Re: [LEAPSECS] Comments on Civil Time decision tree

From: William Thompson <William.T.Thompson.1_at_GSFC.NASA.GOV>
Date: Tue, 27 Sep 2005 16:10:55 -0400

Randy Kaelber wrote:
> On Mon, Sep 26, 2005 at 02:33:00PM -0400, Daniel R. Tobias wrote:
>>I would suppose that such a space probe would have little need to be
>>synchronized with earthly solar time, and thus might be best off
>>operating on TAI, with any adjustments to UTC for the sake of humans
>>observing it on Earth being done at the Earthly end of things.
> That's the way we do it for interplanetary stuff now. Data from
> spacecraft are typically returned in spacecraft clock time (SCLK, which is
> pronounced "sclock") and then translated to whatever time base you want it
> in. Right now, the clock on Mars Odyssey (as I type this) should be
> reading 2/0812228033. Dealing with things like leap seconds, local time
> conventions, and other time conversions are all handled here on Earth.

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.

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.

For both of these missions, the Earth-based time values, TAI or UTC, are
calculated and maintained onboard the spacecraft.

Bill Thompson

P.S. Please excuse me if you get this message twice. I was having trouble with
my subscription setup, and wasn't sure if it really went out the first time.

William Thompson
NASA Goddard Space Flight Center
Code 612.1
Greenbelt, MD  20771
Received on Tue Sep 27 2005 - 13:21:46 PDT

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