[Css-csts] FW: Monitored Data CSTS RID "Generation Time for GET"
Margherita.di.Giulio at esa.int
Margherita.di.Giulio at esa.int
Wed Dec 3 17:06:43 UTC 2014
Dear John,
I have been thinking a bit about your options for timestamping of
parameters delivered via a GET return.
In my view the spirit of Mauro Pecchioli?s RID was that, actually, each
individual CSTS parameter data gets its own generation time tag, rather
than the complete PDU and all its contained parameter data. The reason for
this was, that the parameter data may be useful in the context of
troubleshooting analyses implying correlation with spacecraft telemetry
data.
However, the answer to MPs RID should be that it would be very complex to
provide a timestamp to each and every parameter of a ground station, for
the following reasons:
- for every parameter there are various elements involved in its
sampling: from the basic sensor/instrument, to the relevant application
in charge of it ( e.g. an Antenna?s Front-end Controller) and to the
Station Manager application. In this scenario, who would be in charge of
generating the proper time stamp of the parameter? The value may be very
different depending on who in the chain will generate it ( and therefore
it may become unreliable)
- as a consequence of that, ground station parameters can only to a
minor extent be used for troubleshooting of mission or spacecraft
anomalies. At ESA, the application in charge of monitoring and control of
the Ground Station (the Station Computer) does not have the capability to
handle history data ( i.e. data with an associated time stamp). It only
has the real-time picture of the Stations situation at any given point in
time. In case troubleshooting of operational anomalies is required, other
means are normally used ( e.g. log messages, events, etc).
In the light of the above, I spoke to Mauro Pecchioli, and he agreed that
there is no point in requesting such timestamping of individual
parameters. Also, I outlined there is no point in having a single
timestamp for the whole GET return PDU, as this adds very little
information. Mauro agrees to that.
So, I would say that the RID can be rejected (or ?withdrawn by the
author? I am not sure which is most appropriate). Mauro is in copy. He may
confirm if this is ok with him.
Please let me know if this ok with you.
Kind regards,
Margherita
-------------------------------------------------------------
Margherita di Giulio
Ground Station Back-end Section (HSO-GIB)
European Space Agency ESA/ESOC
Robert-Bosch-Str. 5
D-64293 Darmstadt - Germany
Tel: +49-6151-902779
e-mail: Margherita.di.Giulio at esa.int
From: John Pietras <john.pietras at gst.com>
To: "CCSDS_CSTSWG (css-csts at mailman.ccsds.org)"
<css-csts at mailman.ccsds.org>,
Date: 19/11/2014 17:38
Subject: [Css-csts] FW: Monitored Data CSTS RID "Generation Time
for GET"
Sent by: css-csts-bounces at mailman.ccsds.org
CSTSWG colleagues --
You have already received Mr. Pecchioli's reply (copied below) to my
questions regarding his suggestion to add a Generation Time parameter to
the GET Return. He states that for troubleshooting purposes it may be
necessary to have the measurements timestamped to within an accuracy of a
few milliseconds. He also goes on to say that if multiple parameter values
are queried via a single GET invocation, then each value may have to have
an individual timestamp. Here are my thoughts on the matter.
Regarding the need for a timestamp with accuracy to within an few
milliseconds, if we agree that the MD-CSTS should be applicable to his
troubleshooting use case, then we can add an appropriate generationTime
parameter.
However, I don't believe that his assertion that each queried parameter
has its own timestamp in the Return necessarily holds true. We said in our
meeting last week that the assumption is that each parameter value that is
returned is captured at the time that the GET Return is generated. In
other words, every value in the GET Return would have been sampled no more
than some few (TBD) milliseconds before the time that the GET Return was
created. I would make that requirement explicit in the procedure
specification (although at this point I'm not exactly sure what the best
way would be). A follow-on question is, should the MD-CSTS specification
set the specific limit, or simply say that it is an implementation
parameter that must be documented in the PICS Proforma (and then add it to
the PICS Proforma)?
If we do decide to add the generationTime parameter to the GET Return,
should that be only an MD-CSTS extension of the Cyclic Report procedure,
or should it be a general capability of the CSTS SFW Cyclic Report
procedure?
Finally, I have been thinking about the statement that we made that the
MD-CSTS shouldn't require a second Red Book review. However, we are
considering adding this new capability, and we have already agreed to
modify the Cyclic Report procedure to allow on-change-only reporting.
These are significant technical changes, and I don't know that we can
justify the "no Red-2 Agency review" claim.
I welcome your comments on Mr. Pecchioli's reply and my proposed response.
If you have comments, I would like to ask you to provide them (please copy
the CSTSWG) by 1 December so that we can come to a final resolution of
this RID in a timely manner. Thank you.
Best regards,
John
-----Original Message-----
From: Mauro.Pecchioli at esa.int [mailto:Mauro.Pecchioli at esa.int]
Sent: Wednesday, November 19, 2014 4:47 AM
To: John Pietras
Cc: CCSDS_CSTSWG (css-csts at mailman.ccsds.org)
Subject: Re: Monitored Data CSTS RID "Generation Time for GET"
Dear Mr. Pietras,
thanks for contacting me. The main reason why I raised the suggestion to
add the Generation Time in the GetReturn is to harmonise the definition of
this PDU with the TransferData PDU and avoid unnecessary differences in
the associated processing of parameter data on the receiving side.
However, you raise an important point about the 'time-stamping' accuracy
of parameter samples. When using this data for standard monitoring (e.g.
range checking, live visualisation, etc.), I believe that an accuracy in
the order of few seconds is adequate. But I assume that the parameter data
may also be useful in the context of troubleshooting analyses implying
correlation with spacecraft telemetry data, in which case I believe that
an equivalent accuracy in the time-stamping is required. It should be
noted that we typically require an accuracy of a few milliseconds for the
spacecraft telemetry parameter samples time-stamping. This may imply that
each individual CSTS parameter sample is associated to its own generation
time, rather than the complete PDU and all the contained parameter data.
I hope this helps
Best regards
M. Pecchioli
From: John Pietras <john.pietras at gst.com>
To: "mauro.pecchioli at esa.int" <mauro.pecchioli at esa.int>,
Cc: "CCSDS_CSTSWG (css-csts at mailman.ccsds.org)"
<css-csts at mailman.ccsds.org>
Date: 17/11/2014 21:15
Subject: Monitored Data CSTS RID "Generation Time for GET"
Mr. Pecchioli,
Hello, I?m John Pietras. I?m the book editor for the CSTS Monitored Data
Service Red Book. Thank you for reviewing the book.
One of your RIDs, titled ?Generation time for GET?, stated:
?I suggest to add the generationTime parameter also for the GET Return PDU
(similarly to the TRANSFER-DATA PDU). This is useful for the receiving
side to know when the parameter value has been sampled.?
The CSTS Working Group discussed this RID and decided that we need a bit
more information about your intentions for the recommended changes before
we could decide on how to proceed.
Here is a little bit more background information that will hopefully help
us come to a mutual understanding.
In the RID, you use the TRANSFER-DATA as an example of an invocation
carrying a generationTime. However, there is a difference between the
generation time of a TRANSFER-DATA invocation and a GET return, in that
(in some cases,
anyway) a TRANSFER-DATA invocation may be stored in a Recording Buffer,
and so the generation time is disjoint from the actual delivery time. In
contrast, the GET return is always sent in immediate response to the GET
invocation, so the generation time of the parameters can be assumed to be
the time of receipt, minus some time for transmission through the
underlying communications network. Furthermore, since the GET returns are
all delivered in-sequence by the underlying TCP connection, there is no
question as to the sequencing of the returns.
Therefore, adding a generationTime parameter to the GET return would be
needed only if the generation time that can be estimated from the receipt
time (i.e., normally within a second or two) is insufficiently accurate
for an Agency?s (or Mission?s) purposes. Is there is a requirement to know
the generation time with greater accuracy than within 2 seconds of the
receipt time of the return?
The CSTS WG is considering extending the GET return set of parameters to
include a single generationTime parameter that applies to all values
carried in the return . Note that this would be the generation time of the
GET return itself; it would provide no indication of the actual freshness
of each of the parameters being reported. The assumption is that all
values being reported are current values.
If we were to add a generationTime parameter to the GET return, it would
have to be with the understanding that all values being returned in the
GET return are current as of the time that the GET return is generated and
sent. That is, it would not be possible (without major reworking of not
only the MD-CSTS but the CSTS Framework as a whole) to have measurements
taken at various times sitting and waiting to be retrieved via the GET
operation. If a measurements is ?stale? by any appreciable amount of time,
it negates the usefulness of having the generationTime in the GET return.
So, to summarize:
1. To your understanding, is it sufficient to know that the
time
of a measurement to within 2 seconds of its true generation time? If
so, then no change to the MD service would be required.
2. If it is not sufficient, we could put a single
generationTime
parameter in the GET return. This generationTime would apply to all
values in the return, which implies that all values contained in the
return are sampled essentially instantaneously at generationTime.
Would
that suit the purposes for which you envision the generationTime
parameter?
Thank you in advance for your help in refining and resolving this RID.
Best regards,
John Pietras
Principal Engineer
GST, Inc.
7855 Walker Drive, Suite 200
Greenbelt, MD 20770
240-542-1155
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