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Global Positioning System (GPS): How does it Work?

I recently read about the delay of China’s BeiDou mission and how, instead, China chose to launch a fifth satellite as part of their China High-resolution Earth Observation System (CHEOS). This occurred in late August and roughly three weeks later, the failure of the launch was announced. While reading this I began to wonder why China needs its own positioning system anyway – GPS covers the whole world, right? 

I’ll admit I suddenly didn’t remember how GPS worked exactly! This is especially bad for me to confess as I work in surveying, and I should know this. I did know all about it at some point years ago, but along the way, I’ve taken it for granted. I’m sure I’m not the only one, so here is your refresher.

China launched its 5th CHEOS satellite in August. Credit: Xinhua

What is GPS?

GPS or Global Positioning System is a navigation system using satellites, a receiver and calculations to provide information on location, velocity and time for air, sea and land travel. GPS is comprised of at least three satellites (a fourth satellite is often used to confirm the information provided by the other three).

In North America, when we refer to GPS we are actually referring to one type of positioning system: the satellite-based radio-navigation system owned by the United States (US) government which is operated by the US Space Force.

Originally known as NAVSTAR GPS, GPS was created by the US Department of Defense in the 1970s. It was only allowed to be used by the US military until the 1980s when civilians were granted access. Today, the US government still maintains it and makes it freely accessible to anyone with a receiver.

This “US” GPS is part of the global navigation satellite systems (GNSS) and consists of 24 satellites in six Earth-centered orbital planes, orbiting at 20,000 km above Earth, travelling at a speed of 14,000 km/hr.

How does it work?

GPS is made up of three segments that provide location information:

  1. Satellites – circling the Earth, satellites transmit signals to users on geographical position and time of day.
  2. Ground control – the control segment is made up of Earth-based monitor stations, master control stations and ground antennas. There are monitoring stations on almost every continent in the world (everywhere but Antarctica).
  3. User equipment — GPS receivers and transmitters including items like watches, smartphones and telematics devices.
Primary segments of a GPS. Source: Manual of Geospatial Science and Technology

GPS uses trilateration to calculate location, velocity and elevation. Through this process, signals from satellites are collected to output location information. Satellites orbit the Earth sending out mircrowave signals that can be read and interpreted by a GPS receiver on Earth. To calculate location, the GPS device needs to read the signal from at least four satellites. The device can calculate the distance from that device to a single satellite — so reading from just one satellite doesn’t tell us anything about the location of the device. The device will therefore read signals from multiple satellites (from at least four, but usually from six) creating a series of intersecting spheres (where the distance from the GPS device to the satellite equals the radius). Where the spheres intersect will tell us the location of the device. The shape of the spheres actually produces two points of intersection, but the GPS device knows to choose the point closest to Earth.

The intersection of three spheres produces two points of intersection, so the point nearest Earth is chosen. Source: Geotab.com

As you move the GPS device, the radius changes and new spheres are made, providing a new position. Add in the time from the satellite and this data can be used to figure out velocity, calculate the distance to a destination and the estimated time of arrival.

What other navigation systems are there?

As previously mentioned, the GPS is provided by the US government. Since they control it, they can selectively deny access to the system or degrade the service. In 1999 during the Kargil War, the US did deny access to the Indian military. It is partly because of this that other countries or regions have created their own or are in the process of creating one.

Others include:

  • GLONASS – The Russian Global Navigation Satellite System
  • NavIC – The Indian Regional Navigation Satellite System
  • Galileo – The European Union’s Galileo positioning system (close to completion)
  • BeiDou – China’s BeiDou Navigation Satellite System (very close to completion)
  • Quasi-Zenith Satellite System (QZSS) or Michibiki  – Developed by the Japanese government (due to be completed by 2023)

The Future of GNSS

As countries continue to build and improve their positioning systems, their efforts will increase the accuracy, reliability and capabilities of GPS for both business and personal use.

Some predictions for GNSS’s future

  • New uses: for instance with natural disaster prevention or for virus contact tracing (this is already being looked at for the COVID-19 pandemic using cell phone location data).
  • New GPS satellite launches: more efficient technologies will result in improved navigation abilities.
  • Better signal protection for satellites: no more “signal jamming” and the ability to maneuver to cover “dead zones”

We are already seeing evidence of these predictions with China and Europe’s state-of-the-art positioning systems due to be completed later this year. There have also been developments into new uses. For instance, the CBC ran an article earlier this summer sharing news about a new mission called WildfireSat, a collaboration between the Great Lakes Forestry Centre, the Canadian Space Agency, the Canadian Forest Service and Environment Canada. They are developing a new type of satellite to monitor and relay information about wildfires to ground crews in real time.

A GPS satellite. Source: US Army via howstuffworks.com

Taking the time to review the basics of what GPS is and its future refreshed my understanding of GPS. While I was reacquainted with the basics, I certainly learned plenty of new information and I’m sure you did too. A lot is happening in this field, both on Earth and in space, with improvements with cell phone tracking and satellite enhancements. The application of GPS and other geospatial technologies has been expanding and will continue on this path as researchers and scientists further this impressive technology’s function. There is much to look forward to in this field.


Sources

https://www.geotab.com/blog/what-is-gps/
https://en.wikipedia.org/wiki/Global_Positioning_System
https://en.wikipedia.org/wiki/BeiDou
http://www.xinhuanet.com/english/2020-09/12/c_139363535.htm
https://spacenews.com/china-launches-third-gaofen-9-satellite-postpones-beidou-mission/
https://www.cbc.ca/news/canada/edmonton/wildfire-science-technology-gadgets-1.5602809

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