Chapter 10 — WATCH ROUTINES 



The APT PREDICT MESSAGE discussed 

 earlier in this chapter must be referred to for 

 necessary tracking information. 



In order to compute the required data for 

 proper satellite tracking, weather service per- 

 sonnel must utilize an APT tracking board with 

 a transparent orbital overlay and a tracking 

 diagram. These were defined in the preceding 

 terminology and described in chapter 6 of this 

 manual. 



For complete information on the procedures 

 used for the postlaunch preliminary preparation 

 of the tracking board, transparent orbital over- 

 lay, and tracking diagram, and for instructions 

 in completing an APT tracking worksheet, refer 

 to the NOAA Direct Transmission System Users 

 Guide, or the applicable meteorological satel- 

 lite manual. Some of the steps and procedures 

 to be followed in preparation for receipt of 

 the satellite transmission are as follows: 



1. The appropriate tracking diagram Is cen- 

 tered on the tracking board at the location of 

 the ground station. 



2. The reference subpoint track is plotted on 

 the overlay. 



3. The equatorial line is marked on the 

 overlay with the ascending node location for 

 the reference orbit and all other orbits 1 

 through 12. 



4. The equatorial zone of acquisition is de- 

 termined. 



Proper satellite tracking preparation re- 

 quires the use of the correct APT predict 

 message and the determination of which orbits 

 can be tracked. 



When it has been determined which orbits 

 can be tracked, an APT tracking worksheet 

 for each orbit must then be prepared. 



The satellite pictures received on the ground 

 station's recorder are of no value to the weather 

 forecaster unless the picture has been properly 

 gridded. The term "gridding" refers to the proc- 

 ess of drawing longitude and latitude lines on 

 the received picture. 



Recorders, which do the gridding internally, 

 and provide a picture with latitude and longitude 



lines already on it, have replaced earlier 

 models; consequently, the exacting task of man- 

 ual gridding is seldom, if ever, required. Should 

 manual gridding be required, refer to the User's 

 Guide for detailed information. 



SR Data and its Application 



The scanning radiometer can sense reflected 

 and radiant energy. This allows complete cover- 

 age throughout the entire orbit, over the dark 

 areas of the earth as well as the daylight areas. 



The SR infrared facsimile picture looks like 

 a distorted television picture of clouds (fig. 

 10-3). Various shades of gray which appear 

 in these pictures represent effective radiating 

 temperatures, not variations in reflectivity of 

 visible light. The radiating temperature of a 

 body is affected by its radiative properties, 

 as well as by its temperature. Water, ice, and 

 various types of soil have widely varying radi- 

 ative properties which affect the readings of 

 infrared sensors. Since atmospheric tempera- 

 ture generally decreases with altitude, it is 

 possible to make gross inferences about the 

 heights of cloud tops from their temperatures 

 shown by infrared. Only three classes of 

 temperature (shades of gray) may be readily 

 distinguished in the pictures: 



1. White areas show the coldest tempera- 

 tures and therefore represent high clouds or 

 snow-covered areas. 



2. Light gray areas represent moderate 

 tropospheric temperatures and middle cloudi- 

 ness. This usually means ceilings 7,000 to 

 12,000 feet and little or no precipitation. The 

 lighter shade of gray may represent a height 

 difference of as little as 3,000 ft between the 

 tops of middle clouds and the tops of low clouds. 



3. Dark gray areas show relatively warm 

 tropospheric temperatures. However, low cloudi- 

 ness with little vertical development, such as 

 small cumulus, stratocumulus, stratus, and fog, 

 are indistinguishable from background noise in 

 the infrared pictures. The decision as to 

 whether the dark gray area contains cumulus, 

 stratocumulus, fog/stratus, or no cloud can be 

 made easily from the VHRR pictures. On the 

 other hand, the infrared can be used to make 

 decisions about cloud top height that cannot be 

 made from the VHRR pictures. 



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