Q-Flex Satellite Modem Installation and Operating Handbook
7-1
Chapter 7
Modem Concepts
7.1 System Clocking
There are two terms commonly used when referring to the receive buffering, namely
Doppler
and
Plesiochronous
.
Doppler:
Receive Doppler buffering is required because the incoming data from
the satellite is subject to Doppler shift, which occurs in an approximately
24-hour cycle. The Doppler shift is due to the satellite movement
relative to the earth and is directly proportional to its station keeping
accuracy. Towards the end of a satellite’s life when its station keeping is
degraded to save fuel, the Doppler shift on incoming signals increases
and buffers may have to be increased in size. The receive buffer acts
as an elastic store or First-In-First-Out (FIFO) memory gradually
emptying and filling to absorb differences over the 24-hour period.
A design aim should be to have all clock signals within the system
originating from one master source in order to ensure that the buffer
does not underflow or overflow during the 24-hour cycle
Plesiochronous:
This is where there is more than one clock in the system and a buffer is
required to absorb the difference between the clocks. Data is typically
placed into the buffer with one clock and removed with another, causing
continuous filling or emptying of the buffer and resulting in buffer slips
on a regular basis. The period between buffer slips will be constant, and
proportional to the difference between the two clocks. Each buffer slip
will result in a temporary loss of data and therefore this clocking
scheme is not ideal.
The buffer itself is independent of the clocking scheme and will support all methods of
system clocking. With typical earth station clock accuracies, buffers should never need to
be larger than 16ms.
