TABLE l.—Ver if i cation of computed subpoint locations for ESSA-8. Computed values minus APT Predict values 



are given in degi-ees latitude or longitude. 



•"Minutes after ascending node. 



A sample schedule, computed and printed for 

 ESSA-8, is shown in Figure 2. ESSA-8 transmits 

 only during the daytime, when it is following a 

 north-to-south track. Accordingly, the reference 

 time appearing to the left of each set of subpoint 

 tabulations is 36 min later than the time of as- 

 cending node for the orbit indicated. The num- 

 bers labeled "INITIAL DATA" were read from 

 the data card and are printed for verification. 

 The subpoint locations are printed to the nearest 

 hundredth of a degree of latitude or longitude, 

 for each minute from 37 to 56 min after ascending 

 node, which covers the daytime track from about 

 lat. 60' N to lat. 4" N. The subpoint location cor- 

 responding to a picture transmitted by the satel- 

 lite can be found by interpolation, based on de- 

 termination of the time of the picture relative to 

 the time of ascending node. The .schedule includes 

 only those orbits within range of the APT ground 

 station at the La Jolla Laboratory. 



A comparison of computed subpoint locations 

 with those given in APT Predict messages for 

 seven orbits of ESSA-8, completed between April 

 and October 1972, is presented in Table L Sub- 



point locations are given to the nearest tenth of 

 a degree in APT Predict messages, while com- 

 puted values were carried to the nearest hun- 

 dredth of a degree. Differences between com- 

 puted values and corresponding APT Predict 

 values averaged less than 0.06 degree of latitude 

 or longitude. Since ESSA-8 completes an orbit 

 in a little less than 2 hr, the motion of the satel- 

 lite subpoint is about 3 nautical miles per second. 

 An error of 0.05 degree latitude (equal to 3 nauti- 

 cal miles) in subpoint location is equivalent, 

 therefore, to an error of about 1 sec in determin- 

 ing the time of an observation from the satellite. 



LITERATURE CITED 



BOWDITCH, N. 



1958. American practical navigator an epitome of navi- 

 gation. U.S. Navy Hydrogr. Off. Wash., D. C, 1524 p. 

 MENZEL. D. H. (editor). 



1960. Fundamental formulas of physics. Vol. 2. Dover 

 Publ., Inc., N. Y., p. 680-690. 

 RUNCORN, S. K., (editor-in-chief). 



1967. International dictionary of geophysics. Vol. 7. 

 Permagon Press, Lond., p. 80-82, 326-331. 



