What do Kp numbers mean? How does it affect drone flying?
The Kp number is a system of measuring aurora strength. It goes from 0 to 9 (0 being very weak, 9 being a major geomagnetic storm with strong auroras visible).
Kp's of 1-4 are completely safe. At a Kp of 5, you'll have problems perhaps 1% of the time. At a Kp of 6, you'll lose an average of 5% of your locked satellites; at 7, you'll lose roughly one third.
So when you’re looking at the aurora forecast page, you want to see high Kp numbers. The higher the number, the more risk you take on. Anything above (and including) Kp5 is classed as a geomagnetic storm.
Solar activity interferes with GPS signals in two ways, both due to disruptions in the ionosphere.
(1) It decreases the signal-to-noise ratio and affects carrier frequency, causing the receiver to lose lock on some satellites. Instead of 9 satellites, you might lock only 6, or the number might fluctuate from second to second.
(2) It changes the propagation delay through the ionosphere, making GPS positioning inaccurate even if the receiver has all satellites locked.
Even during a major storm, the extent to which you see these problems will depend on many other factors, including your latitude (worse at high latitudes) and the time of day (worse at night). Sometimes everything will be fine. At other times you'll lose lock completely, or appear to have a lock but the position will actually be wrong by hundreds of feet.
The storms can also interfere with radio control signals, or with the on-board electronics.
So therefore, in general it's better to play it safe during solar storms at Kp of 6 and above, even though most of the time you won't notice any issues. At 7 or higher, you're better off sitting inside and flying a simulator.
A great app to download is called UAV-forecast which shows the weather forecast for your location as well as the Kp levels!