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SAC-PDG-SRFOM  September 2015

SAC-PDG-SRFOM September 2015

Subject:

Re: About the use of time stamps for the Reference Frames

From:

"Crues, Edwin Z. {Zack} (JSC-ER711)" <[log in to unmask]>

Reply-To:

SAC-PDG-SRFOM <[log in to unmask]>

Date:

Sun, 27 Sep 2015 18:21:01 +0000

Content-Type:

text/plain

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text/plain (1 lines)

Hi Björn,

Here’s the semantics of the time stamp attribute:

> This value serves as a 'time stamp' that specifies the simulated time (TT) to which the attributes values correspond. It may be used by federates that do not use HLA time management but still need to know when the attributes were valid. (E.g., a plotting federate that isn't time regulating or time constrained would need the time stamp in order to plot time series.)

TT is the Terrestrial Time time scale that corresponds to as close to a dynamical time as we have near the Earth. If the federate is time constrained then you are right, the TT time stamp should be a constant offset from the Federation Execution HLA time stamp. However, it’s not clear to me that an reliable HLA time stamp is available for all time management schemes. Because of this, we’ve included a TT time stamp for all the dynamic data. This is important for systems that rely on models that are tied to the scenario physical time (e.g. planetary positions from and ephemeris database).

If we don’t include a time stamp, we would at least need a TT epoch attribute that specifies that constant offset in at least one universally accessible object instance. Then to determine the TT physical time associated with the data the user would have to have retrieve the HLA timestamp, read or have stored the TT epoch from the object instance that holds it, and then compute the TT time for the data.

The question is, is a reliable time stamp available for all time management schemes? If yes and we choose not to have a time stamp, then the next question would be where is the TT epoch provided?

Does this address your question?

Zack
*******************************************************************
* Edwin Z. Crues, PhD. | Phone: 281.483.2902 *
* Mail Code: ER7 | Mobile: 832.341.9023 *
* 2101 NASA Parkway | FAX: 281.244.6116 *
* NASA Johnson Space Center | Email: [log in to unmask] *
* Houston, Texas 77058, USA | *
*******************************************************************

> On Sep 24, 2015, at 10:53 AM, Björn Möller <[log in to unmask]> wrote:
>
> We had a discussion today about the use of time stamps versus the Reference Frame object class attribute “Time”. I tried to understand why there is a time stamp attribute and at the same time, logical time stamps are delivered as part of Reflect Attribute Value and Receive Interaction callbacks.
>
> Note that if an attribute update or interaction it sent with an HLA time stamp, then it will always be delivered with a time stamp, no matter if the receiving federate is time regulating or constrained. It doesn’t matter if the message is specified as Time Stamp Order (TSO) or receive order (RO).
>
> The above was not true for HLA 1.3 (that dropped timestamps in some cases), but this has now been fixed since HLA 1516-2000.
>
> I got the impression that the HLA timestamps are microseconds since the beginning of the execution (scenario time rate) and that the Time attribute provides physical scenario time. As an example this would mean that if we are simulating a scenario starting at physical time 17 (seconds) then we would have the following state after simulating 0.1 scenario seconds:
>
> HLA timestamp: 100 000
> Time attribute: 17.1
>
> Did I get it right?
>
> Bjorn
>

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