This article describes the mission operations of the FIMS-SPEAR instrument and its host satellite, including operational modes and errors relevant to data interpretation.

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Satellite orbit

FIMS-SPEAR was the primary payload aboard the STSAT-1 microsatellite (Science and Technology Satellite-1, formerly known as KAISTSat-4), which in September 2003 was injected into a 700 km Sun-synchronous orbit at 98.2 deg inclination with an orbital period of 98.5 minutes and a ~34 minute eclipse. The SaTReC Mission Operations Center in Daejeon, Korea was used to control the mission and receive data. Observations for each orbit were scheduled to begin about 360 s after eclipse entry and end about 300 s before eclipse exit. The spacecraft orbited about 14–15 times per day around the earth. One to two orbits/day were devoted for the aurora and airglow observations of the northern nightside aurora, 4–5 orbits/day for communication with the ground station, and 7–10 orbits/day for astronomical observations.

Receiving telemetry

The data taken during the observations were stored in the Mass Memory System (MMS) and downloaded to the ground station two to three times a week. Unstable receiver sensitivity caused errors in the downloaded MMS data packets. The data were received at least twice to minimize the errors.

Pointing knowledge

The three-axis–controlled spacecraft platform used a star tracker to achieve ~5 arcmin pointing knowledge, pointed accuracy of ~6 arcmin, and a stability of ~12 arcmin.

Pointing modes

Pointings avoided the Sun by 45 deg and the spacecraft velocity vector by 60 deg, and were limited to zenith angles of less than 100 deg.

Various pointing modes were used for survey, target, and calibration observations.

Sky survey mode observations were performed by rotating a spacecraft axis such that the SPEAR field of view was swept, in a "push broom" fashion, perpendicular to its long field of view and in a 180 deg great circle from the north to south ecliptic pole via the anti-Sun direction. Following the anti-Sun progression in this way over 1 year would result in a full viewing of the sky with a maximum of overlapped exposure at the ecliptic poles and a minimum of exposure at the ecliptic plane.
Fixed inertial pointing toward specific targets was also performed.
In early operations, a calibration pointing mode made observations of stars or small (≲ 10 deg) fields through a "back and forth'" spacecraft rotation that sweeps the field of view over a limited angle, typically scanning from the equatorial north to south. In principle this mode was expected to provide the most accurate positional data and exposure times for the observation of point sources, but in practice this mode was found to make the attitude tagging at the boundary of the scanning area worse and was avoided in later operations.

Note: pointed observations (including both the fixed inertial pointing and calibration pointing modes above) were conducted on the Vela supernova remnant, the Antlia Supernova Remnant, the Lupus Loop, the Cygnus Loop, the Monogem Ring, RCW 114, G65.3+5.7, the Orion-Eridanus Superbubble, the Loop I/North Polar Spur, GSH 006-15+7, the Spica nebula, the ζ-Ophiuchi H II region, the Taurus region, the Ophiuchus region, the Draco Cloud, the Aquila Rift, the Taurus-Perseus-Auriga Complex, and the Rho Ophiuchi cloud complex. A list of science papers from the mission teams is available here.

Shutter modes

FIMS observations were performed in 1%, 10%, or 100% shutter aperture modes by adjusting the shutter steps depending on the expected flux level of the target objects. The aurora or dayglow was observed using the 1% shutter mode or 10% mode. The shutter was open 100% for the most astronomical observations. A few astronomical observations were made intentionally in the 10% mode (and sometimes unintentionally due to errors in the electronics). In early operations, the AS (AstroPhysics) observation mode was adopted; in the AS mode, the sky observations were performed for ~25 secs by opening the shutter, and then the shutter was closed for 5 secs to measure the detector background. However, the team soon recognized that the AS mode was not as useful as originally expected because of the uncertainties in attitude information, exposure time, and errors in the FIMS data packets. The AS mode was not used after March 2004 except for a few orbits.

Issues

The attitude reconstruction from multiple overlapping observations revealed the existence of time delays in the attitude reporting system. It was found that the attitude information recorded at a time T was, in fact, that measured at T −∆T. The time delay ∆T was found to be between about 2−4 secs in most orbits (up to ∼6 secs). Given that the time synchronization uncertainty between FIMS and the on-board computer was less than 0.4 secs, this time delay in reporting the attitude information was critical. A communication lag between the payload and the WIST (Wide Image Star Tracker) task, which calculates the spacecraft attitude, might have caused this time delay (Kwak & Park 2004, in Korean). That is, it may be that the attitude information system reported the time after the spacecraft’s attitude was calculated, not the time the star tracker measured the star. The time delay was found to vary continuously, even within each individual orbit.
The star-tracker updates were sometimes lost, resulting in an attitude error caused by gyro drift.
Towards the end of the year, star-tracker updates and attitude-knowledge reports were less frequently available.
There was a degeneracy in the shutter mode commanding Moving the shutter from "position 1" to "position 2" used the same motor commands as moving the shutter from "position 3" to "position 4". So if the shutter started in "position 3", it would always end up in the wrong position. The shutter position readout was correct, however. The problem was eventually corrected with a special operation that performed a closed loop to check the position readout.
Rarely, the shutter would stop between positions. There was an analog and a digital shutter position readout, and indeterminate positions were usually detected thanks to the analog readout not matching the digital versions.

While at least 40% of FIMS-SPEAR data suffered from some form of these problems, the teams created successful schemes to recover high-quality data, as described in Data Processing.

End of operations

Starting in November 2004, a problem occurred in the battery of the satellite system. Observations were intermittently attempted, but after May 2005, observations were no longer possible and science operations ceased.

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