Trimble Autolock/VISION Target Lost: What It Means and How to Fix It

Applies to: Trimble S3, S5, S7, S9 (with Autolock or VISION), VX spatial station

What Does Target Lost Mean?

Trimble's Autolock system uses an active infrared beam and a CCD detector array to continuously track a prism as it moves through the field. VISION technology (on S7/S9) adds a video feed with digital target recognition for enhanced tracking. "Target Lost" occurs when the tracking algorithm can no longer maintain a lock on the prism — the signal strength drops below the threshold for reliable tracking, or the prism disappears from the camera/detector field of view.

In Trimble Access robotic mode, a Target Lost event pauses the measurement session. The field crew member must use the remote controller (TSC3, TSC7, T10/T100) to manually re-aim the instrument at the prism or use the search function to re-acquire automatically.

Common Causes of Autolock Target Lost

• Prism passed behind an obstruction — tree, pole, equipment, or another crew member blocked the line of sight momentarily
• Prism face turned away — the prism rotated more than ±15° from the instrument direction (standard flat prisms); use a 360° prism for omnidirectional tracking
• Prism moved too fast — rapid movement exceeds the tracking servo's angular velocity limit
• Distance too great — Autolock tracking range on S5/S7 is approximately 800m under optimal conditions; longer distances increase Target Lost frequency
• Strong competing light sources — direct sunlight into the objective lens, or a second prism nearby confusing the ATR
• Dirty objective lens or prism face — reducing signal strength below the tracking threshold
• Instrument servo fault — the tracking servo cannot keep up with prism movement
• Multipath or vibration — instrument on an unstable setup causing erratic tracking

How to Fix Trimble Target Lost — Step by Step

1. Stop moving the prism pole — keep it stationary while attempting re-acquisition.
2. From the controller (TSC3/TSC7), tap the target icon or use the "Search" function in Trimble Access to initiate an automatic prism search in the last known direction.
3. If the automatic search fails, use the joystick on the controller to manually aim the instrument toward the prism, then trigger a search or lock once aimed approximately at the target.
4. Ensure the prism face is directed toward the instrument. If using a flat prism (not 360°), the prism must be aimed at the total station.
5. Move to a position with a clear, unobstructed line of sight to the instrument. Avoid areas with dense vegetation or equipment between the prism and instrument.
6. Clean the prism face and instrument objective lens if target is being lost repeatedly at close range.
7. If operating in bright sunlight near the instrument, shade the objective lens briefly during lock acquisition.
8. For persistent Target Lost at normal distances, consider switching to a higher-contrast Trimble 360° mini prism or Active Target for better Autolock performance in difficult conditions.

Configuring Trimble Access for Better Autolock Performance

In Trimble Access, adjust the Autolock sensitivity and search radius under Instrument → Instrument Settings → Autolock. Increasing the search radius improves recovery from short obstructions but slows re-acquisition. The "Lock after Tilt" setting can cause frequent Target Lost events on rough terrain — disable it in stable setups.

Related Trimble Errors

Trimble Total Station Servo Failure | Trimble Total Station EDM Fault | Leica ATR Horizontal Tracking Error