Albedo formula_5 (also called blue-sky albedo) can then be given as: Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo. These terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. The albedos of planets, satellites and minor planets such as asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time comprises a major part of the astronomical field of photometry. For small
Surface-to-air missile than aircraft. Virtually all surface warships can be armed with SAMs, and naval SAMs are a necessity for all front-line surface warships. Some warship types specialize in anti-air warfare e.g. "Ticonderoga"-class cruisers equipped with the Aegis combat system or "Kirov" class cruisers with the S-300PMU "Favorite" missile system. Modern Warships may carry all three types (from long-range to short-range) of SAMs as a part of their multi-layered air defence. SAM systems generally fall into two broad groups based on their guidance systems, those using radar and those using some other means. Longer range missiles generally use radar for early detection