photo of Ginga in the clean room

Ginga



* Mission Overview

The third Japanese X-ray astronomy satellite, Ginga, was launched on a Mu-3SII-3 rocket from Kagoshima Space Center on 5 Feburary 1987. The payload consisted of three instruments: a Large Area Proportional Counter (LAC 1.5-37 keV), developed in collaboration with the UK, an All-Sky Monitor (ASM 1-20 keV), and a Gamma-ray Burst Detector (GBD 1.5-500 keV), developed in collaboration with the USA. The 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 below. 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.
Ginga spacecraft diagram showing instrument locations
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.

The satellite operated till November 1991. The Ginga observing program was open to scientists from Japan, the UK, the USA, and a number of European countries. During the mission lifetime Ginga observed about 350 targets including all classes of X-ray sources.

* Instrumentation

The Large Area Proportional Counter (LAC) was the main scientific instrument aboard Ginga. It was designed and built under a Japan-UK collaboration (ISAS, U. Tokyo, Nagoya U., U. Leicester, Rutherford Appleton Lab). It consisted of eight multicell proportional counters for a total effective area of 4000 cm2.
The field of view of the LAC was 0.8 x 1.7 degrees full width at half-maximum (FWHM), with the longer side parallel to the Z-axis, and was defined by honeycomb collimators made of thin stainless steel sheets. The counters were filled with a gas mixture of 70 percent argon, 25 percent xenon, and 5 percent carbon dioxide to a total pressure of 2 atmospheres at 20 degrees Celsius. The effective energy range over which the detection efficiency was more than 10 percent was approximately 1.5-30 keV. The energy resolution was better than 20 percent FWHM at 5.9 keV.
Accepted X-ray events were pulse-height analysed into a maximum of 48 pulse height channels. There were four modes of observation, listed in the Table below. The time resolution was mode-dependent. The highest time resolution available was 0.98 milliseconds, achieved at the expense of spectral information. The detection limit for the LAC was approximately 0.1 milliCrab, or 2 x 10-12 ergs/cm2-sec in the range 2-10 keV.

Mode No. of Pulse Height Ch. Bit Rate No. of Output Signals
High Medium Low
1 48 0.5 s 4.0 s 16.0 s 8, each of
8 counters
2 48 62.5 ms 0.5 s 2.0 s 2, sums of
4 counters
3 12 7.8 ms 62.5 ms 0.25 s 1, sum of all
8 counters
4 2 H 1.9 ms
L 0.98 ms
15.6 ms
7.8 ms
62.5 ms
31.3 ms
2, sums of
4 counters

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 sources.

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.


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