2.1.

Power Distributor, Cooler Drivers, SpaceWire Routers

All of the SXS components except for the SXS-DIST were not powered at launch. The most critical operation immediately after the launch was to reestablish the low pressure of the liquid He by opening the vent valve using a stored command sequence, which required a very tight condition of the rocket being accelerated and the ambient air pressure being $<130\mathrm{Pa}$.¹⁵ The evaporated He is pumped out through the porous plug. In order to start it in a controlled manner, the liquid He needs some gravity while the outside environment needs to be in a low pressure.¹⁵ The resultant valve status change was confirmed in real time, as the satellite was still in the visibility range from the launch site, which was beyond our expectation.

The start of He pumping should be confirmed independently by the He tank temperature and the temperature difference between up- and down-streams of the porous plug due to the start of the vapor cooling [Fig. 1(g)].¹⁵ To have these telemetry items available at the first contact, we powered on two space wire routers (SXS-SWR-A and B) and the SCD by stored commands. The He fill line was also opened. All of these were confirmed successful at the first contact $\sim 45\min$ after the launch.

The other two cooler drivers (PCD and JTD) were powered on during the second contact. The SXS-DIST, SXS-SWR-A and B, and the three cooler drivers (SCD, PCD, and JTD) were continuously operated until the breakup.

2.2.

Cryocoolers and Loop Heat Pipes

The next urgent operation was to start the cryocooling to minimize the temperature increase of the He tank. A delay could cause a significant, or even entire, loss of the liquid He and thus a shortened lifetime of the mission.¹⁶ This was the most difficult part of the operation planning, as we needed to find time in limited contact passes, which were filled with many critical operations when the spacecraft was most vulnerable to accidents. On $\mathrm{Y}+0$, we started two shield coolers (SC-A and B) with 50 W and two precoolers (PC-A and B) with 32-W operation [Figs. 1 and 2(c)]. This was beyond the requirement of starting just SC-A and B at 40 W. As a result, the He tank temperature started to decrease on the first day.

On $\mathrm{Y}+1$, we started the Joule–Thomson cooler (JTC) and ramped up step by step at every contact as planned. However, by the next remote pass, JTC was turned off by an autonomous (AT) function set for one of the JT circuit pressures [Figs. 2(c) and 2(d)]. We soon figured out that this was not a malfunction, but an expected behavior when the working He gas in the circuit was quickly liquefied at a certain temperature. We never experienced this during ground testing with different cooling speeds. This AT function was necessary only for the cryogen-free mode, so we restarted JTC on $\mathrm{Y}+2$ after disabling this AT function.

Since then, the cryocoolers showed a very stable and high performance.¹⁷ In fact, the cooling power was higher than expected presumably because the Dewar temperatures were closer to the equilibrium than ground testing. This allowed us to lower and rebalance the input powers to the coolers. We operated JTC at 14.2 W, which was much lower than the nominal power (25.2 W) on the ground until $\mathrm{Y}+26/27$. By utilizing this extra margin, we increased the power to 14.8 W for JTC and decreased it from 51 to 43 W for PC-A [Figs. 3(c)]. This was because the compressor temperature of PC-A was higher than the others, and the decreased wattage would help to prolong the lifetime with a lower out-gassing rate at a lower temperature.

The SXS had four sets of LHP (SXS-LHP) to support heat dissipation from the shield-cooler compressors (SC-A/B CMP) and their cold heads (SC-A/B CHD). All four units were started successfully with a use of preconditioning and start-up heaters before the ramp-up of SC on $\mathrm{Y}+0$. The two units for CMP worked until the end, but the other two finished working 1.0 and 19.5 h later for SC-A CHD and SC-B CHD, respectively [Figs. 2(i)]. This was expected as the heat input was too small for the LHP to keep working. The LHP was not necessary with a low-heat input at the beginning of the mission, and we left the units as they were.

2.3.

Adiabatic Demagnetization Refrigerator Controller

The ADRC was powered on $\mathrm{Y}+0$ and the thermometers were calibrated for precise temperature monitor and control of the low-temperature stages. The function test was performed for the heat switches (HS) and the ADR magnet current control on $\mathrm{Y}+4$, and the 50-mK control was started on $\mathrm{Y}+5$. Since then, a total of 18 ADR recycles were performed successfully,¹⁴ which are visible in Figs. 3(e)–3(h). The function and the control of the third-stage ADR for the cryogen-free operation were not tested in orbit.

The ADRC also controls the heaters on the optical blocking filters.⁸ The heaters for both the IVCS and outer vapor-cooled shields (OVCS) filters were checked for their aliveness on $\mathrm{Y}+13$. The temperature control of the Dewar main shell (DMS) filter heater was tested for 3.5 h on $\mathrm{Y}+13$. The DMS filter heater control was planned to be restarted permanently a few days before the GV open to protect the filter from contamination.

2.4.

Signal Processing Electronics

On $\mathrm{Y}+2$, we powered on both the analog and digital signal processing electronics (XBOX^2,9 and PSP,¹⁹ respectively). We started the temperature control of the JFET and the bias input to the sensors with XBOX. The CPU program of the PSP was started and the initial parameters were set for the signal processing. Noise measurement (Sec. 3.2) and bias test were performed on the same day with the detector at $\sim 1\mathrm{K}$, confirming no problem. On $\mathrm{Y}+5$, we measured the noise of the detector at 50 mK for the first time in orbit. The detector $I-V$ curve was also measured. No change was found in comparison to the ground results.^3,13

Some PSP parameters needed adjustment in orbit because we did not know the precise noise level and the consumption rate of the data recorder (DR) during the South Atlantic Anomaly (SAA) passages. After the successful first light observation, we lowered the event threshold from 120 to 75 (equivalent to 166 to 104 eV) on $\mathrm{Y}+14$ as the noise level was low enough. The final threshold was planned to be fixed after the GV was open.

On $\mathrm{Y}+4$ to 6, we adjusted the rate of pulse and noise record dumps. The default settings were too high, which resulted in the loss of noise record data between around February 19 19:50 to February 21 3:45 with a full usage of the DR. We also changed the priorities of data when writing into the DR (data with a lower priority are overwritten by those with a higher priority) so that we could record the diagnostic and event data at a higher priority than the others.

2.5.

Filter Wheel

The FWE was the last SXS component to be powered on $\mathrm{Y}+26$. FWE controls the FW and the MXS. For the commissioning of the FW, we executed a rotation by a small angle ($\pm 2\mathrm{deg}$) for the first time on $\mathrm{Y}+27$ and started the weekly maintenance operation on $\mathrm{Y}+29$ and 36 (Sec. 3.2). A large angle (60 deg or 120 deg) rotation to change the filters was not originally planned until it was required during the performance verification phase. However, in order to calibrate the absolute gain of individual pixels before opening the GV and using the MXS,^10,11 we changed the position to illuminate the detector array with x-rays using the ${}^{55}\mathrm{Fe}$ sources on the wheel for 22.3 h on $\mathrm{Y}+30$ and 31 [Fig. 3(d)]. This data set became the only reference for the absolute gain of the array pixels in orbit.

On $\mathrm{Y}+37$, we started a two-week campaign of commissioning the MXS. The high voltage of 11.3 kV was much higher than other high voltages employed in the spacecraft, and we carefully designed a plan to ramp-up with a step-by-step verification. The first day was successful with the lowest voltage 0.5 kV ramp-up. The second day was not started due to the communication loss by the breakup.

3.1.

Automated Responses

Many automated responses for safety are designed and implemented within a component, and the details can be found in papers describing the individual SXS components. Some components assume that others are working properly, especially those responsible for the multistage cooling chain, thus system-wide protections were needed in addition to component-level protections. We thus implemented and tested such automated responses using the autonomous (AT) and request (RQ) functions of the satellite management unit (SMU), or the data handling system.

Using the AT functions, SMU can send a command sequence based on the telemetry it collects from all onboard components. Using the RQ functions, any flight hardware can request the SMU to send a command sequence to itself and the others based on its own judgment. We used AT to protect the SXS hardware based on their own telemetry and RQ to protect the SXS hardware using the spacecraft information (low battery condition and loss of spacecraft attitude control) at a sampling rate of 1 Hz. Table 4 tabulates the AT and RQ functions that were actually implemented. The JFET voltage AT is intended to protect the hardware from He leakage in the guard vacuum, which can be recognized as an excessively high voltage of the heaters that control the JFET temperature at 130 K. The wheel safe hold (WSH) and reaction control system (RCS) safe hold (RSH) RQ is intended to protect from the loss of the Sun, which could come in the line of sight to bring excessive heat input to the system.

Table 4

Automated responses.

3.2.

Routine Operation

As the mission ended before we completed the commissioning of the SXS, only a part of the routine operations was started. Here, we describe the planned routine operations for reference.

3.2.5.

South Atlantic Anomaly passages

During the SAA, the temperature reading becomes noisy and the rate of false events increases. However, all the SXS hardware were designed to require no operations to cope with these changes, which worked as expected. For additional safety, we decided to ramp down the high voltage of MXS during SAA passages. The MXS commissioning was started only one day before the breakup, and the operation of ramping down at SAA ingress and up at SAA egress was tested only very partially.

The extent of the SAA used for operations is defined based on the anti-co count rate of $>3{\mathrm{s}}^{-1}$ above 30 keV for the SXS in the pipeline processing. The map (Fig. 4) is slightly different from the one conventionally used for Suzaku.

Fig. 4

Definition of SAA based on the anti-co count rate of the SXS.