The project involved the construction of an urban metro tunnel section with multiple excavation faces, cross passages, and temporary refuge chambers. Hundreds of workers from different subcontractors entered and exited the tunnel daily through a limited number of portals and shafts.

Supervisors needed to answer questions such as:

• How many people are currently underground?

• Which section is each team working in?

• Has anyone entered a restricted or unfinished zone?

• If an incident occurs, who is closest and how long will rescue take?

Before RTLS, these answers relied on paper logs, radios, and experience.

• Linear, enclosed environment
  The tunnel stretched over long distances with limited branching, making traditional zone-based systems ineffective.

• No GNSS / GPS availability
  All positioning had to work without satellite signals.

• Dynamic construction layout
  Sections changed as excavation progressed, requiring flexible zone definition rather than fixed rooms.

• Emergency response uncertainty
  In drills, rescue teams often knew who was underground but not exactly where.

• Multi-contractor access control
  Different teams were allowed in different sections at different times, but enforcement was manual.

GridRTLS implemented a UWB RTLS architecture optimized for tunnel environments:

1) Section-based UWB anchor topology

UWB anchors were installed along the tunnel at engineered intervals, creating a continuous positioning corridor. Instead of “rooms,” the system divided the tunnel into logical sections (e.g., every 50–100 meters), allowing personnel to be located by section rather than absolute coordinates.

2) Personnel positioning via wearable devices

Workers wore AQM hybrid positioning safety helmets, enabling:

• Continuous tracking underground

• One-button SOS alerts

• Automatic logging of entry/exit and section transitions

3) Safety zoning and access rules

Virtual zones were defined for:

• Active excavation faces

• High-risk areas (blasting preparation, support installation)

• Restricted or unfinished sections

The system generated alerts when unauthorized personnel entered restricted zones or remained beyond allowed durations.

4) Emergency and rescue support

In emergency drills, supervisors could instantly see:

• Total personnel underground

• Distribution by section

• Nearest safe exits and refuge chambers

• Estimated travel distance and time for rescue teams

Phase 1 — Tunnel mapping and section design
The tunnel was segmented into logical sections aligned with construction phases. Anchor density was increased near:

• Working faces

• Cross passages

• Emergency refuges

• Shaft connections

Phase 2 — Infrastructure deployment
UWB anchors were mounted using tunnel-approved fixtures, with communication backhauled via industrial gateways where wired connections were not continuous.

Phase 3 — Wearable rollout and training
Personnel were issued helmets by role and contractor. Training focused on:

• Entry/exit procedures

• SOS usage

• Understanding section-based alarms

Phase 4 — Operational tuning
Zone boundaries and alert thresholds were adjusted as excavation advanced, without relocating hardware—only configuration updates were required.

After deployment:

• Section-level visibility became the standard
  Supervisors could see exactly how many people were in each tunnel section in real time.

• Emergency response planning improved dramatically
  Rescue drills confirmed that locating workers took minutes instead of tens of minutes.

• Unauthorized access dropped significantly
  Automatic alerts replaced manual enforcement, reducing violations and disputes.

• Auditability and accountability improved
  Entry, movement, and dwell time data provided objective records for safety reviews and contractor management.

Overall, the RTLS transformed the tunnel from a “black box” into a managed, data-driven underground workspace.