One of the things to evaluate is accuracy. Accuracy is important for two reasons: you want your UAV to be where it’s supposed to be, and you want to be able to accurately georeference the data you’re gathering with your payload. But (as you will no doubt have already discovered), accuracy isn’t as simple as looking for a number. You’ll have spotted various abbreviations accompanying those numbers — GPS, PPK, RTK, and GCP most commonly, but you may also see GNSS thrown into the mix. In this article, OxTS will explain what these abbreviations mean, and what they mean for your UAV project.
GCPs stands for ground control points, and they are the most inexpensive method of ensuring your data is accurately georeferenced. They are physical targets that you place on the ground, and for which you know the coordinates. Once your UAV has finished its survey, those points can be used to reference the position of your UAV in the global frame. Rtk Survey Instrument
The biggest drawback with GCPs is that they don’t help your UAV know its own position. GCPs only help provide your UAV with a general position reference. So, GCPs aren’t any use if you want your UAV to fly pre-programmed flight plans. For that, you’ll need a solution such as an inertial navigation system (INS) paired with a GNSS receiver. GCPs can also be time consuming to use and cause additional difficulties at the post-processing stage.
GNSS stands for global navigation satellite system, which are systems that use satellite-based radio navigation to provide positioning, navigation and timing anywhere on Earth. The U.S. GPS is one of four GNSS constellations; the other three are the Russian GLONASS, the Chinese BeiDou, and the European Galileo. There are also two regional satellite navigation systems — the Indian NavIC and the Japanese QZSS.
Many UAVs will have a GNSS receiver built in — it’s what enables them to know where they are on the planet, after all. Using GNSS only, most UAVs can get accuracy of 3 m to 5 m. This level of accuracy isn’t too bad for some applications, but not accurate enough if you’re trying to use the position data for mapping activities.
Most UAVs advertise their ability to perform PPK — which stands for post-processed kinematics. It’s a method of squeezing extra accuracy out of your GNSS signal. OxTS has a blog post here that describes how it works.
The main thing to note about PPK is that you can’t use it in real time. UAVs with PPK capabilities can provide data that’s centimeter-level accurate in optimum conditions, but that accuracy can’t be used for navigating the drone itself. It also means that for activities that require centimetre-level accuracy in real time, PPK doesn’t deliver.
RTK is the best you can get when it comes to position accuracy. RTK stands for real-time kinematic, and just like PPK it can use it to obtain centimeter-level accuracy — but, in real time, rather than in post-processing.
For most mobile mapping activities RTK accuracy is the goal, particularly if you’re using a lidar sensor to create georeferenced pointclouds.
Without RTK accuracy for the duration of your lidar survey, your point cloud may be unusable. The additional accuracy RTK offers could be used to tackle more challenging environments — providing that you have the tools to remain with RTK accuracy for as long as possible in the absence of GNSS.
Most off-the shelf UAVs won’t have RTK capabilities built in; however, to get this level of accuracy, it’s likely that you’ll either need to purchase a top-of-the-range UAV or invest in a custom UAV (either built by you, or by a professional company).
If you’re involved in mobile mapping activities, then at the very least you will need PPK capabilities. Without those, you won’t be able to georeference your data with enough accuracy to be of use to anyone.
When considering the difference between PPK and RTK, you need to consider:
In what environment is your UAV operating? Do you need more accuracy than just a GNSS signal (remembering that PPK can only be applied after the survey takes place)?
Is range of no particular importance – or is the payload on your drone sufficiently large that you need to calculate range very carefully? If so, RTK will give your UAV additional accuracy and, therefore, fuel efficiency.
The final word in accuracy: gx/ix PPK and RTK from OxTS
If you read the blog post mentioned above, you’ll know that RTK (and PPK) rely on having an optimal number of satellites visible. If those satellites are lost, then so is RTK lock. That is, unless you use an OxTS INS with gx/ix tight coupling technology. Gx/ix allows our INS devices to maintain RTK and PPK level accuracy even if the number of visible satellites starts to drop. Essentially, it protects the accuracy of your scan for longer — and it is available on the OxTS xNAV650, our UAV-mountable INS.
The OxTS xNAV650 INS combines a best-in-class inertial measurement unit, with a survey-grade GNSS receiver to output highly accurate navigation data (position, heading, pitch and roll). The xNAV650 is used across the world for applications where reliability and accuracy are critical.
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