About the Data (Very important information!)

Last updated: 18 October 2018

This document discusses two topics: the categories of kernels (SPICE data) available to this instance of the WebGeocalc server, and, some information about the names of kernel sets, objects, instruments and reference frames you will need to understand.

The document is rather long, but some of the information contained here could be important—even critical—to your successful use of WGC. Even those already quite familiar with SPICE should read this document.

This document uses some SPICE terminology that might not be familiar to some users. We provide here a bit of a glossary to help out.

There is a WebGeocalc Tutorial linked from the NAIF server's WebGeocalc page, containing a good deal of useful information; WGC users should read this.

Categories of Data

The WGC installation on the NAIF server provides access to four categories of kernels.

Archived kernels, contained in a project's SPICE delivery to the NAIF Node of the Planetary Data System. Some of these archives are for JPL-managed projects, and some are for other NASA or foreign projects. Most of these archive collections are "accumulating," meaning additional data are added on typically three- or six-month centers. These are available by selecting the kernel set name(s) as described below under "Using Named Kernel Set Selection."
The end date of archived kernel sets is typically six months or more earlier than today! Hover your cursor over the name of an archived kernel set to see the applicable start and end dates.

Mission operations kernels, produced by flight teams for projects operated at JPL, and also for some ESA projects (e.g. MEX, Rosetta, TGO). Also associated with this collection is an eclectic and incomplete set of kernels from pre SPICE era missions such as Voyager. New kernels for ongoing missions could be added very frequently—even daily. These are available only by using manual kernel(s) selection as described below under "Using Manual Kernel Selection."

Generic kernels, existing independent of any particular mission. Examples are planet and satellite ephemerides (SPK), planet and satellite cartographic constants (PCK) including Digital Shape Kernels (DSKs), various tracking station topocentric locations (SPK) and frames (FK), high-precision orientation for the earth and the moon (binary PCKs) and the latest leapseconds kernel (LSK). NAIF has organized the generic kernels into four collections; there is some duplication across them. These kernel sets are available using the kernel set names shown here:

"Solar System Kernels"
"Ground Stations Kernels"
"Latest Leapseconds Kernel"
"Latest Planetary Constants Kernel"

Class programming lessons kernels—those lessons used in the classes taught by NAIF and also found under the "Self-training" link on the NAIF home page. With one of these kernel sets loaded you can use WGC to do some—but not all—steps in five of those instructional programming lessons. Click here for instructions for using WGC to do portions of the lessons.

Access to kernels in all three categories of data is provided in the "Kernel selection" drop-down menu at the top of each calculation page. Ease of use varies considerably: read on!

If you select multiple kernel sets, or if you manually select kernels in addition to selecting one or more kernel sets, there is a possibility of having "competing kernels" that could lead to use of unexpected (not wanted) data in your WGC calculation. Be sure to read the section below on "Selecting multiple kernel sets," most especially the information on competing kernels.

Pre-loaded Kernels

In order to make use of WGC a bit more straightforward, a WGC installation may load some kernel sets automatically during start-up. Typically this would include some of the most commonly used generic kernels, such as leap seconds (LSK) and planetary and satellite ephemerides (SPKs). One can see which if any kernel sets have been pre-loaded during WGC start-up by looking at the "Kernels Selected" panel on the right-hand side. (You could un-load any of these kernel sets by clicking on the "X" symbol found to the right of the kernel set name.)

Using Named Kernel Set Selection

The first and third categories—project archived kernels and generic kernels—are the easiest to use within WGC. This is because NAIF has created one or more meta-kernels for each mission in the mission archive collection, and one meta-kernel for each of the generic kernel collections. Selecting the mission name or one of the four generic kernel sets in the kernel set drop-down menu causes the corresponding meta-kernel(s) to be loaded, which in turn loads all of the underlying "real" kernels: the user need not try to find and load individual kernels. This is a BIG help!

The time span covered by any of these kernel collections is shown, if applicable, in a "tool tip" floater box displayed when your cursor hovers over the kernel set name. (Not available when a mouse is not available.) Take note that in some cases there are "predict" ephemeris data extending well past the end date of the reconstruction data (also called "actual" or "definitive") mission data.

If you would like to see exactly which kernels are contained within a given kernel set's meta-kernel(s) you can look at these locations on the NAIF server.

For the various mission archive meta-kernels:
The mission archive meta-kernels are contained inside the mission archival data sets. Start looking here:
https://naif.jpl.nasa.gov/naif/data_archived.html
Then click on the "Volume FTP Link" for that archive, then on "extras," and then on "mk."
Some missions, such as Stardust, have only one meta-kernel, although there are usually multiple versions. Look at the latest version. Other missions, such as Cassini, have a series of meta-kernels, usually covering one calendar year. There may be multiple versions of each yearly meta-kernel. Look at the latest version for the year or period of interest.

If you wish to learn more about an individual kernel in a mission archive, you will find those kernels starting here:
https://naif.jpl.nasa.gov/naif/data_archived.html
Then click on the "Volume FTP Link" for that archive, then on "data," and then on the kernel type of interest. Note that each kernel type folder contains an "*info.txt" file that outlines the folder contents and describes the kernel naming schema.

For the Solar System Kernels meta-kernel:
https://naif.jpl.nasa.gov/pub/naif/pds/wgc/mk/
Click on the latest version of "solar_system_vxxxx.tm".
If you wish to look at or fetch an individual kernel in the Solar System Kernels collection, look here:
https://naif.jpl.nasa.gov/pub/naif/pds/wgc/kernels/

For the Ground Stations meta-kernel:
https://naif.jpl.nasa.gov/pub/naif/pds/wgc/mk/
Click on the latest version of "ground_stations_vxxxx.tm".
If you wish to look at or fetch an individual kernel in the Ground Stations Kernels collection, look here:
https://naif.jpl.nasa.gov/pub/naif/pds/wgc/kernels/

For the Latest Leapseconds meta-kernel:
https://naif.jpl.nasa.gov/pub/naif/pds/wgc/mk/
Click on the latest version of "latest_lsk_vxxxx.tm".

For the Latest Planetary Constants meta-kernel:
https://naif.jpl.nasa.gov/pub/naif/pds/wgc/mk/
Click on the latest version of "latest_pck_vxxxx.tm".

Selecting multiple kernel sets

You may select two or more mission kernel sets. This feature allows you to, for instance, find times when the Mars Odyssey orbiter was in view of the Spirit rover. If you do select multiple kernel sets, be aware that it's possible a kernel from one set will "compete with" a kernel from the other set. Read here for information on competing kernels.

It is possible to select one or more mission kernel sets and one or more manually specified kernels. Read more about "Manual" kernel selection below. It is also possible to select one or more mission kernel sets and one or more of the generic kernel sets.

Using Manual Kernel Selection

Should you find it necessary to work with one or more individually selected kernels, most likely found in one of the mission operations collections on the NAIF server, that is possible using the "Manual" option of the Kernel selection drop-down menu. There are several likely motivations for doing this.

To have access to "predictive" kernel data available only in the mission operations kernel set for the mission of interest.
To make use of kernel data that have not yet made it into a mission's archive.
To use kernels from past missions that have not yet made it into the PDS SPICE archive.

This "Manual" approach is usually more difficult to use as compared to the methods described earlier because:

The mission operations collections typically contain every single kernel of every type. Unless you are quite familiar with the kernel naming schema(s), or unless you have been told to examine a specific named kernel, you may get lost in these vast collections.
When the "Manual" kernel loading option is used, the auto-complete feature for fill-in boxes is not available.

Selecting the "Manual" option and then pressing the "Choose kernels..." button will display a dialogue box opened in the folder containing all mission operations folders and the "generic_kernels" folder. You'll need to drill down to find the kernel of interest. You can select multiple kernels at a time from within a single folder by using your browser's multiple selection method (e.g. on a Mac hold down the Shift key while clicking on multiple kernel names).

If you find it necessary to use the "Manual" method and you wish to first examine a particular kernel, you can look here on the NAIF server:

https://naif.jpl.nasa.gov/pub/naif/

from where you can select the mission of interest, or the "generic_kernels" folder, and then drill down to find the kernel of interest. For text kernels you can simply click on the kernel name to examine its contents. For binary kernels (those normally having the letter "b" as the first character in the file extension) you will have to download the kernel and use one of the SPICE Toolkit utility programs such as brief or ckbrief to obtain a contents summary, or use the utility named commnt to read any meta-data (comments) included inside the kernel.

Understanding Names

The SPICE system makes wide use of names to identify objects and other items used in computations. Some names are well known, but many are rather obscure. The information below should help you navigate through SPICE names used in WGC.

Available Names

When using a mission kernel set or any of the generic kernels sets, the corresponding meta-kernel(s) are loaded automatically, and the names of the objects (bodies), frames and instruments available from the loaded meta-kernel(s) are used in the auto-fill feature for appropriate input boxes. If you'd like to see a listing of ALL of the names available for a given fill-in box to help you in making your choice, this can be accomplished by typing a blank in the fill-in box.

Kernel Set Names (Mission names)

Mission names are often used in specifying the SPICE data collection—the "Kernel selection"—you wish to work with. Some of these names are rather well known, but some may not be. One could "Google" any mission name to find out more.

All Kernel set names available to WGC are shown in the drop-down menu. Note that this menu has a scroll bar on the right side—scrolling is needed to view all kernel sets.

When a specific mission or generic kernel set is selected as the Kernel selection to be used, an auto-complete feature becomes operable in fill-in boxes; this helps a great deal. Unfortunately that feature cannot be made to work under "Manual" kernel selection.

Object Names (Target, observer or body names)

Solar system objects are often used in computations. Some fairly obvious examples are "Mars," "Phobos" and "Sun." But there are well over one hundred natural satellites of planets, and many hundreds of comets and asteroids that could be used in SPICE computations. With new objects being discovered all the time it is impossible to maintain a current and complete list of names for objects that could be used in WGC calculations.

If you use only mission archive kernel sets, the names of all available objects will appear in the various drop-down menu lists where an object name must be selected as "target" or "observer." The same is true for any of the generic kernel sets: Solar System, Ground Stations, Latest Leapseconds or Latest Planetary Constants. However, this auto-complete feature is not available for any manually selected kernels. For manually selected kernels, the user must have outside knowledge of an available object name of interest. Many such names may be found in the NAIF IDs Required Reading document, as well as inside any SPICE Toolkit. Note, however, that this document gets updated only when a new Toolkit is released, which is not very frequently.

But finding a name in "NAIF_IDs" has no bearing on if data for that particular object is contained inside the kernel you have manually loaded. The real answer to this is a hoped for WGC enhancement: the ability to run, from within WGC, a SPICE Toolkit utility that will show you what objects are included in the manually loaded kernel.

Reference Frame Names

Selecting or entering a reference frame name could be one of the more difficult aspects of using WGC. The frame selection process can be complicated because of the several types of reference frames usually available, not to mention the sometimes large number of frames of some types. The selection process is aided somewhat by the auto-complete feature tied to the Reference frame fill-in box; type in some of the letters associated with a desired reference frame name and any matching frame names will pop up. But as already stated, if you are using manually selected kernels, the auto-complete feature is not available. NAIF's Frames Required Reading document provides much information about the reference frame subsystem. Some details follow, below.

Inertial frames

Several inertial reference frames that are built into the SPICE system are always available, such as J2000 (also known as ICRF and EME2000) and ECLIPJ2000. The J2000 frame is the most frequently used frame in SPICE computations.

Body-fixed frames

A number of body-fixed frames, specified by the International Astronomical Union (IAU), are often available: these are what one uses when interested in computations tied to a planet's, satellite's, comet's or asteroid's orientation (sometimes called the spin state). These frames have a name style of "IAU_body-name," such as IAU_MARS or IAU_TITAN.

Instrument frames

The names of mission-specific reference frames associated with a given mission are available through the auto-complete feature of the "Reference frame" fill-in box. There may be a plethora of reference frame names available, most of which will not be familiar to you. The solution is to examine the mission's Frames Kernel (FK). The mission's FK is available in the project's SPICE archive on the NAIF server, appearing under this location:

https://naif.jpl.nasa.gov/naif/data_archived.html

Select the archive name in the "Volume FTP Link" column, then select the archive's "data" folder and then the "fk" folder. For example, if you are interested in using the Cassini VIMS instrument and you're wondering what reference frame name to use, you would look here:

https://naif.jpl.nasa.gov/pub/naif/pds/data/co-s_j_e_v-spice-6-v1.0/cosp_1000/data/fk/

and open the latest Cassini mission frames kernel—the file named "cas_v40.tf" (or a later version).

Instrument Names

Some WGC computations require specification of an instrument name. The names of instruments associated with a specified mission are available through the auto-complete feature of an Instrument fill-in box. However, there may be many more instrument names than you would expect, and most of the names are based on instrument acronyms that may be rather cryptic. The best way to find out more information about any instrument name used in WGC is to look at that instrument's Instrument kernel (IK) and the mission's Frames Kernel (FK) in the Kernel set collection selected by you. These IKs and FK are available in the project's SPICE archive on the NAIF server, appearing under this location:

https://naif.jpl.nasa.gov/naif/data_archived.html

Select the archive name in the "Volume FTP Link" column, then select the archive's "data" folder and then the "ik" or fk folder. For example, if you are interested in using the Cassini VIMS instrument and you wonder what "CASSINI_VIMS_RAD" means as an instrument name, you would look here:

https://naif.jpl.nasa.gov/pub/naif/pds/data/co-s_j_e_v-spice-6-v1.0/cosp_1000/data/ik/

and open the VIMS instrument kernel—the file named "cas_vims_v06.ti" (or a later version).