size by the same amount (4.6dB and a reduction in antenna area by 

 the same amount (4.6dB = a reduction of 1.7 in antenna diameter). 

 Hence, for the single channel case, the required diameter becomes 



2.5 meters/1.7 = 1.47 meters and for the 100-channel loading case, 



4.6 meters. 



These values can be used as points on two curves, one curve for a 

 single channel loaded DCS, and the other for a fully loaded system. 

 Points for the other types of preamplifiers can be plotted, and a 

 smooth curve drawn between them. 



Figure 2 shows a family of curves giving the required antenna 

 diameter as a function of system noise temperature for various 

 numbers of simultaneous active channels. In using these curves 

 the following conditions should be noted. 



1. The abscissa (noise temperature) is the overall 

 receiving system noise temperature and is the sum 

 of the preamplifier noise temperature and the 75 K 

 cosmic noise. The ordinate gives the required antenna 

 gain. 



2. A 3dB margin, as well as miscellaneous losses of 3dB, 

 are included. 



3. All calculations are based on the receiver being located 

 at the subsatellite point. 



For convenience. Tables 1 and 2 give the margin obtained with some 

 typical preamplifiers and antenna sizes of 3 and 10 meters. A 

 wide variation in the available margin is evident. 



All users must bear in mind that the National Environmental Satellite 

 Service (NESS) cannot guarantee that usage of the DCS will be 

 limited to any maximum number of platforms, and therefore cannot 

 guarantee a minimum signal level at the Earth's surface. Users 

 must, therefore, carefully consider the consequences of lost or 

 inaccurate data against the costs of building a better ground 

 station. 



V. DEMODULATOR/DECODER 



To recover the data, the output of the receiver must be demodulated 

 and decoded. A separate demodulator/decoder should be used for 

 each DCS channel in use, although a single antenna-receiver 

 combination can be used to drive a number of them. 



It is recommended that the demodulator/decoder be purchased from 

 a manufacturer experienced with the GOES DCS. Four problem areas 

 must be considered; these are: 



1. The Spin/Phase Noise : As described earlier, the spacecraft 

 rotation causes spin modulation which appears as noise at 



57 



