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IV. NEOWISE Data Processing

IV.1. Overview

IV.1.a. The WISE Science Data System

Science data processing for NEOWISE is carried out using version 7.5 of the WISE Science Data System (WSDS) developed and operated at IPAC at the California Institute of Technology. The WSDS is a hardware, software, operations, and analysis system that converts raw NEOWISE image data packets, engineering telemetry and ancillary information into the calibrated Single-exposure Images, Source Database, Moving Object Tracklets, and ancillary data and metadata products and user documentation.

The functional block diagram showing the elements and major interfaces of the WSDS is shown in Figure 1. NEOWISE processing utilizes the Ingest, Scan/Frame, and Moving Object Data Reduction Pipelines, QA, FPG and Archive subsystems. The high-level functions of these subsystems are described below. The Multiframe Data Reduction Pipeline that coadds Single-exposure Images and performs deep source extraction and characterization is not utilized for regular NEOWISE processing.

WSDS v7.5 incorporates software and calibration product updates that are tuned for the observed characteristics of the NEOWISE data and flight system performance. Calibration updates include: instrumental calibrations such as bad and noisy pixel masks, responsivity corrections, readnoise and gain maps; point spread functions for source extraction; photometric zero-point offsets; focal plane distortion maps and band-band offset and scale relationships. Instrumental and photometric calibrations are also updated periodically during the survey in response to changing payload and flight system characteristics. WSDS v7.5 also takes advantage of the AllWISE Source Catalog that provides a deep reference for the inertial mid-infrared sky for photometric calibration and for trending various performance parameters. NEOWISE-specific WSDS updates are summarized in IV.1.b below.

WSDS v7.5 also incorporates all relevant improvements and updates from prior WSDS versions such as those used for processing the original WISE mission 3-Band Cryo data (WSDS 4.0), the original mission NEOWISE Post-Cryo data (WSDS v5.0), and the AllWISE data processing (WSDS v6.0).

Figure 1 - Functional block diagram for the WISE Science Data System (WSDS). The elements crossed out in red, the Multiframe Pipeline and the Level 3 archive, are not used for NEOWISE science data processing.

IV.1.a.i. Ingest

The WSDS INGEST subsystem receives science data packets, engineering telemetry and ancillary information from the NEOWISE Mission Operations System, and assembles from them the Level 0 FITS-format images and metadata. Following their generation, INGEST then stages the Level 0 images and metadata for pipeline processing. INGEST operations are initiated automatically following the receipt of raw image, housekeeping and ancillary data that are sent following each NEOWISE downlink pass. The INGEST subsystem is described in IV.2 of the All-Sky Release Explanatory Supplement.

IV.1.a.ii. Pipelines

The WSDS PIPELINES subsystem converts Level 0 imaging data and metadata into calibrated images, extracted source lists, and moving object tracklets and produces corresponding metadata. The PIPELINES subsystem for NEOWISE is comprised of two main components: the Scan/Frame Pipeline and the WISE Moving Object Pipeline System (WMOPS).

The WSDS Multiframe Pipeline that combines individual Single-exposure images and performs deep source extraction is not run as part of NEOWISE data processing. Therefore, there is not a NEOWISE coadded Image Atlas or deep Source Catalog like there was for the AllWISE and All-Sky data releases.

IV.1.a.iii. Final Product Generation (FPG)

For NEOWISE, the FPG subsystem is primarily an operational step in which the data products are characterized and documented, copied to the Archive, and prepared for public release.

IV.1.a.iv. Quality Assurance (QA)

The QA subsystem examines the output and collects metadata from the WSDS PIPELINES and generates concise reports summarizing science data quality at each stage of data processing. These reports are tabulated in web-based compilations that provide the capability to drill down to detailed image, graphical and tabular data.

Elements and operations of QA subsystem are described in section IV.6 of the All-Sky Release Explanatory Supplement. Updates to QA for NEOWISE processing are described in IV.4.

IV.1.a.v. Archive

The Archive subsystem is a multi-tier system that stores and serves raw and processed mission data and metadata. A component of the Archive subsystem is integrated into the WSDS processing system and a component is integrated into the NASA/IPAC Infrared Science Archive (IRSA). The elements of the Archive subsystem in the WSDS are described in section IV.7 of the All-Sky Release Explanatory Supplement. Updates to the Archive subsystem for NEOWISE processing are described in IV.5.

IV.1.b. Synopsis of the WSDS Improvements for NEOWISE

The WSDS updates made for NEOWISE processing focused on optimizing source extractions and calibrations for the characteristics of the NEOWISE data, and on optimizing moving object tracklet detection. Changes were made that improved instrumental, photometric and astrometric calibration, and facilities were added to track and handle additional metadata to help monitor flight system performance.

IV.1.b.i. Ingest System

IV.1.b.ii. Scan/Frame Pipeline

IV.1.b.iii. Moving Object Pipeline System (IV.3)

IV.1.b.iv. QA (IV.4)

IV.1.b.v. Archive (IV.5)

IV.1.c. Basic Data Processing Terminology

orbit - An orbit refers to the observations or block of survey data acquired between the times NEOWISE observed the north ecliptic pole (NEP). The NEOWISE orbital period is approximately 95.3 minutes. Orbit number 1 was defined to start with the fictitious NEP passage immediately prior to WISE launch on 14 December 2009. Thus, WISE was launched into orbit number 1. The NEOWISE Reactivation survey began on 13 December 2013, with the spacecraft in orbit number 22106.

For the purpose of computing the scan number the orbit number is considered to have two halves. The first half of the orbit going from the NEP to SEP has an integer orbit number. The second half going from the SEP to NEP has 0.5 added to the orbit number, hence there can be odd-numbered scans.

scan - A scan refers to the observations or block of survey imaging data acquired between two flight system maneuvers, such as reorientation of the instrument boresight near the ecliptic poles, or slews to acquire TDRSS for data downlink contacts. Because maneuvers often took place near the ecliptic poles, a scan is associated with the image data from approximately a half-orbit. However, scans can cover less or more than half of the arc between ecliptic poles.

Scans are identified in the Single-exposure image headers, Single-exposure source database and metadata tables by the scan_id. The scan_id has the general form SSSSSx, where SSSSS is the five-digit scan number, and x is a letter assigned based on a sequential count of scans started in a specific half-orbit. Up to and including scan number 99799, SSSSS is twice the orbit number, and x="a" unless there is a second scan in the same half orbit, in which case x="b". Following scan 99799, SSSSS rolls over to 01000, and is two times the orbit number minus 98800, and x="r" unless there is a second scan in the same half orbit, in which case x="s". The first scan of the NEOWISE Reactivation survey is 44212a which is the first scan after the NEP crossing in the 22,106th WISE/NEOWISE orbit. Scan 01000r is the first scan after the NEP crossing in the 49,400th WISE/NEOWISE orbit.

single-exposure - Single-exposure refers to the unit of data acquisition carried out by NEOWISE while the sky is frozen onto the focal plane detectors, and to the set of imaging data collected during this data collection. A Single-exposure is acquired approximately every 11 seconds during NEOWISE scans.

frame - A frame is the image data acquired in a single band from a NEOWISE Single-exposure. The W1 and W2 frame exposure time is 7.7 sec. Because scans have different lengths, the number of frame exposures acquired per scan varies. A histogram of the distribution of the number of frames per scan during the first nine years of the NEOWISE survey is shown in Figure 2.

frameset - The set of two images (frames), one in each band (W1, W2), that are acquired simultaneously during each NEOWISE single-exposure. The same region of the sky is imaged in both bands. NEOWISE does not collect images in the W3 and W4 bands.

Figure 2 - Histogram showing the distribution of the number of frames acquired per scan during the first nine years of the NEOWISE Reactivation survey (binsize= 2 frames/scan).

Last update: 14 March 2023

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