Editing Spacecraft Event Time (section)

==SCET versus Earth time==

Since signals between the spacecraft and Earth are limited to the speed of light, there is a delay between the time an event happens on the spacecraft (such as the transmission of data taken from an instrument reading) and the time that a signal reporting the event reaches Earth.  Similarly, there is a delay between when instructions are sent from Earth and when the spacecraft receives the instructions.  The length of delay is related to the distance between the sending and receiving points.  Failure to take this delay into account could result in inaccurate data or mistakes in spacecraft control.

===Calculating SCET===

Determining the Spacecraft Event Time involves taking the time at Earth and adding or subtracting the signal travel time, depending on whether the signal is being sent to or received from the spacecraft.  For events transmitted from the spacecraft to Earth, the SCET of an event on the spacecraft can be defined as equal to the ERT (Earth-Received Time) minus the OWLT (One-Way Light Time).<ref>Basics of Space Flight Glossary; JPL/NASA</ref>  For events transmitted from Earth to the spacecraft, the calculation is TRM (transmission time<ref>{{Cite web |title=Synonyms for Transmission Time |url=http://www.thesaurus.net/transmission%20time |website=Thesaurus.net |language=en}}</ref>) plus OWLT.  For example, if a signal were received on Earth at exactly 11:00 UTC from a spacecraft showing that it had just completed a maneuvering thrust, but the spacecraft was four light-hours away from Earth (the distance of the [[New Horizons]] spacecraft at one point as it approaches [[Pluto]]), the SCET time of the thrust maneuver would have been four hours earlier, at 07:00.

'''Spacecraft Event Time''' in [[Coordinated Universal Time|UTC]] is also known as '''Orbiter UTC''', and Earth-received time as '''Ground UTC'''.

===Spacecraft control===

Since it takes time for a radio transmission to reach a spacecraft from [[Earth]], the usual operation of a spacecraft is controlled with an uploaded command script containing SCET markers to ensure a certain timeline of events. Because of the delay between the sending of instructions from Earth and their receipt and execution by the spacecraft, real-time commanding of robotic spacecraft is done rarely: usually only in response to an emergency event, when changes in spacecraft operations must be made as soon as possible.  For example, a spacecraft could be instructed to go into [[Safe mode (spacecraft)|safe mode]] to protect it during a [[Coronal mass ejection|coronal mass ejection (CME)]] from the [[Sun]].