A few SPICE terms used in WebGeocalc documentation are explained here. But don't forget that many of the fill-in boxes, radio buttons and check boxes have associated HELP viewable by clicking on the blue circle having a white question mark in the center.
These are not intended to be rigorous definitions; they are meant to provide the SPICE context of the term.
- Aberration Correction - Specifies the types, if any, of corrections to be used in computing certain geometric quantities: stellar aberration, one-way light travel time, or both. See pages on aberration correction in the tutorial named "05_concepts."
- Competing kernels - See the discussion on competing kernels.
- Coordinate System - A coordinate system specifies the method of locating a point within a reference frame. Examples are Cartesian coordinates and spherical coordinates. See the "State representation" help provided inside WGC. Also see the coordinate system conventions portions of the SPICE tutorial named "05_concepts."
- Instrument - The name of a science instrument on-board a spacecraft. More generally this is the name of one viewing port of the named instrument, if the instrument has two or more optical paths or "look directions." Sometimes a high-gain antenna or other spacecraft structure is treated like an instrument because of the computational possibilities this treatment affords. One can read the Instrument Kernel (*.ti) for the named instrument to learn more about instrument ports. The web page named "About the Data" provides directions for finding such individual kernels.
- Kernel set - This term usually refers to a collection of kernels that have been aggregated by (within) a meta-kernel. There is one exception—that of "Manual." If you select Manual as the "Kernel set" you will be pointed to the NAIF server's entire kernel collection; from here you can select individual kernels or meta-kernels. But using the Manual selection method can be problematic if you are not familiar with the various collections of kernels available on the NAIF server, and most particularly, with the numerous kernel naming schemas used there.
- Load - "Loading" (a kernel) means to make that kernel (that SPICE data file) available for use in a SPICE-based program such as WebGeocalc.
In WebGeocalc kernels are most normally "loaded" not one-by-one, but rather by loading a "meta-kernel" that names a set of individual kernels. When the meta-kernel is loaded, all the "real" kernels it names are loaded into the program.
One may also unload a kernel or meta-kernel to make that file's data no longer available to the program. Unloading a kernel or meta-kernel is rarely needed, but can be used to change kernel priority when kernel competition exists. (See elsewhere regarding competing kernels.)
- Meta-kernel - A meta-kernel is a SPICE text-style kernel that names, or points to, one or more "real" kernels that will be loaded when the meta-kernel itself is loaded. Using a meta-kernel is a convenient way of aggregating and quickly loading a set of kernels that should be used together.
- Observer - Both "observer" and "target" can be any real object (e.g. a spacecraft, planet, or satellite), a pseudo-object (e.g. solar system or planet system barycenter, or a Lagrange point), or a location (e.g. a tracking station) whose state (position and velocity) is specified by a SPICE SPK file that has been loaded into WGC.
- Reference Frame - A reference frame (often referred to simply as "frame") is a set of three mutually orthogonal unit-length vectors, coupled with a location called the frame's center. A frame may be "inertial," meaning it is non-rotating, or non-inertial, meaning it does rotate.
The most commonly used reference frame in SPICE is the inertial frame named "J2000." Within SPICE this frame name is used to mean the International Celestial Reference Frame (ICRF). (The difference between J2000 and ICRF is very small.) A popular synonym is the Earth Mean Equator and Equinox of J2000 frame.
The most commonly used non-inertial reference frames are body-fixed frames having name style like "IAU_MARS." These body-fixed frames are tied to the named body, and so rotate with that body.
See the pages regarding reference frames in the tutorial named "05_concepts."
- Target - Both "target" and "observer" can be any real object (e.g. a spacecraft, planet, or satellite), a pseudo-object (e.g. solar system or planet system barycenter, or a Lagrange point), or a location (e.g. a tracking station) whose state (position and velocity) is specified by a SPICE SPK file that has been loaded into WGC.
- Text and binary kernels - Two styles of kernels exist in SPICE: "text" and "binary." Text kernels consist entirely of ASCII characters. They may be read or modified using any text or word processing tool. (This capability is not relevant to WGC users.) Binary kernels are binary files, containing little ASCII information. Binary kernels are produced using only SPICE APIs (functions).
Whether a kernel is text or binary is generally not of importance to a WGC user. (But read about text- and binary-style PCKs below.)
- Text- and binary-style planetary constants kernels - The Planetary Constant Kernel (PCK) is unique in that both text and binary versions exist.
The text style is most prevalent. Text-style PCKs contain size, shape and orientation data for planets and their satellites, and for a few comets and asteroids. These data are mostly taken from a document produced by the International Astronomical Union. Binary style PCKs exist ONLY for the earth and the moon. Binary PCKs contain only orientation data for those bodies—size and shape data for the earth and moon must come from a text style PCK. The earth and lunar orientation data provided in binary PCKs are of much greater accuracy than the earth and lunar orientation available in a text style PCK.
- Topocentric frame - A topocentric frame is one located at the surface of an object such as the earth or Mars. Deep space communication stations (e.g. DSS-14) typically have a topocentric frame defined for the antenna (e.g. DSS-14_TOPO). A spacecraft lander or rover (e.g. MER-1) might have a topocentric frame (e.g. MER-1_TOPO) defined for the landing location.