Laser Pulses at the Speed of Light
A LiDAR system emits short laser pulses — typically 5-10 nanoseconds long — and precisely times their round-trip journey to the ground and back. At the speed of light, each nanosecond of timing error corresponds to 15 cm of range error, demanding sophisticated electronics. Modern systems fire 100,000 to 2,000,000 pulses per second, building dense point clouds of the terrain, vegetation, and structures below. This simulation traces a single pulse from emission through atmospheric propagation to surface interaction and return detection.
The Laser Link Budget
Whether a return pulse is detectable depends on the laser link budget: the balance between emitted energy and losses along the path. Energy spreads with the square of distance (beam divergence), is attenuated by atmospheric scattering and absorption, reduced by the surface reflectance at the laser wavelength, and collected by the finite receiver aperture. Each factor is multiplicative, so doubling altitude cuts return energy by roughly four times — making high-altitude surveys significantly more challenging.
Footprint and Resolution
The laser beam diverges as it propagates, illuminating a circular footprint on the ground. The footprint diameter d = 2H × tan(θ/2) determines the spatial resolution of the measurement — features smaller than the footprint are averaged together. Narrow-divergence systems (0.2 mrad) from 1000 m altitude produce 20 cm footprints capable of resolving individual tree branches, while wider beams provide smoother but coarser terrain models.
Multiple Returns
A single laser pulse can produce multiple returns as it penetrates vegetation canopy. The first return comes from the treetop, intermediate returns from branches and understory, and the last return from the ground. Full-waveform systems digitize the entire return signal, enabling detailed analysis of vertical structure. This capability makes airborne LiDAR uniquely powerful for forestry, archaeology beneath canopy, and flood modeling where bare-earth elevation is needed.