🛰️ DAY 8: GNSS RTK vs PPK – DEEP DIVE

Real-Time Kinematic vs Post-Processing Kinematic for Hydrographic Positioning
Instructor: Engr. Rokib Hossain | River Engineering Solutions
🏠 Course Homepage

📖 Today's Outline

📡 GNSS Positioning Basics

Raw GNSS (GPS, GLONASS, Galileo, BeiDou) positions have an accuracy of about 2–5 meters due to atmospheric delays, satellite clock errors, and orbital errors. To achieve centimetre‑level positioning for hydrographic surveys, we use differential techniques that compare the rover (vessel) measurements with a reference station.

Two dominant methods are Real‑Time Kinematic (RTK) and Post‑Processing Kinematic (PPK). Both use carrier‑phase measurements, but the timing of correction application differs.

📌 Key concept: RTK applies corrections in real time. PPK stores raw data and applies corrections after the survey. Both can deliver 1–2 cm accuracy under good conditions.

⚡ Real-Time Kinematic (RTK) – How It Works

RTK workflow:

  1. A base station (rover or permanent CORS) broadcasts corrections (RTCM messages) via radio, NTRIP, or satellite.
  2. The rover receives corrections in real time (latency <1 second).
  3. The rover resolves integer ambiguities and computes a centimetre‑level position instantly.
  4. Your acquisition software logs the RTK‑fixed position directly.

Advantages:

  • ✅ Real‑time QA/QC – you see the fix status and accuracy immediately.
  • ✅ No post‑processing required – ready‑to‑use data after survey.
  • ✅ Ideal for dynamic positioning (dredging, construction).

Limitations:

  • ❌ Requires continuous real‑time communication link (radio, cellular, or satellite).
  • ❌ Limited range from base station (typically ≤10 km for radio; cellular/NTRIP unlimited but depends on internet).
  • ❌ Can lose fix due to obstruction or baseline length → then reverts to float (decimetre level).
📡 [Diagram: RTK setup with base station, radio link, and rover on vessel]
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⏳ Post-Processing Kinematic (PPK) – How It Works

PPK workflow:

  1. Rover records raw GNSS observations (carrier phase, pseudorange, Doppler) at high rate (e.g., 1–20 Hz).
  2. Base station (or nearby CORS) records raw observations simultaneously.
  3. After the survey, you import both rover and base raw data into post‑processing software (e.g., RTKLIB, GrafNav, POSPac).
  4. The software resolves ambiguities and computes a precise trajectory (centimetre level).
  5. Processed positions are merged with sonar data.

Advantages:

  • ✅ No real‑time communication link needed – ideal for remote areas or poor cellular coverage.
  • ✅ You can use data from permanent CORS stations (e.g., nationwide network) after the survey.
  • ✅ Better handling of gaps – software can bridge short outages using forward/backward smoothing.
  • ✅ Allows precise reprocessing with improved satellite orbits (IGS final products).

Limitations:

  • ❌ No real‑time feedback – you only know if the fix succeeded after processing.
  • ❌ Requires additional step of post‑processing and integration (more office time).
  • ❌ If base data is missing or corrupted, the whole survey may be degraded.
📊 [Diagram: PPK workflow – rover logs raw, base logs raw, then post‑processing]
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⚖️ RTK vs PPK – Head‑to‑Head Comparison

FeatureRTKPPK
Correction timingReal‑timePost‑processed
Communication link requiredYes (radio, NTRIP, satellite)No (only to download base data later)
Maximum baseline (typical)≤10 km for radio; >50 km with NTRIP≤30–50 km (depends on software; can be longer with precise orbits)
Real‑time QA/QC availableYes – see fix statusNo – processed after survey
Susceptibility to outagesLoses fix immediately, may not recoverCan bridge short gaps using smoothing
Typical horizontal accuracy1–2 cm + 1 ppm1–2 cm + 1 ppm (similar)
Office workloadLow – ready to useHigher – need post‑processing and merging
Ideal forReal‑time construction, dredging, small areas with good commsRemote areas, large surveys, historical data reprocessing
💡 Pro tip: Many modern receivers support both RTK and raw logging simultaneously. You can run RTK for real‑time guidance and store raw data for PPK backup or improvement.

🌊 Which One for Hydrography?

In hydrographic surveying, the choice depends on project constraints:

  • Use RTK when: You need immediate depth correction, have reliable 4G/NTRIP coverage, baseline is short, and you want to minimize office processing. Common for port surveys, dredge monitoring, and inland waters near cities.
  • Use PPK when: You work in remote coastal areas without cellular or radio coverage, you need the highest precision using IGS final orbits, or you are surveying large offshore areas where a single base station must cover long distances (e.g., 30–40 km).
  • Use both: Log raw data while using RTK. If RTK loses fix, you have a fallback. Some software (e.g., Applanix POSPac) can even combine RTK and PPK into a hybrid solution.

Many national hydrographic offices recommend PPK for primary positioning because of its robustness and ability to reprocess with refined satellite data.

📡 Correction Sources (NTRIP, Base Station, Satellite)

For RTK, corrections can come from:

  • Local base station – you set up a GNSS receiver on a known point, broadcast via radio or WiFi. Best accuracy (short baseline).
  • NTRIP (Network RTK) – uses a network of CORS stations to model errors (e.g., VRS, FKP). Requires internet. Good for large areas without a local base.
  • Satellite‑based RTK (e.g., Trimble RTX, NovAtel TerraStar) – corrections via L‑band satellite. No local base or internet needed but often has a subscription fee and slower convergence (10–30 min).

For PPK, you can use:

  • Your own base station data (post‑survey).
  • Public CORS data (e.g., from nation‑wide networks like NGS CORS in USA, EUREF in Europe).
  • Precise satellite orbits (IGS final products) to improve long baseline solutions.
📡 Important: Always verify the vertical datum of the base station – it must match your survey vertical reference (MSL or ellipsoidal height) or you need a geoid model.

🔗 External Resources & Software

Explore these tools for RTK/PPK processing and CORS data:

ResourceDescriptionLink
RTKLIB – open‑source PPK softwareFree software for post‑processing GNSS raw data (supports many receivers)rtklib.com
NGS CORS (USA)Download base station data from hundreds of stationsngs.noaa.gov/CORS
EUREF Permanent NetworkEuropean CORS dataepncb.oma.be
Applanix POSPacCommercial PPK/INS processing softwareapplanix.com
🔗 Authority linking: Links to NOAA, EUREF, and RTKLIB increase credibility.

📋 Homework & Fieldwork Checklists

📘 Day 8 – Homework Checklist

  • ✅ Read the entire Day 8 post.
  • ✅ Visit the NGS CORS map and find the nearest CORS station to your location. Note its distance.
  • ✅ Download RTKLIB and try converting a sample RINEX file (provided by instructor or online).
  • ✅ Write a paragraph comparing the pros and cons of RTK vs PPK for a 30 km offshore pipeline route survey.
  • ✅ Share this post with #GNSS #RTKvsPPK.

🛠️ Fieldwork Readiness Checklist – GNSS Positioning

  • ✅ Before survey: Decide whether to use RTK, PPK, or both. Check communication links for RTK.
  • ✅ Set up base station (if local) and measure antenna height accurately. Log raw data (for PPK backup).
  • ✅ For RTK, verify that you receive a "FIX" solution with PDOP <2.5 before starting line.
  • ✅ For PPK, ensure rover logs raw data at minimum 1 Hz (5–10 Hz preferred) with a valid RINEX header.
  • ✅ Note the base station coordinates (ITRF or local datum) and the geoid model to be used.
  • ✅ After survey, download base data (if using CORS) and process ASAP to verify quality.
🖨️ Pro tip: Use the Print / Download PDF button to keep these checklists in your field binder.

❓ Frequently Asked Questions

🔹 Can I achieve centimetre accuracy with single‑frequency GNSS? ▼
No, centimetre accuracy requires dual‑frequency or multi‑frequency receivers to resolve carrier‑phase ambiguities. Single‑frequency can only achieve decimetre to metre level.
🔹 What is the maximum baseline for RTK? ▼
With UHF radio, typically 5–15 km depending on antenna height and terrain. With NTRIP (internet), it can work at any distance as long as the network provides corrections (but errors increase with baseline). For reliable centimetric accuracy, most RTK networks work within 30–50 km of a reference station.
🔹 How does PPK handle loss of lock? ▼
PPK software can resolve integer ambiguities again after re‑acquisition. If the outage is short (few seconds), forward/backward smoothing often restores a fixed solution. Long outages may degrade to float.
🔹 Do I need a base station for PPK? ▼
Yes, you need raw observations from a base station (yours or a permanent CORS) to difference errors. Without a base, you get standalone accuracy only.
🔹 Can PPK be used for real‑time navigation? ▼
Not directly, because processing is done after the fact. However, you can run a parallel RTK solution for real‑time steering and later replace positions with PPK for final deliverables.

✅ Action Items & Internal Linking

🎯 Your Tasks for Today

  • 📌 Step 1: Bookmark the Course Homepage and the NGS CORS page.
  • 📌 Step 2: Download the checklist PDF using the button at the top.
  • 📌 Step 3: Explore RTKLIB's documentation to understand post‑processing workflow.
  • 📌 Step 4: Leave a comment on this post: "In my area, I would prefer ____ because ____."
  • 📌 Step 5: Proceed to Day 9: Offsets & IMU using the link below.
◀ Day 7: SBES vs MBES Day 9: Offsets & IMU ▶
🔗 Internal linking note: Day 7 and Day 9 are correctly linked. This builds a strong internal network for SEO.

👨‍🔧 Engr. Rokib Hossain | River Engineering Solutions

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