Each GECAM satellite payload contains 25 gamma-ray detectors (GRDs), which can detect gamma-rays and particles and can roughly localize the Gamma-Ray Bursts (GRBs). GRD was designed using lanthanum bromide (LaBr3) crystal as the sensitive material with the rear end coupled with silicon photomultiplier (SiPM) array for readout. In aerospace engineering design of GRD, there are many key points to be studied. In this paper, we present the specific design scheme of GRD, the assembly and the performance test results of detectors. Based on Monte Carlo simulation and experimental test results, the specific schematic design and assembling process of GRD were optimized. After being fully assembled, the GRDs were conducted performance tests by using radioactive source and also conducted random vibration tests. The test results show that all satellite-borne GRDs have energy resolution <16% at 59.5 keV, meeting requirements of satellite in scientific performance. The random vibration test shows that GRD can maintain in a stable performance, which meets the requirement of spatial application.

The design and performance of GRD onboard the GECAM satellite

The Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a pair of microsatellites (i.e., GECAM-A and GECAM-B) dedicated to monitoring gamma-ray transients including the high-energy electromagnetic counterparts of gravitational waves, such as gamma-ray bursts, soft gamma-ray repeaters, solar flares, and terrestrial gamma-ray flashes. Since launch in 2020 December, GECAM-B has detected hundreds of astronomical and terrestrial events. For these bursts, localization is the key for burst identification and classification as well as follow-up observations in multiple wavelengths. Here, we propose a Bayesian localization method with Poisson data with Gaussian background profile likelihood to localize GECAM bursts based on the distribution of burst counts in detectors with different orientations. We demonstrate that this method can work well for all kinds of bursts, especially extremely short ones. In addition, we propose a new method to estimate the systematic error of localization based on a confidence level test, which can overcome some problems of the existing method in the literature. We validate this method by Monte Carlo simulations, and then apply it to a burst sample with accurate location and find that the mean value of the systematic error of GECAM-B localization is ∼2.°5. By considering this systematic error, we can obtain a reliable localization probability map for GECAM bursts. Our methods can be applied to other gamma-ray monitors.

https://iopscience.iop.org/article/10.3847/1538-4365/acafeb/pdf

GECAM Localization of High-energy Transients and the Systematic Error

The Gravitational wave highly energetic Electromagnetic Counterpart All-sky Monitor (GECAM) is dedicated to detecting gravitational wave gamma-ray bursts. It is capable of all-sky monitoring over and discovering gamma-ray bursts and new radiation phenomena. GECAM consists of two microsatellites, each equipped with 8 charged particle detectors (CPDs) and 25 gamma-ray detectors (GRDs). The CPD is used to measure charged particles in the space environment, monitor energy and flow intensity changes, and identify between gamma-ray bursts and space charged particle events in conjunction with GRD. CPD uses plastic scintillator as the sensitive material for detection, silicon photomultiplier array as the optically readable device, and the inlaid Am-241 radioactive source as the onboard calibration means. In this paper, we will present the working principle, physical design, functional implementation and preliminary performance test results of the CPD. As a result, the energy range of electron, gamma-ray detection efficiency and dead time are tested to be better than the indexes required through the ground calibration experiment. 

/pdf/the-design-and-performance-of-charged-particle-detector-2c7cy0qj.pdf

The design and performance of charged particle detector onboard the GECAM mission

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is mainly designed to detect Gamma-Ray Bursts associated with gravitational wave events. The lanthanum bromide (LaBr 3 ) scintillator with two types of doping (5% Ce 3 + only, 5% Ce 3 + co-doped with 1.5% Sr 2 + ) are used in the Gamma-Ray Detectors (GRDs) of GECAM. Here, by analyzing the on-ground calibration data, we find different characteristics of these two doping types of LaBr 3 scintillator, including the Energy-Channel (E-C) relation, photon non-proportional response, energy resolution, as well as the electron non-proportional response which is derived from photon response and Monte Carlo simulation with Bayesian inference method. The results also show that the photon scintillation light yield and energy resolution can be improved by co-doping Sr 2 + in LaBr 3 :Ce 3 + scintillator, owing to a direct inhibitory effect on the electron non-proportional response. By taking the electron non-proportional response into account, we find that the simulated spectrum is in good agreement with the measured one, including Compton structure, escape peak of Lanthanum and Bromine as well as full energy peak, indicating that the electron non-proportional response is an essential factor in the energy response of scintillator detector thus should be considered in the data analysis. 

Electron non-linear light yield of LaBr<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e899" altimg="si52.svg"><mml:msub><mml:mrow /><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:math> detector aboard GECAM

Massive global Low Earth Orbit (LEO) constellations have been developed rapidly with the characteristics of larger number and shorter orbital periods of LEO satellites in recent years. The increasing requirements of real-time orbit determination and emergency data transfer in full arc can hardly meet the traditional strategies based on regional ground stations. An integrated communication and navigation service architecture based on the third phase of the BeiDou system (BDS-3) Global Short Message Communication (GSMC) is proposed, which enables global coverage and random access. The GSMC downlink single-frequency signal is used for real-time orbit determination, and is further combined with B1C navigation signal for real-time dual-frequency high-precision orbit determination. Considering fast moving of LEO satellites and short burst of uplink messages, the uplink BDS-3 satellite selection strategy based on carrier-to-noise ratio, and uplink high-precision compensation of frequency shift are adopted to increase the communication success rate. The service architecture proposed can improve the precision of the autonomous orbit determination and enhance the capability of real-time command transmission of LEO satellites even without the traditional ground stations involved. Integrated communication and navigation service has been validated by simulation and on-board BDS-3 GSMC test data. The precision of single-frequency real-time orbit determination can reach 2.86m, and the precision of dual-frequency can reach 0.26m. The average time delay of communication is about 11.45s, and the success rate is about 99.8%. The method may provide alternative means of real-time high-precision orbit determination and emergency data transfer of LEO satellites.

Integrated Navigation and Communication Service for LEO Satellites Based on BDS-3 Global Short Message Communication

The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is proposed to explore the significant opportunity of the gravitational wave multi-messenger astronomy by monitoring the gravitational wave high-energy electromagnetic counterpart in all-sky. Based on the satellite science and observation requirements, we propose the design philosophy and schemes of the GECAM satellite system, including the system configuration and main technical parameters, observation requirement analysis, and satellite orbit, attitude, and operation mode design. Further, we summarize the technical characteristics and solutions of satellites, including the satellites all-sky field of view burst source location technology, multi-constraint satellite pointing design, the real-time downlink telemetry technology of trigger information based on the Beidou-3 global position reporting system, the integrated design electronics with low cost and high reliability, and the electromagnetic compatibility design of highly intensive satellite. The GECAM satellite can achieve an all-sky field of view, high sensitivity, good positioning accuracy, and wide energy coverage through system design and optimization. Also, it has the capability of on-orbit trigger positioning and quasi-real-time downlink of detection alerts, satisfying the comprehensive requirements of the detection of gravitational wave gamma-ray bursts and other high-energy transient sources.

/pdf/gecam-satellite-system-design-and-technological-1wcvpea8if.pdf

Wen Chen

Papers

GECAM satellite system design and technological characteristic