Dannt, There is a short briefing on NTP in space on the project page. The first use of NTP in space was on an AMSAT satellite in the early 80s. NTP has been used on Shuttle missions via UHF and TDRSS relay to NASA Goddard servers. I have Shuttle data that show more problems with the laptop running NTP than with NTP itself. Shuttle astronauts are really busy people and don't have time to explain occasional step adjustments.
On symmetry, etc. There are both gravitational and velocity corrections relative to both Earth and solar system barycentric time amounting to some 15 ms, but current space missions don't worry much about that. The mission times are relative to a clock onboard the spacecraft; nothing else matters. Our Earth-Mars simulations didn't worry about these effects either. I suspect disciplining an orbiter/rover clock to these corrections will be dominated by the frequency noise of affordable oscillators. Maybe not. On accuracy. Ham radio EME "Moonbounce" experiments can calibrate Moon range to the millimeter, so there is no reason why NTP could not achieve comparable accuracy as on Earth. There is some talk about using GPS "backwards" for navigation on the Moon, but considerable question about workable signal levels. Proposed designs use some combination of orbiters and fixed sites which could also be used to distribute NTP time. The vehicle we designed for NASA/JPL consists of a standard issue NTP implementation with a front end that simulates motion in space. The simulator uses ephemeris data from JPL to produce state vectors, but the positions have some uncertainty resulting in some 10-ms lightime error. The real case with spacecraft navigation remains to be determined. My real worry is what rover timekeeing is like when the temperature ranges from 0 to 50 below C and when things get shut down in a duststorm. Dave Danny Mayer wrote: > Unruh wrote: > >>[EMAIL PROTECTED] (Danny Mayer) writes: >> >> >>>Unruh wrote: >>> >>>>"David L. Mills" <[EMAIL PROTECTED]> writes: >>>> >>>> >>>>>Brian, >>>>>The longest delay NTP response was from the Moon, as simulated at JPL. >>>>>Next step is the Mars orbiters and then the rovers. JPL discovered the >>>>>max distance threshold had to be increased to handle the Moon delay. >>>> >>>>Thats only a few sec. >>>>And it is symmetrical. Could probably still get usec resolution. >> >>>Of course it's not symmetrical. You of all people should know better >>>than that. >> >>The speed of light differs in different directions? >>And if the response is fast, the distance has not changed much either >>(In a msec the moon moves about a meter, which is 3nsec, way below any >>computer resultion)and that is amost all perp to the earth.Ie, the >>difference in distance during the response time of the remote computer is >>negligible. If you aim for the moon when the moon is just setting and you >>are on the equator, the difference in path lengths is about 5usec, again, a >>completely negligible amount. >> >>Now of course the synchronization will not proper time on the moon, but the >>sync of the earth clock. > > > And neither the moon or other is moving along their geodesic? > > Danny _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
