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GPS | What it is and how it works

A visual introduction to the Global Positioning System

Laurie Stott

on 18 November 2016

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Transcript of GPS | What it is and how it works

GPS is an acronym which stands for the “Global Positioning System”.

This long string of nouns describes a satellite radio based system that can provide people, wherever they may be, with the ability to determine their position anywhere (well, almost anywhere) on earth.
What it is and how it works
GPS is a group or 'constellation' of satellites that orbit the earth at a very high altitude. They use radio signals to transmit their position and the time the signal was sent. The system was developed in the 70s by the US military. It took almost 20 years before it was readily accessible to the general public. After less than a decade of use, it became an essential tool for many different sectors. Today it is a system that is relied upon for navigation, surveying, recreation, asset tracking, the study of plate tectonics and the list goes on....

The GPS system relies upon line of sight (or straight lines from one thing to another) radio waves from the satellites to hand held computers and receivers. Since you could not look from the sky or outer space into a cave or a building, GPS does not work in areas where there is no line of sight to the satellite constellation. For this reason, GPS is not very useful at the bottom of steep ravines or on the north side of mountains and steep topography in the Northern Hemisphere

The space segment consists of 24 satellites positioned in six earth-centered orbital planes with four operation satellites and a spare satellite slot in each orbital plane. The system can support a constellation of up to thirty satellites in orbit.
GPS Satellites orbit the earth from a very high altitude.
Due to the height or distance from the earth's surface and rate at which they travel, time for the satellites is faster than clocks on earth. About 1.7 seconds per century faster and this is significant enough to cause issues with their ability to provide positions on earth. They have to be regularly adjusted for the system to work. Satellites carry four very accurate 'atomic' clocks. Only one is in use at any one time. With these clocks, the GPS satellites have the most accurate timing systems that exists today.
Interesting Fact
GPS satellites transmit
the time the message was transmitted
satellite position at time of message transmission

They receive signals from the Military Control Center. This is the Control Segment of the System. No other user or device can communicate with the satellites.
On the ground, the civilian user receives the signal from the satellites.
What is GPS anyways?
Where and when did it come from?
You won't see GPS satellites cruising by in the night sky
Because of the constantly changing 'constellation' of satellites, satellite coverage varies and there will better times than others to use the system -particularly for navigation and survey purposes.
There are 6 orbital planes. Each satellite completes one orbit every 12 hours
Recreation units are great for navigation and rough estimates of location
High end survey units like the Trimble Pathfinder Pro allow for high accuracy mapping and detailed data collection
An accurate position requires a minimum of four satellites.

GPS receivers calculate their position using the time and the satellites location sent in the signal from the GPS satellites.

The length of time it takes each signal to reach the receiver provides the information needed to calculate distance or range to each satellite and triangulate the position of the receiver on the ground.

The fourth satellite corrects the time error between the highly accurate satellite clocks and the less accurate receiver clock.
With GPS we can navigate from A to B or find a specific point like the tennis courts if we have coordinate or location
We can map points, like a tree, polygons around a fields or other 2D features. We can also map lines or linear tracks, like a trail or stream.
Ionosphere and atmosphere also adversely affect GPS signals and in areas of high ionospheric activity, GPS is not useable.
Near the horizon, signals travel through more atmosphere. This causes more noise and error in satellite signals and so we don't use any satellite below an elevation of 15 degrees.
Here are a few links to help you learn more.
Wikipedia GPS page

US Coast Guard Navigation Center

BC RISC GPS Standards and Guidelines

Presentation created by Laurie Stott | Images and Graphics sourced from:

Earth image
http://earthobservatory.nasa.gov/Newsroom/BlueMarble/Images/globe_west_2048.jpg By NASA [Public domain], via Wikimedia Commons

Aurora Borealis (Ionosphere)
http://commons.wikimedia.org/wiki/File%3AS123_Aurora.jpg; By NASA [Public domain], via Wikimedia Commons

Starry Night
By ESO/S. Brunier [CC-BY-3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

GPS Constellation -animated
By El pak at en.wikipedia (Transferred from en.wikipedia) [Public domain], from Wikimedia Commons.

Theory of Relativity
http://commons.wikimedia.org/wiki/File:Spacetime_curvature.png?uselang=en-gb. Created by User:Johnstone using a 3D CAD software package and an image of planet earth from NASA's Galileo spacecraft.
And so,
the Global Positioning System (GPS) is a very complex and useful tool for positioning and navigation and is widely used by both the private and public sector. If you plan to use the GPS system, it is important to understand the way in which it works and the issues and limitations that exist with its use.

Experience and training will allow you to use it effectively for both recreation and work related activities. As a survey tool, it is an invaluable tool for the inventory, monitoring and management of natural resources.

Einstein's Theory of Relativity applies to GPS Satellites!
So, what does the GPS look like?
GPS is made of three main parts:
(1) Space;
(2) Control; and
(3) the User or the person who holds and uses the device that captures the signal from the GPS unit.
If there are lots of obstacles to the south, like these trees or building connection to satellites may be blocked or limited
There are some limitations and GPS has some issues. Multi-path from trees and buildings affects accuracy and electrical equipment and lines can affect equipment and accuracy as well.
Full transcript