Surveying Through Solid Walls

A good metrology system in the automated environment must. (1) measure the three dimensional position and attitude of any component to a reasonable degree of accuracy; (2) acquire these data fairly rapidly; and (3) be capable of making reliable measurements anywhere on site. A significant limitation for existing surveying systems (including GPS and laser based technologies) is the requirement for direct line-of-sight between the reference and roving surveying elements. Construction sites, as a rule, are highly unstructured environments and clutter is the norm. Recent fundamental research has been undertaken at NIST to develop a real-time Non-Line-of-Sight measurement system capable of tracking to 10 mm accuracy despite the presence of walls, ceilings, floors, and other obstructions. The new system determines the distance from a series of reference transmitters which broadcast pulse-synthesized baseband electromagnetic signals. Digital signal processing techniques are used to recover the time of flight (and hence distance) and to compensate for errors introduced as a result of material penetration and re-transmission of the signal. Results of the most recent series of NIST laboratory investigations leading towards a practical construction metrology system are discussed.