Quantum and Evolution Series Installation and Operating Handbook
8-1
Chapter 8
Modem Concepts
8.1 Doppler, Plesiochronous Buffering and 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 satellites 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 a elastic store or FIFO (First In First Out memory) gradually
emptying and filling to absorb the difference over the 24 hour period. If
the clock signals within the system all originate from one source, then
the buffer will never be exceeded - it will simply slowly fill and empty
over this 24 hour cycle.
Plesiochronous:
This refers to buffering needed when there is more than one clock in the
system and a buffer is required to absorb the difference between the
two clocks. Typically data is placed into the buffer with one clock, and
removed with another, thus causing a gradual
continuous
filling or
emptying of the buffer, causing slips on a regular basis. The period
between buffer slips (when it is totally full or empty) will be constant,
and proportional to the difference between the two clocks.
A buffer will provide both functions - it simply depends on how the system clocking is
arranged whether it provides Doppler buffering, or a combined Doppler and Plesiochronous
buffering (if there are two clocks in the system). Typically a buffer used for a
Plesiochronous system is set larger than a Doppler only buffer, as it has to accommodate a
steady filling/emptying plus a superimposed daily cycle.
The slips, both underflows and overflows are displayed as part of the status information.
Approximately equal counts indicate the buffer is too small to accommodate the incoming
Doppler shift (2ms is usually sufficient). One count significantly higher than the other
indicates that the buffer is slipping due to a large difference in the clock from the satellite
and the buffer output clock. With typical earth station clock accuracies, buffers never need
be larger than approximately 16ms.
