the demodulator output. Unfortunately, the switching rate 

 of the electronically despun satellite antenna is fairly 

 close to the data rate; therefore, the decoder must be able 

 to differentiate between valid data and spin noise. 



Manchester Coding : The DCS uses Manchester coding to transmit 

 data through the system. This type of coding has the 

 advantage that the clock rate is imbedded in the data stream 

 and can be recovered from the data by the decoder. It is, 

 therefore, not necessary to transmit the clock information 

 separately or to impose extreme stability requirements on 

 the oscillator at the remote platform. 



Manchester coding may be described in terms of digital logic 



as the Exclusive OR function between the clock and data. 



The Truth Table for the Exclusive OR function is given below: 



TRUTH TABLE 

 EXCLUSIVE OR 



. DATA 



As can be seen, the output is zero if the clock and data are both 

 one or zero, but is one if the clock and data are opposite. The 

 advantage of this may be seen in the timing diagram (Figure 3). 

 Regardless of the input data, the Manchester-encoded data will 

 change state once and only once during each input data bit. These 

 changes may be used to synchronize the decoder clock. In this 

 manner the remote platform clock information will be carried along 

 with the data. 



3. Pattern Sensitivity : As stated above, the data collection 

 platforms use a Manchester Code to transmit data. The 

 decoder must be capable of extracting the clock rate from 

 the data in the presence of the spin noise described above. 

 Furthermore, it must be able to do this with any possible 

 data bit pattern. The GOES DCS has standardized on the use 

 of ASCII code with odd parity. The decoder, therefore, must 

 be able to accommodate data having as few as two "bit 

 transitions" per 8-bit character. Otherwise, loss of 

 synchronization will permit errors to be received until 

 the decoder resynchronizes . Unfortunately, consecutive 

 zeros can represent valid data from some types of platforms. 

 Some of the electronic techniques which can be used to 

 minimize the spin modulation noise complicate the clock 

 slip problem. 



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