Comprehensive Comparison: Precise Point Positioning (PPP) Methods

This report documents the evolution of GNSS measurement techniques from basic statistical averaging to professional-grade post-processing and real-time RTK solutions. All experiments were conducted using a fixed roof-mounted antenna and u-blox ZED-F9P/ZED-X20P receivers.

1. Overview of Evaluated Methods

Method Title Core Technique Data Source
1 Statistical Averaging Long-term mean calculation (24h) Autonomous GNSS
2 CSRS-PPP Service Cloud-based post-processing Global Ephemerides
3 RTKLIB (Local) Manual post-processing Local Base (APOS)
4 NTRIP (Hardware) Internal RTK Engine Real-time NTRIP Stream
5 RTKNAV (Software) External RTK processing Real-time NTRIP Stream

2. Detailed Method Analysis

Method 1: Precise Point Positioning with Averaging

This “brute force” approach relies on the law of large numbers. By averaging data over 24 hours, local ionospheric fluctuations are partially smoothed out.

  • Performance: Achieved a precision within a 20 cm radius.
  • Key Insight: Changing the dynModel to “stationary” did not significantly improve the result. The absolute offset compared to corrected methods remained over 1 meter.

Method 2: CSRS-PPP & ECTT Transformation

Utilizes the Canadian Spatial Reference System (CSRS) for professional post-processing.

  • Workflow: Collect RINEX data -> Upload to CSRS -> Wait for “Final” orbit products -> Transform coordinates.
  • The Transformation Factor: Results are delivered in ITRF. For European accuracy, the ECTT tool must be used to convert to ETRF, accounting for tectonic plate drift (approx. 2.5 cm/year).

Method 3: RTKLIB with Local Correction (APOS)

The most precise “offline” method using local reference stations from the Austrian BEV (APOS service).

  • Technique: Manual calculation using rnx2rtkp and .obs files from both the rover and a nearby base station (e.g., WIEN00AUT).
  • Results: Extremely tight clustering with a maximum deviation of only 26 mm.
  • Comparison: After transformation, it aligned within 11 cm of the CSRS-PPP result.

Method 4: NTRIP over gpsd (Hardware RTK)

A real-time solution where the u-blox ZED-F9P processes RTCM3 correction data internally.

  • Workflow: Direct NTRIP stream via gpsd to the receiver.
  • Stability: High percentage of Status: FIXED (Q=1).
  • Precision: The average value was only 3.9 cm away from the high-precision results of Method 3.

Method 5: Software-Based RTK (rtknavi_qt)

Utilizes the RTKlib software suite to perform the heavy lifting of RTK calculations on a host PC rather than the chip.

  • Hardware: Conducted with the u-blox ZED-X20P.
  • Observations: It required approximately 30 minutes to achieve a “FIX.”
  • Control: Offers the most granular control over navigation systems (GPS, Galileo, BDS) and satellite selection.

3. Comparison Table

Feature Method 1 Method 2 Method 3 Method 4 Method 5
Real-Time No No No Yes Yes
Complexity Low Medium High Medium High
Accuracy (Relative) ~20-40 cm < 5 cm < 3 cm ~5 cm ~5-10 cm
Ref. System WGS84 ITRF (Global) ETRF (Local) ETRF ETRF
Data Effort Zero RINEX Upload RINEX + Base NTRIP Login NTRIP + Config

4. Final Conclusion & Recommendations

  1. For Static Surveying: Method 3 (Post-processing with local APOS data) is the gold standard, providing the highest repeatability and millimeter-level precision.
  2. For Daily Use: Method 4 (NTRIP into F9P) is the most efficient. It provides professional “Fixed” solutions in real-time with minimal software overhead.
  3. Critical Factor: When comparing results over time (e.g., 2023 vs 2026), always perform a coordinate transformation (ITRF to ETRF). Without it, continental drift will be misinterpreted as measurement inaccuracy.

(1) PPP - Precise Point Positioning with averaging
(2) PPP with gpsrinex, CSRS-PPP and ECTT
(3) PPP with RTKlib and local correction
(4) PPP with NTRIP source for u-blox GNSS receiver over gpsd
(5) PPP with NTRIP source and rtknavi_qt