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Quick Start: CLAS PPP-RTK

This guide walks you through running CLAS PPP-RTK positioning with MRTKLIB, from required files to your first solution.

CLAS (Centimeter Level Augmentation Service) is a satellite-based augmentation service broadcast via QZSS L6D. It provides ionospheric, tropospheric, and satellite bias corrections that enable centimeter-level PPP-RTK positioning within Japan.

Prerequisites

  • MRTKLIB built successfully (cmake --build build)
  • Test data extracted:
Bash
tar xzf tests/data/claslib/claslib_testdata.tar.gz -C tests/data/claslib/

Required Files

CLAS PPP-RTK requires several support files beyond the usual observation and navigation data. All files below are bundled with MRTKLIB.

File TOML Key Purpose
clas_grid.def files.cssr_grid CLAS grid point definitions covering Japan. Required — without this, no corrections are applied.
clas_grid.blq files.ocean_loading Ocean tide loading coefficients at each grid point.
igu00p01.erp files.eop Earth rotation parameters for coordinate frame transformation.
igs14_L5copy.atx files.receiver_atx Antenna phase center model (receiver and satellite).
isb.tbl files.isb_table Inter-system bias table mapping receiver types to CLAS reference.
l2csft.tbl files.phase_cycle L2C quarter-cycle phase shift correction per receiver type.

All bundled files are located in tests/data/claslib/.

ANTEX File: Use IGS14, Not IGS20

CLAS corrections are based on the ITRF2014 reference frame. You must use an IGS14-based ANTEX file (e.g., igs14.atx or igs14_L5copy.atx).

The latest IGS20-based ANTEX (igs20.atx) uses ITRF2020, which introduces millimeter-to-centimeter frame differences that degrade positioning accuracy.

Post-Processing

Run with Bundled Test Data

Bash
mrtk post -k conf/claslib/rnx2rtkp.toml \
  -ts 2019/08/27 16:00:00 -te 2019/08/27 17:00:00 -ti 1 \
  -o output_clas.pos \
  tests/data/claslib/0627239Q.obs \
  tests/data/claslib/0627239Q.nav \
  tests/data/claslib/2019239Q.l6
  • Observation file (.obs / .bnx) — RINEX or BINEX from a GNSS receiver in Japan
  • Navigation file (.nav) — Broadcast ephemeris
  • L6 correction file (.l6) — CLAS L6D data (extracted from QZSS L6 signal)

Expected Output

After ~1 minute of convergence, the solution should reach Fix (Q=4):

Text Only
$GNGGA,160131.00,3908.109031,N,14107.972285,E,4,08,...
  • Q=4 (Fix) — Ambiguities resolved, centimeter-level accuracy
  • Q=5 (Float) — Convergence in progress
  • Q=1 (SPP) — Initial convergence or missing corrections

Typical fix rate for this dataset: >99% (after initial convergence).

Real-Time Processing

For real-time CLAS PPP-RTK, use mrtk run with live receiver streams. See Real-Time CLAS Reference for architecture details, stream configuration, and performance data.

File Replay (Testing)

To test real-time processing with recorded data:

Bash
mrtk run -s -o conf/claslib/rtkrcv.toml

The bundled rtkrcv.toml is pre-configured for file replay using the test dataset with time-tag synchronization.

Live Receiver

For a live u-blox F9P + D9C setup, use the template config:

Bash
mrtk run -s -o conf/claslib/rtkrcv_ubx_clas.toml

Edit the [streams.input.*] sections to match your receiver connections.

Key Configuration Points

These settings are critical for correct CLAS PPP-RTK operation. Getting any of them wrong will silently degrade or prevent Fix solutions.

Frequency: Must Be l1+2

TOML
[positioning]
frequency = "l1+2"   # REQUIRED for CLAS

CLAS provides E1 and E5a corrections but does not provide Galileo E5b phase bias. Using l1+2+3 (nf=3) adds the E5b slot, which has no valid bias correction, causing false geometry-free cycle slips on Galileo and dropping fix rate from >99% to ~67%.

Ionosphere: est-adaptive (Not dual-freq)

TOML
[positioning.atmosphere]
ionosphere = "est-adaptive"   # CLAS provides iono corrections
troposphere = "off"           # CLAS provides tropo corrections

CLAS supplies ionospheric and tropospheric corrections via the grid. The Kalman filter estimates residual ionospheric delays adaptively. Do not use dual-freq — that discards the CLAS ionospheric information.

Tidal Correction: CLAS Grid-Based

TOML
[positioning.corrections]
tidal_correction = "solid+otl-clasgrid+pole"

This uses the CLAS grid points for ocean tide loading interpolation, rather than the station-specific BLQ method.

ISB and Phase Shift: Enabled with Tables

TOML
[receiver]
isb = true
phase_shift = "table"
reference_type = "CLAS"

CLAS corrections are referenced to a specific receiver type. The ISB table and phase cycle table correct for inter-system biases and L2C quarter-cycle shifts between your receiver and the CLAS reference.

Receiver Type

Set [positioning.clas].receiver_type to match your GNSS receiver (e.g., "Trimble NetR9"). This value is used for ISB table lookup. Check isb.tbl for supported receiver types.

Grid Selection Radius

TOML
[positioning.clas]
grid_selection_radius = 1000   # meters

Controls how far from the rover position to search for CLAS grid points. 1000 m is appropriate for most locations within Japan.

Japan Coverage Only

CLAS grid corrections are only available within Japan. Processing observations from outside the coverage area will fail silently (grid lookup returns no results).

Bundled Configuration Files

File Description
conf/claslib/rnx2rtkp.toml Post-processing, single-channel L6
conf/claslib/rnx2rtkp_vrs.toml Post-processing, VRS-RTK mode
conf/claslib/rtkrcv.toml Real-time, single-channel L6 (file replay)
conf/claslib/rtkrcv_2ch.toml Real-time, dual-channel L6 (file replay)
conf/claslib/rtkrcv_ubx_clas.toml Real-time, u-blox F9P + D9C (live)
conf/claslib/rtkrcv_sbf_l6d.toml Real-time, Septentrio SBF + L6D
conf/claslib/rtkrcv_rtcm3_ubx.toml Real-time, RTCM3 + UBX L6

Troubleshooting

All Solutions Are SPP (Q=1)

  1. Missing grid file — Check that files.cssr_grid points to a valid clas_grid.def
  2. Wrong frequency — Ensure frequency = "l1+2", not l1+2+3
  3. No L6 data — Verify the L6 correction file path and format

Fix Rate Lower Than Expected

  1. nf=3 instead of nf=2 — Most common cause. See Frequency above.
  2. Wrong ANTEX — Using igs20.atx instead of igs14.atx introduces frame errors.
  3. Receiver type mismatch — Check [positioning.clas].receiver_type matches isb.tbl.

clas grid file error

The grid file path is invalid or the file doesn't exist. Check the files.cssr_grid setting. Relative paths are resolved from the working directory, not from the config file location.

Next Steps