Length of stay.
Line of scrimmage.
Line of sight.
Optical LOS. Line of vision. A direct imaginary line between two points, as though it were from the center of the eye to the center of the object viewed. A direct non-guided path in the form of a straight line between a transmitter and receiver, uninterrupted by physical matter other than that suspended in the atmosphere. Opaque objects such as mountains, buildings, and trees interrupt optical LOS, as does the horizon over long distances due to the natural curvature of the Earth. Optical LOS is critical in free space optics (FSO) transmission systems and high-frequency radio system. Optical LOS is always preferable in radio systems, even those operating at low frequency.
Radio LOS. A direct nonguided path between a transmitting antenna and a receiving antenna.The criticality of LOS is sensitive to the radio frequency (RF) employed.Very low frequency (VLF), and low frequency (LF) signals tend to be travel between the Earth and the ionosphere. LF and medium frequency (MF) signals propagate as ground waves, which tend to follow the curvature of the Earth. Signals at the high end of the MF range and in the high frequency (HF) range benefit from ionospheric refraction, a phenomenon in which the density gradient in the atmosphere acts like a lens and tends to bend radio beams back towards the Earth. At very high frequencies (VHF) and above (i.e., ? 30 MHz) true optical LOS is considered essential, absent special modulation techniques combined with space division multiplexing techniques such as multiple input/multiple output (MIMO). See also electromagnetic spectrum, ground wave, HF, ionosphere, LF, MF, MIMO, near-LOS, NLOS, radio, refraction, RF, space division multiplexing, VHF, and VLF.