The only way one can read and make use of SPICE data (kernels) is by using SPICE APIs (subroutines) incorporated into a program you have written, or by using a SPICE-aware program you have obtained from elsewhere–one
which incorporates SPICE Toolkit APIs.
If you are intending to use someone else's SPICE-aware tool
If the tool provider has also provided SPICE kernels, be sure those kernels are appropriate and the best for your specific use–don't simply assume they are correct for your needs. Remember that SPICE kernels should contain embedded descriptive metadata, usually referred to as "comments," that might help you select the appropriate kernels. For binary kernels, any included comments can be read using either the "commnt" or "spacit" utility program included in each Toolkit. For text kernels (ASCII files) one can read the included comments by opening the kernel in any text display or text editor tool.
Except for the new Digital Shape Kernel (DSK), every binary kernel has a start and an end time. Toolkit utilities "brief," "ckbrief" and "spacit" can help you determine those times. Some binary kernels also have data gaps between the start and end times. "Brief" and "ckbrief" can help you determine if any gaps exist. If you execute "brief" or "ckbrief" with no other items on the command line, HELP information will be displayed. All three utilities have a User's Guide.
Text kernels have no encoded time bounds. The latest PCK is generally good for any epoch, although a very few PCK data items are said to apply for a specific time span. The lastest SCLK for a given mission should usually be used, except for some planning activities. LSKs can expire only on Jan 01 or July 01 of any year.
The user is advised to ALWAYS read the embedded comments in every kernel to fully understand applicability issues.
See the tutorials for each kernel type for further inforrmation.
If you are intending to write your own SPICE-aware tool
All of the above points are important. In addition you'll need to learn about the capabilities of SPICE Toolkit APIs in order to select the few of those you'll need to incorporate in your program, and you'll need to understand the operational details of those APIs. Some of the SPICE tutorials and the documents "Most Used APIs" and "Permuted Index" can help you find APIs that might meet your needs. The source code "header" included with each API provides usage details and examples. All of this documentation is available from the NAIF website, and all but the tutorials are included in each Toolkit package.
Because a user's programmatic interface to SPICE data is always through SPICE Toolkit APIs, there is no need for users to obtain and read SPICE kernel Software Interface Specifications (SISs).