Understanding how the Global Positioning System (GPS) works

Subject: Science and tech

Sec: Space tech

Context:

In today’s era, from civilians to the military, from precision scientific studies to urban planning and disaster risk estimation, GPS has significantly changed our expectations of where we are and our sense of place.

About GPS

The U.S. Department of Defence started the GPS programme in 1973 and launched the first satellite in 1978.
The modern GPS satellite constellation consists of 24 satellites moving around the earth in six orbits.
Each satellite completes two orbits in a single day.

Three main components of GPS:

The space segment:
- The space segment consists of the 24 satellites.
- The six orbits they occupy are all 20,200 km above the earth, and each orbit has four satellites at all times.
- In this configuration, anyone on the earth will be able to ‘see’ at least four satellites at a time, which is a crucial requirement.
The control segment:
- The control segment consists of a global network of ground-based control stations and antennae that track the 24 satellites, make sure their performance is as expected at all times, and transmit commands.
- The services provided by the GPS system are designed to meet the Standard Positioning Service (SPS) performance standard.
- The SPS standard tells application developers and users anywhere in the world what they can expect from the GPS system.
- The control segment ensures these commitments are kept.
The user segment:
- The user segment pertains to the use of GPS in various sectors and applications.
- The major sectors include:
  - agriculture, construction, surveying, logistics, telecommunications, power transmission, search and rescue, airtravel, meteorology, seismology, and military operations.

How does GPS work?

Each GPS satellite continuously broadcasts a radio signal containing information about its location in orbit, operational status, and the time at which the signal is emitted.
The signals are transmitted at the L1 and the L2 frequencies at 50 bits/second.
The signals are encoded with code-division multiple access.
This allows multiple signals to be transmitted in the same channel and for a receiver to be able to disentangle them.
There are two encoding types:
- The coarse/acquisition mode, which civilians can use to access coarse GPS data, and the precise mode, which is encrypted and is for military use.
- Being an electromagnetic signal, the radio waves travel at the speed of light.
- On your smartphone, a GPS receiver picks this signal up and uses it to calculate its precise distance from the satellite.

Other countries having GNSS

Such systems are currently operated by Australia, China, the European Union, India, Japan, South Korea, Russia, and the U.K. .
Of these, Russia’s GLONASS, the E.U.’s Galileo, and China’s BeiDou systems are global.

India’s satellite system:

India mooted its own Indian Regional Navigation Satellite System in 2006, later rechristened Navigation with Indian Constellation (NavIC).
Its space segment consists of seven satellites: Three in geostationary orbits and four in geosynchronous orbits.
As of May 2023, the minimum number of satellites (four) could facilitate ground-based navigation.
The master control facilities are located in Hassan in Karnataka and Bhopal in Madhya Pradesh.
The NavIC satellites use rubidium atomic clocks and transmit data in the L5 and the S bands, with newer satellites also transmitting in the L1 band.
They include a messaging interface that can receive messages from control stations and transmit them to specific areas, like warning fishers about being close to international borders, etc.
India also operates the GPS-Aided Geo Augmented Navigation (GAGAN) system, which was developed by the ISRO and the Airports Authority of India.

Current Scenario

In 2021, according to one estimate, there were 6.5 billion Global Navigation Satellite System (GNSS) devices installed worldwide.
The figure is expected to rise to 10 billion by 2031.