Technology
Spacecraft
 Adaptive
Rain Fade Compensation
The ACTS system was designed to compensate for fades in the
communications signals due mostly to rain so that communications
will not be lost except during the deepest of fade events.
This technique, adaptive rain fade compensation, is used in
the Baseband Processor mode of operation and involves reducing
the burst rate of the transmissions as well as adding forward
error correction coding to the signal. The combined effect
adds 10 dB's of margin, which is sufficient to ensure communications
through light to moderate rains. This technique ensures that
only those terminals experiencing fade are provided additional
protection.
The Baseband Processor mode terminals, such as a T1VSAT,
were designed with 5 dB of clear weather margin on the uplink
and 3 dB on the downlink. Terminals experiencing fade were
dynamically provided 10 dB fade more of link margin for protection.
When rain fades of sufficient depth were detected, the burst
rate of the terminal was reduced by a factor of four to improve
the link margin by 6 dB. Simultaneously, a rate 1/2 convolutional
coding was added to the signal to provide 4 dB’s more
of performance gain. Both of these happened automatically
and transparently to the user. Also, this compensation was
added independently on the uplink or downlink as is needed.
That is, the uplink may have been faded but not the downlink
at a particular location, so only the uplink was compensated.
Of course, there’s a trade-off in adding the compensation.
By reducing the burst rate and adding the coding, the TDMA
frame efficiency is greatly reduced.
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