Formatting Units of Measure | Set Animation Time | Time Standards

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.

You can sort the Dimension List by selecting Alphabetical or Most Common (default); the Most Common option arranges the dimensions with respect to how frequently they are used in STK.

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 provide data in a certain dimension using a selected unit of measure. STK assigns each item requiring a value 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 enter a value of 60 and then change 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).

Unit settings

Unit settings are currently set separately at three levels: the scenario level, the report and graphing level, and with Connect. These three settings are independent of each other. If you needed to set the unit values to represent Gregorian LCL (LCLG) time for a scenario analysis, you would make the changes from the scenario level on the Basic/Units page, reflecting a global setting. This would modify the way STK displays units for the Animation Time Clock. Any objects that are within the scenario would inherit these settings as their new default. However, the data providers within the report and graphing units do not inherit these parameters. You would need to set these within the Reports and Graph Manager separately. This is also true when using Connect.

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 menu items 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.

Option Description
Temporary The settings specified here remain in effect until you click Apply, OK, or Cancel on the page from which the format dialog was referenced.
for all <ObjectClass> objects Settings are associated with a particular STK Object class; apply the settings specified here to all instances of the specified class in the active scenario.
remove for all <ObjectClass> objects This removes the settings specified here from all instances of the specified class in the active scenario.
for only <ObjectClass> <ObjectName> Apply the settings indicated here to the specified object in the active scenario.
remove for only <ObjectClass> <ObjectName> Remove the default settings for the specified object in the active scenario.

Additional options available in the Format Dialog are:

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 are used to fill out the minimum width specified.
Require Sign If ON, this adds a sign (plus by default) to values entered.
Default Minimum Width If ON, values will display using the default minimum width of two (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 are carried out to the default minimum precision of six 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.

Set animation start time

You can set the animation start time to the time displayed in a field by clicking the button attached to that field and selecting Set Animation Start 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, so GPS Time = TAI - 19 seconds.

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 deduced 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.

Zulu time is synonymous with UTC.

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

Local Time is civil time maintained with an offset considering the observer's longitude as defined by the computer hosting STK. 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.

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 of the US do not recognize daylight or summer time.

STK determines local time and handles daylight time as follows:

  • In Microsoft Windows, local time conversions to UTC time now use the time zone rules stored as part of the Windows operating system, in its registry. When STK is computing a local time, it applies the Windows time zone rules to determine the local-to-UTC conversion, including the offset for Daylight Saving Time in effect at that time.

  • For Linux, STK does not determine if daylight savings time should be applied based on the epoch requested. Instead, it will use the computer's UTC offset (including the Daylight Savings Time offset) when STK was started and apply that constant offset between local and UTC to all date calculations for all times. If your scenario is outside your current local time, where the local-to-UTC offset is different during the scenario times than the offset determined at STK startup, the local time might be off for some or all of your calculations.

Terrestrial Time (TT) is a theoretically ideal time at the Earth geoid. A practical realization is TT = TAI + 32.184 seconds. The quantity ΔT, where ΔT = TT - UT1, is the difference between this ideal time scale and the rotation of the Earth. TT is also 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.

The following table describes the DateFormat dimension.

For a list of all units of measure in the STK units system and how to use those units in STK, see Units System.

Comments on the Date Format units of measure

Unit Comment
EpSec, EpMin, EpHr, EpDay, EpYr

Epoch Seconds, Epoch Minutes, Epoch Hours, Epoch Days, and Epoch Years

Epoch is calculated as time elapsed relative to the scenario epoch.

The minimum and maximum EpSec values are -1e11 and 1e11, respectively. Conversions that represent EpSec values as a string use these min/max values.

LCLG, UTCG

Gregorian Local / UTC

This is the Local / Coordinated Universal Time date and time displayed in Gregorian format, for example 1 Jul 2007 12:00:00.000.

LCLJ, UTCJ

Julian Local / UTC

This is the Local / UTC date and time displayed in day of year format (182/07 12:00:00.000). The leading two digits of the year are assumed to be 19 for YY values of 60-99 or greater and 20 for YY values of 00-59.

LTZG

Local Time Zone Gregorian

If no offset is provided, then this is interpreted as a local time. On a GUI, it would be converted into the form with the correct offset for that time zone. If you provide an offset (+ or - hh:mm), STK applies it as a constant offset from UTC for the whole scenario. You do not have to use an offset matching the current time zone, but STK still displays time in current local time zone representation. An example with the offset included is 25 Jan 2021 12:01:00.000-05:00.

UTCJFOUR

Julian4 UTC

This is UTC date and time displayed in day-of-year format, with four digits representing the year (182/2007 12:00:00.000).

JDate

Julian Date

This equals 2451544.5 plus the number of elapsed UTC days since the epoch of AD January 1, 2000, 0 hours of the Gregorian calendar (2454283.000000). The addition of 2451544.5 correlates the zero epoch at noon of November 24, 4713 BC.

UTC date conversions with this format may not appear accurate before 1972 because UTC days were not defined using leap seconds. Also, time during a leap second may display incorrectly due to an incorrect assumption regarding the length of UTC day when using this format.

JDateOff

Julian Date Offset

This is computed as the Julian Date minus the user-specified offset. Modified Julian Dates can be represented by specifying an offset of 2,400,000.5.

MisElap

Mission Elapsed Time

This is the time elapsed relative to a user-specified epoch. Enter the epoch in text field. The elapsed time is presented in terms of days, hours, minutes, and seconds (100/12:30:05.00).

If you use this option, it is best to enter the dates in the Time Period and Animation pages before changing the DateFormat to Mission Elapsed. This is because Mission Elapsed Time is measured relative to the existing scenario epoch.

GMT

Greenwich Mean Time

This is the UTC date and time displayed as the day of year, integer seconds into the day, and the year (182/43200 2007). This format has limited precision and a confusing nomenclature, and is therefore not recommended for general use.

ModJDate

Modified Julian Date

This is the UTC date and time defined by a fixed offset from the Julian date. It sets the beginning of days at 0 hours, instead of 12 hours, and reduces the number of digits in day numbering. The modified Julian date is obtained by subtracting 2,400,000.5 from the Julian date.

JED

Julian Ephemeris Date

This is a measure of elapsed TDT days, starting from noon on November 24, 4713 BC, of the Gregorian calendar, in the TT time scale. TT (formerly ET) coincides with UT around AD 1902 and was ahead of UT by ~64 seconds in AD 2000. A TT day is 86400 SI seconds in duration.

JDTDB

TDB is a Julian date representation

JDTDB is a Barycentric Dynamical Time displayed in Julian Date format.

TDTG

Gregorian TDT

TDTG is Terrestrial Time (formerly known as Terrestrial Dynamical Time) displayed in Gregorian calendar format.

TDBG

Gregorian TDB

TDBG is a Barycentric Dynamical Time displayed in Gregorian format.

TAIG

Gregorian TAI

TAIG is International Atomic Time displayed in a Gregorian calendar format.

TAIJ

Julian TAI

TAIJ is International Atomic Time displayed in a Julian day-of-year format.

GPSG

Gregorian GPS

GPSG is GPS time displayed in Gregorian calendar format.

GPS

GPS time

GPS time is natively expressed as elapsed time since the GPS epoch of 6 January 1980 00:00:00:00 UTC. The elapsed time is presented in terms of weeks and seconds into the week (1434:43214.000).

GPSZ

GPS Z count

GPSZ is GPS time expressed as elapsed time since the GPS epoch of 6 January 1980 00:00:00:00 UTC. The elapsed time is presented in terms of Z counts (1.5 second increments) (578217609.333). This differs from normal GPS convention, where the Z count is represented as the number of whole weeks since the GPS epoch and the number of 1.5 second increments into the week.

EarthEpTU

Earth canonical time

This is the time elapsed since the scenario epoch, measured in Earth canonical time units, where one time unit equals .

SunEpTu

Sun canonical time

This is the time elapsed since the scenario epoch, measured in Sun canonical time units, where one time unit equals .

DD/MM/YYYY

This is UTC date and time displayed with the indicated format using hours, minutes, and seconds to represent the time of day (01/07/2007 12:00:00.000).

YYDDD

UTC date displayed in SGP4 Epoch date format using the last two digits of the current year, the whole day of the year, and the fractional day of the year (07182.50000000). The leading two digits of the year are assumed to be 19 for YY values of 60-99 or greater and 20 for YY values of 00-59.

YYYY/MM/DD

This is UTC date and time displayed with the indicated format using hours, minutes, and seconds to represent the time of day (2007/07/01 12:00:00.000).

YYYY:MM:DD

This is UTC date and time displayed with the indicated format using hours, minutes, and seconds to represent the time of day (2007:07:01:12:00:00.000).

YYYYDDD

This is UTC date displayed using the year, the whole day of the year, and the fractional day of the year (2007182.50000000).

YYYYMMDD

This is UTC date and time displayed with the indicated format and the fractional day of the month (20070701. 50000000).

ISO-YD

This is UTC date and time formatted as YYYY-DDDThh:mm:ss.sss, where YYYY is the 4 digit year, DDD is the three-digit day of the year, hh is the two-digit hour, mm is the two-digit minute, and ss.sss is a floating point representation of seconds into the minute, where the precision is specified via the time precision setting.

ISO-YMD

This is UTC date and time formatted as YYYY-MM-DDThh:mm:ss.sss, where YYYY is the four-digit year, MM is the two-digit month, DD is the two-digit day of the month, hh is the two-digit hour, mm is the two-digit minute, and ss.sss is a floating point representation of seconds into the minute, where the precision is specified via the time precision setting.

ISO-LTZ

ISO Local Time Zone

This is LTZG date and time (see above) formatted as YYYY-MM-DDThh:mm:ss.sss+hh:mm, where YYYY is the four-digit year, MM is the two-digit month, DD is the two-digit day of the month, hh is the two-digit hour, mm is the two-digit minute, and ss.sss is a floating point representation of seconds into the minute, where the precision is specified via the time precision setting. The end +hh:mm captures the optional offset input, where "+" really means "+ or -". An example with the offset included is 2021-01-25T12:00:00.000-05:00.

MJDsUTC

Day Count UTC

This is the UTC date and time displayed as MJDN:secIntoDay, where mjdn is the modified Julian day number (the integer part of the Modified Julian Date) and secIntoDay is the seconds since the start of the day (54282:43200.0)

MJDsTAI

Day Count TAI

This is the TAI date and time displayed as MJDN:secIntoDay, where MJDN is the modified Julian day number in the TAI time scale and secIntoDay is the seconds since the start of that day (54282:43233.0)

You can enter data in STK using any available unit of measure. When you select another unit or date format, STK converts the values you originally entered to new values in the new units.