GPS

The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible to anyone with a GPS receiver.

The GPS program provides critical capabilities to military, civil and commercial users around the world. In addition, GPS is the backbone for modernizing the global air traffic system.

Basic Concepts:

A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include

·         the time the message was transmitted

·         satellite position at time of message transmission

The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using the speed of light. Each of these distances and satellites' locations define a sphere. The receiver is on the surface of each of these spheres when the distances and the satellites' locations are correct. These distances and satellites' locations are used to compute the location of the receiver using the navigation equations. This location is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units show derived information such as direction and speed, calculated from position changes.

In typical GPS operation, four or more satellites must be visible to obtain an accurate result. Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites.

Applications:

Civilian

•        Cellular telephony: Clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The U.S. 

Federal Communications Commission (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.

•        Clock synchronization: The accuracy of GPS time signals (±10 ns) is second only to the atomic clocks upon which they are based.

•        Navigation: Navigators value digitally precise velocity and orientation measurements.

•        Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to compute phasors.

•        Robotics: Self-navigating, autonomous robots using a GPS sensor, which calculate latitude, longitude, time, speed, and heading.

•        Surveying: Surveyors use absolute locations to make maps and determine property boundaries.

•        Tectonics: GPS enables direct fault motion measurement in earthquakes.

Military

•        Navigation: GPS allows soldiers to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the Commanders Digital Assistant and lower ranks use the Soldier Digital Assistant.

•        Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile.

•        Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles, precision-guided munitions and Artillery projectiles.

•        Search and Rescue: Downed pilots can be located faster if their position is known.

•        Reconnaissance: Patrol movement can be managed more closely.

•        GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System.

Other Systems

Other satellite navigation systems in use or various states of development include:

•        GLONASS – Russia's global navigation system. Fully operational worldwide.

•        Galileo – a global system being developed by the European Union and other partner countries, planned to be operational by 2014 (and fully deployed by 2019)

•        Beidou – People's Republic of China's regional system, currently limited to Asia and the West Pacific

•        COMPASS – People's Republic of China's global system, planned to be operational by 2020

•        IRNSS – India's regional navigation system, planned to be operational by 2012, covering India and Northern Indian Ocean

•        QZSS – Japanese regional system covering Asia and Oceania