Loss in signal power. Electromagnetic signals tend to weaken, or attenuate, over a distance. Some of the signal is absorbed and converted to thermal energy as it interacts with the physical matter between the transmitter and receiver. Some of the signal is absorbed at the molecular level, and some of the signal is emitted and scattered in all directions, some of it at different frequencies.Twisted-pair copper wire systems attenuate electrical signals due to factors including the interaction of the signal with the copper in the conductors as the described by the level of resistance or impedance in the wire, and the tendency of the signal to radiate, or spread out, from the wire. Signal attenuation occurs in terrestrial radio systems due to interaction with the physical matter in the air and the tendency of the signal to disperse, or spread out. Attenuation is a relatively minor issue with respect to satellite radio systems, at least with respect to signal propagation in the vacuum of space, where there is no physical matter to interact with the signal.The portion of the satellite link that travels through the atmosphere is very much subject to attenuation, however. Attenuation also affects fiber optic systems, as some optical energy is absorbed at the molecular level, some is converted to thermal energy, some is dispersed, and some suffers frequency shifts. In some fiber optic systems, some amount of optical energy can be lost in the cladding that surrounds the crystalline core. (Note: Glass actually is not crystalline, but is an extremely viscous fluid.) Attenuation is sensitive to carrier frequency. In electrical and radio systems, for example, higher-frequency signals generally attenuate more than lower-frequency signals. The same phenomenon generally holds true in fiber optic systems, as well, although the measurement is in wavelengths, rather than frequencies, i.e., longer wavelength signals (lower frequency) signals attenuate less than shorter wavelength (higher frequency) signals. All else being equal, the impacts of attenuation increase with distance, and can become so severe over a long distance that the receiver cannot interpret the signals correctly. A variety of measures can be employed to overcome the effects of attenuation. Most commonly, amplifiers and regenerative repeaters are placed on circuits. The level of attenuation is described as insertion loss and is measured in decibels (dB) or decibels per kilometer (dB/km). See also amplifier, dB, dB/km, frequency, gain, insertion loss, repeater, and wavelength.