Operating Principles

Although most commercially-available Airborne LiDAR Systems use a pulsed laser source, there are other operating modes of laser-based remote sensing systems. For example, a laser system can be characterized as a continuous wave (CW) laser system that transmits a continuous signal, and ranging is determined by modulating the intensity of the laser light. In such a system, a sinusoidal signal is received with a time delay. The travel time is directly proportional to the phase difference between the received and transmitted signal. Pulsed laser systems, on the other hand, transmit a series of laser pulses and measure the round-trip time of each laser pulse that scattered back to the optical receiver. The distance (or range) to the target is determined by the one-way time of flight of the laser pulse multiplied by the speed of light.

(1) Laser. The laser ranging unit in airborne laser scanning will include the actual laser; the transmitting and receiving optics; and the receiver with its detector, time counter and digitizing unit.

(2) Laser Wavelength. For topographic mapping using airborne laser scanning, where high energy pulses are required to perform distance measurements over long ranges, only certain types of solid-state, semiconductor, and fiber lasers have the specific characteristics – ability to produce high intensity collimated beams – that are necessary to carry out these operations. Nearly all airborne topographic LiDAR systems that use solid-state crystalline material such a neodymiumdoped yttrium aluminum garnet (Nd:YAG) lasers operate in the near-infrared wavelength range (typically 1064 nm). Fiber lasers (sometimes referred to as glass lasers) operating at or near 1550 nm have also been routinely used, though these systems operate at lower power levels and cannot reach the same operating altitudes as the 1064 nm laser sensors. Lasers have also been developed to operate at 905 nm, but are not very popular for airborne LiDAR applications due to their lowintensity returns over saturated sediments. Another class of lasers operates at the frequencydoubled blue-green wavelength of 532 nm. These sensors are typically used in bathymetric and topobathymetric applications because the green-wavelength laser is able to penetrate through the water column under certain conditions; see Chapter 7 for more details.

(3) Pulse Energy, Pulse Width, and Beam Divergence. The pulse energy, measured in micro Joules (μJ), is simply the total energy of the laser pulse. Pulse duration, measured in nanoseconds (ns), is typically defined as the time during which the laser output pulse power remains continuously above half its maximum value. Beam divergence, measured in milliradians (mrad), refers to the increase in beam diameter that occurs as the distance between the laser instrument and a plane that intersects the beam axis increases. The pulse energy of topographic LiDAR systems are typically low (10-100 μJ) to allow for a tightly focused beam with low beam divergence that is also eye safe. Bathymetric LiDAR systems have pulse energies up to 7 mJ, which are typically much higher than the near-infrared lasers used in topographic applications. The higher power is needed for the laser pulse to penetrate through the water column to map the bottom. The bathymetric sensors with very high laser pulse power also have a large footprint so that the energy is spread across a larger area for eye-safety reasons. The pulse width determines the range resolution of the pulse in multiple return systems (explained below), or the minimum distance between consecutive returns from a pulse. Traditionally, pulse widths for topographic systems have been in range of about 10 ns. This means that there is a “blind spot” of about 1 meter along the laser path behind each received LiDAR return. Newer laser technology has enabled the use of much shorter pulse widths (1-2 ns) for topographic and topobathymetric applications. For topobathymetric applications, a short pulse width laser enables the separation of a return from the water surface and bottom in very shallow water depths. This limits the effective measurement depth to >0.5m for threshold detect topobathy LiDAR systems. 

(4) Pulse Repetition Frequency (PRF). The PRF, measured in kHz, is the number of pulsesemitted by the laser instrument in 1 second. Older instruments emitted a few thousand pulses per second. Modern systems can support frequencies of 400 kHz and newer technologies are now enabling 2 lasers channels to be used in conjunction with the same scanning mirror, thereby producing effective PRF of 800 kHz. Many systems allow different settings for the PRF. This is usually done to allow the systems to fly at different flight altitudes. The PRF is directly related to the point density on the target. For example, a system operating at 167 kHz from the same flying altitude will have higher number of returns than when operating at 100 kHz. Equivalently, a high PRF system can generate desired return densities by operating on an aircraft that flies higher and faster than an aircraft carrying a lower PRF system, thereby reducing flying time and acquisition costs when weather conditions allow for higher flying altitudes.