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How does a GPS receiver work out where I am?

Industry Articles October 13, 2020

The short and oversimplified answer to how a GPS receiver works out where you are is:

  • It calculates how long it takes the signal from each satellite it can see to arrive.
  • It multiplies that time by the speed of light to calculate the distance to each satellite.
  • It then calculates its position relative to no fewer than three satellites using trilateration.
  • Because the receiver knows the precise position of each satellite when the signal was sent, it can translate its own relative position into an Earth-based co-ordinate system.

GPS: Distance is speed x time

Of course, this explanation causes an explosion of questions in most minds. How can a receiver calculate the travel time of the signal unless it knows when it was sent—and how could it possibly know that? What is trilateration? How does the receiver know where a satellite was when it sent the signal? And that’s just the beginning.

The good news is you don’t need to understand any of this in order to use GPS. However, there’s no such thing as too much information, so if you’re curious to understand a little more about how it works (which might help you when you’re collecting measurements for work), carry on reading our related articles:

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The GPS signal

The hardest part of understanding GPS is the signal itself. This article explains the key things you need to know.

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Finding satellites

The first thing a receiver needs to do when it’s powered up is look for satellites. How does it find them? We explain here.

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Working out the range to a satellite

Each satellite in the GPS constellation transmits a unique C/A code, which appears to be random noise, but is in fact a deterministic sequence. How do we use this to work out the range to a satellite? We explain here.

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Trilateration: How distance measurements help to work out location

Once we know the distance to each satellite, how do we use this information to help work out our location? Click here to learn about trilateration.

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SPS

The term SPS stands for Standard Positioning Service. Learn how SPS provides the lowest accuracy GPS position measurements here.

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What are differential corrections or DGPS?

Most of the error in an SPS measurement comes from the fact the receiver has no way of actually knowing how the speed of the satellite signal has been affected as it passed through the ionosphere. Explore ways to correct these errors here.

Read more...

What is RTK?

RTK stands for Real-Time Kinematic and is another technique that improves the accuracy of GPS position measurements. Read this page to learn why it’s one of the hardest to understand and the most intensive to implement.

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What are the limitations of GNSS?

Does DGPS or RTK most improve the accuracy of velocity measurements? GNSS systems don’t drift and can achieve high levels of accuracy, so what are the limitations of GNSS systems? Find out here.

Read more...
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