Generally, when we talk about GPS, it is thought to be a map application on a smart device, such as mobile phones, vehicles, or GPS devices. This shows a destination point, distance, and time to reach the point.
GPS is an abbreviation of Global Positioning System, which is a set of three different components:
Space, Ground control, User equipment
Space consists of Satellites that orbit the Earth and transmit radio signals to GPS receivers.
Ground control is made up of Earth-based monitor stations, master control stations, and ground antennas. Ground Control’s job is tracking and operating the satellites in space and monitoring transmissions. There are monitoring stations on almost every continent in the world, including North and South America, Africa, Europe, Asia, and Australia.
User equipment or GPS receivers including all devices on the earth which are equipped with GPS such as watches, vehicles, smartphones, and telematics devices.
The most popular and commonly used navigational satellite systems in the United States are NAVSTAR or GPS. In addition to the United States, there are several countries like Russia (GLONASS), India (IRNSS), China (BeiDou), and Europe (GALILEO) for example, who have their navigational satellite systems as well. The systems belonging to China and India are not worldwide systems because they are revolving in a geosynchronous orbit above their countries.
In this passage, we will discuss the functions of the NAVSTAR systems, simply referred to as GPS. It is also quite common for most phones and devices to have futures of both GPS and GLONASS as navigation systems.
The first time that the U.S. Department of Defense put the satellites to use was only for military purposes. Soon after, they were made available for civilian use in the 1980s.
The GPS Satellite System specification
The GPS consists of 31 satellites orbiting the Earth; 24 of them are core satellites and the rest are spare and serve as emergency replacements when something happens to others. The Satellites need constant maintenance and sometimes some repairs. After all, they only last about 10 years.
These 24 satellites are circling on orbital planes of a constellation with a constant distance of 20,000 km from the earth and with a speed of 14,000 km/h.
The satellite’s main power supply is provided by solar energy and they also have backup batteries onboard that can be used in case of a solar eclipse.
The GPS Ground control is the global ground station on earth that tracks the GPS satellites, monitors their transmissions, performs analyses, and sends commands and data to the constellation. These stations are located on all continents.
How does GPS work?
GPS satellites circle the Earth twice a day in a precise orbit. Each satellite constantly transmits a unique radio signal with orbital parameters and extremely accurate timestamp (obtained through atomic clock on-board the satellites), that allow GPS devices to decode and compute the precise location of the satellite.
In GPS positioning systems two basic scientific concepts are used: trilateration (not triangulation) and
Distance ( = Speed * time). As we know, the speed of a radio signal is light speed.
2-D or 3-D Trilateration is a method that can determine the position by knowing the distance from 2 or 3 Geographical coordinates.
As is mentioned, the radio signal which is sent from the satellite to the GPS receiver provides two important pieces of information such as the instantaneous, the exact time of signal transmission, and the orbital coordinates of the satellite.
The GPS receiver uses the above relationship to calculate the distance to each satellite. Knowing the distances of at least three satellites will allow us to determine the position of GPS receivers on Earth.
First, let’s explain three-sided surveying in two dimensions (2D three-sided surveying). The distance from the first satellite of the GPS receiver is R1 and the distance from the second satellite is R2. Imagine two circles with the first and second satellites in the center and radii R1 and R2.
Being on the surface of the earth, the position of the GPS receiver is the intersection. This considers the earth as the intersection with the third circle. If you put the three circles together, they will be in the same place as the GPS receiver.
2D three-range surveying is the calculation of latitude and longitude positions on a map.
In 3-D trilateration, it’s the same, but there’ll be spheres instead of circles. 3-D position includes latitude, longitude, and altitude geographical coordinates. In the three-dimensional world, instead of two satellites, we need three satellite measurements.
In the three-dimensional world, the intersection point of three spheres gives two points. Just like in the previous case, using the Earth as the fourth surface we find the correct point in the below picture.
Let’s consider two important complex issues here:
The time in the GPS satellite is based on the precise time of atomic clocks that exist on them, while the receivers have regular quartz clocks. This Time difference will cause a huge error in GPS calculations. Having information from the fourth satellite solved this error. Therefore for calculating the location and time of a GPS receiver we need four satellites’ radio signal information.
Einstein’s general theory of relativity
Another complication is based on Einstein’s general theory of relativity; In space far from gravity, the time of the motion for an object is faster than that for an object on the ground.
At an altitude of 20,000 kilometers above the Earth, the satellites experience one-quarter of the Earth’s gravity, thus, according to Einstein’s general relativity theory, the clock should accelerate by about 45 microseconds per day. This causes 38 microseconds to offset every day in the atomic clock.
To compensate for this, a theory of relativity equation is integrated into the computer chips and adjusts the rates of the atomic clocks. Without this application of the theory of relativity, the GPS would have produced an error of 10 kilometers every day.
GPS satellites have atomic clocks that keep the most precise time, but it would be impossible to install these clocks in every receiver. Satellites’ atomic clocks get 38 microseconds ahead of ground clocks every day.
GPS not only determines the most precise location of people and things but also sends a time signal accurate to less than 10 billionths of a second. Power grids, banking systems, and mobile networks all use GPS for precisely timestamped transactions to operations from synchronized call transfers.
Uses of GPS
Global Positioning System GPS is a powerful and dependable tool for businesses and organizations in many different industries.
Surveyors, scientists, pilots, boat captains, first responders, and workers in mining and agriculture are just some of the people who use GPS daily for work. They use GPS information for preparing accurate surveys and maps, taking precise time measurements, tracking position or location, and navigation. GPS works at all times and in almost all-weather conditions.
- There are five main uses of GPS:
- Location — Determining a position like a car locator,
- Navigation — Getting from one location to another.
- GPS Tracking — Monitoring object, pet, School Bus Tracking, Trailer Tracking, or personal movement.
- Mapping — Creating maps of the world.
- Timing — Making it possible to make precise time measurements.
But GPS can also be really useful for scientists and not just in ways you might expect, like by helping them track animals. Scientists can also use GPS to help predict how bad a tsunami might be, to warn people about flash floods, or measure how dry a forest gets during a drought all by taking advantage of two basic ideas.
GPS doesn’t require an internet connection.
GPS does not require internet or cell phone signal. However, with their help, the GPS launch can be significantly accelerated. Satellite location information can be downloaded from the Internet, rather than downloading it directly via satellite, which is very slow. Such GPS systems are called assisted GPS.
So, the next time you track your food delivery or navigate your car, please keep in mind how important the theory of relativity, developed by Einstein, and the other mathematical ideas are behind GPS.
Author: Sherry Mosleh IoT Development Engineer@ Arshon Technology Inc.