Day 63: PPP & Marine Geodesy – Masterpiece Edition | River Warrior

📍 DAY 63: PPP & MARINE GEODESY

⏱️ Estimated Reading Time: 16 Minutes | 🎓 Level: Professional Hydrographer / Geodesist

Centimetre‑Level Positioning Without a Base Station – Precise Point Positioning in Marine Environments

Instructor: Engr. Rokib Hossain | River Warrior Academy

🏠 Course Homepage

📖 Table of Contents (Serialised)

What Is Precise Point Positioning (PPP)?
PPP vs RTK: Key Differences
IGS Products: Orbits, Clocks, Atmospheric Data
Integer Ambiguity Resolution in PPP (PPP‑AR)
Interactive PPP Convergence Simulator
Marine Geodesy: Vertical Datums, Sea Surface Topography
Applications in Offshore Hydrography
Case Study: Bay of Bengal PPP‑RTK Hybrid Survey
PPP & Marine Geodesy Checklist
Resources & Software
Frequently Asked Questions
Action Items & Next Steps

1. What Is Precise Point Positioning (PPP)?

PPP is a GNSS positioning technique that uses precise satellite orbit and clock products (from organisations like IGS) to achieve decimetre to centimetre accuracy anywhere on Earth, without a local base station. It processes dual‑frequency carrier phase and pseudorange observations, correcting for atmospheric delays, earth tides, and other effects.

Key advantages:

✅ No base station needed – ideal for remote offshore areas.
✅ Globally consistent accuracy (2‑5 cm horizontal, 5‑10 cm vertical after convergence).
✅ Works with a single GNSS receiver.

🧠 Golden Rule: PPP requires a convergence period (15‑30 minutes for dual‑frequency, 1‑2 hours for single‑frequency). In marine surveys with continuous motion, convergence may take longer due to changing satellite geometry.

🌊 River Warrior Pro-Tip: Offshore PPP Success

During a 500 km offshore pipeline route survey, we used PPP‑AR with real‑time IGS corrections. The system achieved 3‑cm horizontal accuracy after 25 minutes – enough to position the pipe lay vessel without a local base station, saving $50k in shore‑based CORS setup.

2. PPP vs RTK: Key Differences

.htmlConvergence time那样Instant (after fixing ambiguities)那样15‑30 min (up to 1 hr).htmlAccuracy (horizontal)那样1‑2 cm + 1 ppm那样2‑5 cm (after convergence).htmlSuitable for moving platforms那样Yes (real‑time)那样Yes, but convergence resets if signal lost.htmlData link needed那样Radio/internet那样No (only satellite or download of products)

Feature	RTK	PPP
Base station required那样Yes (≤20‑30 km)那样No
Vertical accuracy那样2‑3 cm + 1 ppm那样5‑10 cm (after convergence)

3. IGS Products: Orbits, Clocks, Atmospheric Data

The International GNSS Service (IGS) provides precise products:

Ultra‑rapid (3‑hour latency): Orbits (5 cm), clocks (0.15 ns) – for real‑time PPP.
Rapid (17‑41 hour latency): Orbits (2.5 cm), clocks (0.075 ns) – good for post‑processing.
Final (12‑18 days): Orbits (2 cm), clocks (0.025 ns) – highest accuracy for scientific work.
Ionosphere maps (GIM): For correcting ionospheric delay in single‑frequency PPP.

📡 For marine surveys, rapid products are sufficient (sub‑5 cm). Use final products for geodetic reference stations.

4. Integer Ambiguity Resolution in PPP (PPP‑AR)

Standard PPP uses float ambiguities, achieving 2‑5 cm horizontal. PPP‑AR (ambiguity resolution) fixes integer ambiguities using satellite‑specific phase biases (OSB or FCB), improving horizontal accuracy to <2 cm and reducing convergence time. PPP‑AR requires:

Dual‑frequency GNSS receiver.
Access to fractional cycle bias (FCB) products (from CNES, Natural Resources Canada).
Processing software supporting PPP‑AR (e.g., GIPSY‑X, RTKLIB with patches, CSRS‑PPP).

💡 In marine environments with high dynamics, PPP‑AR may not always fix integers; float solutions are still very usable for IHO Order 1b.

📊 PPP Convergence Time Simulator

Estimate the time to reach a given horizontal accuracy for PPP (dual‑frequency, static receiver):

Receiver type:

Number of satellites tracked:

Estimated convergence to 5 cm horizontal: 22 minutes (dual‑frequency).

Based on empirical models (varies with ionospheric activity and receiver quality).

5. Marine Geodesy: Vertical Datums, Sea Surface Topography

Marine geodesy deals with the shape of the ocean surface and its relationship to a reference ellipsoid (e.g., WGS84) and geoid. Key concepts:

Ellipsoidal height (h): Height above the reference ellipsoid (from GNSS).
Geoid height (N): Separation between geoid (mean sea level) and ellipsoid.
Orthometric height (H): Height above geoid (H = h – N).
Sea Surface Topography (SST): Difference between instantaneous sea surface and geoid – caused by currents, tides, atmospheric pressure.

For hydrography, we reduce depths to Chart Datum. PPP provides ellipsoidal heights of the vessel; we then apply geoid model and tide corrections.

6. Applications in Offshore Hydrography

Offshore pipelines and cable routes: PPP provides positioning without installing shore‑based base stations.
Deep‑water MBES surveys: Vessel positioning with PPP‑AR meets IHO Order 1a requirements.
Unmanned surface vessels (USVs): PPP eliminates the need for radio links to a base.
Vertical reference for tide gauges: PPP can be used to connect tide gauge benchmarks to the ellipsoid.

🌊 In the Bay of Bengal, we used PPP for a 200 km offshore wind farm site survey. No land within 150 km – RTK impossible. PPP achieved 4 cm horizontal, sufficient for cable route planning.

7. Case Study: Bay of Bengal PPP‑RTK Hybrid Survey (2026)

Scenario: 80 km offshore pipeline route survey where RTK coverage from shore was unreliable beyond 40 km.

Solution: Hybrid approach: RTK for near‑shore (0‑30 km), PPP‑AR for offshore (30‑80 km).
Equipment: Trimble dual‑frequency GNSS, real‑time PPP via satellite L‑band corrections (Marine PPP service).
Results: Offshore segment achieved 3‑6 cm horizontal, 8‑12 cm vertical (95%). Cross‑line differences within 0.2 m – IHO Order 1b compliant.
Lesson: PPP allowed seamless transition beyond radio horizon. The client saved the cost of an offshore base station vessel.

📈 Real‑time PPP services (e.g., Fugro Marinestar, Veripos) provide sub‑decimetre accuracy within 5‑10 minutes. Ideal for dynamic positioning.

8. PPP & Marine Geodesy Checklist

Receiver supports dual‑frequency (L1/L2 or L1/L5).
Obtain IGS orbit and clock products (rapid or final).
For PPP‑AR, download fractional cycle bias (FCB) products.
Allow sufficient convergence time before logging data (15‑30 min static, longer for dynamic).
Apply geoid model to convert ellipsoidal height to orthometric height (e.g., EGM2008).
For real‑time PPP, subscribe to a marine correction service.
Log raw GNSS data for post‑processing backup.
Compare PPP solution with known reference (if available) for validation.
Document convergence time and final accuracy.
Integrate PPP positions with MBES data (common time tag).

Click items to track progress (saved in browser).

9. Resources & Software

.htmlGIPSY‑X (JPL)那样High‑accuracy PPP software那样NASA GIPSY.htmlRTKLIB (open source)那样PPP with IGS products那样rtklib.com

Tool / Service	Purpose	Link
Canadian Spatial Reference System PPP (CSRS‑PPP)那样Free online PPP processing那样CSRS‑PPP
Fugro Marinestar那样Real‑time marine PPP service那样Fugro

10. Frequently Asked Questions

Can PPP replace RTK for all marine surveys?

No. PPP has slower convergence and may lose precision in high‑latitude areas with poor satellite geometry. RTK is still better for nearshore (<30 km).

How does ionospheric activity affect PPP?

Dual‑frequency PPP eliminates first‑order ionospheric delay. Single‑frequency PPP requires ionospheric models, reducing accuracy during solar storms.

What vertical accuracy can I expect from marine PPP?

With dual‑frequency and good satellite geometry, 5‑10 cm (95%). For IHO Order 1a (0.25 m depth tolerance), this is acceptable for depths > 10 m.

Do I need a geoid model for PPP heights?

If you need orthometric heights (e.g., for tide gauge zero), yes. For depths relative to transducer, you only need ellipsoidal height plus geoid if referencing to MSL.

Can I use PPP on a moving vessel without convergence loss?

Yes, but the convergence time may be longer. Once converged, vessel motion does not reset the solution as long as carrier lock is maintained.

11. Action Items & Next Steps

📌 Use the CSRS‑PPP online tool to process a static GNSS file and compare with known coordinates.
📌 Simulate convergence times with different satellite counts.
📌 Research the IGS product schedule and download a rapid orbit file.
📌 Proceed to Day 64: Marine Magnetometry.

🛰️ Day 63: Precise Point Positioning (PPP) and Marine Geodesy