What is GPS?
The GPS which stand for Global Positioning System is a satellite-based navigation system that includes 24 satellites deployed in space which is used to provide location, velocity and time information. It was developed by the US Department of Defense, and is completely free for the public. However, there are accurate alternatives available in many countries nowadays. Modern receivers simultaneously make use of GPS and other navigation systems, to get the most accurate position.
The satellites that a GPS system uses are deployed in space about 12,000 above from the earth’s surface and they orbit the earth once every 12 hours at an extremely fast pace of roughly 7,000 miles per hour. The satellites are evenly spread out so that four satellites are accessible via direct line-of-sight from anywhere on the globe to find the exact location of particular object.
GPS has already become an integral part of our lives. GPS is really an interesting technology. It uses a system of 24 satellites continuously orbiting the Earth and requires at least four satellites to track your location. It uses at atomic clock and the time error of your mobile phone is also a matter of great concern. Moreover, Albert Einstein’s theory of relativity plays an important role in GPS technology. Finally, a real life application for the theory of relativity.
What is Telecommunication Technology?
What are the uses of GPS?
In this era of technology, GPS is a powerful and important tool for different individuals and organizations. Some individuals like Surveyors, scientists use GPS on a daily basis for work. They use GPS information for preparing accurate surveys and maps, taking precise time measurements, tracking location, etc. GPS works well almost in every situation or in all weather conditions.
There are some main uses of GPS:
How does GPS Work?
Each GPS satellite broadcasts a message that includes the satellite’s current position, orbit, and exact time. A GPS receiver combines the broadcasts from multiple satellites to calculate its exact position using a process called triangulation. Three satellites are required in order to determine a receiver’s location, though a connection to four satellites is ideal since it provides greater accuracy.
What is triangulation?
Triangulation is a process used to determine location of a radio transmitter by measuring either the radial distance, or the direction of the received signal from two or three different points.
It is a complicated network. Let’s simplify and understand the GPS technology logically. Suppose you want to find out you friend’s location, and your friend have a mobile phone which has an integrated GPS receiver. Now GPS uses a mathematical technique called trilateration to locate your friend’s position.
What is trilateration?
Trilateration refers a mathematical technique used by a global positioning system (GPS) device to determine user position, speed, and elevation. It applies the geometry of circles, spheres, and triangles to constantly receiving and analyzing radio signals from multiple GPS satellites. By which a GPS device can calculate the precise distance or range to each satellite being tracked.
Using some engineering techniques, the satellites measure the distance between the object and the satellites. Here one satellite finds the distance between your friend and that satellite. So, he should be somewhere within the circle that first satellite covers. Similarly, the second satellite also finds the distance between your friend and that satellite as well.
So he should be on those circles as well. Now, there are, two known distances from two satellites. With two signals, the precise position could be any of the two points where the circles intersect. Now there is a small issue. There are many intersection points. So, which is his final position? Then third satellite helps to reveals his true location where all three circles intersect. Now you can see the exact location of your friend.
How the distance between you and the satellite is measured?
All the satellites are equipped with a very accurate atomic clock. The satellite sends an intermittent radio signal down to Earth. This radio signal will contain the exact time the signal was sent, and the position of the satellite. Assume the receiver also has a very accurate clock. The receiver on Earth receives the signal. Since radio waves travel at the speed of light, your receiver receives the signal after certain time duration.
By finding out the difference between the sent and received times, and multiplying it by the speed of light, you will be able to find out the distance between you and the satellites. Since the satellite has already sent you its coordinate, you can easily build a sphere around the satellite’s center point and find out your position as explained before.
One thing to note here is that the time measurement has to be very accurate. Even an error of microseconds will give an error in the range of kilometres, since the speed of light is so huge. Here comes the main issue. Your receiver does not have a highly accurate clock. Your mobile phones or laptops work on crystal clocks that are not accurate when compared to atomic clocks. Having an atomic clock in a smartphone is simply impractical. You can easily see how inaccurate our smartphone clock is compared to an atomic clock by checking the time settings.
What is time offset?
The difference between actual time and the time measured by our smartphone is called time offset. This time offset will cause a huge error in GPS calculations.
Why there four satellites are needed to find exact location?
The time offset of our smartphone with all three of the satellites is the same. Since, all satellites keep the same time. The time offset value of our smartphone becomes the new unknown. This means, apart from the three special coordinates, we have to solve the time offset value of our receiver as well. For that we need an extra satellite measurement to solve this fourth unknown, and that is why we need four satellites to measure our location. This way we avoid the need of an atomic clock in our mobile device. If we check our current GPS constellation, it will be clear that at least four satellites can see our location at any point in time.
More problems with GPS
Beside these, here we have one more issue to solve. Even with all these advanced technologies, this GPS system will not give us the right location. Here comes the importance of Einstein’s theory of relativity. Time is not absolute; it depends upon many other factors.
According to the theory of special relativity; a fast moving clock will slow down. The atomic clocks, which are moving at speed of 14,000 kilometres per hour, will slow down by seven microseconds every day due to this. At an altitude of 20,000 kilometres above the Earth, the satellites experience one quarter of the Earth’s gravity. Thus, according to Einstein’s general relativity theory, the clocks will tick slightly faster.
In this case, it moves around 45 microseconds faster every day. This means a net 38 microseconds offset is created every day in the atomic clock. To compensate for this, a theory of relativity equation is integrated into the computer chips, and it adjusts the rates of the atomic clocks. Without this application of the theory of relativity, the GPS would have produced an error of 10 kilometres every day.
At the end,
You may have a question in your mind that does GPS require an internet connection? Then answer is NO. GPS does not require an internet, or cell phone signal. However, with their help, GPS startup can be greatly speed up. Satellite location information can be downloaded by the internet, rather than direct satellite downloads, which are very slow. Such GPS systems are known as assisted GPS.