GPS is the acronym of Global Positioning System, first placed into orbit of the U.S. Department of Defense (DoD). It is a satellite-based navigation system containing a constellation of 27 satellites, of this 24 Earth- orbiting satellites are in actual work and remaining 3 are used as backups in case for emergency or any satellites fails. Initially GPS was implemented for military navigation purpose but soon it came in for civilian applications.

The GPS space assembly of 24 satellites are 12,000 miles above us, orbiting the Earth at a speed of 7000 miles an hour. They are steady moving objects making a couple of orbits every day. GPS satellites are solar-powered, but they are also provided with backup batteries to drive the satellites in situation like solar eclipse. These batteries compensate the absence of solar energy. The orbits are arranged in such a way that atleast 4 orbits are visible in the sky at anytime and anywhere on Earth. Each satellite is empowered with small rocket boosters to drive them in the right path.

NAVSTAR is the name of the official U.S. Department of Defense GPS. In 1978, the first GPS satellite was launched. In 1994, the network of 24 satellites in GPS space was achieved. Each orbit has a life time of about 10 years. After which they are replaced by launching a new orbit, constantly. An orbit weighs around 3000- 4000 pounds and about 17 feet across with extended solar panels. The power required for signal transmission is about 50 watts. . The signals from the GPS satellite travel by line of sight, which means they will pass through clouds, glass and plastic but cannot pass through solid substances such as buildings and mountains. A GPS receiver make use of these signals from alteast 4 satellites to calculate the location on Earth.

How GPS Works

The GPS is based on the Principle of Triangulation. The 24 satellites moving in a precise orbit transmit signals to the Earth surface. Each GPS satellite is provided with atomic clock. The transmitted signals indicates the location and the current time of the satellite. All the satellites synchronize in operations, hence they transmit the repeating signals at the same instant. The signals from different satellites reaches the GPS at different times indicating the far distance from the GPS receiver. The signals are then triangulated by the GPS receiver. Essentially, the GPS receiver calculates the time taken by the satellite to transmit the signal. This reveals the distance between the GPS receiver and the satellite. Similarly the distance between few other satellites are also calculated. This helps to locate the User’s position on the electronic map.

Initially, GPS receiver must lock signals from atleast 3 satellites to locate 2D position which is latitude and longitude. Signals from 4 or more satellites must be locked to be more specific in locating the position. 3D location includes latitude, longitude and altitude. Once done with the location, GPS receiver unit is further provides with other information like track movement, speed, trip distance, destination distance, time for sunset and sunrise etc. It is also possible to locate the exact street number if the GPS receiver locks active signal from many satellites.

GPS Signal System

There are two of low power radio signals transmitted by the satellites, known as L1 and L2. Generally L1 signal is used for civilian purpose which has a frequency of 1575.42MHz in the UHF band. Essentially we have to understand three types of information transmitted in the signal. They are pseudo random code, almanac data and ephemeris data. Each one of this information is essential to trace out the satellites available for the user’s view and locate their position on Earth. The validity of this information varies accordingly. Validity of Almanac data is about many months compared to Ephemeris data which is valid only for few hours.

Some technical terms are considered to indicate the validity of the information. When all data is considered invalid it is termed as Factory Start. If Almanac data is current and Ephemeris data expired, then it is Cold Start. When both Almanac and Ephemeris data are current, it is Warm Start. The pseudo random code is an I.D code. It reveals the fact about the satellite which transmits the information. The satellite information page displays the number attached to the signal bar. So that we come to know which of the satellite is transmitting the information.

The Almanac data describes the orbital arrangement of the satellites. The Almanac data from each satellite explains its orbital location. This helps the GPS receiver to expect the view of the possible satellites in the local sky. The User can then track only those available satellites and forget about the others, which are out of view. Though the data is valid for many months, it is not very precise. The Ephemeris data describes the GPS receiver about the position of each satellite at any time throughout the day. Each satellite broadcasts its ephemeris data revealing only its own orbital information. Ephemeris data is very precise and valid for 4 hours. The information is transmitted at regular intervals.

Factors Affecting GPS Signal

When the signal passes through the atmosphere, it slows down. Mainly Ionosphere and Troposphere delays the signal. A built-in model in the GPS system calculates the time of delay to correct this error. When signal gets reflected on objects, the travel time of the signal gets increased causing errors. This is known as signal multipath. The GPS receiver clock is not as accurate as atomic clock provided to each satellite. This may lead to receiver clock errors. Ephemeris errors can occur due to inaccuracies in the Ephemeris data. Accuracy mainly depends on the number of satellites visible in the sky. Signal reception can be blocked by large buildings electronic interference and other factors. Essential fact is that the GPS unit does not work underwater, underground or indoor.

Satellite geometry refers to relative position of the satellites at any time. When satellites are relatively positioned at wide angles to each other, then it is known as Ideal geometry. When it is tightly grouped or placed in a line, then there exists poor geometry. The accuracy of the signal can be improved by not imposing Selective Availability, which was once done by the U.S. Department of Defense, to avoid adverse effects. When SA is turned off, intentional degradation of the signal can be avoided. Thus GPS is best alternative for maps and guides. It can be used while traveling and prevent the usage of navigation maps and avoid getting lost. Anglers can use GPS to identify the hot fishing spot.