Troubleshooting: Forward/Reverse Separation

📌 Problem

A spike in Forward/Reverse Separation during GNSS post-processing typically indicates inconsistency between the forward and backward trajectory solutions. 

This issue can be observed in post processing trajectory reports:

• Trajectory Report Review for TrueView 3DIS (Applanix)
• Trajectory Report Review for TrueView 3DIS (CHCNav)
• Trajectory Plot Review for TrueView 3DIS (TruNav)

🔍 Root Cause(s)

This can happen for several reasons:

1. Poor Satellite Geometry (High PDOP)
   • If the number of visible satellites drops or they are clustered in one part of the sky, the solution becomes less stable, causing divergence between forward and reverse runs.
2. Cycle Slips or Loss of Lock
   • Interruptions in carrier-phase tracking (due to obstructions, multipath, or signal interference) can introduce discontinuities that affect one direction more than the other.
3. Multipath Effects
   • Reflections from nearby surfaces (buildings, vehicles, water) can distort measurements, leading to inconsistent ambiguity resolution between forward and reverse processing.
4. Ambiguity Resolution Issues
   • If integer ambiguities are resolved differently in forward vs. reverse processing, the separation will spike. This often happens in challenging environments or when the baseline is long.
5. Data Gaps or Low-Quality Observations
   • Missing epochs or noisy data (e.g., due to poor antenna setup or interference) can cause one direction to interpolate differently than the other.
6. Atmospheric Disturbances
   • Rapid changes in ionospheric or tropospheric conditions can affect phase measurements unevenly across the two processing directions.
7. Incorrect or Inconsistent Constraints
   • If constraints (e.g., fixed points, antenna heights) are applied differently or incorrectly, forward and reverse solutions will diverge.

🧠 How to Diagnose

• Check satellite visibility plots and PDOP graphs for the time of the spike.
• Review cycle slip reports and ambiguity resolution status.
• Compare residuals and quality indicators for forward vs. reverse runs.

📬 Need Help?

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