Satellite images, or pictorial representations of
satellite-sensed information, are some of the most
frequently used tools in the fields of meteorology and
As a Navy or Marine Corps observer, one of your
primary duties will be to acquire satellite imagery. You
may also be required to process the imagery to better
display features of interest to the analyst. Later, as you
begin to analyze meteorological and oceanographic
situations, you will use satellite imagery as one of your
most important sources of information.
In this chapter, we begin with an explanation of
some of the basic terminology used to describe satellite
orbits and satellite tracking. Next, we introduce
environmental satellite programs, and then describe
the various types of environmental satellites and
explain their purposes. We then discuss some of the
most common types of satellite imagery, and acquaint
you with a few basic imagery enhancement techniques.
We complete the chapter by taking a brief look at some
of the equipment and methods that you will use to
acquire and process satellite imagery.
LEARNING OBJECTIVES: Define basic
terminology used in relation to satellite orbits
and satellite tracking.
Before you can effectively acquire and use satellite
imagery, it is important that you become familiar with
some basic satellite terminology.
Environmental satellites orbit the earth at various
altitudes. Some environmental satellites operate lower
than 800 kilometers (500 statute miles), while others
operate as high as 35,800 kilometers (22,300 statute
miles). To stay in orbit, lower altitude satellites must
orbit faster than higher altitude satellites. As a result,
satellites in orbit at 800 kilometers complete an orbit in
a little over 100 minutes, while satellites in orbit at
35,800 kilometers require 24 hours to complete an
The inclination angle of a satellites orbit is the
angle the satellites path makes as the satellite crosses
the equator (fig. l-1). This term is usually referred to
as the satellite inclination.
Satellites that have an inclination of 0 degrees
circle the earth over the equator in an equatorial orbit.
When a satellite in an equatorial orbit moves from west
to east in the same direction that the earth rotates, its
speed and altitude may be adjusted so that it is always
located in a stable orbit over the same position on the
equator. Satellites in these orbits are called
geostationary, earth-synchronous, or geosynchronous
since they are stationary relative to their position over
the equator. Their fixed location provides continuous
coverage of the same area over a 24-hour period.
As shown in figure 1-1, satellites with high orbital
angles generally cross over the polar regions and are
called polar-orbiting satellites. These satellites orbit
the earth about 14 times a day and provide global
coverage every 12 hours. A single orbit of a polar-
orbiting satellite is composed of an ascending node,
which is the period of time when the satellite is
traveling from south toward the north, and a
descending node, which is the period of time when the
satellite is traveling from north toward the south.
The position directly under a satellite on the
surface of the earth is called the satellite subpoint or
nadir, while the track of the satellite subpoint along the
surface of the earth is called the satellite path.
Now lets consider some additional terms used in
satellite orbits and satellite tracking.
Because the earth rotates, each time a polar
orbiting satellite crosses the equator, its position is
further west than its position on the previous orbit.
This change in position is called the nodal increment
(fig. l-2). The total time it takes the satellite to
complete an orbit is called the nodal period. The term
epoch refers to a specific reference point in a satellites
Most polar-orbiting environmental satellites use a
nodal increment and a nodal period that keep pace with
the rotation of the earth and keep the satellite path
crossing the equator at the same local mean time