Definitions

A few definitions relating the Time subsystem:

Time representations

Many different representations of time have been defined. The default representation at JPL is et. This stands for Ephemeris Time. SPICE, and thus MaxQ, represent time as Ephemeris Time.

Ephemeris

In astronomy Ephemeris refers to data describing the state (position and velocity) of an object over time.

Ephemeris Time

Ephemeris Time represents the time variable of a trajectory. Often Ephemeris Times are given in “Seconds past J2000”. MaxQ serializes SEphemerisTime values equivalently, and values are displayed in the Unreal Editor the same way. Many Blueprint actions accept an SEphemerisTime variable as et.

Epoch

A particular value for Ephemeris Time is referred to as as an Epoch.

J2000 Epoch

The J2000 epoch means “12:00 Terrestrial Time January 1, 2000”.

Ephemeris Period

In MaxQ, an Ephemeris Period is the difference in time between two Epochs. Data type SEphemerisPeriod is used for Ephemeris Periods. Internally this is serialized as “Seconds”. A number of mathematical operators allow addition of Ephemeris Periods to Ephemeris Times, etc.

MaxQ Blueprint Actions

A number of Blueprint actions are available to manipulate time variables.

Time Initializations

String-To-SEphemerisTime initializations utilize str2et but do not intrinsically handle errors (although an error may remain signaled within SPICE, causing the next SPICE call to diagnose it).

Time Conversions

SEphemerisTime-To-String conversions utilize et2utc. SEphemerisTime converts to/from Double types as Seconds past J2000. Similarly converting SEphemerisPeriods to/from Double types gives simply Seconds.

Constants