IV. NEOWISE Data Processing

Note - The identifiers for numbered asteroids that were observed by NEOWISE prior to mjd=59500 (13 October 2021) or after mjd=59950 (6 January 2023) may contain the MPC number, object name, and the designation. (e.g., (2643) Bernhard 1973 SD). The identifiers for numbered asteroids observed between mjd=59500 and mjd=59950 will contain only the MPC number (e.g., (2643)).

• A - Asteroid, numbered
• U - Asteroid, unnumbered
• C - Comet, periodic
• N - Comet, non-periodic, and Interstellar
• P - Planet
• S - Planetary Satellite

Note that the interstellar objects 1I/'Oumuamua 2017 U1 and 2I/Borisov 2019 Q4 were assigned a classification of t='N' for the purposes of the NEOWISE Known Solar System Object processing.

N.B. - The true observation mid-point times for NEOWISE W1 and W2 detections are slightly earlier than the times given by the mjd value (see I.2.c.iii.1). For source detections made on images acquired before June 23, 2015 11:29:30.47 UTC (mjd=57196.47882488), the observation times are approximately 1.57±0.25sec earlier than the mjd values. For source detections after that date, the observation times are approximately 0.57±0.25 sec earlier than the mjd value.

Scan_id is a six-character string that identifies a block of survey exposures that are 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 take place near the ecliptic poles, the scan number is roughly associated with a half-orbit's worth of images. However, scans can cover less or more than half of the arc between ecliptic poles.

The scan_id has the general form SSSSSx, where

• SSSSS is a zero-filled, five-digit integer that is related to the NEOWISE orbit number. The orbit number increments each time the flight system passes the north ecliptic pole (NEP). Orbit number 1 was defined to start with the fictitious NEP passage immediately prior to WISE launch. Up through scan number 99799, SSSSS is twice the orbit number. Following scan number 99799, SSSSS rolls over to 01000, and is twice the orbit number, minus 98800. Scan numbers following the rollover are also distinguished by having different values of x.


• x   is a letter assigned based on a sequential count of scans started in a specific half-orbit. For scans up to and including 99799, x="a" unless there is a second scan in the same half-orbit, in which case x="b". For scans after 99799, x="r" unless there is a second scan in the same half-orbit, in which case x="s".

The first scan whose data are used in the NEOWISE Data Release is scan 44212a. This was the first scan after the NEP crossing in the 22,106th WISE/NEOWISE orbit. The final NEOWISE Reactivation scan is 64272r, the first scan after the NEP crossing in the 81,536th orbit.

Frame_num has the general form MMM, a zero-filled three-digit number.

The frameset number is computed based on time bins which count from the scan start time:

where

• int[] means truncate to an integer
• frame_utc is the nominal frame exposure center UTC (when the boresight crosses the image center) in seconds
• dt_beg is 4.4s. This is a nominal offset to approximate when the exposure began
• scan_start_utc is the scan start time in seconds
• bin_size is 10s.

For example, a frame with a start time 1234 secs after the scan start time would have a frame number of 124.

N.B.

• Frame_num is not a frameset sequence number
• As a result of using a 10 sec bin, every 11th frame skips a number, so it is normal to see breaks in the frame numbering and it doesn't indicate missing data.
• Because frameset numbering is based on the predicted scan start time from the survey plan, and because frameset data may not start being acquired until well after the start of the scan, frameset numbers often do not start at 1 for a scan.

CAUTION: A spatial association between the predicted SSO position and the position of a NEOWISE Single-exposure extraction does not necessarily mean that this is a NEOWISE detection of the SSO. The Single-exposure Source Database contains a significant number of spurious detections of noise excursions, transient pixel events and image artifacts. Thus the spatial coincidence between the SSO position and NEOWISE extraction may be a chance alignment with a spurious extraction or an inertial background astrophysical source. See II.3.c for suggestions on how to identify reliable SSO detections.

This column is "null" if there is not a NEOWISE Single-exposure extraction spatially associated with the predicted position of the SSO.

Source_id has the general form SSSSSxMMM-IIIIII, where

• SSSSSx is the scan_id,
• MMM is the frameset number within the scan, frame_num, and
• IIIIII is the zero-filled, six-digit source number, src, within the frameset.