GPS - Global positioning system | Bridge Equipment onboard the ships

How accurate is GPS?
GPS device accuracy depends on many variables, such as the number of satellites available, the ionosphere, the urban environment and more.

Some factors that can hinder GPS accuracy include:

Physical obstructions: Arrival time measurements can be skewed by large masses like mountains, buildings, trees and more.
Atmospheric effects: Ionospheric delays, heavy storm cover and solar storms can all affect GPS devices.
Ephemeris: The orbital model within a satellite could be incorrect or out-of-date, although this is becoming increasingly rare.
Numerical miscalculations: This might be a factor when the device hardware is not designed to specifications.
Artificial interference: These include GPS jamming devices or spoofs.
Accuracy tends to be higher in open areas with no adjacent tall buildings that can block signals. This effect is known as an urban canyon. When a device is surrounded by large buildings, like in downtown Manhattan or Toronto, the satellite signal is first blocked, and then bounced off a building, where it is finally read by the device. This can result in miscalculations of the satellite distance.

A brief history of GPS
Humans have been practicing navigation for thousands of years using the sun, moon, stars, and later, the sextant. GPS was an advancement of the 20th century made possible by space-age technology.



GPS technology has been used globally throughout history. The launch of Russia’s Sputnik I satellite in 1957 ushered in the possibility of geolocation capabilities and soon after, the U.S. Department of Defense began using it for submarine navigation.



In 1983, the U.S. government made GPS publically available, but still kept control of the available data. It wasn’t until 2000 that companies and the general public gained full access to the use of GPS, eventually paving the way for greater GPS advancement.



For more on the history and development of GPS, see our post on the History of GPS satellites and commercial GPS tracking.

Global Navigation Satellite Systems (GNSS)
A GPS is considered to be a Global Navigation Satellite System (GNSS) — meaning it is a satellite navigation system with global coverage. As of 2020, there are two fully operational global navigation satellite systems: the U.S. navigation signal timing and ranging (NAVSTAR) GPS and Russia’s Global Navigation Satellite System (GLONASS). The NAVSTAR GPS consists of 32 satellites owned by the U.S. and is the best-known and most widely-utilised satellite system. Russia’s GLONASS consists of 24 operational satellites with three remaining as spares or in testing.


Illustration of GLONASS, GPS and Galileo Satellites.



Other countries are also racing to catch up. The EU, for example, has been working on Galileo, which is expected to reach full operation capacity by the end of 2020. China is also building the BeiDou Navigation Satellite System, with 35 satellites planned to be in orbit by May 2020. Japan and India are also well on their way with their own regional systems, the Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigation Satellite System (IRNSS), respectively.

GPS vs GNSS Devices
Though GPS is a subset of GNSS, receivers are differentiated as GPS (meaning GPS-only) or GNSS. A GPS receiver is only capable of reading information from satellites in the GPS satellite network, while the typical GNSS device can receive information from both GPS and GLONASS (or more than these two systems) at a time.



A GNSS receiver has 60 satellites available for viewing. While a device only needs three satellites to determine its location, accuracy is improved with a larger number of satellites. The chart below shows an example of the number of satellites available (shown in green), along with its signal strength (height of the column), to a GPS receiver. In this case, 12 satellites are available.



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