EphemerisCache

class EphemerisCache(body_coeffs, body_ids, center_ids)

Bases: object

Chebyshev ephemeris for a set of bodies, read out of the loaded SPK kernel or kernels. Positions only. Build it with scarabaeus.utils.EphemerisCache.build_ephemeris_cache.

A record covers \([t_{mid} - t_{rad},\ t_{mid} + t_{rad}]\). With \(\tau = (t - t_{mid})/t_{rad} \in [-1, 1]\) and \(c_k\) the coefficients of one axis, the coordinate is the Chebyshev series

\[x(t) = \sum_{k=0}^{N} c_k\, T_k(\tau),\]

summed by Clenshaw’s recurrence [1]:

\[b_k = c_k + 2\tau\, b_{k+1} - b_{k+2}, \quad b_{N+1} = b_{N+2} = 0, \qquad x(t) = c_0 + \tau\, b_1 - b_2.\]

A body is chained through its centers down to the solar-system barycenter, \(\mathbf{r}_b = \mathbf{r}_{b/c(b)} + \mathbf{r}_{c(b)}\), and a relative position is the difference of two such positions, \(\mathbf{r}_{t/o} = \mathbf{r}_t - \mathbf{r}_o\).

Parameters:

body_coeffslist of list of tuple

Chebyshev records (t_mid, t_rad, x, y, z) for each body across the cached interval, where x, y and z are coefficient lists.

body_idslist of int

SPICE IDs in the same order as body_coeffs.

center_idslist of int

SPICE IDs of each body’s center. Necessary for chaining between frames for bodies that are not the primary, e.g. Phobos.

Methods

get_relative_position(target_id, observer_id, t)	Position of target_id relative to observer_id at t, in km, in the SPK native frame (J2000).
get_relative_positions(target_ids, ...)	Positions of target_ids relative to observer_id at t, in km, in the SPK native frame (J2000), as an (n, 3) array with one row per target.

References

[1]

C. W. Clenshaw, “A note on the summation of Chebyshev series,” Mathematical Tables and Other Aids to Computation, vol. 9, no. 51, pp. 118-120, 1955.

get_relative_position(target_id: int, observer_id: int, t: float) → List[float]

Position of target_id relative to observer_id at t, in km, in the SPK native frame (J2000). Both bodies must be in the cache.

get_relative_positions(target_ids: List[int], observer_id: int, t: float) → ndarray

Positions of target_ids relative to observer_id at t, in km, in the SPK native frame (J2000), as an (n, 3) array with one row per target. All bodies must be in the cache.

ids: List[int]

Building a cache

build_ephemeris_cache(body_ids, t0: float, tf: float)

Build a Rust EphemerisCache for body_ids over [t0, tf].

Parameters:

body_idslist of int

SPICE IDs to cache. The FIRST entry is the primary/observer body.

t0, tffloat

Interval to cache, in ephemeris seconds past J2000 (TDB). Callers should include any epoch buffer they need.

Returns:

EphemerisCache

Cache covering body_ids plus any center bodies required for chaining, appended after them (see EphemerisCache.ids).

Raises:

NoLoadedSPKError

If no SPK kernel is loaded.

ValueError

If a requested body has no segment in any loaded SPK.

RuntimeError

If a segment has an unsupported SPK type (only types 2 and 3 are readable as Chebyshev position records).

exception NoLoadedSPKError

Bases: Exception