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P300H P300 Series Modem Installation and Operating Handbook Page 140
8 APPLICATION NOTES
8.1 DOPPLER & PLESIOCHRONOUS BUFFERING
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 slowing filling and emptying 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 (-Ve) and Overflows (+ve) are displayed on the Detailed Rx Status screen.
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.