Multipath
Multipath means the same radio pulse arrives at the receiver more than once. One copy may be the direct path. Later copies bounced off rack faces, a mezzanine, a mast, a dock door, or another vehicle. For RTLS the risk is not extra energy. It is that the location engine locks a late copy and treats it as the range.
Warehouses are built from reflectors. Selective pallet racking is a steel grid. Mezzanines add a metal plane over the travel lane. Forklifts are metal boxes that park in the worst RF slots because that is where the charger or the last free bay is. RSSI systems (classic Wi-Fi or coarse BLE) smear under that. UWB short pulses can separate arrivals in time, which is why vendors call it robust in metal. Separation only helps if the first path is still there to pick.
Multipath and NLOS often arrive together. NLOS is the blocked direct path. Multipath is the extra delayed copies. Either one stretches a 30 cm lab error bar into meters on the aisle. How to score that on your layout is in ask for a coverage map, not 30 cm on the slide.
Key Components
Direct copy: The shortest geometric path, present only under line of sight.
Reflected copies: Later arrivals from steel, stock, floors, and vehicles. Each extra meter of path is extra nanoseconds of delay, which the engine can misread as extra range.
First-path detector: UWB and similar stacks try to take the earliest usable peak, not the strongest. Strongest is often a bounce off a nearby rack.
Time-varying scene: Doors, parked fleet, and fill level change which copies exist. A clean survey at mid-shift can be a smear at shift-end charging.
Applications in Manufacturing and Logistics
Multipath is why a live pin walks through a rack beam or sits in aisle 11 while the truck is in aisle 12. Conventional faces are only a few meters apart. A one-to-two meter range bias is an adjacent-aisle error. Pick confirmation and no-go alerts fail first. Spaghetti and zone occupancy usually still work if the pin stays in the aisle.
Dock doors are a special case: a moving metal reflector. Measure occupancy with the door open and closed. An open-door demo is not a closed-door plant.
Benefits and Challenges
Knowing multipath exists keeps you from buying one accuracy number for the whole roof. You densify anchors, raise them, change tag mount, or drop the job to zone-level in the worst pocket. You do not 'tune away' physics with a filter and call the brochure proven.
The survey cost is real. You must mount the tag as operations will live with it and walk hostile aisles with inventory in place. Average centimeters hide aisle jumps. Count those jumps.
Related Terms
Blocked direct path is NLOS. Clear path is LOS. Distance still comes from time of flight on UWB. The plant document that shows where bounce wins is a coverage map. Indoor location as a category is IPS / RTLS.
Frequently Asked Questions
Is multipath the same as interference from Wi-Fi? No. Wi-Fi coexistence is a different issue (UWB usually sits at the noise floor). Multipath is copies of your own pulse arriving late after bouncing in the hall.
Why is the strongest signal the wrong one? A nearby rack can reflect more energy than the weak direct path around a load. Taking the loudest peak then overstates range.
Does more anchors always fix it? More LOS anchors help. More anchors that only see bounces can agree on a wrong position. Geometry and first-path quality matter more than count alone.
Can software filtering remove multipath? Filters smooth jitter. They do not restore a missing direct path. If the first path is gone, the pin is a guess until coverage or the job spec changes.