The X-ray Imaging and Spectroscopy Mission (XRISM) is an X-ray astronomy satellite successfully launched in September 2023. The satellite carries two X-ray telescopes with a focal length of 5.6 meters. One of these is Resolve, which consists of an X-ray Mirror Assembly (XMA) and a microcalorimeter array, providing a relatively narrow field of view (FoV) of 3.05 arcminutes squared. The other is Xtend, which consists of an XMA and an X-ray CCD camera, offering a large FoV of 38 arcminutes squared. Due to Resolve’s limited FoV, pointing accuracy is crucial for stable observation. The observation aimpoint, defined as the position on the focal plane where an on-axis target is located, was verified within a few arcseconds of the center of the array after the satellite’s boresight correction. It was also confirmed to be suitable, with no significant irreversible shift detected over approximately half a year. In the commissioning phase, all measurable requirements for pointing accuracy were confirmed to be met. The absolute pointing determination accuracies are less than 20 arcseconds for both instruments. The aimpoint shift and its temperature dependence were also assessed for each detector. The aimpoint shifts of both instruments in each observation have a good correlation on the X-axis, but not on the Y-axis in the detector coordinates. Resolve’s Y-axis shift clearly depends on the base panel temperature, on the order of a few arcseconds, which can be ignored for the absolute control accuracy and effective area. The sharp PSF core with an FWHM of approximately 10 arcseconds and arcsecond-scale relative determination accuracy enable Xtend to achieve good image reconstruction performance.
KEYWORDS: Global Positioning System, Clocks, Vacuum, Temperature metrology, Data conversion, Calibration, Satellites, Data modeling, Space operations, Physics
We report the results from the ground and on-orbit verification of the XRISM timing system when the satellite clock is not synchronized to the GPS time. XRISM carries a GPS receiver which synchronizes the main satellite clock to the GPS time but in a rare case that the satellite fails to receive the GPS signal, the clock runs freely and its frequency changes depending on its temperature. In this case, we correct the time drift considering the temperature dependency of the clock frequency measured in advance. To confirm that the accuracy of the time assignment in the GPS unsynchronized mode satisfies the requirement (within a 350 us error in the absolute time, for the satellite bus system plus ground system), we have performed the ground and on-orbit tests. In the thermal vacuum test performed in 2022, we obtained the GPS unsynchronized mode data and the temperature versus clock frequency trend. Comparing the time values assigned to the data and the true GPS times when the data were obtained, we confirmed that the requirement was satisfied in the temperature condition of the thermal vacuum test. We also simulated the variation of the timing accuracy in the on-orbit temperature conditions, using the Hitomi on-orbit temperature data and the dependency of the timing error on the temperature gradient obtained in the thermal vacuum test. We found that the error remained within the requirement over ∼ 300000 s without any time calibration data. After the launch, we performed on-orbit tests in 2023 September and October as part of the bus system checkout. The temperature versus clock frequency trend was found to remain unchanged from that obtained in the thermal vacuum test and the observed time drift was consistent with that expected from the trend.
The XRISM is the newly born X-ray satellite led by JAXA and NASA in collaboration with ESA, aiming to perform high-resolution spectroscopy of many astronomical X-ray objects. In the era of multi-messenger astronomy, where observations are performed in various wavelengths and include neutrino and gravitational data, it is important for the observatories to assign precise time of photons. To achieve the science goals of the XRISM mission, an absolute timing accuracy of 1.0 ms is required for the Resolve. The timing system, including both onboard instruments and off-line data-processing tools, is designed to meet this requirement. Following the lessons of the previous X-ray mission of Hitomi, comprehensive list of items that affect the accuracy of the timing are listed together with the timing error budget. During the system design and verification phases on the ground, all elements are controlled and verified to be within the budgets at the component level. After the launch of the satellite on 7 September 2023, in the initial commissioning phase, the overall timing performance of the timing system is scheduled to be confirmed to satisfy the timing requirements using a millisecond pulsar. The XRISM spacecraft carries the GPS receiver and the timing system uses the GPS signals in the nominal operation mode. In this presentation, we summarize the detailed design of the timing system of the XRISM, and the results of the timing verification tests both on ground and in orbit in the nominal operation mode. Detailed results on the failure mode of the GPS receiver will be presented in another presentation.
Fast timing capability in x-ray observation of astrophysical objects is one of the key properties for the ASTRO-H (Hitomi) mission. Absolute timing accuracies of 350 or 35 μs are required to achieve nominal scientific goals or to study fast variabilities of specific sources. The satellite carries a GPS receiver to obtain accurate time information, which is distributed from the central onboard computer through the large and complex SpaceWire network. The details of the time system on the hardware and software design are described. In the distribution of the time information, the propagation delays and jitters affect the timing accuracy. Six other items identified within the timing system will also contribute to absolute time error. These error items have been measured and checked on ground to ensure the time error budgets meet the mission requirements. The overall timing performance in combination with hardware performance, software algorithm, and the orbital determination accuracies, etc. under nominal conditions satisfies the mission requirements of 35 μs. This work demonstrates key points for space-use instruments in hardware and software designs and calibration measurements for fine timing accuracy on the order of microseconds for midsized satellites using the SpaceWire (IEEE1355) network.
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