NAIF’s Approach to SPICE Kernel Archive Preparation¶

NAIF’s approach to creating PDS4 SPICE archives can be summarized by this statement:

All SPICE data for a given mission are archived as UNIX text and binary files(1) in a single, accumulating archive on a single virtual volume(2) having the same directory structure(3), the same set of meta information files(4), data file labels with the same structure(5), and archive documents with the same structure(6) as all SPICE archives produced by NAIF(7).

Each of the points identified in this statement using a number in parentheses is briefly explained below:

All SPICE data are archived as UNIX text and binary kernels, meaning that all text kernels have lines terminated by <LF> (line feed) only and all binary kernels are in LTL-IEEE (little-endian or IEEE LSB) binary format.

Following this requirement ensures consistency across all SPICE archives and usability of the data for both future users and the archive distribution and manipulation tools at the NAIF Node of the PDS. If conversion of text kernels with lines terminated by <CR><LF> (carriage return and line feed) and binary kernels in BIG-IEEE (big-endian) format to the required format is needed, NAIF provides a utility program called BINGO, available on the “Utilities”s page on the NAIF web site.

All SPICE data for a given mission get archived in a single accumulating archive bundle on a single virtual volume.

There is no need to break SPICE data for a given mission into multiple bundles based on the mission phases (as was done by NEAR), kernel types (as was done in other PDS3 data sets produced by other nodes), data producers, different observers (spacecrafts) in a mission or in any other way. Doing so leads to duplication of data, extra effort in preparing multiple marginally different copies of meta-information files, and, most importantly, complicates the lives of future users who, in most cases, prefer to get all SPICE data for a mission from one place.

The Bundle name for this single archive is usually set to <sc>_spice where <sc> is the spacecraft acronym. For example for MAVEN, maven_spice, for ExoMars 2016, em16_spice, and so on.

All SPICE bundles – even those for the missions that have ended – are accumulating, which means that if/when additional SPICE data become available, these data are added to the same archive. For on-going missions the data that is being added is usually covering the next period of time; for past missions the data that is being added usually provides a better trajectory solution, a better attitude estimate, an instrument parameters update, or fixes liens. New additions are called releases or increments. SPICE bundles do not have a specific file to record releases, nevertheless the version of the bundle label is equivalent to the bundle release. Checksum files in the Miscellaneous collection are also a good resource to understand the files included in each release.

Once in the archive, no files are removed from it or replaced with different versions with the same name. Instead, new files superseding already archived files are added to the archive. For kernels, they are given distinct names and the fact that they supersede previously archived kernels is reflected in the meta-information files (SPICEDS and meta-kernel files).
All SPICE archives have the following PDS-compliant directory structure:
<sc>_spice           bundle root directory
 |
 |-- document        contains the SPICE archive description file
 |
 |-- miscellaneous   contains the miscellaneous collection subdirectories
 | |
 | |-- checksum      contains checksum files
 | +-- orbnum*       contains orbit number files
 |
 +-- spice_kernels   contains kernel type subdirectories
   |
   |-- ck            contains binary Camera-Matrix kernels (CKs)
   |-- dbk*          contains binary database kernels (DKBs)
   |-- dsk*          contains binary digital shape kernels (DSKs)
   |-- ek*           contains text event kernels (EKs)
   |-- fk            contains text frame kernels (FKs)
   |-- ik            contains text instrument kernels (IKs)
   |-- lsk           contains text leapseconds kernels (IKs)
   |-- mk            contains meta-kernels (MKs)
   |-- pck           contains text and/or binary planetary constant kernels (PCKs)
   |-- sclk          contains text spacecraft clock kernels (SCLKs)
   +-- spk           contains binary spacecraft kernels (SPKs)
where the <sc> part is formed as described above (for example, for MAVEN it is maven_spice). Elements with * are present if needed.

The miscellaneous directory may contain additional subdirectories to store additional value-added, non-kernel files such as ORBNUM files.

The document directory may (but does not have to) contain additional subdirectories for additional documents or document sets.
All SPICE data have the following meta-information files, most of which are required by PDS standards:
bundle_<sc>_spice_v???.xml                                  PDS4 bundle label
readme.txt                                                  readme in text format
document/collection_document_v???.xml                       document collection labels
document/collection_document_inventory_v???.tab             document inventory tables
document/spiceds_v???.html                                  SPICE archive description document
document/spiceds_v???.xml                                   SPICE archive description label
miscellaneous/collection_miscellaneous_v???.xml             miscellaneous collection labels
miscellaneous/collection_miscellaneous_inventory_v???.tab   miscellaneous inventory tables
miscellaneous/*/*.xml                                       XML labels, 1 per checksum and ORBNUM
spice_kernels/collection_spice_kernels_v???.xml             SPICE kernels collection labels
spice_kernels/collection_spice_kernels_inventory_v???.tab   SOUCE kernels inventory tables
spice_kernels/*/*.xml                                       XML labels, 1 per kernel
where the <sc> part is formed as described above (for example, for MAVEN it is maven_spice). Elements with ??? have one file per archive release with ??? replaced with the document version number.
All SPICE kernels included in all SPICE archives are labeled with PDS4 XML labels.

Templates for different PDS Information Model versions of SPICE kernel labels (and other labels) are provided with the NAIF PDS4 Bundler.

All SPICE archives include a SPICE Description document (SPICEDS) that provides all the required information to describe in detail the SPICE archive.

Examples of different SPICEDS that can be used as references are provided with the NAIF PDS4 Bundler.

While experts on PDS standards can (and did during peer-reviews) find a number of things about SPICE archives that need improvement or even correcting, NAIF continues to carry on with the archiving approach that it established and polished over 20+ years of creating PDS3 SPICE data sets and over 5+ years of creating PDS4 SPICE archive bundles. Applying this approach without major deviations results in archives that truly look and feel the same from mission to mission. This helps both the users of the data who can count on finding archives with the same structure, and the NAIF node staff who in most cases are the people providing expert advice about SPICE kernel archives.

For the reasons noted above, please carefully follow the instructions provided in this chapter and use the NAIF PDS4 Bundler software package.