![]() Its units of measurement are different from SCET. SCLK is not as constant and stable as the UTC-derived SCET. SCLK has a nearly-direct relationship with SCET: it is the best possible on-board estimate of SCET. SCLK, Spacecraft Clock is the value of a counter onboard a spacecraft, described further in Chapter 11. SCET, Spacecraft Event Time is the UTC time onboard the spacecraft. Its reference points are the center of the Earth and the immediate position of a spacecraft or the center of a celestial body. OWLT varies continuously as the spacecraft's distance from the Earth changes. Knowledge of OWLT is maintained to an accuracy of milliseconds. OWLT, One-Way Light Time is the elapsed time it takes for light, or a radio signal, to reach a spacecraft or other body from Earth (or vice versa). TRM, Transmission time is the UTC time of uplink from Earth. At 5:00 pm at a particular location on Mars, the Sun would be low in the west. ![]() Around 11:30 am or 12:30 pm at a particular location on Venus, the Sun would be nearly overhead. A planet that rotates more slowly than Earth would have an object in its sky at 1:00 local time move to 2:00 local time in more than an hour of Earth-clock time. Local time on another planet is conceived as the equivalent value of time for the Sun's distance from the meridian, as it is on Earth. Local time is also determined on other planets when needed. Many locations change between standard time and daylight saving time (see below). Its reference point is one's immediate locality: when it is 12:00:00 noon Pacific Time at JPL, it is 20:00:00 UTC, and 13:00:00 Mountain Time in Denver, Colorado. Local time is UT adjusted for location around the Earth in time zones. It is also common to encounter references to UT or GMT when the system actually in use is UTC, for example, "Uplink the command at 1801Z." It is common to see outdated references to GMT, even in currently operating flight projects. Its reference point is also Greenwich, England: when it is noon on the prime meridian, it is noon (12:00:00) UT. UT is not updated with leap seconds as is UTC. It is based on the imaginary "mean Sun," which averages out the effects on the length of the solar day caused by Earth's slightly non-circular orbit about the Sun. UT, Universal Time also called Zulu (Z) time, was previously called Greenwich Mean Time, GMT. Its reference point is Greenwich, England: when it is midnight there on Earth's prime meridian, it is midnight (00:00:00.000000) - "all balls" - UTC. UTC is used by astronomers, navigators, the Deep Space Network (DSN), and other scientific disciplines. The addition or subtraction of leap seconds, as necessary, at two opportunities every year adjusts UTC for irregularities in Earth's rotation. Its rate does not change by more than about 100 picoseconds per day. ![]() It is based upon carefully maintained atomic clocks and is highly stable. UTC, Coordinated Universal Time, is the world-wide scientific standard of timekeeping.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |