Re: [LEAPSECS] what is broken?

From: Fred Patt <>
Date: Thu, 21 Sep 2000 10:47:36 -0400

Good questions, not simple to answer.

Recall that I stated that multiple systems were affected. I have some
knowledge, not necessarily hands-on familiarity, with at least a few. One
heavily-used example is the Goddard Trajectory Determination System (GTDS),
which has produced orbit data for many NASA spacecraft and has roots going back
to at least the mid 1970's. Another is the U.S. Space Command SGP4 program,
which (in its older, public-domain version) is used as the basis for several
packages which use NORAD orbital elements as inputs. Our system (the SeaWiFS
Processing System) is less than 10 years old, and has a remaining lifetime of
perhaps another 10 years; but it too has code borrowed from an earlier system.
I guess my point here is that most of the systems at Goddard used for orbit
generation rely on legacy code at some level, so system lifetime is a difficult
quantity to measure.

UTC is, and has been, the standard time system for spacecraft at NASA, to the
best of my knowledge. I don't know what the standard was prior to 1972, since
that predates my involvement, but my guess would be an earlier UT variant. Nor
do I know specifically why UTC was chosen over TAI, since I don't have access to
that level of policy-making.

Practically speaking, though, it's not hard to understand. NASA coordinates
spacecraft support activities which include a global network of tracking
stations, and also exchanges information with a myriad of other organizations
world-wide. UTC is the most widely recognized time standard, and simplifies
these sorts of
communications. The NORAD elements mentioned above also use UTC. The one
downside is the bit of pain it causes every 1-to-2 years, when time stands still
but the satellites keep moving.

I hope this is helpful.

Fred Patt

Steve Allen wrote:
> On Tue 2000-09-19T15:58:49 -0400, Fred Patt hath writ:
> > I can provide such an example.
> Thank you. Now may we discuss?
> > At NASA/Goddard Space Flight Center, we run satellite orbit models which typical
> > generate ephemeris data on 1-minute intervals, synchronized with UTC. Our
> > spacecraft data streams are likewise time-tagged to UTC.
> What is the lifetime of this system?
> Why was UTC chosen rather than TAI?
> Was it merely that UTC was more conveniently available than TAI?
> If TAI were as conveniently available as UTC, would the same choice be
> made again?
> --
> Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064
> Voice: +1 831 459 3046
> PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93

* Frederick S. Patt
* SAIC General Sciences Corporation
* Code 970.2, Goddard Space Flight Center
* Greenbelt, MD 20771
* Vox 301/286-5723 * Fax 301/286-0268
* Email
Received on Thu Sep 21 2000 - 07:59:54 PDT

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