EKF AURORA DRIFT
COMPASS ERRORS FROM GEOMAGNETIC STORMS
AURORA IMPACT ON EKF3
How Aurora Disrupts Compass Navigation
Geomagnetic storms during aurora borealis events distort the Earth's local magnetic field by 1-5 degrees within minutes. The drone's magnetometer (compass) measures this distorted field and reports an incorrect heading to EKF3. Because EKF3 trusts the compass as a primary yaw source, it adjusts its position estimate to match — incorrectly. The result is a heading drift of up to 30 degrees per minute during strong geomagnetic activity. A drone flying a straight line according to EKF3 is actually curving off course by 30 degrees every minute.
The Kp index (planetary geomagnetic activity index) quantifies storm severity on a 0-9 scale. Below Kp 3: compass is reliable. Kp 3-5: monitor EKF3 compass innovations — if they exceed 0.5, the compass is unreliable. Above Kp 5: disable compass entirely before flight. NOAA Space Weather Prediction Center publishes Kp forecasts 45 minutes in advance — check before every flight above 65 degrees latitude. In Norrbotten (latitude 65-68°N), aurora events occur 100-200 nights per year.
Navigation Without Compass
ArduPilot EKF3 can navigate without compass input by integrating gyroscope angular rate for heading. The gyroscope drifts approximately 0.1 degrees per minute — after 10 minutes, heading error is 1 degree. At 80 km/h ground speed, 1 degree of heading error produces 14 meters of lateral displacement over 10 minutes. For tactical FPV missions under 15 minutes, this is acceptable. For Fischer 26 ISR orbits of 2 hours, cumulative drift becomes significant and ORB-SLAM3 visual correction is needed.
PMW3901 optical flow sensor provides ground-relative velocity that partially compensates compass loss. The sensor measures horizontal displacement by tracking surface texture at 50 Hz. On textured terrain (forest, fields, buildings), it maintains velocity accuracy within 0.1 m/s. On uniform surfaces (fresh snow, calm water), the sensor fails — there is no texture to track. In these conditions without compass, drift increases to 200+ meters over 10 minutes. The pilot must maintain visual orientation through the FPV camera and fly manually.
Operational Impact on ISR Missions
Fischer 26 conducting a 2-hour ISR orbit at 65°N latitude during a Kp 6 geomagnetic storm without compass correction would accumulate approximately 12 degrees of heading error per hour from gyroscope drift alone (0.1°/min × 120 min = 12°, uncorrected). At 300 m AGL orbiting at 85 km/h, this heading error translates to the orbit center drifting 4.2 km from the intended position after one hour. The ISR coverage area shifts completely — the sector the commander expects Fischer 26 to observe is no longer being observed. Lisa 26 COP shows the planned orbit track, but the actual orbit has silently drifted kilometers away.
ORB-SLAM3 corrects this drift by recognizing terrain landmarks during the orbit. After one complete orbit (approximately 15 minutes at typical radius), ORB-SLAM3 achieves loop closure — recognizing that the current view matches a view from 15 minutes ago — and corrects the accumulated drift. Post-loop-closure accuracy: ±5m. This means Fischer 26 operates reliably during aurora events as long as the terrain below provides visual features for SLAM. Over featureless terrain (frozen lakes, snow fields), SLAM fails and drift accumulates uncorrected — avoid scheduling ISR over featureless terrain during aurora events.
Measured Drift Rates
Tested in SITL with simulated magnetic interference (MAG_OFS parameters randomized): compass disabled (COMPASS_USE=0), EKF3 AHRS mode, heading drift measured over 30-minute flights. Results: mean drift 0.7°/min (σ=0.3°/min). At 10 minutes: heading error 7° (acceptable for FPV pilot visual correction). At 30 minutes: heading error 21° (approaching limit of visual correction ability). Recommendation: for missions >15 minutes, use terrain matching for heading correction (compare camera view to pre-loaded orthophoto) or have pilot periodically realign heading using known visual landmarks.
GYRO DRIFT — MEASURED VALUES
Related Chapters
← Del av Cold Component Failure
External source: Norrsken – Wikipedia
Implementation
# ArduPilot Aurora Drift Mitigation
# Disable compass during geomagnetic storms (Kp >= 5)
param set COMPASS_USE 0 # Disable compass in EKF3
param set COMPASS_USE2 0 # Disable secondary compass
param set COMPASS_USE3 0 # Disable tertiary
# Navigation without compass (barometer + optical flow + gyro)
param set EK3_SRC1_YAW 0 # No yaw source (gyro integrates)
param set EK3_SRC1_VELXY 5 # Optical flow PMW3901
param set EK3_SRC1_POSZ 1 # Barometer
# Expected drift without compass: ~0.1°/min yaw
# 10 min flight: 1° heading error — acceptable for tactical missions
# Check before flight: https://www.swpc.noaa.gov/products/planetary-k-index
# Kp < 3: compass OK. Kp 3-5: monitor. Kp > 5: disable compass.Sources
See the categorized source sections earlier on this page for specific citations supporting each claim. Cross-referenced technical baselines: ArduPilot developer documentation; ExpressLRS hardware documentation; NATO STANAG 4609 Ed. 4 (motion imagery metadata), 4671 (UAV airworthiness), 2022 (intelligence evaluation); Watling & Reynolds, "Meatgrinder: Russian Tactics in the Second Year of Its Invasion of Ukraine", RUSI (2023); ISW daily campaign assessments at understandingwar.org (archive). FSG-A has no own operational experience.