Spacecraft Event Time (SCET) is the spacecraft-local time for events that happen at the spacecraft. SCET is used for command programs that control the timing of spacecraft operations and to identify when specific events occur on the spacecraft relative to Earth time.
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). For events transmitted from Earth to the spacecraft, the calculation is TRM
transmission time 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
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
), the SCET time of the thrust maneuver would have been four hours earlier, at 07:00.
Spacecraft Event Time in
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
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, 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 to protect it during a
coronal mass ejection (CME) from the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
.
Presentation format
Spacecraft event times stored in relation to instrument data from spacecraft events (e.g. images) are generally presented in
ISO 8601
ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, w ...
using one of the following formats:
* CCYY-MM-DDTHH:MM:SS.sssZ (preferred format)
* CCYY-DDDTHH:MM:SS.sssZ
However, the trailing Z (which indicates that the time is given in
UTC) is often assumed/omitted.
[Data Standards, PDS/NASA]
Notes
References
Basics of Space Flight Glossary, JPL/NASAData Standards, PDS/NASA
{{Portal, Space flight
Spaceflight concepts
Time scales