1.2.9 What Data Error Probability Can the DCS User Expect? 



Under reasonable conditions, the user can expect to obtain a bit 

 error probability of 10 ^ or better, using data received at the 

 CDA and provided to the user at the Central Distribution Facility 

 (CDF) at Camp Springs, Maryland. 



Various factors affect the data quality. Some of these are determined 

 by the GOES system design, and some are related to instantaneous 

 spacecraft usage and propagation conditions. Appendix D treats the 

 considerations involved in reception at Earth terminals other than 

 the CDA. 



The spacecraft down-link power at S-band is shared between the CDF 

 response and occasional TIROS-N data relay transmissions. As the 

 number of DCP ' s increases, somewhat less power is available to each 

 simultaneously active channel. Furthermore, simultaneous operation 

 of the VISSR subsystem reduces the total power available to the DCP/ 

 TIROS-N down-link transmission by 4 to 5 dB. Sufficient power margin 

 is provided in the system design to accommodate full anticipated loading 

 under the above worst-case power sharing conditions when utilizing the 

 CDA or an equivalent performance ground terminal. 



Other factors that can affect data quality ^ are the DCP ' s geographic 

 location with respect to the spacecraft, as losses are slightly higher 

 at the "Earth's edge"; multipath propagation, in which DCP signals 

 reflected from the ground, ocean, etc., interfere with direct signals; 

 and ionospheric scintillation, which may occasionally produce greater 

 than normal path attenuation. Maintenance of the DCP, adjustment of 

 the DCP antenna positioning, and provision of an unobstructed path 

 to the GOES are essential to minimum-error performance. 



1.2.10 Can a User Achieve Direct Data Readout From the GOES 

 Satellite? 



Any user, with discretion, may implement a data collection ground 

 receiving terminal and thereby achieve direct readout of the S-band 

 reply data from the GOES satellite. This secondary data collection 

 terminal enables a user to be independent of the primary CDA data 

 collection receive terminal. However, the user will be required to 

 adhere to the channel assignments and schedules coordinated for the 

 GOES system by NESS. The design constraints and characteristics 

 required of such a secondary user-owned data collection terminal are 

 presented in Appendix D. 



At the CDA, the present demodulators are sensitive to data patterns, 

 and some characters of the ASCII set cause difficulty. This 

 problem is being remedied. 



- 8 - 



