Indoor and Urban Positioning Challenges

GPS signals, transmitted at roughly 20,200 kilometers and received at extremely low power, cannot reliably penetrate building materials such as concrete, steel, and low-emissivity glass, meaning indoor GPS positioning is generally unavailable or highly inaccurate; in dense urban settings, tall buildings create what is commonly called the "urban canyon" effect, blocking direct line-of-sight to many satellites while simultaneously generating strong multipath reflections off glass and steel facades, which can degrade horizontal accuracy from the typical few meters to tens of meters or produce entirely erroneous position fixes.