2. The satellite and the scientific instruments

2.1. The scientific payload

The configuration of the scientific payload and the energy bands covered by the different instruments are presented in Fig. 1 (click here) and Fig. 2 (click here) respectively. The wide band capability is provided by a set of instruments co-aligned with the Z axis of the satellite (Fig. 1 (click here)), Narrow Field Instruments (hereafter NFI) and consisting of (Table 1 (click here)):

• MECS (Medium Energy Concentrator Spectrometers): a medium energy (1.3-10 keV) set of three identical grazing incidence telescopes with double cone geometry (Citterio et al. 1985; Conti et al. 1994), with position sensitive gas scintillation proportional counters in their focal planes (Boella et al. 1996 and references therein).
• LECS (Low Energy Concentrator Spectrometer): a low energy (0.1-10 keV) telescope, identical to the other three, but with a thin window position sensitive gas scintillation proportional counter in its focal plane (Parmar et al. 1996 and references therein).
• HPGSPC, a collimated High Pressure Gas Scintillation Proportional Counter (4-120 keV, Manzo et al. 1996 and references therein).
• PDS, a collimated Phoswich Detector System (15-300 keV, Frontera et al. 1996 and references therein).

Perpendicular to the axis of the NFI and pointed in opposite directions there are two coded mask proportional counters (Wide Field Cameras, WFC, Jager et al. 1996 and references therein) that provide access to large regions of the sky in the range 2-30 keV. Each WFC has a field of view of (FWHM) with a resolution of 5'.

Finally, the four lateral active shields of the PDS will be used as monitor of gamma-ray bursts with a fluence greater than about in the range 60-600 keV, with a temporal resolution of about 1 ms.

Each instrument (the four NFI and the two WFC's) is controlled by a dedicated computer which has, in particular, the task of performing on board pre-processing of the scientific data according to the acquisition mode required by the specific observation. The basic mode is the Direct mode, in which each single event is transmitted with the full information. For sources fainter than about 0.3 Crab ( in the 2-10 keV range), the telemetry (see next Section) is sufficient to support this mode for all instruments simultaneously. For brighter sources Indirect modes allow to reduce the telemetry by producing on-board spectra, images and light curves with a large choice of parameters. The scientific data packets produced by each instrument are received by the main on-board processor (see next Section) on the basis of the overall telemetry occupation and instrument priority programmable from ground.

 
 

Figure 1: BeppoSAX scientific payload accommodation

 
 

Table 1: BeppoSAX instruments

2.2. The spacecraft and the other subsystems

The main characteristics of the spacecraft are presented in Table 2 (click here). BeppoSAX is a three axes stabilized satellite, with a pointing accuracy of 1'. The main attitude constraint derives from the need to maintain the normal to the solar arrays within from the Sun, with occasional excursions to for some WFC observations. Due to the low orbit (see next section) the satellite will be in view of the ground station for only a limited fraction of the time. Data will be stored onboard on a tape unit with a capacity of 450 Mbits and transmitted to ground every orbit during station passage. The average data rate available to instruments is about 60 kbit/s, but peak rates of up to 100 kbit/s can be retained for part of each orbit.

With the solar panels closed, the spacecraft is 3.6 m in height and 2.7 m in diameter. The total mass amounts to 1400 kg, with a payload of 480 kg. The structure of the satellite consists of three basic functional subassemblies:

• the Service Module, in the lower part of the spacecraft, which houses all the subsystems and the electronic boxes of the scientific instruments;
• the Payload Module, which houses the scientific instruments and the Star trackers;
• the Thermal Shade Structure, that encloses the Payload Module.

The primary sub-systems of the satellite are:

• The Attitude Orbital Control System (AOCS), that performs attitude determination and manoeuvres and operates the Reaction Control Subsystem in charge of orbit recovering. It includes redundant magnetomers, Sun acquisition sensors, three star trackers, six gyroscopes (three of which are for redundancy), three magnetic torquers and four reaction wheels, all controlled by a dedicated computer. The AOCS ensures a pointing accuracy of 1' during source observations and manoeuvres with a slew rate of per min.
• The On Board Data Handler (OBDH) is the core for data management and system control on the satellite and it also manages the communication interfaces between the satellite and the ground station. Its computer supervises all subsystem processor activities, such as those of each instrument, and the communication busses.
• Other subsystems are devoted to managing the power generated by the solar panels and battery energy storage (Electric Power and Solar Array Subsystem), to raising the orbit in case of decay below 450 km (Reaction Control Subsystem), to controlling the temperature of the satellite (Thermal Control Subsystem) and to ensure communication with the Ground Station (Telemetry Tracking and Command Subsystem).

 
 

Figure 2: Energy coverage of BeppoSAX instrument

  
Table 2: BeppoSAX: spacecraft main characteristics

 
Table 3: BeppoSAX: launch, orbit