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NoiseThe problem of noise occurs when the signal is corrupted. This happens when other signals are added to it (often heard as a hiss or buzz), or it is reflected off a building and interference between the original wave and the reflected wave causes the waves to cancel out. These events can produce either a varying amplitude or such a low amplitude that the information can no longer be read. At Surrey Space Centre the problem of noise is solved by using 'packet
switching'. The digital data (1s and 0s) are assembled into groups
of bits. The first group of bits indicate which satellite is sending
the message and where it is for (e.g. UOSAT 2 to Mission Control). Then
there's a series of bits that say what packet is being sent. Next come
the actual data. Finally there's a checking procedure. The satellite
sends a 'cyclic redundancy check code' or a parity code. This is a code
word consisting of 2 bytes related to the data sent. If the code word
is recognised then the data have not been corrupted. If the code word
is not readable then the data are probably also corrupted and Mission
Control sends a reply saying 'send again' until the data are read correctly. |
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