Package org.orekit.time

This independent package provides classes to handle epochs, time scales, and to compare instants together.

See:
Description

Interface Summary

TimeScale	Interface for time scales.
TimeStamped	This interface represents objects that have a AbsoluteDate date attached to them.
UTCTAILoader	Interface for loading UTC-TAI offsets data files.

Class Summary

AbsoluteDate	This class represents a specific instant in time.
ChronologicalComparator	Comparator for TimeStamped instance.
DateComponents	Class representing a date broken up as year, month and day components.
DateTimeComponents	Holder for date and time components.
GPSScale	GPS time scale.
TAIScale	International Atomic Time.
TCGScale	Geocentric Coordinate Time.
TDBScale	Barycentric Dynamic Time.
TimeComponents	Class representing a time within the day broken up as hour, minute and second components.
TimeScalesFactory	Factory for predefined time scales.
TTScale	Terrestrial Time as defined by IAU(1991) recommendation IV.
UTCScale	Coordinated Universal Time.

Package org.orekit.time Description

This independent package provides classes to handle epochs, time scales, and to compare instants together.

The principal class is AbsoluteDate which represents a unique instant in time, with no ambiguity. For that purpose, the ways to define this object are quite strict.

The easiest and most evident way is to define an instant with an offset from another one. Orekit defines 6 reference epochs. The first 5 are commonly used in the space community and the last one is commonly used in the computer science field.

• Julian Epoch: -4712-01-01 at 12:00:00, TTScale
• Modified Julian Epoch: 1858-11-17 at 00:00:00, TTScale
• Fifties Epoch: 1950-01-01 at 00:00:00, TTScale
• GPS Epoch: 1980-01-06 at 00:00:00, UTCScale
• J2000 Epoch: 2000-01-01 at 12:00:00, TTScale
• Java Epoch: 1970-01-01 at 00:00:00. TTScale

The second definition, which could be the source of some confusion if not used with care, is by giving a location (a date) in a specific time scale. It is of prime importance to understand the various available time scales definitions to avoid mistakes. Orekit provides 5 of the most important ones:

• TAIScale: International Atomic Time,
• TTScale: Terrestrial Time as defined by IAU(1991) recommendation IV. Coordinate time at the surface of the Earth. It is the successor of Ephemeris Time TE. By convention, TT = TAI + 32.184 s,
• UTCScale: Coordinated Universal Time. UTC is related to TAI using step adjustments from time to time according to IERS (International Earth Rotation Service) rules. These adjustments require introduction of leap seconds. Some leaps are already known and predefined in the library (at least from 1972-01-01 to 2009-01-01) and other ones can be supported by providing UTC-TAI.history files somewhere in the directory hierarchy pointed to by the orekit.data.directory property,
• TCGScale: Geocentric Coordinate Time. Coordinate time at the center of mass of the Earth. This time scale depends linearly from TTScale.
• GPSScale: Global Positioning System reference scale. This scale was equal to UTC at start of the GPS Epoch when it was 19 seconds behind TAI, and remained parallel to TAI since then (i.e. UTC is now offset from GPS due to leap seconds). TGPS = TAI - 19 s.

Once it is built, an AbsoluteDate can be compared to other ones, and expressed in other time scales. It is used to define states, orbits, frames... Classes that include a date implement the TimeStamped interface. The ChronologicalComparator singleton can sort objects implementing this interface chronologically.

Author:

L. Maisonobe