Global Positioning System (GPS)

The Global Positioning System (GPS) is a satellite based navigation system that reveals the accurate location of its user through a GPS enabled unit.

Since its invention, the Global Positioning System (GPS) has proved to be an extremely useful tool available at man’s disposal. The GPS is a network of 24 satellites that orbit around the earth twice at least daily. The system helps in discovering a new location, identifying an existing one, or guiding a person to reach a certain location. Originally, the system was developed by the U.S. Department of Defense as an integral part of their defense technology. The first ever satellite for this purpose was launched in February 1978. Later on in the year 1994, a full constellation of 24 GPS satellites orbiting the earth was achieved. The service remained privately used by the U.S. military until 1996, when President Clinton declared it open for the civilians as well.

Basically, GPS is designed to answer the following questions accurately and simultaneously:

  • "Where am I?"
  • "Where am I going?"
  • "Where are you?"
  • "What's the best way to get there?
  • "When will I get there?"    

Working of GPS

A GPS device works by capturing the signals transmitted from the GPS satellites in orbit. GPS receivers take this information and use triangulation to calculate the user's exact location. The GPS receiver records the time when a signal is sent to it by a particular satellite. At the same time, the receiver also records the time of the signal reaching it. The receiver now compares these two to derive the actual time difference, which tells the receiver how far the satellite is from it. Next, the receiver receives signals from other satellites as well and measures the respective distances. This then enables the receiver to reveal the exact location of the user on its screen, marked by a blip, symbol, etc.

A GPS receiver can reveal the 2D (latitude and longitude) location of a person, only when it’s locked onto at least three satellites. If the receiver is able to get in contact with four or more satellites, it can then reveal the 3D (latitude, longitude and altitude) position of a person. Upon precisely determining the location of a user, the receiver can also calculate other information such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time, etc. A GPS receiver is accurate to within 15 meters of a location on average.

GPS Signals

The signals transmitted by the GPS satellites are classified into two frequencies, namely L1 and L2. The frequency available for civilian use is L1, which measures 1575.42 MHz in the UHF band. The GPS signals travel through a host of objects such as clouds, glass, plastic, etc. However, they cannot pass through tall solid objects such as buildings, mountains, etc. To avoid such a situation, a receiver normally stays in contact with multiple satellites so that if one or two get blocked by an object, it can receive signals from the other satellites.

A typical GPS signal can be broken down into three bits of information, which are:

  • Pseudorandom code: The pseudorandom code is indicative of the identity of the satellites that are transmitting their signals. These codes are special numbers that are designated to each satellite for easy identification.
  • Ephemeris data: The ephemeris data contains important information about the signal strength and health of a satellite. It also contains information like the current date, time, etc. The ephemeris data is vital for GPS receiver in determining the location of its user.
  • Almanac data: The almanac data tells a GPS receiver about the particular position of a satellite, at a given point of time. It also provides the receiver with other useful information such as the orbital information of the satellite in question, and that of every other satellite in the constellation.

GPS Stats

The constellation of 24 GPS satellites is about 12,000 miles above land, and these are constantly orbiting around the earth at a speed of 7,000 miles per hour. They weigh about 2,000 pounds and measure up to 17 feet in width. Having a transmitting power of merely 50 watts or so, these satellites are equipped with small rocket boosters that help them to fly in the correct path. Each satellite is built to last for 10 years and is powered by solar energy radiated from the sun. In the event of a solar eclipse, these satellites are kept in motion and function by providing them with backup batteries onboard. The official name given for these satellites by the U.S. Department of Defense is NAVSTAR. Replacements of these satellites are constantly being built and launched into the orbit as and when required.

The Working of GPS

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