AbsoluteDate with Benefits: AbsoluteDateExt

The AbsoluteDateExt is a thin wrapper around the already powerful (but not too python friendly) Orekit AbsoluteDate class. AbsoluteDate uses the underlying Java class for its operations, but offers some useful tools like ordering help through magic classes (such as __lt__(), __ge__() etc.), overloaded plus and minus operators and time units support. It is a subclass of AbsoluteDate and does not introduce any new parameters. Therefore, it can be safely used in the Orekit functionalities.

An Orekit AbsoluteDate object can be converted to AbsoluteDateExt by simply creating a new object, that is a deep copy of the old AbsoluteDate object (in this example abs_date):

abs_date_ext = AbsoluteDateExt(abs_date)

Time Units Support

satkit makes extensive use of the units (thanks to pint package), and the AbsoluteDateExt class can easily work with units for the methods that are commonly used with AbsoluteDate. The following is an example for the shiftedBy() method.

>>> date1 = AbsoluteDateExt(2020, 7, 11, 00, 0, 0.0, TimeScalesFactory.getUTC())
>>> date2 = date1.shiftedBy(-240.0)  # normal float (interpreted as seconds)
>>> date3 = date1.shiftedBy(+4 * u.min)  # Quantity
>>> date4 = date1.shiftedBy(+120)  # int

As can be seen, the “extended” version of the AbsoluteDate can now work with units (saving a lot of unnecessary conversion errors), can default to “seconds” when no unit is supplied (just like AbsoluteDate does) and, unlike AbsoluteDate, it can now handle int input.

Similar support exists for AbsoluteDateExt.durationFrom() and {py:meth}.AbsoluteDateExt.isCloseTo methods. Therefore, the output of the AbsoluteDateExt.durationFrom() is a Quantity object, expressing time properly with units.

Adding and Subtracting Time

AbsoluteDateExt brings addition and subtraction support. Therefore, the following can be conveniently written, adding and subtracting durations from a date to compute another date:

>>> date1 = AbsoluteDateExt("2020-04-13T00:00:00.000", TimeScalesFactory.getUTC())
>>> 
>>> date2_add = date1 + (-240.0)  # normal float
>>> date3_add = date1 + 4 * u.min  # Quantity
>>> date4_add = date1 + 120  # int (interpreted as seconds)
>>> 
>>> date2_sub = date1 - 4 * u.min  # Quantity

Continuing with the same example, a date can be subtracted from another, resulting in a duration.

>>> dt2_sub = date2_add - date1

Sorting Time Objects

The AbsoluteDateExt class enables sorting of lists of time objects. For example, the following generates a list of time objects and then sorts it:

sorted_list = sorted([date1, date2, date3, date4])