irfpy.spice.timescale

Information about conversion between timescales.

3. Time scales

There are several definitions in time scales. The page https://www.stjarnhimlen.se/comp/time.html summarizes in a good way.

Scale

SPICE

Note

TAI

Y

Based upon the atomic second as defined by the ``oscillation of the undisturbed cesium atom

UTC

Y

Most frequently used time. irfpy is based on. UTC-TAI should be integer nubmer of seconds

UT

(Ambiguous due to multiple definition of UT? maximum difference <0.03s in between)

UT0

Raw, uncrrected UT.

UT1

UT0 corrected for polar wandering. Usually UT means UT1

UT2

UT1 corrected for seasonal variations in the Earth’s rotatioanl speed. It is considered obsolete

ET

Y

Ephemeris time, used 1960-83. Replaced in 1984 by TDT and TDB.

TDT

Y

Terrertrial Dynamical Time. Used 1984-2000. TDT=TAI+32.184. Replaced by TT in 2001. Timescale of ephemerides from the Earth’s surface.

TDB

Y=ET

Barycentric Dynamical Time. Time scale of ephemeroids referred to the baricentre of the solar system. Diff from TDT a few mill-sec.

TT

Terrestrial time. Replacing TDT/TDB in 2001. Practically, it can be the same as TDT (and thus ET and TDB)

TCG

TCB

Practically, the relation ET=TDT=TDB=TT can be used.

                                                 ET 1960-1983
                                                TDT 1984-2000
 UTC 1972-  GPS 1980-    TAI 1958-               TT 2001-
----+---------+-------------+-------------------------+-----
    |         |             |                         |
    |<------ TAI-UTC ------>|<-----   TT-TAI    ----->|
    |         |             |      32.184s fixed      |
    |<GPS-UTC>|<- TAI-GPS ->|                         |
    |         |  19s fixed  |                         |
    |                                                 |
    <> delta-UT = UT1-UTC                             |
     | (max 0.9 sec)                                  |
-----+------------------------------------------------+-----
     |<-------------- delta-T = TT-UT1 -------------->|
    UT1 (UT)                                       TT/TDT/ET

In SPICE, you can use unitim function to convert between systems. In addition, str2et can be used to convert datetime object to et.

>>> import datetime
>>> t0_utc = datetime.datetime(2019, 1, 5, 12, 35, 40)
>>> print(t0_utc)
2019-01-05 12:35:40

>>> import spiceypy as spice

>>> from irfpy.spice import timescale, localize
>>> localize.furnsh_tls()

>>> et = spice.str2et(t0_utc.strftime('%FT%T'))
>>> print(et)
599963809.1840458

The ET is converted to TDT (=TT).

>>> tt = spice.unitim(et, 'ET', 'TDT')
>>> print(tt)
599963809.184

TAI is also obtained.

>>> tai = spice.unitim(et, 'ET', 'TAI')
>>> print(tai)
599963777.0

From the TAI, one can get UTC via ET as

>>> tai = 0
>>> et = spice.unitim(tai, 'TAI', 'ET')
>>> t_py = et2datetime(et)
>>> print(t_py)
2000-01-01 11:59:28
irfpy.spice.timescale.download_lsk(url=None, localname=None, update=False)[source]

Try to download LSK file from the NAIF’s server.

The downloading file is https://naif.jpl.nasa.gov/pub/naif/generic_kernels/lsk/latest_leapseconds.tls

The downloaded file name becomes “latest_leapseconds.tls” in the tempfile.gettempdir()` unless specified by ``localname keyword. The file name is returned.

irfpy.spice.timescale.datetime2et(t)[source]

Python’s datetime is converted to ET

>>> import datetime
>>> t = datetime.datetime(2013, 5, 10, 4, 30, 25, 123456)
>>> print(t)
2013-05-10 04:30:25.123456
>>> from irfpy.spice import localize
>>> localize.furnsh_tls()
>>> et = datetime2et(t)
>>> print(et)
421432292.3088006
irfpy.spice.timescale.et2datetime(et)[source]

ET is converted to python’s datetime

>>> from irfpy.spice import timescale, localize
>>> import spiceypy as spice
>>> localize.furnsh_tls()

>>> et = 421432292.3088006
>>> t = et2datetime(et)
>>> print(t)
2013-05-10 04:30:25.123456