You login to a food delivery app in your smartphone to order your favourite pizza. Doorbell rings within 15 min and you hear a voice, “mam, your pizza is ready for delivery”. Wait! What just happened? Are you in Hogwarts school of magic? Or is there any magic wand inside your smartphone? It’s the built in GPS device in your smartphone. Ancient sailors navigated with the help of stars, islands and even the size of waves. It was only around the 14th century, magnetic compasses came into usage. They were widely used for marine navigation in Europe and China. But in olden days, navigation was only about finding the right direction towards the destination. Travelers couldn’t pinpoint their location with reference to common units. We came to know more about the world only when we started understanding the global coordinate systems involving longitudes and latitudes. Modern positioning and navigation systems involving GPS were developed for the US military. First navigation satellite was launched in 1978. By 1993 it was ready for operation with the constellation of 24 satellites. To utilise its full potential in various fields, GPS was allowed for civilian applications from the 1980s.
Initially, the USA had the policy of ‘selective availability’. This policy controlled the availability and quality of service to the civilian users as well as other countries. However, this policy was later discontinued in 2000. Today it is used for numerous purposes including navigation, telecommunication, gaming, food delivery, fishing, geo fencing and many adventure activities. GPS ground station, satellites and receiver are the 3 components of the GPS navigation system. GPS receivers use radio waves emitted by the constellation of satellites called ‘Navstar’ to determine the positional coordinates. Basic constellation consists of 24 orbiting satellites which function as a reference system for GPS. There are six independent orbits housing 4 satellites each. A few additional satellites are positioned in this constellation to provide backup for the active one’s. This constellation ensures that any given point on the earth is covered by at least 3 to 4 satellites at a given time. Trilateration is the principle behind the working of GPS. It involves signals from at least 3 satellites at a time. These satellites are positioned in the middle earth orbit at an altitude of around 20,000 km. Radio signals transmitted by the satellites travel at the speed of light. Receiver in your GPS device listens to the radio signals sent by these navigation satellites. Receiving time of signal multiplied by its speed, gives the distance between the receiver and satellites. This Distance can also be considered as the radius of a sphere of which satellite is the centre point. Even though we get the distance at this stage, we can’t tell where we are on the earth’s surface. As per the trilateration concept, the receiver can be anywhere within the sphere of the satellite. To overcome this problem, we need 2 more satellites. Now the junction point of spheres of 3 satellites provide your coordinates on the earth. Accuracy of this result can be improved by adding data from additional satellites. The latest GPS receivers have an accuracy upto 30 cm. In navigation applications, this process is happening continuously at preset time intervals. As time is an important parameter for distance calculation, even a minor change in time may result in miscalculation of distance in kilometers. To avoid this problem, GPS satellites use atomic clocks that have accuracy up to one billionth of a second. Many other countries have started developing their own navigation systems independent of the GPS. These were meant for privacy and security reasons. Following is the list of navigation systems developed by other countries. GLONASS by Russia, BeiDou by China, Galileo by European Union. Japan and India have developed their own regional navigation systems covering their territories. Can you imagine a world without GPS? Leave your opinion in the comment section.