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What are the limitations of GNSS?

Industry Articles October 13, 2020

Does DGPS or RTK most improve the accuracy of velocity measurements?


Limitations of GNSS - Carrier wave frequency shift velocity accuracyIt’s a common misconception that GPS velocity measurements are made by measuring the time taken to move between two consecutive position measurements. This is not how velocity is measured—and is also the reason why position accuracy has no effect on velocity accuracy.

To work out velocity, GPS receivers actually use Doppler shift. So they look at the frequency shift in the carrier wave from each satellite and calculate a velocity vector using that information. So it doesn’t matter if your receiver is in SPS mode or RTK Integer, the velocity measurements will be equally accurate.



What are the limitations of GNSS?

GNSS systems, on the whole, are pretty awesome. They’re easy to use, don’t drift and can achieve high levels of accuracy. However, they’re not perfect. For starters, to reach their full potential they need a clear and uninterrupted view of the sky. That’s great if you’re working in the middle of a field, but if you’re trying to survey city streets, or working under bridges and in tunnels, at best you get reduced accuracy, and at worst you get no measurements at all.

GPS Satellite tunnel

Also, while a GPS receiver is great at capturing position and velocity measurements it’s not much good if you’re also interested in roll, pitch and yaw. A dual-antenna system helps solve some of those problems, but still doesn’t provide a complete picture. And for extremely dynamic manoeuvres, there is simply not the resolution required.

When you combine an inertial navigation system (INS) with GPS to create a GPS-aided INS (also written as GPS+INS), you solve the problems described above, and at the same time you also solve the problems associated with un-aided inertial navigation systems (particularly the problem of drift). This combination is at the heart of OxTS’ precision measurement solutions. And this is how our customers are able to measure with confidence – using OxTS’ market-leading technologies to measure position, orientation and dynamics in all environments.

This is one of the articles in our ‘What is GNSS?‘ series.

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