Feature: Navigation
GPS correction methods and determination of precise location
By Aaron Nathan, CEO and Founder, Point One Navigation T
he global positioning system (GPS) has revolutionised how humans and machines navigate on Earth, with many organisations incorporating
this resource in their systems, products and applications. Autonomous vehicles, robots, logistics fleets and emergency response systems all use GPS to precisely locate objects on the Earth’s surface and use optimised routes. Tis list is set to grow as new services and systems are developed and introduced every day. Yet, GPS signals can suffer in accuracy
and from delays, influenced by various atmospheric and technological effects. Tis calls for correction methods, of which there are several, each with a unique set of advantages and considerations.
GPS operation GPS is a satellite-based navigation system that provides precise positioning and timing information anywhere on Earth.
Te system consists of a constellation of satellites, ground control stations and GPS receivers. GPS satellites continuously transmit signals that contain information about their position and time. A GPS receiver picks up these signals and calculates the time it took for them to travel from the satellite to Earth. Combining this calculation with the speed of light, the GPS receiver can then determine its distance from the satellite. By repeating this process and
intersecting the distance measurements from multiple satellites, the receiver can then triangulate its precise position on the Earth’s surface.
Top five reasons for GPS signal inaccuracies Understanding signal errors and what causes them is important when selecting the best GPS correction method for a particular application. Various factors contribute to errors in GPS signals,
50 July/August 2024
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including ephemeris inaccuracies, satellite clock discrepancies, ionospheric disturbances, tropospheric conditions and biases between different satellite systems. To be able to choose the right correcting method and achieve precise and reliable positioning, distinguishing between these sources of error is critical.
1. Ephemeris errors One of the inputs a GPS receiver uses to calculate its location is ephemeris data, or the information about the satellite’s orbital parameters and its position in space. Inaccurate or outdated ephemeris data leads to errors in determining the satellite’s location, affecting the accuracy of the GPS positioning calculations. In other words, if the GPS does not know exactly where the satellite is, or uses an incorrect location, the signals it receives will not be interpreted accurately.
2. Satellite clock inconsistencies Despite their well-known accuracy,
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