(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.
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