It may seem trivial, just turning the drone on, waiting for a ‘Green’ GPS light, and then off you go collecting data and doing other cool things. But, what happens when your aircraft suddenly “forgets” where it is, or perhaps suddenly “thinks” it is somewhere completely different?
It can be easy for us to simply assume that GPS is always “good” or maybe you occasionally glance at the number of satellites that your controller is reporting and say to yourself “Oh, it says I have more than ‘X’ satellites, that should be plenty!” But, how many times have we heard of a drone, or maybe have had it happen ourselves, simply flying away in what seems to be a random direction? The dreaded “Fly-Away”. Why would it do that? Here are a few people who asked the same question!
Unfortunately a true “Fly-Away” is exactly what the name suggests and the aircraft may never be seen again, so it can be difficult to say exactly WHY it happened, after all there are many complex systems involved, but one culprit could very well be the navigation solution that the GPS/GNSS chip is providing to the brain of the drone. It isn’t hard to imagine this happening since I am sure many of us have also had our own car GPS systems become confused about what road we are on, or assume we just exited a freeway when in reality we didn’t. The issue can come down to the precision of the GPS solution. In other words, how well does the equipment know its own position? If it suddenly thinks it is somewhere else on the earth then its reaction may be to try and correct itself by flying back to the “correct” location.
Notice the GPS information in DroidPlanner simply showing the number of satellites being used to provide the 3D position fix
Without getting too technical about how GPS/GNSS satellites work, we should just realize that each satellite’s signal is traveling both FAR and FAST. There are plenty of opportunities for the signals to become delayed, distorted, reflected, bounced, etc. Even assuming the signal did reach the receiver in an ideal manner, you would still only expect to be able to triangulate a position with as much certainty as you have for the positions of the satellites themselves. All these small errors can just keep adding up, so it is critical to try and minimize them.
There are systems in place to constantly monitor and correct those signals in an effort to ensure that we are all getting top-shelf answers from our GPS devices, but there are still situations that can cause your position to be more or less uncertain such as the surrounding terrain blocking signals from certain satellites. If you are a subscriber to the “more is always better” school of thought, it’s important to realize that access to “LOTS” of satellites isn’t always a reliable indication of GPS quality either, since those satellites may not always be well spread across the sky, and instead may be “clumped” together with respect to your receiver. It is always best to consider triangulation sources which are “spread out” more, and in the case of GPS this means that having 4 satellites directly above you wouldn’t necessarily provide as good of a result as 4 satellites spread across the sky.
One type of metric that can be used to quantify how well your GPS knows its position is commonly called the Dilution of Precision (DOP). This is a number (lower the better) used to represent how well the position is known based on geometries between the receiver and all of the satellites. There are different types of DOP such as PDOP, HDOP, GDOP, etc. but HDOP is important for UAS because it is the ‘Horizontal’ element and will reveal how well known the position is in the horizontal plane. Some UAS autopilots/flight controllers won’t even allow the aircraft to be armed if this value is higher than some set safety threshold.
One thing that our company has been doing for a LONG time is figuring out exactly where satellites are as well as accurately predicting where they are going to be. This allows for a highly accurate prediction of what you might expect the GPS landscape to look like before you actually put the aircraft into the air.
It can be as easy as glancing at the chart or seeing the min/max values of HDOP predicted for your flight area or along a specific flight path. You could equate this to checking a weather forecast, perhaps. Just a quick check to make sure the HDOP is going to be within acceptable limits for the duration of your planned flight. After all, why risk having issues, especially ones that may be out of your control. Sometimes simply waiting a few minutes for the positions and availability of the satellites to change can result in much better circumstances for satellite-based navigation.
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