site  also  contains  transmission  schedules  and transmitting frequencies for various United States and foreign-operated   satellites. REVIEW  QUESTIONS Q7. What is the purpose of a satellite atmospheric sounder? Q8. What is the main advantage of geostationary satellites? Q9. Which GOES satellite provides imagery over all of South America? Q1O. What type of satellite is the NOAA 14? Q11. What  are the main advantages of polar-orbiting satellites? Q12. What is the average swath width of a polar- orbiting satellite? Q13. Which  organization  is  responsible  for  providing near real-time DMSP environmental  imagery to the  fleet? Q14.     Which  geostationary  satellite  will  provide imagery  for Spain and Portugal? SATELLITE IMAGERY LEARNING  OBJECTIVES:  Recognize  the particular   advantages   of   imagery   from geostationary  satellites  and  polar-orbiting satellites. Define   spatial   resolution, radiometer, electromagnetic wave, and albedo. Define   the   terms   visual,   infrared,   near infrared,  and water vapor  as they relate to satellite imagery. Recognize the advantages of visual, infrared, and water vapor imagery. The   pictures   or   images   available   from environmental satellites vary, depending on the type of satellite and the type of sensor in use. Geostationary satellites continuously "look" at the same geographical area of the earth. However, the image area is centered on  the  satellite  subpoint  on  the  equator.  At  the subpoint, clouds are seen from directly overhead. Further away from the subpoint, clouds seen in the image are viewed from an angle, and feature distortion occurs.    Cloud  cover  is  often  overestimated  toward image edges because the sensor is actually viewing the clouds from the side. Near the horizon, the image is considered unusable due to distortion. Polar-orbiting satellites are in much lower orbits than geostationary satellites; therefore, the satellite can only see a limited portion of the earth as the satellite   sensors   scan   from   horizon   to   horizon. Because of the acute view angle near the horizon, the satellite image near the horizon is usually of little value and is usually not processed or displayed by receiver station  equipment. IMAGERY  RESOLUTION Satellite  sensors  designed  to  produce  pictures  or images of earth, its oceans, and its atmosphere are very different from the cameras used to take a photograph. They are more like a video camera, only much more specialized.   These   scanning   sensors   are   called radiometers, and instead of film, an electronic circuit sensitive only to a small range of electromagnetic wavelengths measures the amount of energy that is received.  Satellites  may  can-y  several  different  image sensors, each of which is sensitive to only a small band of energy at a specific wavelength. The radiometer used by the TIROS-N and POES series satellites is known  as  the  Advanced  Very  High  Resolution Radiometer (AVHRR) and contains many types of sensors. Satellite sensors scan across the surface of the earth in consecutive scan lines along a path normal to the direction of travel of the satellite. As the sensor moves  through  a  scan  line,  it  very  rapidly  measures energy levels for only a very small portion of the earth at  a  time.  Each  individual  energy  measurement  will compose a single picture element or  pixel of  the  overall satellite  image.  The  sensor  then  assigns  an  intensity level from 0 to 256 for each pixel. The size of the area (field-of-view) scanned by the sensor determines the spatial  resolution  of  the  overall  image.  Thus,  the smaller the area scanned for each pixel, the higher the spatial resolution. Some sensors may scan an area as small as 0.5 km across (high resolution), while others scan areas as large as 16 km (low resolution). When composed into an image, smaller pixels allow the image to be much clearer and show greater detail. Clouds and land boundaries appear better defined. If objects are smaller than the sensor resolution, the sensor averages the brightness or temperature of the object with the background. Normally, the sensors aboard satellites are able to provide better resolution for visual imagery than for infrared imagery. DMSP satellites have very high-resolution capabilities in both visual and infrared. 1-9