High Interference

High interference sites are locations where the GPS signal environment is actively hostile, not just partially degraded. The sky overhead may still be visible, but the signals reaching your aircraft's antenna are a contaminated mix of direct satellite contact and reflected, refracted, or re-radiated signals. The result: signal bars show strong, lock indicator is green — but the position being reported is not accurate. The failure is invisible until you check your data.

Key Rule

If aircraft position drifts while stationary before arming, wait 10 minutes. If it hasn't settled, return the next morning before 08:00. A pilot who arms and flies on a drifting position will produce corrupted data — and won't know it until 48 hours later.

High interference has two primary sources, and most high interference sites involve both simultaneously.

Physical reflection — large steel-framed buildings, hospital complexes, university structures, dense commercial blocks reflect GPS signals off their facades
Your aircraft receives both the direct signal from the satellite and the reflected version arriving a fraction of a second later from a different direction
Electromagnetic interference — broadcast towers, mobile network infrastructure, high-voltage powerline corridors, rail electrification systems
Some of this radiation overlaps with GPS frequency bands and degrades the receiver's ability to isolate satellite signals cleanly
Hospital campuses combine both sources — dense multi-wing building reflections + concentrated medical equipment EMI
A site can exhibit high interference even with a clear sky view — the problem is signal quality, not satellite geometry

Recommended Time Windows

06:00 – 08:30 — ✅ Only recommended window for survey — satellite geometry + EM at peak, noise at minimum

08:30 – 10:00 — ⚠️ Marginal — proceed only if position holds steady and drift is under 2m in 2 minutes

10:00 – 15:00 — 🚫 Do not fly survey — GPS at worst, EM interference at peak. Consistent data quality failure window.

15:30 – 17:00 — ⚠️ Inspection only — visual documentation acceptable. Not for survey deliverables.

After sunset — 🚫 Do Not Fly

Primary Failure Mode

Invisible positional error. The aircraft completes the mission without triggering any warnings. The pilot has no indication anything went wrong. The processed dataset returns 48 hours later showing errors that cannot be corrected in post-processing.

Knowledge Check

1. What is the most reliable on-site indicator that you are at a high interference site?

2. Why can a site have high GPS interference even when it has a completely clear sky view?

3. You arrive at a hospital campus site at 09:45. Position drift is 5m over 2 minutes. What should you do?

4. What makes the 15:30–17:00 secondary window acceptable for some work at high interference sites?