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Figure  1-1.Satellite  inclination.
TYPES OF ENVIRONMENTAL SATELLITES

Aerographers Mate, Module 03-Environmental Satellites and Weather Radar
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(LMT) during each orbit. These types of orbits are called  sun-synchronous orbits—they are synchronized with the movement of the sun across the earth’s surface. For instance, an orbit may be calculated so that the satellite path crosses the equator on the descending node 2 hours after sunrise on each orbit. The  ascending  and  descending  nodes  may additionally be identified by the relative time of day, such   as   day-ascending,   day-descending,   night- ascending or night-descending node (fig. l-3). When an ascending node crosses the equator at a particular relative time, the descending node of the same orbit will cross the opposite side of the earth at a LMT about 12 hours opposite the ascending node LMT (plus one- half the nodal period). For example, if a satellite has a sun-synchronous orbit with a day-ascending node, then the descending node on the other side of the earth will be a night-descending node. To simplify the situa- tion, only the relative time of day of the ascending node  is  referenced.  For  example,  an  environmental satellite  known  as  a  day-ascending  satellite  will always be over the sunlight portion of the earth when moving north, and it will always be over the dark side of the earth when traveling south. Night-ascending satellites will move northward over the dark side of the earth and southward over the sunlight side of the earth. The  National  Oceanic  and  Atmospheric Administration  (NOAA)  normally  maintains  at  least two  operational  polar-orbiting  satellites.  One  is  in  a sun-synchronous  morning  orbit  and  the  other  is  in  a sun-synchronous afternoon orbit. Thus, each satellite provides two images every 24 hours (one day image and one night image), producing a total of four images a day over any given area. When a satellite achieves orbit around the earth, the orbit is rarely a perfect circle. Most orbits are actually elliptical and they change over time because the earth is not a perfect sphere; it flattens over the poles  and  bulges  near  the  equator.  The  gravitational pull of the earth, sun, and moon also plays a role. When the satellite comes closest to the earth, the satellite is said to be at perigee, and when it is farthest away from the earth, the satellite is said to be apogee. When at Figure 1-3.—Day-ascending and night-ascending polar orbits. 1-3







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