Re: [LEAPSECS] Comments on Civil Time decision tree

From: William Thompson <>
Date: Tue, 27 Sep 2005 17:46:44 -0400

Randy Kaelber wrote:
> On Tue, Sep 27, 2005 at 03:56:07PM -0400, William Thompson wrote:
>>The spacecraft that I've had experience with coordinate the spacecraft clocks
>>with Earth-based time standards.
>>The Solar and Heliospheric Observatory (SOHO) spacecraft (which at a distance of
>>0.01 A.U. can be considered to be on the edge of interplanetary space)
>>synchronizes its onboard clock to TAI time, expressed as the number of TAI
>>seconds since 1 January 1958. The spacecraft operators keep track of the clock
>>drift, taking into account the approximately 5 second light travel time, and
>>periodically uploads new clock frequency parameters to keep the onboard clock in
>>sync with TAI to within a specified requirement.
> I suppose I should've prefaced with an "in my experience". :-)
>>The upcoming Solar Terrestrial Relations Observatories (STEREO) go in orbit
>>around the Sun, and are thus definitely interplanetary. They also use the JPL
>>SPICE system, and thus spacecraft SCLK files, like other interplanetary
>>missions. The STEREO clocks will be synchronized to UTC, including an
>>adjustment for leap seconds.
> Ugh. Is there a compelling science or operations reason to try to synch
> this clock with terrestrial times on-board that I'm just ignorant about?
> It sounds like a more work and more things that can break, versus just
> profiling the on-board clock and making SCLK kernels to map back to
> terrestrial time frames. Maybe the exact times aren't so important? With
> Odyssey and THEMIS, if we're off by a second, it's a 30 pixel along-track
> offset error in our images, so we're pretty obsessed with knowing exactly
> when we start and stop imaging.

On the contrary, exact times are extremely important in the STEREO project.
Images from the two spacecraft are supposed to be synchronized to each other to
better than a second. (The images from one spacecraft are actually delayed
relative to the other to take into account the different solar distances of the
two spacecraft--the amount of delay is periodically uploaded from the ground.)
The accuracy requirement for the delivery of UTC to the instruments is +/- 0.410

Each instrument team commands its own instrument directly. Time-tagging is done
via UTC. I sincerely doubt that the individual instrument commanding
workstations know anything about SCLK files. The orbit and attitude files are
provided in SPICE format, which is probably a first for these teams, and I've
been leading the effort to learn how to use the SPICE kernels.

I don't have the number in front of me, but I believe that the timing
requirements for the SOHO spacecraft is even more stringent, on the order of 0.1
seconds. This is to support the helioseismology instruments. SOHO doesn't use
SPICE, and thus does not have an SCLK file.

The telemetry timestamps from both SOHO and STEREO are in TAI and UTC
respectively, referenced in both cases to 1-Jan-1958. For the STEREO
spacecraft, which uses UTC, one has to interpret this as the number of non-leap
seconds, as is done with Unix/NTP time. There's a one-second period of
ambiguity on the STEREO spacecraft whenever a leap second is inserted, and time
critical operations will be avoided during that second.

SOHO uses TAI time onboard the spacecraft, but all ground operations are done in
UTC time. I wrote the software that many of the instrument teams use to convert
between UTC and TAI in their data analysis software, as well as some of the
commanding software.

> I can see SOHO using a geocentric time. It's relatively close to Earth and
> holds a more or less constant distance from it. It seems that STEREO is
> going to have a more complicated relationship with a geocentric coordinate
> frame.

I'm not sure what the extra complication is. For both missions, one has to deal
with a significant light travel time, much larger than the required time
accuracy. The process of taking this into account is essentially the same, no
matter where you are in the solar system, and no matter whether you feed the
results back to the spacecraft, or simply take it into account on the ground.

Bill Thompson

William Thompson
NASA Goddard Space Flight Center
Code 612.1
Greenbelt, MD  20771
Received on Tue Sep 27 2005 - 14:48:29 PDT

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