In addition to natural noise described above, there is' a source of 

 noise peculiar to the SMS/GOES family of satellites, called spin 

 modulation or "spin mod." 



The SMS/GOES satellites are spin stabilized, which means they must 

 spin continuously to maintain correct attitude. The spin rate 

 is nominally 100 revolutions per minute (rpm). 



The antenna elements for the communications system are located 

 around the periphery of the satellite. As the satellite spins, 

 it is necessary to continuously switch the output of the trans- 

 mitter to those elements that are facing the Earth. This is 

 called an "electronically despun antenna." 



As an antenna element on one side of the satellite is passing out 

 of view of the Earth and is being switched off, an antenna element 

 on the opposite side of the satellite is coming into the Earth's 

 view and is being switched on. It can, therefore, be visualized 

 that the electrical center of the antenna rotates around the 

 satellite in a direction opposite to the rotation of the satellite. 

 Therefore, the antenna pattern is always pointing toward the Earth. 



The spin mod noise is peculiar to the electronically despun 

 antennas that are used on the SMS-1 and 2, and GOES-1, 2, and 3 

 satellites. If the phase center of the antenna were located 

 on the spin axis of the satellite, the problem would not exist. 

 A more detailed descriptin of this source of noise is given in 

 Appendix B. 



At the time this report is being written, it is expected that the 

 next generation of GOES satellites (GOES-D, E, and F) will use 

 a mechanically despun antenna. With this type of antenna, the 

 antenna mechanically spins in a direction opposite to the spin 

 direction of the satellite, so that the antenna appears stationary 

 with respect to the Earth. As there will be no switching of 

 antenna elements, the phase center of the antenna will remain 

 constant and there will be no spin mod. This will improve the 

 operation of the system by reducing the noise against which the 

 desired signal uust compete. The spin-mod has been measured as 

 40° peak-to-peak for Earth stations located on the same 

 longitude as the satellite and 70° peak-to-peak for Earth 

 stations located on the horizon at the Equator, 



Unfortunately, the desired signal information is also phase 

 modulated on the carrier so that spin mod caused by the antenna 

 switching appears as noise along with the desired information. 

 This noise is in addition to the natural noise described previously. 



It is therefore necessary to have a higher carrier-to-noise ratio 

 (CNR) than would otherwise be required. 



The CNR actually needed depends on the highest bit-error rate 

 (BER) that is considered acceptable for the user. Considering 



55 



