only one channel transmitting, the entire 2.5 watts would be 

 available to this one channel. If two channels were transmitting 

 simultaneously, the 2.5 watts would be shared equally between both 

 channels. The power in each channel will be reduced by 3dB. 

 (Note: A reduction of 3dB is equivalent to a numerical factor of 

 one-half). Although there are 183 possible channels, statistically 

 it is very highly unlikely that all 183 will ever be transmitting 

 simultaneously. A more reasonable number of channels transmitting 

 simultaneously might be 100, which will give a 20dB per channel 

 reduction in signal level from 2.5 watts. (Note: A 20dB reduction 

 corresponds to a numerical reduction of 100). 



The situation is not ideal, however. The satellite uplink receiver 

 puts out noise which is rebroadcast by the transmitter along with 

 the desired signals. This rebroadcast noise requires transmitter 

 energy that would otherwise be available for useful signals. The 

 amount of energy converted into noise is equivalent to approximately 

 10 simultaneous signal channels. This amounts to a reduction of 

 lOdB in the useful, available power when a small number of channels 

 are active. (Note: A reduction of lOdB is equivalent to a numerical 

 reduction of one-tenth). When a large number of channels are active, 

 say 100 or more, this noise component tends to be suppressed. The 

 available energy is then equally distributed among the active 

 channels. The power reduction for 100 channels is only 20dB instead 

 of 30dB. (Note: 20dB is equivalent to a numerical value of 100 

 while 30dB is equivalent to 1000). This is discussed in more detail 

 in Appendix A. 



From the above it can be seen that available power per channel can 

 vary from about 0.25 watts when only one channel is active to about 

 0.25 watts when 100 channels are transmitting simultaneously. This 

 is a lOdB variation in the available received energy and can occur 

 at random. 



Another factor to be considered is that, beginning with the GOES-2 

 satellite, a low-power mode is used operationally. In this mode 

 the total satellite transponder output power is 0.4 watts or a 

 reduction of BdB from the 2.5-watt level. (Note: 8dB is a numerical 

 reduction by one-sixth). However, the DCS signal will actually be 

 reduced by only about 2dB (40 percent); this is because VISSR and 

 S-VISSR are not transmitted in the low-power mode so, except for 

 telemetry which is small, the entire transponder power is then 

 available to the DCS. It is expected that the low-power mode will 

 be used during the predictable eclipse periods. 



The free-space loss between isotropic antennas from synchronous 

 altitude to the subsatellite point is 188dB; the free-space loss 

 to Earth's edge is increased to 189. 3dB. The satellite trans- 

 mitting antenna gain is 2dB less at Earth's edge than at the sub- 

 satellite point. In addition, the radiation pattern of the 

 satellites favors the Northern Hemisphere, so that there is an 

 additional loss of about 2dB at the Earth's edge in the Southern 

 Hemisphere. The signal level that can be expected at Earth's 

 edge in the Southern Hemisphere is, therefore, approximately 



