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GPS 101
An Introduction to the Global Positioning System
What is GPS?
GPS stands for Global Positioning System. It is a space-based, satellite navigation system, owned and operated by the United States of America, which provides users with positioning, navigation, and timing services. The system consists of three segments: the space segment, the control segment, and the user segment. The space and control segments are directly operated, maintained, and developed by the US Air Force (USAF) on behalf of the Department of Defense.
  • Space Segment — The space segment is made up of a constellation of at least 24 satellites that constantly transmit one-way signals, providing the position of each satellite and the time. Currently, there are more than 30 operational satellites, with a few decommissioned satellites held in reserve.
  • Control Segment — The control segment is a world-wide network of monitor and control stations that maintains the satellites in their proper orbits. It also tracks the GPS satellites, uploads data and software updates, and maintains the health and status of the entire constellation. The operational hub of this network is at Schriever Air Force Base east of Colorado Springs.
  • User Segment — The user segment of the system is your GPS receiver, which receives GPS signals and uses the received information to calculate its position and the time.
A Brief History
The United States Department of Defense (DoD) started developing GPS in 1973, for military use. Although the USAF began launching GPS satellites in 1978, the GPS receivers of the day were too bulky and heavy for a person to carry. Portable military GPS receivers called Manpacks appeared in 1988. These backpack-mounted receivers weighed 17 pounds and cost about $45,000 each. Civilian access to GPS began as early as 1983, primarily with GPS receivers that had to be transported in vehicles. Over time, the receivers became smaller and cheaper. The first commercially available, handheld GPS receiver was introduced in 1989. Although GPS was originally developed for the US military, it became a national resource and openly available to the public in 1996 when President Clinton required the DoD to turn off a feature that limited positional accuracy for public use. Today, GPS receivers are highly miniaturized and inexpensive, often found as microchips in everyday devices such as cell phones, automobiles, and laptops.
How GPS Works
GPS satellites continuously broadcast radio signals toward the Earth. Your GPS receiver "listens" for these signals and uses the information from four or more satellites to determine its own location. The GPS satellites orbit the Earth twice a day. The orbital paths of all the satellites ensure that at least five satellites are visible from any point on the globe at any time. The signal from each satellite contains data indicating its location and the current time. Each satellite contains a very precise atomic clock in order to provide accurate timing data.
With the signals from the satellites, your GPS receiver uses four-dimensional trilateration to calculate its own position in three dimensions and the correct time. To do this, your GPS receiver needs to know:
  • The distance between it and each satellite
  • The location of each satellite
Distance
To determine the distance to a satellite, the GPS receiver multiplies the amount of time it took the signal to arrive by its speed. A repeating digital code, called the pseudo-random code, in every satellite’s radio signal allows the GPS receiver to figure out when a portion of the signal left the satellite. The codes are unique to each satellite. Although radio signals travel at the speed of light in a vacuum, there is a miniscule reduction in speed due to propagation in the Earth’s atmosphere. Using atmospheric correction data, your GPS receiver knows the speed of the signal.
Location
Your GPS receiver knows the positions of every GPS satellite in the sky because it acquires almanac and ephemeris data from the satellites:
  • Almanac data provides coarse orbit and status information about all the satellites in the constellation. Every GPS satellite transmits the same almanac.
  • Ephemeris, or orbital position data, is a table of values that gives the exact position of a satellite in the sky at a specific time. Each satellite transmits its own ephemeris as a set of coefficients for published equations.
GPS Accuracy
GPS receivers are very accurate and can find a location as close as 10 feet. However, there are some conditions that can lead to inaccuracy:
  • Geometry — The geometry of satellites refers to where the satellites are located, relative to each other and your GPS receiver. If all the located satellites are in the same area, the receiver’s calculated position may be slightly off. However, if all four satellites are in North, South, East, and West locations, then the calculated position will be very accurate.
  • Blockage/Urban Canyon — Blockage occurs when a GPS signal is blocked in a vehicle or in a parking garage. Buildings and mountainous terrain can also block the satellites’ reception. GPS can be inaccurate when you are in an urban setting with many tall buildings, also known as an urban canyon. Dense foliage, such as in a forest with heavy tree cover, can block GPS signals. It is also difficult to use GPS from inside a building because the materials used in its construction may block or degrade GPS signals.
  • Multipath — Multipath occurs when a radio signal reflects off an object and travels further than intended. This confusion of signals due to reflection could make GPS position inaccurate.
  • Weather — Clouds, fog, rain, and snow do not noticeably degrade GPS signals. GPS was designed for the military as an all-weather navigation system, and it is used to guide aircraft and ocean-going vessels through bad weather and zero-visibility conditions. A common, weather-related issue that does affect the accuracy of a GPS receiver is poor weather-sealing that allows moisture and other contaminants to come in contact with wires and electronic components. It is also possible for a layer of water or ice covering the antenna to affect GPS signal reception.
Telenav and GPS
Telenav has created applications that utilize GPS technology to provide a variety of useful features for both personal and business purposes. When you make a request for directions or information based on Location Based Services (LBS), the signal goes from your handset through a cell phone tower to the Telenav servers. Telenav immediately pulls up relevant information and sends it back through the cell phone towers to your handset, delivering voice- and on-screen driving directions, maps, and business data. Telenav uses LBS for cell phone apps, in-car navigation, and vehicle- and asset-tracking, and delivers this information with real-time GPS location, all to make life easier for customers. We encourage you to take a tour through our web page at www.telenav.com to see what Telenav and GPS can do for you.