Software Architecture and Specifications of Chandrayaan
1 Introduction:
This Data Product and Archive Software Interface Specification for the Solar X-ray Monitor (XSM) instrument on-board the orbiter of Chandrayaan-2 mission describes the XSM data products, product file definition, generation of data products, and PDS4 archive organization. It provides the details of XSM data archived and disseminated from ISRO Science Data Archive (ISDA) at Indian Space Science Data Center (ISSDC). This document serves as a reference for scientists who intend to utilize the XSM data by providing them an overview of the instrument, data product definitions, etc. Some acronyms used in the document and a list of reference documents are given below. Rest of this document is organized as follows. Section 2 provides a description of the XSM instrument, observation plan, and calibration. The data product definition and generation of products with XSM Data Analysis Software (XSMDAS) are described in section 3 and section 4 provides the organization of the XSM PDS4 archive and file nomenclature. Formats of the data products and sample XML label are provided in appendix.
XSMDAS: XSM Data Analysis Software The XSM Data Analysis Software (XSMDAS) is designed to process the level-0 data to level-1 (raw) and level-2 (calibrated) data products. It is composed of individual modules that cater to specific functionality and is meant to be used by the scientific users for the analysis of XSM data. All calibration information of the XSM required for the analysis are included in a calibration database (CALDB) described in the next section, which is used by the XSMDAS modules. xsml1gen Level-0 pld file Science file xsmhkextract xsmcalsa xsmgtigen xsmgenspec xsmgenlc HK parameter file Sun Angle file GTI file Spectrum & ARF Light curve Key: Level-0 files Level-1 files Level-2 files XSMDAS modules SPICE Kernels CALDB Figure 3.1: Work flow of the XSM Data Analysis Software (XSMDAS) showing its modules and their input and output files. Figure 3.1 shows the flowchart of the XSMDAS where individual modules and their input and output files are shown. The modules that generate level-1 files do not have any user-configurable parameters and also do not use any calibration information. Hence, these are executed only at June 2021 Page 13 Chandrayaan-2 XSM Data Products and Archive Software Interface Specification the POC to generate level-1 data files, which are to be made available to the users. On the other hand, the modules for generating level-2 science products require various user inputs as well as the calibration data and hence the users may need to execute these modules with specific inputs to meet their requirements. Thus, these modules of XSMDAS are made available, along with the first release of data sets from ISDA. Subsequent revisions of the software, if necessary, will also be provided from time to time. XSMDAS is composed of eight modules; three each for the generation of level-1 and level2 data products and two additional modules for adding spectra and generating XML labels corresponding to the data product files. A brief description of the algorithm employed and the available options for all the modules are given below. A complete list of user input parameters is given in the user guide distributed along with the software. Help files can also be accessed with the xsmhelp command. A. Modules for level-0 to level-1 processing 1. xsml1gen: Combines raw payload data frames corresponding to one day of observation from multiple level-0 files and generates a single FITS file, ignoring corrupted frames, if any. 2. xsmhkextract: Extracts the house keeping parameters of XSM from the header of the data frames and creates a table with these parameters. 3. xsmcalsa: Using SPICE kernels with the orbit, attitude, and clock information of the spacecraft and other auxiliary information, computes the instantaneous angle subtended by the Sun vector with respect to the instrument bore-sight for the time stamps correponding to level-1 science file generated by xsml1gen. It utilizes user-level application program interfaces provided with the SPICE toolkit. A FITS table with the polar (θ) and azimuthal (φ) angles of the Sun with respect to XSM instrument reference frame (see figure 2.2) is the output of this module. B. Modules for level-1 to level-2 processing 1. xsmgtigen: Creates good time intervals (GTI) during which the observations are useful for scientific analysis based on the house keeping parameters and the Sun angle parameters. This module also provides a provision to incorporate additional user-defined GTIs. 2. xsmgenlc: Generates the light curve from the level-1 data for a specified energy range, correcting for the effective area variation with angle. The generated light curves gives count rates for on-axis observations with XSM. There is an option to generate a light curve for any energy range with a time bin size as multiples of one second and to generate a light curve for the three pre-defined coarse energy ranges with a time bin size in multiples of 100 ms. 3. xsmgenspec: Generates the gain corrected 512 channel PI spectrum from the raw 1024 channel PHA spectral data for the required duration. The output can be either a timeintegrated spectrum (type-I PHA file) or a time series of spectra (type-II PHA file) with June 2021 Page 14 Chandrayaan-2 XSM Data Products and Archive Software Interface Specification a specified time bin size. For each PI channel, apart from the counts, statistical errors computed from Poisson distribution and systematic errors from the ground calibration are also recorded in the spectrum file. Ancillary response file (ARF) corresponding to the spectrum incorporating the effective area is also generated along with the spectrum. C. Additional modules 1. xsmpds4gen: This module generated the PDS4 XML label associated with each data product file. It reads the FITS product file and generates the respective XML label using the appropriate template XML file. The attributes that had placeholder values are replaced with the actual values based on the contents of the FITS file. 2. xsmaddspec: This tool provides the facility to add together multiple spectrum files generated by xsmgenspec to create a single spectrum output. The module takes care of the propagation of statistical and systematic errors of the channel-wise counts while adding the spectra together. It also combines the ancillary response files that correspond to the individual spectra and provide an output ARF file to be used with the added spectrum. 3.5 XSM Calibration Database (CALDB) Various calibration parameters such as gain, spectral redistribution, and effective area, which are required for the analysis of XSM data, are derived from extensive ground calibration experiments. These data are stored in a set of FITS files that form the calibration database (CALDB) of XSM, which is accessed by the XSMDAS modules to generate the products. The XSM CALDB design follows a philosophy similar to the HEASARC’s CALDB system2 meant for X-ray astronomy missions. Each aspect of calibration data is stored in individual FITS file BINTABLE extensions with appropriate structure. In case of any update in a particular calibration data, a new file with the same format as the previous one is included in the new release while retaining the old versions. A calibration index file (FITS format) included in each CALDB release keeps a record of the file names with the latest calibration data of each type and includes the names of older versions of the same data with their quality flag marked as bad. The index file also defines the validity start and end time for each file, thus allowing multiple good files of the same type valid over a different range of times. This is particularly useful if some of the calibration parameters (e.g., gain) varies over the mission duration and different set of values are to be used for observations at different times. With such a design of the CALDB system, the calibration file names are not hard-coded in the software. XSMDAS accesses the index file of the CALDB and identifies the appropriate calibration data file based on the observation time of the data being processed. This allows updates of the CALDB without any updates in the data analysis software unless there is a change in format or addition of new types of calibration files. The XSM CALDB includes six types of calibration files other than the calibration index file, the contents of which are briefly described below. 2 https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/caldb_intro.html June 2021 Page 15 Chandrayaan-2 XSM Data Products and Archive Software Interface Specification (i) Gain: This file includes the gain and offset of the XSM over a grid of TEC current and interaction position in the detector. (ii) Effareapar: This file includes the parameters required for the computation of effective area, defined over an x-y grid of position of the Sun in the field of view of XSM. (iii) RMF: Redistribution Matrix File defining the spectral redistribution function in Pulse Invariant (PI) channels over a grid of incident energies. (iv) RSP: Response files including redistribution and on-axis effective area for use with area scaled spectra. Two such files where one correponds to without Be filter and the other with Be filter. (v) Syserror: Channel-wise systematic errors to be included in the spectrum are recorded in this file. (vi) Ebounds: This file defines the nominal energy range for the PI channel definition. (vii) Abscoef: This file includes the X-ray absorption coefficients of materials that form the entrance window (Be), dead layer (SiO2), and the detector (Si) obtained from NIST. The XSM CALDB is also made available during the data release. Further updates in the calibration if any would also be released as subsequent versions of the CALDB. 4 XSM PDS4 Archive Organization The archival of data from payloads of Chandrayaan-2 follow PDS4 standard. PDS4 information model 1.11.0.0 is adopted for generation of archive of all instrument data sets including XSM. For XSM, the POC will create instrument archive bundle in PDS4 compliant format, which will be uploaded to ISSDC for archival. In this section, the mission level archive organization, XSM bundle structure and collections, nomenclature, and dissemination plan are discussed.
4.1 Mission Level Archive Organization:
Figure 4.1 shows the mission level archive organization. Under isda archive, mission (ch2),
and instrument host orbiter (cho) directories are included. Under the orbiter directory, context
products and applicable xml schema files are stored. The instrument bundles are organized
under the respective mission operation phase, primarily the normal operation phase (nop). Like
xsm, directories of other instruments with the respective bundle are placed under nop directory.
The context directory includes the context products as defined in the PDS4 data provider’s
manual. Context products for the mission, instrument host or the orbiter, and all instruments
are included. Each of them is an XML file with a unique LID which includes description of
the respective component like mission or instrument. Data and other products in the archive
are referenced to these context products by referring to their LIDs in the product labels. LIDs
for the context products of Chandrayaan-2 mission, Chandrayaan-2 orbiter, and XSM are listed
below:
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Chandrayaan-2 XSM Data Products and Archive Software Interface Specification
isda
context nop xml_schema
cho
ch2
xsm
Figure 4.1: Mission level archive organization
• urn:isro:isda:context:investigation:mission.chandrayaan2
• urn:isro:isda:context:instrument host:spacecraft.ch2orbiter
• urn:isro:isda:context:instrument:xsm.ch2orbiter
The XML schema directory includes the applicable XML schemas for the PDS4 labels in the
archive.
4.2 XSM Instrument Bundle
All data products and other products such as documents related to XSM are included in an
instrument bundle. The XSM instrument bundle then includes multiple collections for each
type of products like data, document, calibration, and miscellaneous. Figure 4.2 shows the
organization of individual collections under the xsm instrument bundle.
The data collection includes day-wise raw and calibrated data products and labels of XSM.
Documents related to XSM data are included as document collection and calibration collection
has the required calibration information. The XSMDAS is included under the miscellaneous
collection. The instrument level bundle and association to collections are defined in the bundle xsm.xml file under the main instrument directory. For easy reference, a readme file with the
bundle organization is also included there. Details of the contents under each collection is given
in the next section.
4.2.1 LID Definition
Every product in a PDS archive is assigned with a unique logical identifier (LID). The products also have a version identifier (VID) so that multiple versions of the same product can be
uniquely identified. LIDs are defined following the standards defined in the PDS4 data provider’s
handbook and the Chandrayaan-2 archive conventions.
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Chandrayaan-2 XSM Data Products and Archive Software Interface Specification
xsm
calibration data documents miscellaneous
yyyy
mm
dd
raw calibrated
Figure 4.2: XSM instrument bundle organization
In the case of XSM, the LIDs for the instrument bundle, collections, and basic products are
defined as per the below convention:
• Bundle LID: The bundle LID for XSM is defined as
urn:isro:isda:ch2 cho.xsm
where ch2 cho.xsm is the unique bundle id.
• Collection LID: These are formed by appending the collection id to the bundle LID
defined above. Each collection under the XSM bundle has a unique id and is defined in
table 4.1.
• Product LID: LIDs for basic products are formed by appending the unique product id
to the collection’s LID as:
urn:isro:isda:ch2 cho.xsm:
.png)
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