CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS
CCSDS 130.2-G-3 Page 5-2 September 2015
5.4 CAN COP-1 BE APPLIED TO DEEP SPACE COMMANDING?
Yes. In deep space, however, a special technique is used to transmit a sequence of Transfer
Frames reliably over a space link with a very long round-trip light-time delay.
Reliable transmission uses the Sequence Controlled Service of COP-1, which depends on a
feedback loop via the telemetry link. The COP-1 sender retransmits a missing Transfer
Frame and all the subsequent Transfer Frames, but only after it has received a report of the
missing frame, or after its timer has expired. Usually the value of the COP-1 timer is set to
the expected round trip delay, but for deep space this can leave the uplink unused for long
periods. Performance can be improved by transmitting a sequence of Transfer Frames
multiple times, without waiting for the round trip light-time delay to elapse. Two methods
are available:
– the value of the timer used by the COP-1 Sequence-Controlled Service can be set to a
lower value, so that retransmission happens earlier;
– the Synchronization and Channel Coding Sublayer has an option for multiple
transmissions of a Transfer Frame.
The two methods can be used separately or in combination (for details, see reference [20]).
At the receiving end, COP-1 can deliver the SDUs (user data) contained in the received
sequence of Transfer Frames to the receiving user in sequence and without duplication using
the standard procedure of COP-1, even though the sequence of Transfer Frames has been
transmitted multiple times.
5.5 CAN COP-1 BE APPLIED TO BLIND COMMANDING?
Yes. It is sometimes necessary to transmit commands when telemetry is not available. In
such cases, the Expedited Service of COP-1 (see 4.4.2.3) can be used, because it does not
need the feedback loop via the telemetry link that is required for proper operations of the
Sequence Controlled Service. If the Expedited Service is used, however, there is no
guarantee that all transmitted data is delivered to the receiving user. Since transmitted
Transfer Frames are always protected by the error correction/detection capability of the
Synchronization and Channel Coding Sublayer, data delivered to the receiving user has a
very high probability that it is error-free, but there may be gaps in the data because Transfer
Frames containing errors not correctable by the Synchronization and Channel Coding
Sublayer are discarded and not retransmitted.
Alternatively, the Sequence-Controlled Service of COP-1 can be used with the multiple
transmissions described above (see 5.4). At the receiving end, the Sequence-Controlled
Service delivers the SDUs (user data) contained in the received Transfer Fram
es without
duplication, even though the Transfer Frames have been transmitted multiple times. For a
deep space mission, this can be more efficient than using the Expedited Service.