Harbour & Airport

Two environments with failure modes found nowhere else. Harbour and waterfront sites where water surface multipath is the primary GPS challenge, and airport proximity sites where airspace compliance is the governing factor — not GPS at all. Both are common in the Sydney market. Both require specialist awareness before you arrive on site.

Key Rule

Harbour: position launch so open water is BEHIND the antenna, not in front. Fly before 09:00 before sea breeze builds. Airport: confirm authorisation FIRST — before assessing weather, before setting up equipment, before anything else. Do not assess conditions at an airport proximity site until you have confirmed you are authorised to fly.

Water is one of the most reflective surfaces for radio frequency signals. A calm harbour surface reflects GPS signals with high efficiency in a horizontal direction — directly into the path of an aircraft antenna flying above or near the waterline.

GPS signals from satellites near the horizon travel at a low angle relative to Earth's surface — when they pass over water, a portion reflects off the surface and continues upward
Your antenna receives both the direct signal from above and the reflected signal from below, arriving from different angles with a slight time delay
A flat, calm harbour surface under high midday sun produces maximum reflection intensity — signal angle and surface reflectivity align worst-case
A choppy surface under an early morning low-angle sun produces less coherent reflection — this is why the 06:00–09:00 window matters
Water reflections are omnidirectional and vary with surface conditions — more unpredictable than building multipath which is directional
Warmer water evaporates more actively — water vapour close to the surface slightly refracts GPS signals in summer

Recommended Time Windows

06:00 – 09:00 — ✅ Best — low sun angle, minimal water reflection intensity, sea breeze not yet building

09:00 – 14:00 — ⚠️ Workable — multipath increasing as sun rises, sea breeze building, monitor wind closely

14:00 – 16:00 — ⚠️ Wind risk — sea breeze at daily peak, verify conditions before committing

15:30 – 17:30 — ✅ Good secondary — sun dropping, reflection reducing, wind often easing

Any time without CASA authorisation (airport sites) — 🚫 Illegal — Do Not Fly regardless of conditions or experience level

Any time with current authorisation (airport sites) — ✅ Authorised — proceed, check NOTAMs on the day

Primary Failure Mode

Harbour: position accuracy that looks acceptable from the ground but degrades over open water at lower altitudes. Sea breeze arriving faster than expected, forcing mission postponement mid-setup. Airport: regulatory non-compliance — flying without authorisation in controlled airspace. Not a data quality failure — a legal and safety failure.

Knowledge Check

1. What makes water surface multipath different from building-based multipath in urban environments?

2. What is the recommended launch position at a harbour or waterfront site?

3. You arrive at a harbour site at 10:30 on a clear summer day. Wind is 18 km/h from the south. What is the most significant risk for the next 2 hours?

4. You are planning a job at a site 4km from Bankstown Airport. What is the first thing you must confirm?

5. Why does flight altitude matter more at harbour sites than at most other environment types?