day, approximately 23 hours and 56 minutes.

With this orbital configuration and number of satellites, a user at any location on Earth will have at least four satellites in view 24 hours per day

Control Segment

The Control Segment consists of the master control station (MCS), located at Falcon Air Force Base in Colorado Springs, Colorado; remote monitoring stations, located in Hawaii, Diego Garcia, Ascension Island, and Kwajalein; and uplink antennas, located at three of the four remote monitor stations and at the Master Control Segment.

The primary task of the operational control segment is tracking the GPS satellites in order to determine and predict satellite locations, system integrity, behavior of the satellite atomic docks, atmospheric data, the satellite almanac and other considerations.

This information is then packed and uploaded into the GPS satellites through the S-band link. The four stations can track and monitor the whereabouts of each GPS satellite 20 to 21 hours per day. Land-based and space-based communications connect the remote monitoring stations with the MCS.

User Equipment

GPS user equipment includes the Receiver sets that can range from simple devices that provide only basic positioning information to complex multichannel units that track all satellites in view and perform a variety of functions. Most GPS receivers consist of three basic components viz. antenna, receiver-processor unit and control/ display unit.

• Antenna receives the signal and, in some cases, has anti-jamming capabilities
• Receiver-processor unit converts the radio signal to a useable navigation solution
• Control/display unit displays the positioning information and provides an interface for receiver control.

How GPS works

As we studied above, GPS is a network of 24 satellites that orbits the Earth twice a day, transmitting signals back to earth. A GPS receiver locks onto signals from three or more satellites and determines its location, using a method called trilateration. The receiver calculates the difference between the time a satellite sent a signal and the time the system received it. Using the information gathered from several signals, the receiver triangulates the exact position. It can even determine how fast one is going and how long it will take to reach one’s destination.  GPS receiver compares the time a signal was transmitted by a satellite with the time it was received.

The time difference tells the GPS receiver how far away the satellite is.

• Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map. This is the fundamental principle behind GPS.
• A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement.
• With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude and altitude).

Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

GPS receiver is only a receiver, without any transmitting capability. The satellites do not contain any databases about the locations or anything. They contain highly precise atomic clocks which generates some code which it keeps transmitting to the earth. The GPS receiver gets that code from multiple satellites which is slightly time-shifted due to difference in the distances of satellites. Using this difference the receiver calculates the longitude and latitude.

What are the GPS Signals?

GPS is a constellation of 24 satellites. It transmits two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

Standard Positioning System and Precision Positioning Systems in GPS

GPS was originally developed as a military force enhancement system and for exclusive use of military. However, GPS has also demonstrated a significant potential to benefit the civil community in an increasingly large variety of applications. In an effort to make this beneficial service available to the greatest number of users while ensuring that the national security interests of the United States are observed, two GPS services are provided. The Precise Positioning Service (PPS) is available primarily to the military of the United States and its allies for users properly equipped with PPS receivers. The Standard Positioning Service (SPS) is designed to provide a less accurate positioning capability than PPS for civil and all other users throughout the world.

Global Navigation Satellite System (GNSS)

A satellite navigation system with global coverage may be termed a global navigation satellite system or GNSS. The United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS as of now.
China is making its Beidou navigation system and is working towards making it a complete GNSS in near future. Similarly, Galileo is the GNSS of European Union, currently in initial deployment phase, scheduled to be fully operational by 2020 at the earliest.
Russia is on advance stage of achieving full coverage by its GLONASS system. GLONASS has achieved 100% coverage of Russia’s territory.
It has 22 operational satellites, short of the 24 satellites needed to provide continuous global coverage, and is expected to be completed during 2011.
The GLONASS satellites designs have undergone several upgrades, the latest is GLONASS-K.
Please note that India is pursuing space cooperation with Russia currently on joint lunar exploration; development of small experimental satellite for space science studies; use of Russian global navigation satellite system (GLONASS); and preliminary studies for human spaceflight.
The Integration of India’s Regional Navigational Satellite System with Russia’s GLONASS constellation will facilitate reliable and enhanced performance in satellite based navigation, in a seamless manner through dual system receivers.