Page 139 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecrafr Applications
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The maximum transmission rate for the satellite is:
TR,,, = fJl0 = 6 x lo8 bps = 600 Mbps

which indicates that simultaneously the link could handle:

TR,,/TR (one call) = 25,000 calls

assuming that the baseband information for each call can be combined
into a single continuous bit stream. The methods used to accomplish
this are beyond the scope of this text, but notice that the maximum peri-
od for each bit on the carrier wave must be much less than that for each
call separately:

tb (one call) = t$n = 4.2 x sechit
tb (link) = 1.4 fb”cont~9 = l/TR,,, = 1.7 x sechit

This result reveals that less than two nanoseconds can be allotted for
each bit transmitted!

The following sections describe the basic digital modulation methods
and their similarity to the analog modulation techniques discussed earlier.

Amplitude Shift Keying (ASK). In this modulation method, the carrier
wave is either transmitted (representing a digital “one”) or not transmitted
(signifying a digital “zero”) each bit period. The relationships between the

CARRIER
WAVE

Figure 5-12. Amplitude shift keying modulation. The amplitude of the carrier
wave is used to represent the digital baseband signal.