Kp is a 3-hour planetary index from 0 to 9 that measures how disturbed Earth's magnetic field is. Strong geomagnetic storms can degrade satellite positioning, but NOAA's own impact scale first mentions navigation problems at Kp 7, and the effects are strongest at high latitudes. So a flat "never fly above Kp 5" rule is too strict for most mid-latitude pilots and too relaxed near the auroral zone.
What Kp actually measures
According to NOAA's Space Weather Prediction Center (SWPC):
- Kp comes from magnetometers. Each one records the largest swings in the horizontal part of the magnetic field over a 3-hour window.
- The planetary value averages readings from 13 observatories between 44° and 60° geomagnetic latitude, north and south.
- The scale runs from 0 to 9. Around 1 is calm, and 5 or more counts as a geomagnetic storm. Values are often written in thirds, such as 5−, 5 and 5+.
- NOAA publishes a near-real-time estimate and a 3-day forecast in 3-hour blocks. The definitive Kp comes later from the German Research Centre for Geosciences (GFZ).
Kp describes the magnetic field, not your GPS. It is a useful proxy because the same solar activity that shakes the magnetic field also disturbs the ionosphere, the layer that GNSS signals pass through.
How storms affect drone GNSS
NOAA explains the chain like this:
- The ionosphere slows and bends satellite signals. Receivers correct for a typical, quiet ionosphere.
- During storms, extra electrons make that correction wrong. Single-frequency receivers, accurate to about a metre in calm conditions, can see errors of tens of metres or more in severe storms.
- When the ionosphere is very disturbed, receivers can lose lock on satellites altogether.
- Storm effects are mostly a high-latitude problem, although major storms reach mid latitudes too.
- Near the magnetic equator, a separate effect called scintillation is worst just after sunset. It is part of the normal day-night cycle and not driven by storms, so Kp will not warn you about it.
For a drone, a degraded fix can mean a wandering hover, a less accurate return-to-home point, or, depending on the model, a switch to a mode without position hold.
NOAA's G scale
NOAA links Kp to its geomagnetic storm scale:
| Kp | NOAA level | Satellite navigation, per NOAA | Typical aurora reach | Storm days per 11-year cycle |
|---|---|---|---|---|
| 5 | G1 Minor | not listed | high latitudes | about 900 |
| 6 | G2 Moderate | not listed | about 55° geomagnetic | about 360 |
| 7 | G3 Strong | intermittent problems may occur | about 50° geomagnetic | about 130 |
| 8, including 9− | G4 Severe | degraded for hours | about 45° geomagnetic | about 60 |
| 9 | G5 Extreme | may be degraded for days | about 40° geomagnetic | about 4 |
Myth: "Never fly above Kp 5"
This rule of thumb is common on drone forums. The facts:
- Kp 5 is G1, the lowest storm level. NOAA lists no satellite-navigation effect at G1 or G2.
- G1 conditions show up on roughly 900 days per solar cycle. A blanket Kp 5 cut-off would ground mid-latitude pilots often, for an effect NOAA does not list at that level.
- In northern Norway, Iceland or Alaska the opposite applies. The auroral zone sits much closer, so a Kp 4 or 5 deserves more attention there than it does in Madrid.
Kp is not useless, it just needs context. That context is latitude.
Myth: Kp means the same everywhere
Kp is one planetary number, but its effects depend on geomagnetic latitude, which is measured from the magnetic pole, not the geographic one. NOAA's aurora lines in the table above are in geomagnetic degrees for this reason.
The geomagnetic north pole sits over the Canadian Arctic. As a result, North America is "further north" magnetically than Europe at the same map latitude. On a simple dipole model, Minneapolis at 45°N comes out at roughly the same geomagnetic latitude as London at 51.5°N.
Altiwind adjusts Kp by geomagnetic latitude before scoring it:
| Geomagnetic latitude | Example areas | Adjustment |
|---|---|---|
| 60° and above | auroral zone: northern Norway, Iceland, Alaska | +1 |
| 50° to 60° | UK, Germany, southern Canada | 0 |
| 40° to 50° | Spain, Italy, much of the US | −1 |
| below 40° | Florida, Canary Islands, most of the tropics | −2 |
So a Kp 5 in Madrid scores as 4, while a Kp 5 in Tromsø scores as 6.
How Altiwind uses Kp
- Now: the current Kp, adjusted for your spot, with a "GPS: normal" label at an adjusted 4 or less and "GPS: may be degraded" from 5.
- Score: adjusted Kp of 4 or less is within your limits, 5–6 is marginal and 7 or more is outside your limits.
See Kp index for drones for how it shows in the app, or check the live value with the Kp index tool.
When the label says "may be degraded"
- Wait for a solid satellite lock before take-off, and check the home point is recorded.
- Keep the drone closer and away from obstacles, so a drifting hover has room.
- Be ready to fly without position hold. Practise that on a calm day first.
- Remember that Kp is only one of the checks. Wind, gusts and visibility usually matter more; the go/no-go score shows all of them together.
Sources
- NOAA SWPC, Planetary K-index
- NOAA SWPC, NOAA Space Weather Scales (G1–G5 effects, aurora latitudes and frequency per cycle)
- NOAA SWPC, Space weather and GPS systems
FAQ
What Kp is too high for flying a drone?
It depends on where you are. With Altiwind's defaults, an adjusted Kp of 5–6 is marginal and 7 or more is outside your limits. The adjustment ranges from +1 in the auroral zone to −2 below 40° geomagnetic latitude.
Does Kp 5 affect GPS in the UK or Germany?
NOAA's scale lists no satellite-navigation effect at G1 (Kp 5), and these areas get no adjustment in Altiwind. The app still shows "GPS: may be degraded" at an adjusted 5, as a prompt to check your satellite lock.
Why is Altiwind's Kp different from NOAA's number?
It isn't. Altiwind shows NOAA's Kp, then scores a latitude-adjusted value so the colour reflects your spot.
Is a high Kp the same as an aurora forecast?
They are linked. A higher Kp pushes aurora towards lower latitudes, which is why NOAA's aurora lines follow the G scale. The same latitude effect is why Altiwind weights Kp by where you fly.