Sunday, March 8, 2009

Daylight Savings Time 2009

Every fall and every spring, across most of the United States, Europe ("Summer Time"), and Canada, we all go through the "spring forward, fall back" time confusion as our time-keeping system shifts. Just that change is confusing enough for most of us.

But computing time in international space flight and international astronomy involves multiple time zones, time types, and computations. There's an entire galaxy of different time-keeping schemes, including Spacecraft Event Time, Round-Trip Light Time, and other specific measurements.

Here's an over-view from NASA:

UTC, Coordinated Universal Time, is the world-wide scientific standard of timekeeping. It is based upon carefully maintained atomic clocks and is highly stable. Its rate does not change by more than about 100 picoseconds per day. The addition or subtraction of leap seconds, as necessary, at two opportunities every year adjusts UTC for irregularities in Earth's rotation.

UTC is used by astronomers, navigators, the Deep Space Network (DSN), and other scientific disciplines. 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. The U.S. Naval Observatory website provides information on the derivation of UTC.

Universal Time also called Zulu (Z) time, was previously called Greenwich Mean Time, GMT. 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 is not updated with leap seconds as is UTC. Its reference point is also Greenwich, England: when it is noon on the prime meridian, it is noon (12:00:00) UT.

It is common to see outdated references to GMT, even in currently operating flight projects. 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."

Local time is UT adjusted for location around the Earth in time zones. 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. Many locations change between standard time and daylight saving time. Local time is also determined on other planets when needed.

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. 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. Around 11:30 am or 12:30 pm at a particular location on Venus, the sun would be nearly overhead. At 5:00 pm at a particular location on Mars, the sun would be low in the west.

TRM, Transmission time is the UTC time of uplink from Earth. 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). Knowledge of OWLT is maintained to an accuracy of milliseconds. OWLT varies continuously as the spacecraft's distance from the Earth changes. Its reference points are the center of the Earth and the immediate position of a spacecraft or the center of a celestial body.

time enroute

SCET, Spacecraft Event Time is the UTC time onboard the spacecraft. It is equal to TRM + OWLT. ERT is equal to SCET + OWLT.

SCLK, Spacecraft Clock is the value of a counter onboard a spacecraft, described further in Chapter 11. SCLK has a nearly-direct relationship with SCET: it is the best possible on-board estimate of SCET. SCLK is not as constant and stable as the UTC-derived SCET. Its units of measurement are different from SCET.

Tracking and predicting the exact relationship between SCLK and SCET is accomplished by analyzing telemetered SCLK values and trends with respect to the UTC-derived SCET, and regularly producing and applying a SCLK/SCET coefficients file which tracks the gradual drift of SCLK versus SCET.

RTLT, Round-Trip Light Time is the elapsed time it takes for a signal to travel from Earth, be received and immediately transmitted or reflected by a spacecraft or other body, and return to the starting point. It is roughly equal to 2 x OWLT, but not exactly, because of the different amount of distance the signal must travel on each leg due to the constant motions of both Earth and spacecraft.

For reference, RTLT from here to the Moon is around 3 seconds, to the sun, about 17 minutes. Voyager 1's RTLT at this writing in October 2000 is over 22 hours and increasing roughly an hour per year.

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