CHAPTER 1 ENVIRONMENTAL  SATELLITES INTRODUCTION Satellite images, or pictorial representations of satellite-sensed information, are some of the most frequently used tools in the fields of meteorology and oceanography. 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. SATELLITE  TERMINOLOGY 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 orbit. The  inclination  angle  of  a  satellite’s  orbit  is  the angle the satellite’s 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 let’s 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 satellite’s orbit. 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 1-1