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 frame. -- Randy Kaelber [EMAIL PROTECTED] Scientific Software Engineer Mars Space Flight Facility, Department of Geological Sciences Arizona State University, Tempe, Arizona, USA
