The MAVEN SPICE Data Archive
========================================================================
Last update: November 15, 2021
Introduction
============
This document describes the MAVEN SPICE archive, containing
observation geometry and other ancillary data in the form of SPICE
System kernel files for the MAVEN spacecraft and its instruments and
targets. It describes the archive directory structure, kernels
included in the archive, their naming conventions, and provides
pointers to the SPICE software and documentation.
Table of Contents
=================
Introduction
Table of Contents
Overview
Errata
Archive Contents
Kernel Types
Archived Kernel Details
SPK Files
PCK Files
IK Files
CK Files
LSK Files
SCLK Files
FK Files
MK Files
Miscellaneous Files
Checksum Files
Orbit Number Files
File Formats
SPICE Software and Documentation
Contact Information
Cognizant Persons
Overview
========
This archive includes the complete set of MAVEN SPICE data files
(kernel files), which can be accessed using SPICE software. The
SPICE data contain geometric and other ancillary information needed
to recover the full value of science instrument data. In particular
SPICE kernels provide spacecraft and planetary ephemerides,
spacecraft and instrument orientation, instrument mounting
alignments, and data needed for relevant time conversions. Data in
the SPICE kernel files must be accessed using the software called
the SPICE Toolkit produced and distributed by the Navigation and
Ancillary Information Facility (NAIF) Node of the Planetary Data
System.
This archive includes data covering from MAVEN launch, 2013-11-18,
through the end time of the latest spacecraft orientation file
supplied in the archive. Until the end of the spacecraft lifespan
this archive will be accumulating with new data added according to
the project archiving plan.
Back to Table of Contents
Errata
======
1. No IK file for the STATIC instrument was available at the time of
the initial archive release. It was added to the archive in the
release 2 (August 2015).
2. All XML labels in the releases 1 (May 2015) and 2 (August 2015)
referred to the PDS4 information model version 1.4.0.0 while
setting the Primary_Result_Summary purpose attribute to the
"Observation Geometry" value, which was added to the model only
in version 1.5.0.0. This made these labels non-compliant. During
preparation of the release 3 (November 2015) the information
model version and references to the model schema and schematron
files in the offending labels were changed from 1.4.0.0 to
1.5.0.0 to make them compliant.
3. This document is a simple HTML document. Providing documents in a
simple mark-up format was allowed by earlier versions of the PDS4
Standards but became prohibited in later versions starting summer
2015. Since the document was compliant at the time when its first
version was released, it will continue to exist in the archive
as an HTML document.
4. Some of the daily IUVS CK files used as input in generating the
"v01" merged IUVS CKs covering years 2014-2016 were incomplete,
missing 10-20% of data. When this was discovered in May 2017,
all merged IUVS CKs for these years were regenerated using the
complete daily CKs, and were added to the archive as "v02" files,
fully superseding "v01" files.
5. Some of the reconstructed CK spacecraft and post-deploy Articulating
Payload Platform (APP) files present data gaps. When these gaps are
significant (when they have a duration of several hours), these data
gaps are covered by the corresponding predicted CK files. In the
current release these gaps are present on 2014-12-08, 2014-12-26,
2015-03-03, 2015-06-30, 2021-01-14, 2021-01-15, and 2021-09-27.
6. The XML labels of the ancillary products have the reference_type
attribute of the Internal_Reference association within the
Reference_List class of the Context_Area class pointing to the latest
archive description document set to "ancillary_to_data" which is not
correct. The correct value "ancillary_to_document" could not be used
in these labels because it is not available in the PDS4 information
model (IM) 1.5.0.0 used by this archive. This value was added to IM
only in the version 1.14.0.0.
7. The Miscellaneous collection was only added to the bundle in the
release 26 (August 2021) and therefore the ORBNUM tables, checksum
tables, and collection inventories and their labels that correspond
to previous releases were added irregularly in that release. This is
reflected in the bundle label of that particular release that contains
additional Bundle_Member_Entry classes for these miscellaneous
collections.
Back to Table of Contents
Archive Contents
================
Files in this archive are organized into a set of subdirectories
below the top-level directory. The following diagram shows the
structure and content of these directories. The directory names are
enclosed in square brackets ([]).
[top-level-directory]
|
|- bundle_maven_spice_v*.xml Bundle PDS4 label, one or more versions.
|- readme.txt Bundle top-level readme file.
|
|-[spice_kernels] Directory containing SPICE kernels.
| |
| |- collection_spice_kernels_v*.xml PDS4 labels for Spice Kernels collections.
| |- collection_spice_kernels_inventory_v*.tab
| | Spice Kernels collections' inventories.
| |
| |-[ck] Directory containing CK files.
| | |
| | |-- *.bc Binary CK files.
| | +-- *.xml PDS4 labels for CK files.
| |
| |-[fk] Directory containing FK files.
| | |
| | |-- *.tf Text FK files.
| | +-- *.xml PDS4 labels for FK files.
| |
| |-[ik] Directory containing IK files.
| | |
| | |-- *.ti Text IK files.
| | +-- *.xml PDS4 labels for IK files.
| |
| |-[lsk] Directory containing LSK files.
| | |
| | |-- *.tls Text LSK files.
| | +-- *.xml PDS4 labels for LSK files.
| |
| |--[mk] Directory containing MK files.
| | |
| | |-- *.tm Text Meta-kernel files.
| | +-- *.xml PDS4 labels for MK files.
| |
| |-[pck] Directory containing PCK files.
| | |
| | |-- *.tpc Text PCK files.
| | +-- *.xml PDS4 labels for PCK files.
| |
| |-[sclk] Directory containing SCLK files.
| | |
| | |-- *.tsc Text SCLK files.
| | +-- *.xml PDS4 labels for SCLK files.
| |
| +-[spk] Directory containing SPK files.
| |
| |-- *.bsp Binary SPK files.
| +-- *.xml PDS4 labels for SPK files.
|
|-+-[miscellaneous] Directory miscellaneous files.
| |
| |- collection_miscellaneous_v*.xml PDS4 labels for miscellaneous collections.
| |- collection_miscellaneous_inventory_v*.tab
| | Miscellaneous collections' inventories.
| |
| |-[checksum] Directory containing checksum files.
| | |
| | |-- *.tab Text checksum files.
| | +-- *.xml PDS4 labels for checksum files.
| |
| +-[orbnum] Directory containing orbit number files.
| |
| |-- *.orb Text orbit number files.
| +-- *.xml PDS4 labels for orbit number files.
|
+-[document] Directory containing documents.
|
|- collection_document_v*.xml PDS4 labels for document collections.
|- collection_document_inventory_v*.tab
| Document collections' inventories.
|
|- spiceds_v*.html This file and its earlier versions.
+- spiceds_v*.xml PDS4 labels for this file and its earlier versions.
Back to Table of Contents
Kernel Types
============
Different SPICE kernel types are used to store different kinds of
ancillary data.
SPK (Spacecraft Planet Kernel) files contain ephemerides (position
and velocity) for spacecraft, planets, satellites, comets and
asteroids as well as for moving or fixed spacecraft and instrument
structures. SPK files are located under the spice_kernels/spk
directory of this archive.
PCK (Planetary Constants Kernel) files contain certain physical,
dynamical and cartographic constants for target bodies, such as size
and shape specifications, and orientation of the spin axis and prime
meridian. PCK files are located under the spice_kernels/pck
directory of this archive.
IK (Instrument Kernel) files contain instrument parameters relevant
for computing an instrument's geometry such as field-of-view
definitions, CCD and optical distortion characteristics, and internal
timing parameters. IK files are located under the spice_kernels/ik
directory of this archive.
CK (C-matrix Kernel) files contain time varying orientations for
spacecraft, spacecraft structures, and articulating science
instruments. CK files are located under the spice_kernels/ck
directory of this archive.
LSK (Leapseconds Kernel) files contain the leapseconds and the
values of other constants required to perform a transformation
between Universal Time Coordinated (UTC) and Ephemeris time (ET),
which is also known as Barycentric Dynamical Time (TDB). LSK
files are located under the spice_kernels/lsk directory of
this archive.
SCLK (Spacecraft Clock Kernel) files contain on-board clock
calibration data required to perform a transformation between
Ephemeris time (ET) and spacecraft on-board time (SCLK). SCLK files
are located under the spice_kernels/sclk directory of this
archive.
FK (Frame definitions Kernel) files contain information required to
define reference frames, sources of frame orientation data and
connections between these frames and other frames supported within
the SPICE system. The science instrument frame definitions provided
in the FK files include mounting alignment information for the
instruments. FK files are located under the spice_kernels/fk
directory of this archive.
MK (Meta-Kernel) files list sets of related SPICE kernels that
should be used together, providing an easy way to make data from
these kernel sets available to a SPICE-based application by loading
meta-kernels into the program using the high level SPICE data loader
routine FURNSH. MK files are located the spice_kernels/mk
directory of this archive.
Back to Table of Contents
Archived Kernel Details
=======================
The subsections below provide details, such as file naming
conventions and specific details, for each of the kernel types
included in this archive.
Back to Table of Contents
SPK Files
SPK (Spacecraft Planet Kernel) files contain ephemerides (position
and velocity) for spacecraft, planets, satellites, comets and
asteroids as well as for moving or fixed spacecraft and instrument
structures.
This archive includes binary SPK files containing the following
kinds of ephemeris data:
- planetary ephemeris SPKs
These SPKs contain ephemeris data for the Solar System
planet barycenters, and Sun, Mercury, Venus, Earth, and Moon.
They are produced by the JPL Solar System Dynamics group
and are named DEXXX, e.g. DE430.
- Martian satellite ephemeris SPKs.
These SPKs contain ephemeris data for Mars, Phobos, Deimos,
Sun, Earth barycenter, and Earth. They are produced by the
JPL Solar System Dynamics group and are named MARXXX, e.g.
MAR097.
- reconstructed spacecraft trajectory SPKs.
These SPKs contain the reconstructed MAVEN spacecraft
trajectory determined by the MAVEN Navigation (NAV) team,
JPL.
- structures SPKs
These SPKs contain fixed locations of selected spacecraft
structures and science instruments. They are produced by
NAIF based on the MAVEN engineering documentation.
The SPK files are provided under the spice_kernels/spk
directory and are named according to the following schemas:
- planetary ephemeris SPKs
deVVVs.bsp
where
VVV is the ephemeris version
s (optional) indicates that the original file
coverage was shortened to reduce the file
size.
- Martian satellite ephemeris SPKs
marVVVs.bsp
where
VVV is the ephemeris version
s (optional) indicates that the original file
coverage was shortened to reduce the file
size.
- reconstructed spacecraft trajectory SPKs
maven_PHASE_rec_YYMMDD_yymmdd_vV.bsp
where
PHASE is the mission phase:
cru for cruise and MOI
orb for all orbital phases
YYMMDD is the file coverage start date
yymmdd is the file coverage stop date
V is the version; the file with the latest
version supersedes all similarly named
files with earlier versions.
- structures SPKs
maven_struct_vVV.bsp
where
VV is the version; the file with the latest
version supersedes all similarly named
files with earlier versions.
The most detailed description of the data in each SPK file is
provided in metadata included inside the comment area of the file.
This information can be viewed using the utility program COMMNT
included in the NAIF Toolkit.
Back to Table of Contents
PCK Files
PCK (Planetary Constants Kernel) files contain certain physical,
dynamical, and cartographic constants for target bodies, such as
size and shape specifications, and orientation of the spin axis and
prime meridian.
This archive includes generic text PCK files produced by NAIF,
containing data from the reports of the IAU Working Group on
Cartographic Coordinates and Rotational Elements. The generic text
PCK version 00010 contains data from the 2009 report.
The PCK files are provided under the spice_kernels/pck
directory and are named according to the following scheme:
- generic text PCKs
pckVVVVV.tpc
where
VVVVV is the version; the file with the largest
number is the latest, but the user of PCK
data must decide if the latest version,
or an earlier version, is appropriate
for his/her analysis task.
The most detailed description of the data in a text PCK file is
provided in metadata included inside the description area of the
file. This information can be viewed using any text editor program.
Back to Table of Contents
IK Files
IK (Instrument Kernel) files contain instrument parameters relevant
for computing an instrument's observation geometry such field-of-view
definitions, CCD and optical distortion characteristics, and internal
timing parameters.
This archive includes text IK files for all MAVEN instruments
except MAG and LPW, for which common instrument geometric parameters
normally stored in IKs are not applicable, and for the MAVEN
antennae. These IKs were created by MAVEN instrument teams with help
from NAIF.
The IK files are provided under the spice_kernels/ik
directory and are named according to the following scheme:
- instrument and antennae IK files
maven_NAME_vVV.ti
where
NAME is the instrument acronym
ant (for antennae)
euv
iuvs
ngims
sep
static
swea
swia
VV is the version; the file with the latest
version supersedes all similarly named files
with earlier versions, but the SPICE user must
decide if the latest version, or an earlier
version, is appropriate for his/her analysis
task.
The most detailed description of the data in a text IK file is
provided in metadata included inside the description area of the
file. This information can be viewed using any text editor program.
Back to Table of Contents
CK Files
CK (C-matrix Kernel) files contain time varying orientations for
spacecraft, articulating spacecraft structures, and articulating
science instruments.
This archive includes binary CK files containing the following kinds
of orientation data:
- reconstructed spacecraft orientation CKs
These CKs created by NAIF contain the reconstructed MAVEN
spacecraft attitude determined on-board and downlinked in
spacecraft telemetry.
- predicted spacecraft orientation CKs
These CKs created by NAIF contain the predicted MAVEN
spacecraft attitude and angular velocities from the
Attitude Articulation Prediction/Simulation (AtArPS)
Data Return Files (DRF) produced by the Spacecraft
Team, LMCO.
- reconstructed post-deploy Articulating Payload Platform
(APP) orientation CKs
These CKs created by NAIF contain the reconstructed MAVEN
APP attitude computed from APP gimbal angles measured
on-board and downlinked in spacecraft telemetry.
- predicted post-deploy Articulating Payload Platform
(APP) orientation CKs
These CKs created by NAIF contain the predicted MAVEN
APP gimbal rotations and rates from the AtArPS DRF files
produced by the Spacecraft Team, LMCO.
- reconstructed pre-deploy APP orientation CKs
These CKs created by NAIF contain the reconstructed MAVEN
APP attitude computed from pre APP deployment IUVS
alignment calibration performed during cruise by the
IUVS team, CU/LASP.
- reconstructed IUVS mirror orientation CKs
These CKs created by the IUVS team, CU/LASP contain the
reconstructed MAVEN IUVS mirror orientation computed from
from IUVS mirror angles measured on-board and downlinked in
the IUVS housekeeping telemetry.
- nominal SWEA orientation CK
These CKs created by NAIF contain the nominal fixed SWEA
orientation for pre- and post- SWEA boom deploy periods.
The CK files are provided under the spice_kernels/ck
directory and are named according to the following scheme:
- all CKs
mvn_NAME_TYPE_YYMMDD_yymmdd_vVV.bc
where
NAME is the structure acronym
sc for spacecraft
app for APP
iuvs for IUVS
swea for SWEA
TYPE is the data type
rel for reconstructed
rem for reconstructed, merged
pred for predicted
prm for predicted, merged
nom for nominal
YYMMDD is the file coverage start date
yymmdd is the file coverage stop date
VV is the version; the file with the latest
version supersedes all similarly named
files with earlier versions.
The most detailed description of the data in each CK file is
provided in metadata included inside the comment area of the file.
This information can be viewed using the utility program COMMNT
included in the NAIF Toolkit.
Back to Table of Contents
LSK Files
LSK (Leapseconds Kernel) files contain the leapseconds and the
values of other constants required to perform a transformation
between Universal Time Coordinated (UTC) and Ephemeris time (ET),
also known as Barycentric Dynamical Time (TDB).
This archive includes generic text LSK files produced by NAIF,
containing leapsecond information based on the IERS announcements.
The generic text LSK version 0011 contains leap second data up to
and including the July 1, 2015 leapsecond.
The LSK files are provided under the spice_kernels/lsk
directory and are named according to the following scheme:
- generic text LSKs
naifVVVV.tls
where
VVVV is the version; the file with the latest
version supersedes all similarly named
files with earlier versions.
The most detailed description of the data in a text LSK file is
provided in metadata included inside the description area of the
file. This information can be viewed using any text editor program.
Back to Table of Contents
SCLK Files
SCLK (Spacecraft Clock Kernel) files contain on-board clock
calibration data required to perform a mapping between Ephemeris
time (ET) and spacecraft on-board time (SCLK).
This archive includes text SCLK files containing correlation data
for the main MAVEN on-board clock, created by NAIF from the
SCLK_SCET files generated by the Spacecraft Team, LMCO.
The SCLK files are provided under the spice_kernels/sclk
directory and are named according to the following scheme:
- main MAVEN spacecraft clock SCLK files
mvn_sclkscet_VVVVV.tsc
where
VVVVV is the version; the file with the latest
version supersedes all similarly named
files with earlier versions.
The most detailed description of the data in a text SCLK file is
provided in metadata included inside the description area of the
file. This information can be viewed using any text editor program.
Back to Table of Contents
FK Files
FK (Frame definitions Kernel) files contain information required to
define reference frames, sources of frame orientation data and
connections between these frames and other frames supported within
the SPICE system. The science instrument frame definitions provided
in the FK files include mounting alignment information for the
instruments.
This archive includes text FK files defining reference frames for the
MAVEN spacecraft, its structures, and science instruments. These
FKs were produced by NAIF using the spacecraft and instrument design
documentation as well as actual instrument alignment values
determined pre-launch and as the result of in-flight calibrations.
The FK files are provided under the spice_kernels/fk
directory and are named according to the following scheme:
- MAVEN mission FK files
maven_vVV.tf
where
VV is the version; the file with the latest
version supersedes all similarly named files
with earlier versions, although a SPICE user
might decide decide to use an earlier version
if trying to be consistent with some earlier
computed results.
The most detailed description of the data in a text FK file is
provided in metadata included inside the description area of the
file. This information can be viewed using any text editor program.
Back to Table of Contents
MK Files
Meta-kernel (MK) files (a.k.a 'furnsh' files) provide lists of the
archived kernels included in this archive suitable for loading into
a SPICE-based application via the high level SPICE data loader
routine FURNSH. Using meta-kernels makes it easy to load with
one call a comprehensive SPICE data collection for a given period.
The text MK files are provided under the spice_kernels/mk
directory and are named according to the following scheme:
- MAVEN yearly MK files
maven_YYYY_vVV.tm
when
YYYY is the year covered by the kernels listed in
the MK
VV is the version; the file with the latest
version supersedes all similarly named
files with earlier versions.
Multiple versions of a meta-kernel for the same year may exist if
new kernels were added to the archive in a later release and the
meta-kernel for that year had to be updated to incorporate them. The
reasons for adding new kernels include (but are not limited to)
adding new kinds of kernels, adding new versions of existing kernels
containing more complete or more accurate data, and adding new
versions of existing kernels superseding the existing kernels that
had errors. If more than one meta-kernel file for the same year is
present, then the file with the latest version number supersedes all
earlier versions. The latest yearly meta-kernels can be loaded
together sorted by year to provide coverage for more than one year.
(Note that on some environments loading too many kernels at once may
slow down data access.)
Although the meta-kernels were created such that on a UNIX
workstation they can be loaded into a SPICE-based application
directly from the directory in which they reside -- because the
PATH_VALUES keyword is set to '..' and '/' is used as the path
delimiter -- it is recommended that users make local copies of these
files and update the PATH_VALUES keyword in each of them to point to
the actual location of the archive's 'spice_kernels' directory on
their system. Additional changes including replacing '/' and '\' in
all path specifications and converting UNIX line terminators to
terminators native to the user's system may be required to use these
meta-kernels on non-UNIX workstation.
The following guidelines were followed to pick and set priority --
determined by the order in which the kernel files are listed, with
the latest one taking precedence -- for the kernel files included in
these meta-kernels:
- the latest archived versions of LSK, PCK, SCLK, IK, FK, generic
SPKs, fixed offset SPKs, and fixed orientation CKs were included
- the latest reconstructed MAVEN NAV SPKs were included
- the adequate latest predicted CKs were included
- the latest reconstructed CKs were included
Additional description of the kernels sets listed in a text MK file is
provided in metadata included inside the description area of the
file. This information can be viewed using any text editor program.
Back to Table of Contents
Miscellaneous Files
===================
Miscellaneous files provide value added elements that are outside the
scope of the PDS archive requirements. Miscellaneous files included in
this archive are currently limited to checksum and orbit number files.
The subsection below provides details for these files.
Back to Table of Contents
Checksum Files
A checksum file provides a table of MD5 checksums for all files in the
archive as of a particular archive (bundle) version, including
checksums for all previous checksums files and their labels but
excluding the checksum for the checksum file itself and its label.
The checksum files are provided under the miscellaneous/checksum
directory and are named according to the following scheme:
- MAVEN checksum files
checksum_vVVV.tab
where
VVV is the version; the file with the latest
version supersedes all similarly named
files with earlier versions and can be
used to validate the archive in its
entirety.
Back to Table of Contents
Orbit Number Files
An orbit number (ORBNUM) file provides a table of records ordered
by an increasing orbit numbering scheme. The orbit number changes
at every periapsis event and the information contained for each
record includes the orbit number ("No."), periapsis node UTC time
("Event UTC PERI") and SCLK time ("Event SCLK PERI"), apoapsis UTC
time ("OP-Event UTC Apo"). It also includes a few additional items
computed at the time of periapsis event such as planetocentric
subsolar longitude and latitude ("SolLon" and "SolLat") in the Mars
body-fixed frame based on the IAU 2000 Mars rotation model,
planetocentric subspacecraft longitude and latitude ("SC Lon" and
"SC Lat") in the Mars body-fixed frame based on the IAU 2000 Mars
rotation model, spacecraft altitude above Mars ("Alt") relative to
the IAU 2000 Mars ellipsoid, and Sun-Mars distance ("Sol Dist").
The ORBNUM files have been generated during the mission using the
NAIF's ORBNUM utility program for consolidated spacecraft trajectory
SPK files for the orbital phases of the mission.
MAVEN orbit number files have the same names as the corresponding
SPK files but with the extension ".bsp" replaced by ".orb".
The ORBNUM files are provided under the miscellaneous/orbnum
directory and are named according to the following scheme:
- MAVEN orbit number files
maven_orb_rec_YYMMDD_yymmdd_vV.orb
where
YYMMDD is the original SPK file coverage start date
yymmdd is the original SPK file coverage stop date
V is the version; the file with the latest
version supersedes all similarly named
files with earlier versions.
Back to Table of Contents
File Formats
============
All text documents, checksum files, ORBNUM files, and other meta
information files such as descriptions, detached PDS4 labels, and
inventory tables, are stream format files, with a carriage return
(ASCII 13) and a line feed character (ASCII 10) at the end of the
records. This allows the files to be read by most operating systems.
The text kernel files in this archive -- LSKs, PCKs, SCLKs, IKs, FKs,
and MKs -- are UNIX text files, with a line feed character (ASCII 10)
at the end of each line. Binary kernel files -- SPKs and CKs -- are
IEEE LSB binary files. While data files in this archive can be used
'as is' with SPICE toolkits for the majority of supported environments,
converting the files to the binary or text format native to the user's
computer may be required for use of the data with FORTRAN toolkits on
incompatible environments. Refer to the documentation provided with the
SPICE Toolkit to find out whether conversion is needed in each specific
case.
NAIF provides a tool (BINGO) that can convert either binary or text
kernels from one format to another. This means that text files can
have their line terminator changed and binary files can be converted
to the native format of the platform of interest. BINGO is available
from the utilities page at the NAIF Node of the PDS:
https://naif.jpl.nasa.gov/naif/utilities.html
Back to Table of Contents
SPICE Software and Documentation
================================
The latest SPICE Toolkit for a variety of computer platforms,
additional utility programs, documentation, tutorials, and hands-on
lessons are available at the NAIF Node of the PDS:
https://naif.jpl.nasa.gov
specifically:
Toolkit: https://naif.jpl.nasa.gov/naif/toolkit.html
Utilities: https://naif.jpl.nasa.gov/naif/utilities.html
Tutorials: https://naif.jpl.nasa.gov/naif/tutorials.html
Lessons: https://naif.jpl.nasa.gov/naif/lessons.html
Back to Table of Contents
Contact Information
===============================
PDS Navigation and Ancillary Information Facility (NAIF),
MAIL STOP 301-121
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA, 91109-8099
818-354-8136
WWW Site: https://naif.jpl.nasa.gov
Back to Table of Contents
Cognizant Persons
=================
This archive was produced by Boris Semenov and Marc Costa Sitja,
Planetary Data System Navigation and Ancillary Information
Facility Node, Jet Propulsion Laboratory, Pasadena, California.
End of MAVEN SPICE archive description.
Back to Table of Contents