Operating Modes

For each session between ground contacts (18-33 hours) SIXA will be programmed using a series of operation commands sent from the ground station to the satellite. If the device is not in use, it will normally be powered down (off). When switched on, the DPUs perform first a sequence of standard power-up tests. After this SIXA enters the STANDBY operating mode. All service functions are performed in the standby mode. After receiving the parameters in the standby mode, and warming up in the MEASUREMENT mode, the device is ready to start observing or calibration. When SIXA is in the measurement mode, the analog electronics is on. Other OPERATING MODES are DUMPING and DATA RETENTION modes. During, for example, reorientation it is in the data retention mode, refreshing data stored in RAM. Science data is transferred to the satellite in the dumping mode.

There will be a selection of six OBSERVING MODES available: three different SEC (single event characterization) modes, Energy-Spectrum Mode (ESM), Window-Counting Mode (WCM), and Time-Interval Mode (TIM). The control over the modes is shared between two processors, so that one DPU runs the SEC modes, and the other DPU the three other modes. The ESM, WCM and TIM can be run in parallel with one SEC mode (maximum of four modes simultaneously), and any of the modes can be switched off. The starting and ending times of each spectrum are stored with a 10 ms accuracy. The time resolution in SEC, ESM and WCM is about 30.5 $\mu$sec, and in TIM it is 12.21 $\mu$sec.

In the SEC modes the detector acts as a photon counter, and an event list is obtained. Every event is stored in a four-byte word (photon energy, arrival time, and id. of pixel). SEC 1 measures the events in all 19 elements and all energy channels. SEC 2 measures only events in the 7 central elements and within two energy windows. SEC 3 is similar to SEC 2 but the window limits are not applied. The arrival time is stored by a counter with units of about 30.5 $\mu$sec.

In ESM an energy spectrum is obtained. The integration time (10ms, 1 s - 33 h) is specified by the user. During this interval the events in each of the 1024 channels are counted (corresponding to the energy range about 0.5-20 keV). The optimal integration time depends on the source strength. At least several thousand events have to be recorded before a useful spectrum is obtained.

In WCM an X-ray light curve is obtained in two energy bands. The user selects the sampling time (time bin) and the two energy bands within which the events are summed in each of the seven central elements separately. Typically, the energy bands could be 0.5-2 keV (soft), 2-10 keV (medium), 10-20 keV (hard) and 6-7.5 keV (the iron line). For hardware requirements, ESM and WCM are coupled, so that the time bin is a fixed (1/1024, 2/1024, 4/1024, ..., 1) fraction of the integration time in ESM.

In TIM a time interval spectrum is obtained. The seven central elements act as one, and the time intervals between every other event (i.e. t2-tl, t4-t3, ...) caught by them is stored. The time unit for the intervals is 12.21 $\mu$sec, and there are 8192 channels, each corresponding to one time interval (n $\times$ 12.21 $\mu$sec, n=1,2,3,...,8192). When a time interval matching an interval in the range from 12.21 to (8192 $\times$ 12.21) $\mu$sec is registered, the corresponding counter is incremented. TIM mode is therefore suitable for searching short time periodicities in astronomical objects.

WCM and SEC 2 modes use common energy windows defined in the measurement program for each target separately. Energy windows for TIM, however, are independent of these and more permanent, but can also be redefined with a separate parameter command.