[CMC] CMC Action : A-2015-11-10

Nestor.Peccia at esa.int Nestor.Peccia at esa.int
Mon Feb 1 17:34:24 UTC 2016

Dear all,

The CMC placed upon the CESG Chair the above mentioned action to clarify 
the Time Transfer, Clock correlation and Time Synchronisation Projects 
from the ICPA / IOAG Presentation.

You will find below an extract from the IOAG Service Catalogue 2, which is 
self explanatory


Time Services Group. These services allow the calculation of time 
correlation elements and synchronization by means of time distribution.

IOAG Service Group
IOAG Service Types
Space Link Interface
Ground Link Interface
Time Services
Time Synchronization Service
Time Transfer (TT) (*)
·       Clock Correlation (CC) (*)
·       Time Synchronization (TS) (*) 

(*) Provides capability that is more auxiliary and secondary to the main 
Space Internetworking standards. Besides, work-around solution is 
available now.

In an attempt to reduce ambiguity, a specific terminology is used that may 
differ from common practice used in some space flight organizations. The 
term ?clock? means some instrument used to indicate, measure, keep, and/or 
co-ordinate time (on spacecraft this is most often a counter driven by 
some accurate, but free running, oscillator) while the term ?time? means a 
reading of such counter at a point in time and, in particular, also to 
relate that time to a specific universal timescale. Formalized definition 
of all of these terms is the province of the standardization 

Up until now missions have typically used private means for doing clock 
correlation between a free-running spacecraft clock and ?Earth time?, by 
computing the relationship between the spacecraft local clock and some 
universal timescale such as UTC. Typically this is done by recording time 
data at specific events that are observable both on the spacecraft and on 
ground (e.g., leading edge of the first bit of the Attached Sync Marker of 
a clearly identifiable telemetry frame) and then noting the time-tags both 
at transmission from the spacecraft and at reception on ground. A clock 
correlation process is used on the ground to establish the relationship 
between the spacecraft clock and Earth time. This process requires precise 
knowledge of the position spacecraft at the observed event so that the 
propagation delay induced by the signal path can be compensated.

More formally: ?time transfer? is performed by exchanging time data 
formatted according to some agreed ?time code?, while ?clock correlation? 
uses the data exchanged by ?time transfer? in order to determine the 
offset between the clocks at the sending and receiving ends. Clock 
correlation does not imply alignment of clocks, but it may require 
knowledge of clock stability, skew, and drift and most importantly the 
propagation delay induced by the signal path(s) involved in the time 
transfer. Clock correlation may also involve comparison of local clock 
times to some standard high precision clock that accurately reflects a 
common timescale such as UTC.

Synchronization of activities on multiple spacecraft may require time 
synchronization of their clocks to the same universal timescale, e.g. UTC. 
Depending upon the application and the environment, various means may be 
applied to either update the local clock (clock synchronization) or just 
to maintain knowledge of the offsets between the spacecraft clock and the 
universal timescale (clock correlation).
The essential process for time synchronization relies on time transfer. 
The time data are used to calculate the offset between the two clocks with 
or without eventually performing clock synchronization. A generalized 
diagram for this is shown in Figure 4-1.

The IOAG Time Synchronization Service will allow aligning clocks to a 
common timescale, such as UTC and it requires both clock correlation and 
time transfer be performed.
For transferring timing information one of the formats defined in [TCF, 
CCSDS 301.0-B-3 Time Code Formats. Blue Book.] standard shall be used.
The Time Synchronization Service relies on the following standards:
Clock Correlation Procedures [CC]
Time Transfer [TT]
Time Synchronization [TS].

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