[time-nuts] Looking for info about first true radio controlled clock

jimlux jimlux at earthlink.net
Wed Feb 23 15:33:21 UTC 2011


On 2/23/11 5:09 AM, Javier Herrero wrote:
> El 23/02/2011 05:53, jimlux escribió:
>> CCSDS time codes reference NASA 36 bit.. maybe a reference it's in the
>> back of the CCSDS standard.
>>
> First CCSDS.301 issue seems to be January 1987, and references (on the
> 4th issue, Nov 2010) listed at last page seems not very old. CUC
> recommended epoch, as per the standard, is 1 Jan 1958. CUC is longer
> than 36 bit (I have met it with 56-bit, but can be different), and no
> reference to NASA 36-bit code seems to appear there (except if I've
> overlooked it :) )
>
> Last issue can be downloaded here
> http://public.ccsds.org/publications/archive/301x0b4.pdf Some older
> versions are also available.
>

Yeah.. I just looked and saw the same.  Maybe some other document... I 
looked at the deep space 810-005 document, but didn't see it their 
either.  I'll look when I get to work.

Most of the DSN stuff uses CCSDS formats, although the Time Code 
Translator (TCT) boxes at the stations probably can handle NASA36 in 
some form.  The TCT is what provides all the timing signals in a station 
(it also distributes 100 and 10 MHz, and 1pps.. or at least the one in 
the rack in the lab downstairs from my office does.. there is, 
apparently, some variability in these things.. they're mfrd in small 
quantities)

I found a Landsat document from the 80s which mentions that NASA36 and 
IRIG-A were recorded on the tapes, but that the NASA36 track wasn't 
used.  It referenced a Data Format Control Book, Volume 5 (Payload), 
which is presumably some Landsat spec.  and in fact, I just found
landsat.usgs.gov/documents/L7-DFCB-07_Landsat_D_Payload_DFCB.pdf


If you want to hunt around.. I'd look for documents about the early days 
of TDRSS.  There might be a reference to a document that defines the 
Time-of-Day clock.  Back then (and even now), the whole data stream 
flows from one box to another to another to another, and timecode is 
essential to keep the data from multiple flows lined up.  Each box does 
one small function (e.g. demodulator, then decoder, then deframing, 
decommutation, etc.).  It's very, very much driven by legacy 
compatibility with analog patch cables for interconnects, etc. (i.e. it 
might be done by a digital processor today, but back in the 80s, when 
they wanted to replace the analog demodulator, they'd only have the 
budget to do just that piece.. so you'd build a digital rack mounted 
unit(s) that duplicated the interfaces of the thing you're replacing.

You just don't get the budget to do an end-to-end refit, and for that 
matter, figuring out new box level requirements from the top down is a 
huge task.  Much easier to keep the architecture the same, keep the 
interface requirements the same.





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