P300H P300 Series Modem Installation and Operating Handbook Page 140
8 APPLICATION NOTES
8.1 DOPPLER & PLESI OCHRO NO US 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 t he earth and
is directly proportional to its station keeping accuracy. Towards the end of a
satellites life when its s tation keeping is degraded to save fuel, the Doppler shift
on incoming signals increases and buff ers may have t o be increase d in size. The
receiv e b uf fe r ac t s a s a elastic store or FIFO (First In Fi rst Out memory) gradually
emptying and filling to absorb the difference over the 24 hour period. If the clock
signals within t he sy stem all originate fr om 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 w hen 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 pla ced into the buffer with one clock, and removed with another, thus
causing a gradual continuous filling or emptying of the buffer, caus ing slips on a
regular basis. The period between buffer slips (when it i s totally full or empty) will
be constant, and proportional to the difference between the tw o clocks.
A buffer will provide both fu nctions , it simply depends on how the sys tem clocking is arranged whether
it provides Doppler buffering, or a combined Doppler and Plesiochronous buffering (if there are tw o clocks
in the s ystem). Typ i c a lly a buffer use d fo r a P le s i ochronous sy st em 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 buff er output clock. W ith typical earth station
clock accuracies, buffers never need be larger than approximately 16ms.