2.3.2 Conditions Affecting Access and Performance 



Periodic preventive maintenance is scheduled for the DCS well 

 in advance, since no interrogated data collection can be 

 carried out during these periods. 



Twice each year, the Earth's position will be such that the space- 

 craft orbit carries it through the Earth's shadow. Because the 

 spacecraft's prime power source is a solar array, safety considerations 

 may require the transponder to be operated in a low-power mode if 

 at all to conserve secondary power. This results in a satellite 

 transmitter power drop of from 1 to 3 dB. 



DCP up-link response signals, occasional TIROS-N data relay signals, 

 and satellite receiver noise share the down-link power output of 

 the GOES. Increasing the number of active DCP channels thus decreases 

 the power per channel, and also simultaneously reduces the down-link 

 noise. Until the noise contributed by the CDA or other ground 

 terminal receiving system becomes the limiting factor, the net signal- 

 to-noise ratio is nearly independent of channel loading, and bit 

 error probabilities of less than 10~^ may be anticipated, as 

 described in paragraph 1.2.9 on page 8. 



2.4 Command and Data Acquisition 



2.4.1 Response Data and Command/Interrogate Signals 



A UHF Pilot Response signal centered in the platform response 

 frequency band and derived from the CDA station frequency 

 standard is transmitted to the spacecraft transponder along with 

 replies from the deployed platforms. The corresponding signal 

 received at the CDA Station is recovered from the S-band receiver. 

 The recovered signal is applied to the response link receiver. The 

 recovered signal is applied to the response link Automatic Phase 

 Control and Multicoupler Chassis which compensates for spacecraft 

 oscillator variations, and divides the received CDA response signals 

 in order to drive the response channel data demodulators. Forty 

 data demodulators are presently implemented with the capacity to 

 expand at a future date. 



18 - 



