Global Navigation Satellite Systems
Navigation Satellite Systems use satellites to tell you where you are. If it can tell you your location anywhere in the world, it's called a Global Navigation Satellite System (GNSS).
There are four GNSS:
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The United States' Global Positioning System (GPS)
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Russia's Global Navigation Satellite System (GLONASS)
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China's BeiDou Navigation Satellite System (BDS)
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European Union's Galileo
All four systems consist of 24 operational satellites and each system operates similarly. Each satellite sends a signal containing its position and the exact time. The time it takes for this signal to reach your receiver is directly related to the distance between you and the satellite. By comparing the signal's time of transmission with the time it's received, your receiver can determine its distance from multiple satellites to calculate your position.
| Number of Satellites | Position |
|---|---|
| 1 | Your location could be any point on a sphere centered on the satellite. |
| 2 | Your location could be any point on a circle at the intersection of two spheres, each centered on a different satellite. |
| 3 | Your location could be at one of two points at the intersection of three spheres, each centered on a different satellite. One point will have an impossible location or velocity. |
| 4 | Your location and altitude can be calculated. Four satellites allows for more accurate receiver clock synchronization, as your receiver can compare its time to multiple sources. |
GNSS Errors
GNSS positions are accurate to about 15 metres. Inaccuracies in GNSS positions arise due to:
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Signal arrival calculation errors. Your receiver can calculate signal travel time to within 10 nanoseconds. Since signals travel over a large distance at the speed of light, this still represents a position error of about three metres.
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Atmospheric effects. The speed of GNSS signals can vary as they travel through the atmosphere. These error is smallest when the satellite is directly overhead and becomes greater for satellites nearer the horizon where the path through the atmosphere is longer. Atmospheric effects include signal dispersion in the ionosphere and the effect of humidity and pressure in the troposphere.
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Multipath effects. Signals can bounce off objects like buildings and mountains. This is less of an issue in moving vehicles, where the reflected signals result in obviously innacurate positions.
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Ephemeris and clock errors. The satellite's position data (ephemeris) and its internal clock may not be perfectly accurate.
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Dilution of precision (DOP). DOP tells you how much the accuracy of your location is affected by the arrangement of satellites in the sky. When satellites are close together, your location is less accurate (high DOP). When they're spread out, your location is more accurate (low DOP).
| Source | Effect (m) |
|---|---|
| Signal arrival calculation | 3 |
| Ionospheric dispersion | 5 |
| Tropospheric effects | 0.5 |
| Multipath effects | 1 |
| Ephemeris errors | 2.5 |
| Clock errors | 2 |
Receiver Autonomous Integrity Monitoring (RAIM)
Your GNSS receiver needs at least five satellites to perform RAIM or fault detection. RAIM performs consistency checks by comparing the location result from different groups of satellites. The receiver provides an alert if the results are inconsistent.