How GPS technology works

                        

What is GPS and how it works:

GPS or Global Positioning System is the current navigation system used across the globe for navigation purposes. It is also called NavStar and was built by US military. It has been fully operational since 1995 and currently operated by US Space Force. 

In ancient times, people used to look at the stars for navigation. They made themselves to remember the constellation of the stars and used those patterns to locate any places. But, with the evolution of human mind, technology has been being evolved so far. Now, we use navigation technology to locate any objects in the earth. One great invention of navigation technology is GPS.

What is GPS:

The full form of GPS is Global Positioning System. It has mainly three parts- satellites, ground station and receivers. 

Satellites are like stars which are placed at a particular orbit. They contain many electronic devices like cameras, transponders, antennas etc. 

The ground stations receive the signal coming from the satellites and they locate the position of that particular thing.

The receivers are found on the mobile phones, cars or other electronic devices. These receivers can calculate the distance between any particular place or objects and the receiver. itself. 

Currently GPS system has 31 active satellites which were placed inclined to 55 degrees with respect to the earth's equator and orbit around 20,000 km from the earth's surface. The technology used in the GPS is very complex but the concept is very simple.

GPS working principles:

                                 

The  concept of GPS came from the laws of relativity. The satellites of the navigation constellation carry very stable atomic clocks synchronized with each other and ground clocks. 

According to theory of special relativity, moving clocks tick much slower compared to the clocks which are in rest position. So.due to the movement of the satellites with respect to the earth, time dilation occurs and hence their onboard clocks run a few microsecond faster than the clocks on earth surface. That is why the time is needed to be corrected daily. Because without correction, the location of GPS would be off by as much as almost 6 miles per day.

Each satellite in the constellation transmits a carrier wave with modulation which includes a pseudorandom code and a message of Time Of Transmission (TOT). Pseudorandom code is a random binary sequence which is used to decode the message transmitted by transmitter and this code is known to the receiver. If this code is not known to the receiver, then it can never extract the message sent by the transmitter. 

Since the speed of the signal or radio wave is constant and independent to the speed of the satellites hence, time delay between the Time of Transmission (TOT) and Time of Arrival (TOA) become directly proportional to the distance from the satellite to the receiver. The GPS receivers monitors multiple satellites and calculate the precise location of the receiver.

Related Article: NavIC: All you need to know

How many satellites do GPS use:

Actually there are 32 active satellites revolving around the orbit of earth, but we need minimum 4 satellites in Line-of-sight. Because, as explained above, each satellite transmits some radio waves which contains the data of exact time and position of the satellite. 

The radio waves are electromagnetic waves and their speed is constant. So, the time taken between TOT and TOA is multiplied by the speed of light. This is the distance between the satellite and the receiver. By solving four geometric equations the exact position of the receiver can be calculated.

List of GPS satellites:

There are so many satellites launched for navigational purposes. But many of them are retired or damaged. Currently there are 32 satellites which are operational. They are- USA-132, USA-145, USA-150, USA-151, USA-154, USA-166, USA-168, USA-175, USA-177. USA-180, USA-183, USA-190, USA-192, USA-196, USA-199, USA-201, USA-206, USA-213, USA-232, USA-239, USA-242, USA-248, USA-251, USA-256, USA-258, USA-260, USA-262, USA-265, USA-266, USA-289, USA-293 & USA-304.

Satellites from USA-132 to USA-206 were launched by Delta-II rocket which was an expendable launch system. USA-232, USA-239, USA-248, USA-251, USA-260 and USA-293 were launched by Delta-IV M+(4,2) rocket. USA-242, USA-256, USA-258, USA-262, USA-265 and USA-266 were launched by Atlas 401 rocket and USA-289 and USA-304 were launched by Falcon 9 Block 5 rocket. 

What frequency do GPS satellites use:

All satellites in the GPS constellation use the L band. The L1 carrier (1.57542 GHz) is modulated by Coarse-acquisition (C/A) and Precise code (P code) while the L2 carrier (1.2276 GHz) is modulated by only P code. This P code can be encrypted which is called P (Y) code and only available for military equipments.

The L3 (1.38105 GHz) band is used to transmit data from satellite to ground station. This data is used by United States Nuclear Detonation Detection System for locating nuclear detonations in atmosphere or near space.

L4 band (1.379913 GHz) is being studied for additional ionospheric corrections.

L5 band (1.17645 GHz) falls into an internationally protected range for aeronautical navigation. it has been proposed for use as a civilian safety-of-life (SoL) signal.

Uses of GPS:

There are limitless uses of GPS around the world. Some of them are-

1. In astronomy.
2. In automated vehicles.
3. Disaster management.
4. Aircraft tracking.
5. Navigation.
6. Robotics.
7. Sports.
8. Telematics.
9. Phasor measurements.
10. Target tracking.
11. Tactonics.
12. Missile and projectile guidance.
13. Search and rescue.
14. Study of geographical area etc.

In spite of GPS, there are various navigation systems made by different countries. For example, Russia made GLONASS, European Union made Galileo, Japan made QZSS, China made BeiDou and India made IRNSS or NavIC.