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Description of the Ginga Mission
(the name means "galaxy" in Japanese) was the third Japanese X-ray astronomy
satellite. It was launched on 5th February 1987 and re-entered the atmosphere
on 1st November 1991 following the natural decay of the orbit. The main scientific
instrument was the Large Area Proportional Counter
(LAC; Turner et al. 1989) designed
and built by a group in the UK led by Leicester University. The other instruments
on board were the All-Sky Monitor and Gamma-ray Burst Detector. A full description
of the satellite is given in Makino et al.
(1987). During its lifetime Ginga performed over 1000 observations
of approximately 350 different targets, covering all classes of cosmic X-ray sources
satellite was placed into an orbit with perigee of 510 km and apogee of 670 km
and an inclination of 31°. Its orbital period was ~96 minutes. Ginga was approximately
1000 x 1000 x 1550 mm in size. It weighed about 420 kg. The Ginga configuration
is shown in the figure above. The spacecraft was three-axis stabilised by a momentum
wheel and a four-gyro inertial reference system, calibrated by two CCD star trackers.
The spacecraft pointing manoeuvers were carried out with three-axis magnetic torquers.
The time required to move the Z-axis was slow and observation lengths
of less than a day were not practical. Manoevers that rotated around the Z-axis
were made quicker, but this required a suitable alignment of the sources carried
out with three-axis torquers. The pointing accuracy was better than six arc minutes,
while the attitude reconstruction had an accuracy of approximately one arc minute.
The solar panels had to be held within 45 degrees of the sun direction in order
to satisfy power constraints. This constraint limited the portion of the sky observable
by the LAC at any given time of the year, to within a band of +/- 45 degrees wide
along a great circle perpendicular to the sun vector. Data were transmitted at
three different bit rates: 16384 bps (high rate), 2048 bps (medium rate) and 512
bps (low rate). An on-board bubble-memory data recorder with a capacity of 41.9
Mbits could store data for 42.7 minutes at the high data rate, 5.68 hours at the
medium rate, and 22.73 hours at the low data rate. The stored date were played
back during a ground contact at either 65,536 bps or 131,072 bps.
Area Proportional Counter
The LAC experiment, sensitive to X-rays
with energy 1.5-37 keV, consisted of eight collimated co-aligned proportional
counters with a total effective area of approximately 4000 cm² and energy
resolution of 18%; at 6 keV, scaling as E-½
throughout the full energy range. In each counter the anode structure was of a
multi-layer and multi-cell design which provided both gain uniformity and low
internal background through the use of anticoincidence. The high voltage supply
was normally operated at ~ 1830 V, but was reduced occasionally to ~
1745 V to achieve a larger energy range. Steel collimators restricted the field
of view to 1.1 x 2.0 degrees (FWHM); the top and bottom 15 mm were coated with
silver paint to prevent contamination through iron, nickel and chrome fluorescence
lines. The fluorescence line of silver at 22.1 keV can be visible at high energy
but is well away from lines of astrophysical importance and can be used for calibration.
origin and behaviour of the LAC background is described in Hayashida et al. (1989). The main sources
of background include the internal component generated after passage through the
Earth's radiation belts, in particular the South Atlantic Anomaly, the high- and
low-energy particles in the Earth's magnetosphere, and the diffuse Cosmic X-ray
Background (CXB). The first two components generate a background which is a strong
function of time and energy. Summed over the top- and mid-layer electrodes as
well as over the full energy range (1.5-37 keV), this varies between 50 and 100
counts sec-1 . The CXB contributes approximately
18 counts sec-1 to the background, which varies
as a function of position in the sky but is constant in time.
The All-Sky Monitor (ASM) consisted of 2 identical gas
proportional counters, and was sensitive to 1-20 keV. Each counter was equipped
with a collimator which had 3 different fields of view (1° x 45° FWHM).
The aim of the ASM was to create an all-sky survey every 1-2 days to look for
transient events (to alert the LAC) and to collect a long-term record for X-ray
Gamma-Ray Burst Detector (GBD)
The purpose of the
Gamma-Ray Burst Detector (GBD) was to detect gamma-ray bursts in the energy range
1-500 keV with a time resolution of 31.3 msec and high-energy resolution. It was
comprised of two sensors: a proportional counter (PC) and a scintillation spectrometer
(SC). The GBD could also operate as a radiation belt monitor for high particle
backgrounds which could harm the other 2 experiments.