Troubleshooting: IMU Data Failure – FDIR2 Failure Detected

📌 Problem

During GNSS-Inertial Trajectory Processing that utilizes Applanix POSPac, the following warning and subsequent failure may be observed in the processing log:

WARNING : IMU data failure - FDIR2 failure detected
ERROR   : Process error in integration

This is often preceded by additional warnings such as:

WARNING : Continuous primary GNSS position measurement rejections

Processing typically aborts shortly after this condition is detected, and a valid trajectory solution is not produced.

🔍 Root Cause

The FDIR2 failure indicates that POSPac’s internal Fault Detection, Isolation, and Recovery (FDIR) system has determined that the IMU data is no longer consistent with the navigation solution and must be rejected.

Importantly, this condition is most commonly not caused by bad IMU data. Instead, it is typically the result of filter divergence caused by upstream issues, most frequently related to GNSS.

In the majority of cases, the sequence is:

• GNSS position updates are repeatedly rejected
• The solution relies heavily on IMU propagation
• The navigation filter becomes unstable (residuals grow)
• FDIR escalates to FDIR2 and rejects the IMU
• The solution degrades and processing fails

Common root causes include:

• GNSS data inconsistency
  • Incorrect base station coordinates, datum, or epoch
  • Cycle slips, gaps, or poor satellite geometry
  • Inconsistent or missing ephemeris data
• Time synchronization issues
  • Incorrect processing offset between GNSS and IMU
  • Latency or misalignment in timestamps
• Incorrect installation parameters
  • Invalid GNSS-to-IMU lever arm values
  • Incorrect mounting orientation
• Dynamics or configuration mismatch
  • Incorrect vehicle model or processing settings

True IMU data corruption (e.g., gaps or saturation) is a less common cause and should only be considered after GNSS and configuration issues are ruled out.

✅ Probable Resolutions

Follow the steps below to isolate and resolve the issue. Focus on GNSS consistency first, as this is the most common root cause.

1. Investigate GNSS Measurement Rejections

• Review logs for:
  • Continuous primary GNSS position measurement rejections
• Verify:
  • Base station coordinates, datum, and epoch are correct
  • RINEX files are complete and cover the full mission duration
  • Satellite availability is consistent (no large gaps or drops)
  • Ephemeris files match the constellations being processed

Learn more about Troubleshooting: “WARNING: Continuous primary GNSS position measurement rejections”

2. Validate Time Alignment

• Adjust the processing offset (e.g., ±30 seconds) and reprocess
• Ensure GNSS and IMU data streams are properly synchronized

3. Verify Installation Parameters

• Confirm GNSS-to-IMU lever arm values are correct (magnitude and sign)
• Verify mounting angles and sensor orientation

4. Review Processing Configuration

• Ensure the correct vehicle dynamics model is selected
• Confirm processing mode and GNSS settings match the dataset

5. Inspect IMU Data (If Needed)

• Review IMU QC plots for:
  • Data gaps
  • Spikes or saturation in acceleration or angular rate
• If IMU anomalies are reported, investigate hardware or logging issues

🔑 Key Takeaway

FDIR2 IMU failures are typically a downstream effect of GNSS inconsistency or timing misalignment—not a primary IMU failure.

Resolving GNSS measurement rejection issues will usually eliminate the FDIR2 condition and allow successful trajectory processing.

📬 Need Help?

If you're still stuck, please Contact Support for assistance.