Units of Measure

Scenario Units

Scenario Units establish the default settings for all units of measure used in a scenario. These settings are used for display and data input purposes throughout STK. The table on the Units page displays a summary of the units of measure currently set. To see a list of all available units of measure, open the Units page of the Scenario's Basic properties by double-clicking the scenario in the Object Browser and selecting the Units page.

Note: You can override these default settings for a specific field on a particular property page by using the Format Dialog. The Format dialog also enables you to revert back to the default scenario setting for a particular field.

To change the unit of any dimension listed in the table, click CurrentUnit and select the appropriate value from the drop-down list.

Individual Units

Various areas of object properties throughout STK require that you input data in a certain dimension using a selected unit of measure. Each item requiring a value is assigned a default unit. The unit abbreviation that appears next to the specified value in any editable field indicates the unit being used (e.g., 60 sec or 1 Jan 1997 00:00:00.00 UTCG).

You can change the unit of measure used for any option in STK using the button that is part of the editable field. Click to display a list of available units of measure. When you change the unit of measure, STK converts the value entered to its equivalent value in the selected unit. For example, if you entered a value of 60 sec and then changed the unit of measure from seconds (sec) to hours (hr), STK would automatically convert the value from 60 (sec) to its equivalent in hours (0.0166667).

Formatting Units of Measure

To format units of measure to be used in specific instances throughout the active scenario, click , then select Format... from the available units list to launch the Format Dialog.

Use the Apply setting to this field option to select the instances to which the specified formatting should be applied.

Additional options available in the Format Dialog:

Option	Description
Unit Selector	Click to display a list of available units of measure from which you can select the unit that will be applied to the object(s) indicated using the Apply setting to this field option.
Leading Zeros	If ON, leading zeros will be used to fill out the minimum width specified.
Require Sign	If ON, adds a sign (plus by default) to values entered.
Default Minimum Width	If ON, values will display using the default minimum width of 2. The minimum width is the number of places to which a value must be carried out. A decimal point and sign are each counted as an increment of the minimum width.
Default Precision	If ON, values will be carried out to the default minimum precision of 6 decimal places. If OFF, indicate the number of decimal places that values should be carried out.
Notation	Indicate the notation format in which values should be displayed.

Set Animation Time

You can update the animation time in the 2D and 3D Graphics windows to the time displayed in a field by clicking the button attached to that field and selecting Set Animation Time.

Time Standards

International Atomic Time (TAI) is a physical time scale affected by the Earth's gravitational and rotational potential, and deduced from a weighted average of various international frequency standards. Relative weighting is based on the historical stability of the individual standards. TAI is maintained by the Bureau International des Poids et Mesures (BIPM) and is the basis of other time scales.

Global Positioning System Time (GPS) is the background (reference) time scale of that satellite-based navigation system; ideally, it is steered to lag TAI by nineteen (19) seconds, i.e., GPS Time = TAI - 19 s.

Universal Time (UT1) is the angular measure of Earth rotation inferred from observations. Earth-rotation angle provides a sequentially increasing continuum that is everlasting and widely apparent, and serves as the astronomical basis of civil time of day. The angular rate of modern-day UT1 has been defined to closely follow Newcomb's convention for mean solar time, based on the mean motion of the Sun reduced from 19th-century observations.

Coordinated Universal Time (UTC) is a broadcast time standard providing both astronomical time of day and atomic-time interval. UTC is kept within +/-0.9 s of UT1 by the introduction of leap seconds and is therefore a legally recognized proxy for Universal Time in most countries. UTC is always offset from TAI by an integer number of seconds, and is thus a carrier of precision frequency and time interval for broadcast standards based on the SI second.

Note: Zulu time is synonymous with UTC.

Greenwich Mean Time (GMT), generally a synonym for Universal Time and/or UTC, is used in STK as a synonym for Coordinated Universal Time (UTC).

Local Time is civil time maintained with an offset as defined by an observer's longitude. In STK, local time is a synonym for Zone Time, which generally maintains hourly offsets from UTC, and is realized based on the computer system clock configuration.

Note: The local time offset from UTC changes between local Standard Time (LST) and local Daylight or Summer Time (LDT) as determined by the computer hosting STK. In the United States through 2006, local time changed from Standard Time to Daylight Time at 02:00 LST to 03:00 LDT on the first Sunday in April and returned at 02:00 LDT to 01:00 LST on the last Sunday in October. Effective 2007, local time changed from standard time to daylight time at 02:00 LST to 03:00 LDT on the second Sunday in March and returns at 02:00 LDT to 01:00 LST on the first Sunday in November. Some areas do not recognize daylight or summer time, however.

Terrestrial Time (TT) is a theoretically ideal time at the Earth geoid. A practical realization is TT = TAI + 32.184 s. ΔT = TT - UT1 is the difference between this ideal time scale and the rotation of the Earth. TT has also been known as Terrestrial Dynamical Time (TDT) when considered as a coordinate time for geocentric orbits. TT is the successor of pre-relativistic Ephemeris Time (ET).

Barycentric Dynamical Time (TDB) is intended to serve as the independent argument of barycentric ephemerides and equations of motion. It is defined as being linearly related to Barycentric Coordinate Time (TCB), where TCB is further related to TT through a complex sequence of relativistic transformations. The linear relationship between TDB and TCB is chosen such that the rate of TDB closely matches TT for the time span covered by the JPL Development Ephemerides. TDB is sometimes designated as Barycentric Ephemeris Time (Teph) when used as the time scale of the JPL ephemerides.

Comments on selected units of measure

Note: You can enter data in STK using any unit of measure; STK converts the values entered to the option selected here.