About the Data - SPICE
About WGC Data
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. NAIF provide here a bit of a glossary to help out.
There is a WebGeocalc Tutorial containing a good deal of useful information; WGC users really should read this.
Categories of Data
The WGC installation on the ESA SPICE Service server provides access to four categories of kernels (available under the "Kernel selection" drop-down menu of any WGC Calculation)
- ARCHIVED: Archived kernels, contained in a SPICE PDS3 Dataset or PDS4 Bundle delivery to the Planetary Science Archive (PSA). Most of these archive collections are "accumulating", meaning additional data are added on typically three- or six-month basis.
- OPS: Mission operations kernels, produced by ESS or by the Science Operations Centers. New kernels for ongoing missions could be added very frequently—even daily. These usually include all and the most up-to-date information of interest for a given mission.
- PLAN: Mission operations kernels for planning purposes, produced by ESS or by the Science Operations Centers. New kernels for ongoing missions could be added very frequently—even daily and are used for planning purposes for both the SOC's and the Instrument Teams during operations.
- LEGACY: Legacy Kernels, produced by ESS that contain kernels of missions which are in Legacy phase and therefore are not updated anymore (such as Giotto).
- STUDY: Study Kernels, produced by ESS or by the Science Operations Centes. These kernels have a rather 'volatile' nature and might go through important change and updates. These datasets could even be considered 'test' data.
- TEST: Test Kernels, produced by ESS; typically contain test data with very preliminary information for missions which are in early stages of development.
- SUPPORT: Support Kernels, produced by ESS; contain data that has been requested ad-hoc to the ESA SPICE Service, typically for a mission that already has another category of data.
- SPICE CLASS: 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.
- MANUAL: described below under "Using Manual Kernel Selection."
Access to kernels in all nine 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.
Using Named Kernel Set Selection
The first to eight categories —everyone except Manual— are the easiest to use within WGC. This is because ESS has created one or more meta-kernels for each kernel collection. Selecting any kernel set 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 needs not try to find and load individual kernels. This is a BIG help!
The time span covered by any of these kernel collections is not shown. Please note that for archive kernel sets often the end time is several months, or more, behind the current date, because archiving typically lags three to six months behind real time. In a very few cases there are "predict" ephemeris data extending well past the end date of the reconstruction (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 ESS SPICE server.
- FOR THE VARIOUS MISSION ARCHIVE META-KERNELS: The mission archive meta-kernels are contained inside the mission archival data sets. Start looking here. For PDS3 Archives, then click on the "Link to Archive" for that archive, then on "EXTRAS" and then on "MK" and open the latest version of the meta-kernel. For PDS4 Archives, then click on the "Link to Archive" for that archive, then on "spice_kernels" and then on "mk" and open the latest version of the meta-kernel.
- FOR THE VARIOUS MISSION OPERATIONAL META-KERNELS: The operational meta-kernels are published in Git via Bitbucket and are also available at the ESS FTP. In the following link, you have a table available that directs you to both BitBucket and the FTP for each mission once there, you can navigate to the meta-kernel directory and retrieve tha latest version.
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 Express orbiter was in view of the ExoMars Trace Gas Orbiter.
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.
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 ESS 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 PSA SPICE archive.
This "Manual" approach can be rather more difficult to use as compared to the methods described earlier.
- 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).
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.
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, such 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.
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 almost certainly the most frequently used frame in SPICE computations.
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_MERCURY.
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 ESS server.
For example, if you are interested in using the BepiColombo MIXS instrument and you're wondering what reference frame name to use, you would look here: ftp://spiftp.esac.esa.int/data/SPICE/BEPICOLOMBO/kernels/fk/ and open the latest BepiColombo mission frames kernel—the file named "bc_mpo_v18.tf" (or a later version).
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.
For example, if you are interested in using the BepiColombo MIXS instrument and you wonder what "MPO_MIXS-C" means as an instrument name, you would look here: ftp://spiftp.esac.esa.int/data/SPICE/BEPICOLOMBO/kernels/ik/ and open the VIMS instrument kernel—the file named "bc_mpo_mixs_v04.ti" (or a later version).