[evla-sw-discuss] EVLA WIDAR: Delay Model - format and content

[Barry Clark]

There's a lot of content here, and I suspect I'll have much more to say
when I've had time to do some work on these (I'm a bit tied up at the
moment in my other hat as scheduling officer).  However, a couple of 
remarks that occurred to me on a rather cursory reading.

1.  Number of derivatives.  We are working with two derivatives
at the moment.  This seems to work pretty well for VLA antennas tracking
translunar objects.  (And the code can be easily changed to handle NMA 
antennas.)  The cases that probably require more (VLBI and satelite 
tracking) have not had any serious design work done on them, so it's a 
little unclear how we want to handle them.  I strongly suspect that they 
also will turn out to have a number of derivatives set by the application, 
not something decided on the fly.  My preference would be that the number 
of derivatives be moved to a setup message, not sent with every delay model.
(Presuming that it's needed at all, that it's inconvenient to just use
whatever arrives.)

2.  Double.  Why the preference for a hex encoded double, instead of the
native 'double' form?  (Just in case somebody decided to read one of these
godawful things.)
> Each coefficient is 64 bit (8 byte) floating-point number encoded as hexBinary.

3.  Timestamps.  We work internaly in a double precision MJD and fraction
system (LSB is about one microsecond).  Converting that to ISO 8601 form 
is easy of course, but I rather suspect that you also will be working in 
some similar system internally, and it might save a few a little bother at 
both ends just to send that.

4.  Delays.  Who organizes the delay center?  Do I send numbers which might
be negative, or should I add something to them to make them always positive?
If so, what?