Re: [LEAPSECS] Comments on Civil Time decision tree

From: Randy Kaelber <randy_at_MARS.ASU.EDU>
Date: Tue, 27 Sep 2005 13:29:57 -0700

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.

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

Randy Kaelber                              
Scientific Software Engineer
Mars Space Flight Facility, Department of Geological Sciences
Arizona State University, Tempe, Arizona, USA
Received on Tue Sep 27 2005 - 13:33:47 PDT

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