Spaceborne Detection Technology for Assessing Particle Radiation in Highly Elliptical Orbits

Abstract

Satellites traversing highly elliptical orbits (HEOs) encounter more severe radiation effects caused by the space particle environment, which are distinct from those in a low Earth orbit (LEO), medium Earth orbit (MEO), and geostationary orbit (GEO). This study proposed a space environment detection payload technology for assessing the particle radiation environment in HEOs. During ground tests, all technical indicators of the detection payload were calibrated and verified using reference signal sources, standard radioactive sources, and particle accelerators. The results indicate that the space environment detection payload can detect electrons and protons within the energy ranges of 30 keV to 2.0 MeV and 30 keV to 300 MeV, respectively, with an accuracy greater than 10%. The detection range of the surface potential spans from −11.571 kV to +1.414 kV, with a sensitivity greater than 50 V. Furthermore, the radiation dose detection range extends from 0 to 3.38 × 10⁶ rad (Si), with a sensitivity greater than 3 rad (Si). These indicators were also validated through an in-orbit flight. The observation of the particle radiation environment, radiation dose accumulation, and satellite surface potential variation in HEOs can cover space areas that have not been addressed before. This research helps fill the gaps in China’s space environment data and promotes the development of a space-based environment monitoring network.

1. Introduction

The increase in aerospace endeavors, such as the Beidou navigation system [1], manned spaceflight initiatives, and lunar exploration missions, has promoted the proliferation of space assets in China, notably satellites. Consequently, the national economy and societal functions are increasingly dependent on space infrastructure and services. With the increase in the scale and technological sophistication of space activities, the hazards posed by the space environment become more pronounced and critical. The term “space environment” encompasses the physical conditions and their fluctuations across the expanse extending from tens of kilometers above the Earth’s surface to the vicinity of the sun. Its key components include solar electromagnetic radiation, charged particle radiation, plasma, orbital atmosphere, and micrometeoroids [2,3]. The space environment exerts a notable influence on the operational safety and efficacy of both spaceborne and terrestrial systems. It impacts the reliable functionality of aerospace equipment in orbit, the safety of astronauts and aviation operations, and the dependable operation of satellite applications, including navigation, communication, and earth observation systems [4,5,6]. In addition, solar particle events also have a significant impact on the stable operation of terrestrial systems such as radar networks, power grids, and oil pipelines.

Existing technologies for the monitoring of the space environment are applicable to low Earth orbits (LEOs), medium Earth orbits (MEOs), and geostationary orbits (GEOs), with most technologies limited to LEOs [7]. However, the current configuration of space monitoring technologies leaves gaps in coverage between different systems, leaving gaps in coverage between different orbits and resulting in a lack of data for unmonitored space environments. To address this gap, payloads for space environment detection have been installed on China’s inaugural satellite operating within a highly elliptical orbit (HEO), using technologies to measure medium- and high-energy particles (electrons and protons), the radiation dose, and the satellite’s surface potential. This initiative aims to assess the particle radiation environment within this orbit, encompassing inter-orbit spaces that have hitherto remained unmonitored, thus filling the gap in space environment data. Moreover, it is integrated with solar monitoring and space environment surveillance in LEO, MEO, and geosynchronous orbit (GSO), forming a comprehensive space-based monitoring network to observe the space environment for China. The network facilitates comprehensive monitoring along the causal chain, enabling the observation of the entire progression of space hazard events originating from the sun and propagating towards Earth.

2. Application Objective

The satellite navigating HEO spans altitude ranging from approximately 1500 km to 40,000 km. Unlike satellites orbiting in other trajectories, this satellite traverses the Earth’s radiation belt, characterized by dense high-energy charged particles, as well as the Earth’s plasmasphere, which contains dense plasma. It traverses both the inner and outer regions of Earth’s magnetosphere, encountering intricate space environments comprising charged particle radiation, plasma, and magnetic fields. Notably, the medium- and high-energy charged particle radiation in the Earth’s radiation belt poses a significant threat to satellite safety, while space plasma exerts an interference on satellite and onboard equipment operations through surface charging and discharging effects [8,9].

Satellites occupying this orbit spend significant amounts of time traversing the open magnetic field lines near the poles, where shielding by the geomagnetic field is negligible. Consequently, solar wind plasma, medium- and high-energy particles from solar proton events, and plasma originating from magnetic reconnection in the magnetotail and on the dayside magnetopause can directly infiltrate near-Earth space via these magnetic field lines. In comparison to conventional satellite orbits like LEO, MEO, and GEO, HEO therefore presents a more complex and hazardous space environment.

Given the distinctive nature of space particle radiation and its effects in HEO, this study highlights the importance of installing space environment monitoring payloads on satellites operating within this orbit. The aim is to measure charged particle radiation spectra, satellite surface potential, and the particle radiation dose within the orbital space. The following three sections discuss the objectives of this study.

2.1. Space Radiation Environment Disturbance Warnings

Within the magnetosphere, wave–particle interactions play a significant role in particle acceleration and dynamic loss processes. Various waves, including ULF, VLF, and MS, can interact with radiation belt particles, resulting in particle precipitation and pitch angle scattering. Consequently, these events lead to variations in the positions, energies, and types of particle precipitation occurrences.

When large-scale shock waves generated by solar proton events or coronal mass ejection impact Earth, they induce significant fluctuations in the Earth’s magnetic field. These fluctuations cause the Earth’s magnetic field lines to oscillate akin to guitar strings, generating ULF waves. These ULF waves, standing waves in nature, possess the capability to accelerate electrons traversing along magnetic field lines to very high velocities. This phenomenon facilitates the transfer of solar wind energy to “killer electrons” (>100 keV) within the magnetosphere, culminating in the rapid formation of “killer electron storms” in only 15 min [10]. Electrons ranging from 1 to 100 keV serve as “seed” electrons, which, propelled by magnetospheric plasma wave mechanisms, can metamorphose into MeV killer electrons. Consequently, measurements of electrons within this energy range can forecast sudden surges in MeV killer electron flux several tens of minutes to several hours in advance.

Space environment disturbance warnings, thus, rely on measurements of medium-energy electrons and protons in orbital space, the investigation of various wave–particle precipitation mechanisms, the validation of the rapid precipitation loss effects of EMIC waves on killer electrons, and the analysis of the relationship between precipitating particles and solar indicators such as AP indicators.

2.2. Particle Radiation Effect Assessment

The high-energy charged particle environment, commonly referred to as the particle radiation environment, can induce both total dose effects and single-particle effects on satellites. Total dose increases can lead to degradation in material performance and result in the loss or failure of satellite components. Conversely, single-particle effects can trigger errors or system crashes within electronic systems, resulting in operational disruptions, interruptions, or even catastrophic failures, ultimately compromising the entire satellite [11,12].

Most orbital segments of satellites operating in HEO are located at high altitudes and latitudes. Within this region, magnetic field lines can extend to areas with weak magnetic fields distant from the Earth’s center and may directly connect to solar magnetic field lines. This connectivity facilitates the ingress of high-energy protons from the sun, galactic cosmic rays, and even higher-energy heavy ions from galactic cosmic rays into this orbit, thereby creating an exceedingly harsh particle radiation environment.

Based on the radiation environment characteristics inherent to this satellite orbit, the detection of solar high-energy particles, radiation belt protons, galactic cosmic rays, and radiation dosage is thus performed to ascertain the primary particle radiation characteristics, posing significant threats to satellites within this orbit, thereby facilitating the accurate assessment of radiation impacts on satellites.

2.3. Anti-Surface Charging Applications

Within the space particle environment of HEO, hot plasma emerges as a predominant factor. Characterized by high temperature, energy, and flux, hot plasma tends to accumulate on satellite surfaces, inducing surface charging. Additionally, hot plasma exhibits significant anisotropy, resulting in local potential variations across the satellite surface and consequent surface differential potentials.

Upon surface charging, several adverse effects may ensue: first, the increased leakage current in solar cell arrays leads to diminished efficiency; second, pollutants adhere, contaminating optical lenses and impairing surface material performance [13,14]; and third, when surface charging generates a substantial potential difference, satellite surface discharge may occur, resulting in interference, anomalies, and equipment damage, or even satellite failure [15].

Overall, the inaugural Chinese satellite operating in HEO encounters a complex and challenging space environment, and the space environment within this orbit has not been comprehensively examined. Although satellites such as CRRES and Van Allen Probes have detected high-energy electrons, protons, etc., in this orbit, there are some shortcomings, such as the insufficient coverage of radiation bands in high-latitude regions and the insufficient coverage of complete solar activity cycles during the satellite mission time [16,17]. Specifically, charged particle radiation and its detrimental effects on satellite in-orbit safety, such as single-particle effects, radiation dosage, and surface charging/discharging effects, have yet to be investigated. Additionally, the collected space environment data from this orbit are insufficient. To ensure the in-orbit safety of satellites within this new trajectory, targeted space environment detection is therefore imperative. Acquiring firsthand detection data on particle radiation and its effects is crucial for bridging the data gap and enhancing China’s space-based space environment monitoring system.

3. Desired Performance Parameters

The space environment detection payload aboard the satellite operating within HEO provides data concerning charged particle radiation spectra, satellite surface potential, and the total particle radiation dose within the satellite’s operational area. This is based on the physical characteristics of this orbital radiation band, such as the energy of protons, which is mainly concentrated in the range of a few MeV to several hundred MeV, and the energy of electrons, which is concentrated in the range of about 100 keV to several MeV, as well as the radiation effect of space particles. This study also takes into account collaborative observations with other satellites, such as the LEO Fengyun-3 satellite and the GEO Fengyun-4 satellite [18,19], to achieve more efficient regional coverage and data complementarity. The essential technical indicators are outlined in

Table 1. Main technical indexes of particle detection.

Item	Energy Range	Energy Resolution	Detection Direction
High-energy proton	2.5 MeV~300 MeV, 8 energy channels (2.5 MeV~5 MeV~10 MeV~15 MeV~25 MeV~40 MeV~80 MeV~165 MeV~300 MeV)	<15%	−Z
High-energy electron	0.2 MeV–2.0 MeV, 9 energy channels (0.2 MeV~0.3 MeV~0.4 MeV~0.5 MeV~0.6 MeV~0.8 MeV~1.0 MeV~1.3 MeV~1.7 MeV~2.0 MeV)	<15%	X, Y, −Z
Medium-energy proton	0.03 MeV–2.5 MeV, 7 energy channels (0.03 MeV~0.055 MeV~0.102 MeV~0.188 MeV~0.33 MeV~0.65 MeV~1.0 MeV~2.5 MeV)	<15%	X, Y, −Z
Medium-energy electron	30 keV~410 keV, 5 energy channels (30 keV~50 keV~85 keV~144 keV~243 keV~410 keV)	<20%	X, Y, −Z

,

Table 2. Indexes of radiation dose detection.

Item	Measuring Range	Sensitivity	Detection Direction
Total radiation dose	0~3 × 106 rad (Si)	<10 rad (Si)	−Z

and

Table 3. Indexes of surface potential detection.

Item	Measuring Range	Sensitivity	Detection Direction
Surface potential	+1 kV~−10 kV	<100 V	−Z

.

In these tables, the definition of the three detection directions (X, Y, and −Z) is based on the satellite coordinate system, where −Z represents that it is pointing towards the sun, and a description of the indexes can be found in the relevant literature [20,21]. During the ground development phase, detailed calibration was conducted on all technical indexes to ensure that they met the design requirements. The specifics of the parameters measured, the methods used for measurement, and the criteria for evaluation are further described in Section 5 of this study.

4. Payload Detection Technology

The space environment detection payload installed on satellites within HEO comprises various detectors designed to monitor charged particle radiation spectra, the satellite total radiation dose, and surface potential. These detectors include a tri-directional high-energy electron detector (HEED), a tri-directional medium-energy particle detector (MEPD), a single-directional high-energy proton and dose detector (HEPDD), a surface potential detector (SPD), and a shared space environment control unit (SECU).

The HEED and MEPD employ distinct Si semiconductor sensor systems to measure the energy spectra and flux of high-energy electrons, medium-energy electrons, and protons from three spatial directions. The HEPDD utilizes a single Si semiconductor sensor system and PMOS sensors to measure the energy spectra of high-energy protons in a single direction and the total radiation dose within the satellite. Meanwhile, SPD employs capacitance voltage division technology to measure the satellite’s surface charging potential. The detection data from these payloads are collected and processed by their respective electronic control units, transmitted through internal cables to the space environment control unit, and subsequently packaged and transmitted uniformly via the satellite buses.

Figure 1 depicts 3D images of various space environment detection payloads, arranged from left to right. The specific dimensions of each payload are provided in the figure caption, and their detailed descriptions will be provided in the subsequent subsections.

4.1. HEED Detection

High-energy electrons constitute a prevalent component of particle radiation in space and are significant contributors to radiation dose damage to satellites, as well as a primary cause of satellite internal charging and discharging. The tri-directional HEED employs three independent sensor probe systems to measure high-energy electrons in three orbital directions of the satellite. Each sensor probe comprises a collimation system and a semiconductor measurement system, as depicted in Figure 2.

The collimation system serves to restrict the opening angle and geometric factors of the instrument, thereby controlling the influx of particles into the semiconductor measurement system. High-energy electrons penetrate the semiconductor measurement system through the collimation system, depositing energy in the semiconductor detector, which is then converted into voltage signals. By analyzing the amplitudes of these voltage signals, the incident particle energy can be determined. Additionally, a logical operation mode using the FPGA algorithm was devised to effectively categorize particles with differing energies while analyzing the pulse amplitude containing particle energy information [22].

4.1.1. Heed’s Collimation System

A collimation structure is incorporated around each probe component of the high-energy particle detector to constrain the instrument’s detection field of view [23]. Additionally, the collimation structure’s shielding effect reduces the interference ratio of obliquely incident particles, such as high-energy protons and electrons. Utilizing a Geant4 Monte Carlo simulation with the physics list QGSP-BERT [24], the final geometric factors of the instrument were determined by considering the limited field of view of the collimator [25], the actual size of the sensor probe structure, and the shielding conditions of the instrument; these factors serve as the foundation for subsequent data processing.

The collimation system comprises the external structure, anti-scattering structure, and light-shielding layer. The external structure serves as the outermost support and shielding structure of the probe, designed in a cup-shaped configuration with a smaller bottom and a larger top. Additional shielding is applied to the probe’s exterior to minimize the impact of obliquely incident particles on the measurement accuracy. An anti-scattering device is integrated into the high-energy electron probe to prevent the electron scattering from interfering with the measurements. The light-shielding layer of the high-energy electron probe consists of a 15 μm aluminum film, which is capable of blocking protons below 1 MeV and electrons below 30 keV.

4.1.2. Heed’s Sensor System

The semiconductor detector measurement system is a crucial component of the entire instrument’s measurement apparatus. It utilizes a silicon semiconductor stack to form a telescope, enabling the measurement of energy spectra and flux of high-energy electrons. The types and energy of high-energy particles can be determined by analyzing the amplitude of pulse signals generated by the energy deposited by incident particles in different semiconductor detectors of their respective probes. An amplitude analysis, in conjunction with the logical combination of the output signals of the different detectors mentioned above, is employed to obtain the electron energy spectra of high-energy electrons [26]. Figure 3 illustrates the basic principles of the telescope system in HEED.

The function of the semiconductor sensor is to receive charged particles incident on the probe. Existing ion-implanted Si semiconductor sensors offer optimal performance in terms of linear response range, energy resolution, and spatial applicability. In this study, the HEED uses a total of five silicon semiconductor detectors, with the first one having a thickness of 500um and the remaining four having a thickness of 1 mm, and their diameters are all 20 mm. These detectors were all purchased from Micron Semiconductor Ltd. in Brighton, UK, performed well, and are widely used in the aerospace industry. They use N-type high-resistance silicon wafers and form a PN junction through the ion implantation process. Additionally, they are in a state of complete exhaustion while working.

These detectors are tightly installed inside a copper shell, with a 2 mm interval between every two pieces, including a 0.5 mm thick insulation gasket and a 1.6 mm thick sensor package, as shown in Figure 2 above.

4.1.3. Electronic System

The HEED electronic system serves to process and gather high-energy electron signals from each probe. Electronic circuits provide the necessary bias voltage for semiconductor detector operation. The basic operational sequence involves the following steps: After entering the sensor, high-energy electrons generate charge signals, which are then converted into voltage signals by the front-end electronic preamplifier. These signals undergo further amplification by the principal amplifier. Subsequently, voltage pulses traverse the peak hold circuits to generate sampleable peak hold signals. The ADC within the digital circuits samples the signal amplitudes, representing the energy information of the incident particles. The FPGA processor analyzes the amplitudes of the signals sampled by the ADC to determine the energy range to which the incident particles belong. Finally, the FPGA packages the detection data into complete data packets, which are transmitted via the subsystem 422 bus and the SECU interface.

The instrument’s electronic components include the sensor readout system, principal amplifier, high-voltage circuit, data collection and processing circuit (comprising an FPGA, a crystal oscillator, a watchdog, an EEPROM, an SRAM, and an ADC), power supply circuit, and interface circuit (comprising a 422 interface, a telemetry interface, and a power interface). The front-end sensor readout system uses the charge amplifier A250F produced by AMPTEK as the main electronic component [27].

4.2. MEPD Detection

Medium-energy electrons primarily induce the charging and discharging of shallow media (e.g., battery arrays and satellite skins), while medium-energy particles (electrons and protons) serve as sensitive parameters in space environment disturbances, crucial for space weather forecasting [28,29]. By monitoring changes in the space environment through medium-energy particle measurements, timely and effective detection can be achieved, facilitating early warnings for satellites to implement corresponding in-orbit management measures.

Based on the fundamental principles of the “telescope method” in conjunction with “pinhole imaging” [30], the tri-directional MEPD employs Si semiconductor detectors to construct a telescope system for medium-energy particle measurement [31]. Each medium-energy electron and proton detector consists of two probes, with each probe containing three sub-probes. This configuration enables the tri-directional detection of medium-energy electrons and protons in space.

4.2.1. MEPD’s Sensor System

Within each medium-energy electron probe and medium-energy proton probe, three sub-probes are arranged on three faces of a trapezoidal structure in a fan shape. Each sub-probe comprises a collimation system and a semiconductor telescope array. The principle of medium-energy electron or medium-energy proton detection is similar to the HEED described earlier and will not be repeated here.

The MEPD sensor system utilizes two ion-implanted Si semiconductor detectors, each with a thickness of 300 μm and a diameter of 12 mm. They were also purchased from Micron Semiconductor Ltd. in Brighton, UK. The configuration of the medium-energy proton probes is depicted in Figure 4. After passing through the collimation systems of the medium-energy electron and proton probes, the two sensors inside each sub-probe form detection flare angles of 30° and 40°, respectively.

Of the two Si sensors, the first measures the energy loss of energetic particles in the sensor and converts it into electrical signals. The second sensor serves as an anti-coincidence detector, eliminating interference from high-energy particles. By combining signals from the sensors using an amplitude analysis and appropriate logical operating modes, the particle energy spectra and flux can be measured.

4.2.2. MEPD’s Collimation System

During the medium-energy proton measurement, to prevent electrons from interfering with medium-energy protons, the probe collimation system internally employs deflection magnets to effectively divert electrons, thus barring their entry into the detector.

The design of the medium-energy electron probe is similar to that of the medium-energy proton probe, except for the absence of internal deflection magnets. The proton telescope utilizes internal deflection magnets capable of diverting electrons, ensuring clean proton measurement data. Conversely, the electron probe yields mixed measurement data of protons and electrons because of the lack of internal deflection magnets. The proton data measured by the proton telescope must be subtracted from the mixed data acquired by the electron telescope to obtain electron data without errors.

4.3. HEPDD Detection

High-energy protons encountered during satellite operation in HEO originate from three primary sources: high-energy protons trapped within the radiation belt, solar high-energy protons emitted during solar activity, and high-energy protons from galactic cosmic rays. These high-energy protons can engage in nuclear reactions with satellite components, generating secondary heavy nuclei that induce single-particle effects.

The HEPDD comprises two main components: the high-energy proton detection system and the radiation dose detection system. The former measures the energy spectra of high-energy protons during solar activity events, while the latter measures the total radiation dose of this orbit.

The operational principle of high-energy proton detection mirrors that of high-energy electron detection, encompassing three components: a collimation system, a semiconductor detector, and an electronic system. Permanent magnets are employed in the collimation system to deflect electrons to counteract the interference and irradiation effects of high-energy electrons in space on the proton probe [32]. These magnets produce a central magnetic field strength of up to 4200 Gs, effectively neutralizing the influence of electrons below 1 MeV with an efficiency of not less than 95% [31]. Moreover, akin to the HEED, the high-energy proton probe incorporates a light-shielding layer comprising a 15 μm aluminum film, capable of blocking protons below 1 MeV from entering the sensor system.

The sensor system utilizes four ion-implanted Si semiconductor sensors, employing principles and design concepts akin to those of the high-energy electron probe. The thickness of the first one is 500 um, and the following three are 1 mm, all with a diameter of 20 mm. Their other information is the same as that of the HEED. Figure 5 illustrates the sensor probe structure for the HEPDD. Finally, the energy spectra and flux of high-energy protons are obtained through an amplitude analysis using appropriate logical operating modes.

4.4. Radiation Dose Detection

Spaceborne charged particle radiation can induce irradiation effects on satellites and onboard equipment. The radiation dose refers to the cumulative energy deposition of high-energy particles in a material after their interaction with it. Damage to materials and devices by particles mainly occurs through the process of charged particle energy loss within the material. Radiation dose detectors measure the energy loss of particles in materials per unit mass, serving as a crucial parameter for studying the effects of the space environment on materials and devices [33].

In radiation dose meter-based detection, RADFET technology is adopted and PMOS devices are utilized to directly measure the accumulated space radiation dose [34]. The PMOS sensor utilizes a p-channel metal oxide semiconductor field effect transistor with a gate oxide layer thickness of 100 nm and purchased shelf products from Varadis Ltd. in Cork, Ireland. The basic structure of a PMOS sensor is depicted in Figure 6, where the SiO₂ insulation layer between the n-type Si and the gate electrode constitutes the radiation-sensitive region. A notable characteristic of SiO₂ is the presence of internal hole “traps”, with higher trap concentrations at the SiO₂/Si interface. Radiation exposure to SiO₂ produces electron–hole pairs through ionization, and some holes are captured by the internal hole “traps” in SiO₂, resulting in changes in the electrical properties of PMOS. The magnitude of these changes is related to the radiation dose.

Upon irradiation, the induced charges and interface states in the insulation layer (SiO₂) cause changes in the surface potential, namely, changes in the gate voltage. The radiation dose is measured based on the correlation between the gate voltage and radiation dose, which must be obtained through ground calibration.

4.5. SPD Detection

The space plasma environment can cause satellites’ surfaces to become entirely charged, and typically, satellites employ an equipotential design. However, during space environment disturbances, such as geomagnetic eruptions, variations in materials and directions can lead to temporary potential differences on the satellite surface. Monitoring surface potential can furnish data for satellite anomaly diagnosis, suggestions for subsequent engineering enhancements, and a foundation for ground test research [35].

Surface differential charging refers to potential differences between insulators or isolated conductors on the satellite surface and the satellite ground induced by space particle environments. The proposed technical scheme simulates the state of insulator mediums on the satellite surface using satellite surface materials and employs capacitance voltage division detection technology to detect the charging potential of insulation mediums [36]. The basic principles of surface potential detection are depicted in Figure 7.

The SPD comprises quartz glass samples, an electronic circuit processing system, and structural components. The outer layer of the sensor is made of quartz glass, with the inner layer being a circular gold-plated piece connected to core wires. Both the outer and inner layers of the glass can be equivalent to the capacitance (Cs), with specific values determined by material properties. A voltage division capacitor (C1) is connected between the quartz glass and the satellite ground. When the quartz glass (used to simulate the satellite skin) becomes charged, the voltage values at both ends of Capacitor C1 can be measured to derive the charging potential of the quartz glass surface, representing the satellite surface potential.

The sensor surface acquires charge upon the impact of charged particles on the glass surface at the frontmost part of the sensor. As the circular gold-plated area inside the glass forms a capacitor with the sensor surface, the induced surface charging potential is then outputted to the electronic section via the lead wires of the sensor for measurement. The working principle of electronics is as follows: firstly, the stability of the sensor output signal is ensured through the input follower circuit, then the small signal output by the sensor is further amplified through the voltage amplification circuit, and finally scientific data are output through the follower output circuit.

4.6. Data Management

In addition to the aforementioned four detectors, a shared space environment control unit (SECU) is also installed to serve as the data and control center for each space environment detection payload. It adopts a backup design with primary and standby machines and performs a cold backup of the SECU system through satellite platform power distribution control to enhance subsystem reliability. Figure 8 illustrates the internal structure and information flow of the SECU on the satellite space environment detection payload.

5. Calibration Results

Following the development of the space environment detection payload prototype for an HEO satellite, ground calibration experiments were conducted on each payload’s technical indicators to verify and precisely determine the actual detection parameters. Calibration items for high-energy particle detection payloads encompass the energy range, energy resolution, and flux error, while the radiation dose and potential detection mainly involve the range and sensitivity.

Given that different detection payloads measure different physical quantities, diverse calibration methods were employed. Particle detector calibration, for instance, utilizes a combination of calibration and simulation for standard radioactive sources and equivalent signal sources when the ground accelerator beam conditions are not met.

Table 4. Calibration methods and results of space environment detection payload.

Parameter	Method	Expected Results	Achieved Results
High-energy proton	Heavy Ion Accelerator of China Atomic Energy Institute (Protons < 40 MeV); Huairou electron accelerator (20–1600 keV) at CAS; standard radioactive source 207Bi; other energies are analyzed using the combination of equivalent signal generator calibration and simulation analysis	Energy range: 2.5 MeV~300 MeV; Energy resolution: <15%	Energy range: 2.497 MeV–296 MeV; Energy resolution: 5.12%@80 MeV; Flux error: 8.75%
High-energy electron		Energy range: 0.2 MeV–2.0 MeV; Energy resolution: <15%	Energy range: 195.8 keV~2.003 MeV (Direction X), 196 keV~2.004 MeV (Direction Y), 202 keV~1.996 MeV (Direction −Z); Energy resolution: 10.44%@800 keV (Direction X), 10.22%@800 keV (Direction Y), 10.57%@800 keV (Direction −Z); Flux error: 8.75%
Medium-energy proton		Energy range: 0.03 MeV–2.5 MeV; Energy resolution: <15%	Energy range: 30.9 keV~2.503 MeV (Direction X), 30.9 keV~2.505 MeV (Direction Y), 31.0 keV~2.497 MeV (Direction −Z); Energy resolution: 7.52%@72 keV (Direction X), 7.19%@72 keV (Direction Y), 8.82%@72 keV (Direction −Z); Flux error: 8.58%
Medium-energy electron		Energy range: 30 keV~410 keV; Energy resolution: <15%	Energy range: 29.1 keV~410.7 keV (Direction X), 29.1 keV~414.9 keV (Direction Y), 29.5 keV~409.0 keV (Direction −Z; Energy resolution: 9.52%@72 keV (Direction X), 9.65%@72 keV (Direction Y), 9.59%@72 keV (Direction −Z); Flux error: 8.58%
Total radiation dose	Standard radioactive source 60Coγ at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences	Measuring range: 0~3.0 × 106 rad (Si); Sensitivity: <10 rad (Si)	Measuring range: 0~3.38 × 106 rad (Si); Sensitivity: <3 rad (Si)
Surface potential	Static high-voltage source (+30 kV~−30 kV)	Measuring range: +1 kV~−10 kV; Sensitivity: <100 V	Measuring range: 1.414 kV~−11.571 kV; Sensitivity: <50 V

summarizes the main calibration methods and experimental results for each payload.

Among the calibration experiment items for the payloads, the energy resolution of particle detectors assesses the repeatability of instrument measurements for particles at a single energy level. A higher energy resolution corresponds to the instrument having a greater measurement accuracy. In this project, the energy resolution calibration of particle detectors was conducted using the Huairou electron accelerator, with a beam energy range of 20 keV–1.6 MeV. Based on the experimentally obtained data, Gaussian fitting was performed to fit the changing trend of the count ratios between adjacent energy levels, yielding the energy resolution of particles at different energy levels. The related specific principles, methods, and processes of calibration have been reported in previous studies [37,38,39]. Figure 9 presents the Gaussian fitting result for the medium-energy electron (72 keV) in the x-direction. The energy resolution was 9.52%, which met the required specification very well.

The calibration of the radiation dose range was conducted at the Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences. In this experiment, a 20,000-curie ⁶⁰Co standard gamma-ray source was utilized to irradiate the dose sensor, employing a PMOS with a sensitive layer thickness of 100 nm. The dose rate of the standard source was calibrated according to the Chinese dose standard, ensuring accuracy, reliability, and widespread applicability for various radiation dose measuring instruments. Upon the irradiation of the sensor, output signals were generated, transmitted to the dose circuit, and collected and analyzed by the data collection system. Figure 10 illustrates the calibration test plan.

The output response of the PMOS sensor to radiation exposure, indicating the conversion relationship between the change in gate voltage output and the accumulated radiation dose, is represented by the following formula:

Y = A1·X + A2·X² + A3·X³

(1)

where Y represents the radiation dose in rad (Si); X denotes the change ΔV in the gate voltage of the sensor, V; and the conversion relationship coefficients A1, A2, and A3 are determined by fitting the irradiation test data of the radiation source. The experimental data processing and fitting curve of the PMOS sensor are illustrated in Figure 11.

The specific values of the radiation dosimeter within the detection range were calculated using the above formula, determining the startup voltage, amplification factor, and maximum voltage of the instrument. The maximum measuring range of the dose probe is 3.38 × 10⁶ rad (Si).

Overall, the ground calibration of space environment detection payloads demonstrated that all indicators met the requirements. Notably, the radiation dose detection was susceptible to temperature fluctuations, necessitating temperature correction when analyzing and processing in-orbit detection data based on the ground calibration results [40]. Specifically, a relationship curve between changes in threshold voltage and temperature was derived from threshold voltage data and ambient temperature data of the RADFET sensor obtained through in-orbit measurements. By eliminating the influence of the RADFET temperature effects on space radiation dose detection, the threshold voltage of the RADFET sensor solely affected by space radiation at each moment was calculated. Finally, the cumulative space radiation dose was derived from the calibration curve.

6. Conclusions

This article reports China’s first satellite venture into HEO and describes the encountered space environment characterized by more complex and severe particle radiation compared to LEO, MEO, and GEO. The proposed space environment detection payload census technique facilitates the measurement of medium- and high-energy charged particles, the radiation dose, and satellite surface potential across various orientations in space. Throughout the ground testing phase, all payload technical specifications underwent calibration and experimental verification using reference signal sources, standard radioactive sources, and particle accelerators, and notably, all outcomes met the specified criteria. After the satellite’s deployment into space, China acquired detection data on the space particle radiation and its associated impacts within this orbit, effectively filling the data gap.