Hi Zack,

The HLA Logical time can jump directly to any positive number when the federation starts. In practice most federates in a Space FOM simulation will need to support “late joining” anyway, since the Environment will be there all of the time.

While it is easier to have a fixed epoch, maybe there are reasons to be flexible in the standard, and allow for different epochs. The downside with this is that federates will also need to be flexible and be able to support different epochs (and find out which one that is currently in use).

So the bigger picture is, what should the relationship between HLA time stamps and simulation scenario time be? Fixed? Variable in some particular way? Fully flexible? And how should a federate support that?

Bjorn


From: SAC-PDG-SRFOM on behalf of Edwin Crues
Reply-To: SAC-PDG-SRFOM
Date: måndag 28 september 2015 19:58
To: "[log in to unmask]"
Subject: Re: About the use of time stamps for the Reference Frames

Hi Björn,

Number three will only work if the HLA federation execution time stamp can start at a number other than zero (0).  This issue isn’t how far back does the simulation need to start but that the simulation start epoch will almost always be different than whatever is chosen for the SRFOM fixed epoch.  I’d prefer not to tie the HLA time stamp to the simulation scenario time.

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 28, 2015, at 11:12 AM, Björn Möller <[log in to unmask]> wrote:
> The question is, is a reliable time stamp available for all time management schemes?
Yes
> If yes and we choose not to have a time stamp, then the next question would be where is the TT epoch provided? (assume you mean Time attribute)
Three options for TT epoch are:
• This is specified separately in some scenario definition, for example as a start up parameter or a common scenario file. This is error-prone.
• This is specified in a well-known object instance in the federation. This is flexible but requires some start up code.
• This is specified in the SRFOM (fixed). Our chosen time representation, the 64 bit signed integer representation, interpreted as microseconds, corresponds to more than 290 000 years. J2000 may be an option (or J1900). Or Jan 1 1970 for obvious reasons. How far back in time do we need to simulate?
Bjorn
From: SAC-PDG-SRFOM on behalf of Edwin Crues
Reply-To: SAC-PDG-SRFOM
Date: söndag 27 september 2015 20:21
To: "[log in to unmask]"
Subject: Re: About the use of time stamps for the Reference Frames
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




To unsubscribe from the SAC-PDG-SRFOM list, click the following link:
https://discussions.sisostds.org/index.htm?SUBED1=SAC-PDG-SRFOM&A=1