Kernels Inspection

This tutorial will walk you through the basic features and interfaces that allow you to inspect the properties of binary ephemeris kernels.

Available Times

Timescale

SPK and PCK segments support two types of ephemeris timescales, namely, TDB and TCB. The timescale of the loaded kernels can be retrieved as follows:

using Ephemerides 

# Load the kernel
eph = EphemerisProvider("kernel.bsp")

# Retrieve the ID of the kernel timescales
id = ephem_timescale_id(eph)

The retrieved ID is 1 for TDB and 2 for TCB. A value of -1 is returned if the kernels are empty.

Timespan

To retrieve the first and last available time in the ephemeris files associated to a provider object, two functions are available to distinguish between SPK and PCK data:

ephem_spk_timespan(eph)
ephem_pck_timespan(eph)

where eph is an EphemerisProvider instance. Both functions return the minimum and maximum available time in TDB seconds since J2000, as well as a continuity parameter defined as follows:

• 0 no SPK or PCK data is available.
• 1 the quantities of all bodies are available for any time between the first and last time.
• 2 the quantities of some bodies are available on discontinuous time intervals between the first and last time.
• 3 the quantities of each body are available on a continuous time interval between the first and the last time, but not available for any time between the first and last time.

Available Points and Axes

To retrieve the list of NAIF IDs with the points or axes that have available ephemeris data, these function should be called on an EphemerisProvider instance:

ephem_get_points(eph)
ephem_get_axes(eph)

Segment Records

Position and orientation metadata relative to the records loaded in the ephemeris kernels can be retrieved with the following two functions, respectively:

ephem_spk_records(eph)
ephem_pck_records(eph)

Both functions return a vector of Ephemerides.EphemRecordSPK or Ephemerides.EphemRecordPCK ordered by priority, i.e., they use the highest priority records when there are multiple records that could satisfy the same target, center pair for a given epoch.

In particular SPK records contain the following information:

• target: NAIF ID of the target object
• center: NAIF ID of the center object
• axes: NAIF ID of the reference axes
• t_start: start times of each sub-window, in TDB seconds since J2000
• t_end: final times of each sub-window, in TDB seconds since J2000

Similarly, PCK records contain these information:

• target: NAIF ID of the target axes
• center: NAIF ID of the reference axes
• t_start: start times of each sub-window, in TDB seconds since J2000
• t_end: final times of each sub-window, in TDB seconds since J2000

For PCK records, the reference axes ID is set into the center field. Please notice that whenever a gap between the data of a given pair of (center, target) objects is present, t_start and t_end will store the start and end times of each window with available data, respectively.