The following table presents comments on selected units of measure used in ODTK.
|DateFormat||EpSec, EpMin, EpHr, EpDay||
Epoch Seconds, Epoch Minutes, Epoch Hours, and Epoch Days.
Epoch calculated as time elapsed relative to the scenario epoch.
Gregorian Local / UTC.
Local / Universal Coordinated Time date and time displayed in Gregorian format (1 Jul 2007 12:00:00.000).
Julian Local / UTC.
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.
SGP4 Epoch date format
UTC date and time displayed in day-of-year format, with four digits representing the year (182/2007 12:00:00.000).
2451544.5 plus the number of elapsed UTC days since the epoch of AD January 1, 2000 0h of the Gregorian calendar (2454283.000000). The addition of 2451544.5 correlates the zero epoch with noon of November 24, 4713 BC.
Note: 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.
UTC date displayed using the year, the whole day of the year, hours, minutes, and seconds with the hours, minutes, and seconds appearing to the right of the decimal point YYYYDDD.HHMMSS. So 1 Jul 2007 12:30:40 would be represented as 2007182.123040.
Julian Date Offset.
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.
Time elapsed relative to a user-specified epoch. Enter epoch in text field. The elapsed time is presented in terms of days, hours, minutes, and seconds (100/12:30:05.00).
Tip: 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.
UTC date and time displayed with the indicated format and the fractional day of the month (20070701. 50000000).
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).
Greenwich Mean Time.
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.
Modified Julian Date.
UTC date and time defined by a fixed offset from the Julian date. 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.
Julian Ephemeris Date.
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 is ahead of UT by ~64s in AD 2000. A TT day is 86400 SI seconds in duration.
Terrestrial Time (formerly known as Terrestrial Dynamical Time) displayed in Gregorian calendar format.
International Atomic Time displayed in a Gregorian calendar format.
International Atomic Time displayed in a Julian day-of-year format.
GPS time displayed in Gregorian calendar format.
GPS time 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).
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).
Earth canonical time.
Time elapsed since the scenario epoch measured in Earth canonical time units where one time unit equals .
Sun canonical time.
Time elapsed since the scenario epoch measured in Sun canonical time units where one time unit equals .
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).
GPS Z count.
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). Note that this differs from normal GPS convention where the Z count is represented as the number of whole weeks since the GPS epochs and the number of 1.5 second increments into the week.
A Barycentric Dynamical Time displayed in Gregorian format.
|MassUnit||Pounds||1 pound mass is the quantity of matter that would weigh 1 pound at sea level|
|SmallDistanceUnit||All||These units are used to describe distances smaller than 1 meter|
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.
|Unit Format Options|
|Temporary||The settings specified here will 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 object class; apply the settings specified here to all instances of the specified class in the active scenario.|
|remove for all <ObjectClass> objects||Remove 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:
|Unit Selector||Use the button 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 selected, leading zeros will be used to fill out the minimum width specified.|
|Require Sign||If selected, adds a sign (plus by default) to values entered.|
|Default Minimum Width||If selected, 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 selected, values will be carried out to the default minimum precision of 6 decimal places. Otherwise, indicate the number of decimal places to which values should be carried out.|
|Notation||Indicate the notation format in which values should be displayed.|
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.