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 (see below). 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.
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.
ERT, Earth-Received Time is the UTC of an event received at a DSN station.
