In this paper, we present a novel integrated simulation tool that models science yield and mission resources as a function of science payload, mission and system design, spacecraft behavior and modes, and target uncertainties. This provides a comprehensive and self-consistent approach to multi-spacecraft mission formulation around small bodies. We demonstrate this tool for the case of a notional three-spacecraft mission to the near-Earth asteroid Apophis, which will flyby Earth in 2029.

Introduction: The close approach of asteroid (99942) Apophis on April 13, 2029 presents a unique opportunity to achieve breakthrough science and strengthen planetary defense goals. As discussed in [1], low-frequency (VHF) radar observations can probe the interior structure of small bodies, as demonstrated by CONSERT at comet 67P [2, 3], and the planned JuRa low frequency radar on Hera/Juventas at the Didymos system—target of the DART mission. Radar measurements can determine the distribution of monolithic objects and voids within the body at 10’s of meter scale, which are critical for potential deflection and disruption attempts. This is best accomplished by multi-static, low frequency radar [4]. A mission concept to exploit the Apophis opportunity has been developed in a collaboration between NASA/JPL and CNES. The Distributed Radar Observations of Interior Distributions (DROID) mission would rendezvous with Apophis in late Summer 2028, seven months prior to Earth closest approach (ECA) and escort it through the encounter. A possible asteroid flyby on the way would delay arrival to late 2028 but still provide ample time for pre-ECA characterization. DROID’s measurements would determine the interior structure and properties, the body’s shape, morphology and rotation and observe any resolvable changes. DROID provides unique high fidelity in situ data that complements and enhances Earth-based optical and radar observations of Apophis, as well as data collected by OSIRIS-APEX which is due to rendezvous with Apophis 8 days after ECA. As illustrated in Figure 1, DROID’s architecture calls for three spacecraft: an ESPA Grande-class Mothership and two 6U CubeSats. The Mothership carries the CubeSats to Apophis, achieves the rendezvous cruise trajectory, performs high resolution imaging, and acts as a Direct-to-Earth (DTE) node for the constellation. Once Apophis’s physical characteristics (shape, spin, gravity field) are sufficiently characterized, the Mothership deploys both CubeSats, which then insert themselves into coordinated low orbits to perform monostatic and bistatic radar observations. Mission Goals: The DROID mission has two primary goals. The first goal is to understand the interior structure of a rubble pile asteroid and implications for its formation, evolution and response to a deflection attempt. Objectives include determining shape and density, and determining the internal size, distribution, and arrangement of blocks and voids within Apophis. \ DROID’s second goal is to understand how close planetary encounters affect asteroids. DROID will provide critical pre-ECA imagery of Apophis that are necessary for change detection. Objectives include determining if material moves on the surface of Apophis during the Earth flyby, and determining how the spin state of Apophis changes during ECA. Payload: Given the goals above, DROID employs four types of payloads distributed over three spacecraft (Figure 1). Objectives requiring surface imaging are to be met with a narrow-angle camera (NAC) on-board the Mothership spacecraft, whose focal plane is to be based on the Advanced CASPEX detector [5]. Additional wide-angle cameras (WACs) are carried on the two CubeSats for optical navigation. The objective to map internal structure is achieved using the Low Frequency Radar (LFR) on the CubeSats. The LFR is baselined as a version of JuRa (60 MHz), [6], modified to operate in a bistatic mode [1]. Inter-Spacecraft Link (ISL) S-band transponders on all three spacecraft perform data transfer between CubeSats and Mothership, and synchronize the CubeSat clocks for accurate bistatic radar measurement. ISLs are also used with the Mothership’s DTE link to map the gravity field. Mission Architecture: The DROID mission architecture is compatible with either direct launch or rideshare and will utilize heritage bus designs that can achieve the required propulsion performance. DROID’s 3.54 km/s ΔV requirement is similar to that of ESCAPADE, which uses bipropellant propulsion [7], DROID’s reference mission is constrained by a cruise trajectory insertion (CTI) window of about October-November 2027. Details of launch, CTI and cruise are provided in [8]. Operations: DROID arrives at Apophis around August-September 2028 (~December if it performs a precursor asteroid flyby) and executes a 0.30 km/s burn to reduce its relative velocity. During this phase, the Mothership NAC begins preliminary characterization of Apophis’s shape and spin. Approach imaging is then followed-up by several flyby maneuvers used to characterize the gravity field with DTE communication. The Mothership then deploys the CubeSats, which maneuver into 2-5 body radii altitude, sun-synchronous terminator orbits using their own cold gas propulsion. Following CubeSat deployment, the Mothership positions itself in a 9 body radii altitude orbit where it continues its imaging investigations using the NAC. The CubeSats are positioned antipodally with ±15° margin in their relative position and continuously collect both monostatic and bistatic echoes. A 2-body radii altitude orbit will enable mapping of 20% of the 3D monostatic Doppler sampling at 60 MHz [9], within 40 days. Radar data products include: (1) 3D volumetric backscatter via monostatic/bistatic tomographic SAR, (2) average dielectric constant along interior bistatic ray paths with assessment of internal heterogeneity [10]. The configuration of the DROID constellation during ECA and Post-ECA operations is the subject of on-going studies. Major ECA drivers include positioning of cameras to maximize the likelihood of capturing surface changes and mitigating the risk of collisions with potential ejected debris. The major Post-ECA driver is escaping from Apophis orbit to a safe heliocentric orbit prior to depleting propellent in order to avoid any possibility of impacting the asteroid and perturbing its orbit. Acknowledgments: This work is being carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA (80NM0018D0004), and at CNES. ©2022 California Institute of Technology. Government sponsorship acknowledged. References: [1] Herique, A. et al (this meeting). [2] Barbin, Y. et al (1999) ASR 24. [3] Kofman, W. et al. (2007) SSR 128. [4] Haynes, M. et al (2022) LPSC #1295. [5] Bezine, J. et al. (2021) ICSO 118520V. [6] Herique, A. et al (2020) EPSC. [7] French, R. (2019) AIAA SSC. [8] Amini, R. et al (2022) Apophis T-7 #2012. [9] Haynes, M. et al. (2021) ASR 68. [10] Herique, A. et al (2018) ASR 62.

A temperature-dependent analytical model for total-ionizing-dose-induced excess base current in BJTs is proposed. Model captures base current evolution with temperature on irradiated parts. In this work, BJTs are irradiated at room temperature. Base currents are obtained and the concentrations of oxide defects created during irradiation are calculated. Both base current and defect densities resulting from room temperature irradiations are used as inputs to SPICE simulations and the analytical model. Experimental data obtained from measurements at both low and high temperatures on parts irradiated at room temperature are shown to compare well to the simulation results and analytical model over a range of temperatures.

Science rationale: Our knowledge of the internal structure of asteroids entirely relies on inferences from remote sensing observations of the surface combined with theoretical modeling [1]. Is Apophis a rubble-pile, as expected, or a monolithic rock, and how high is the porosity? What is the typical size of the constituent blocks? Are these blocks homogeneous or heterogeneous? If Apophis is bilobed, how does the material differ between each lobe? After many asteroid rendezvous and fly-by missions from different nations, these crucial and yet basic questions remain open. Direct measurements of the deep interior structure and composition are needed to better understand the accretion and dynamical evolution of asteroids in general. These measurements at Apophis in particular will directly improve our ability to understand and predict stability conditions as well as to interpret the response of Apophis to the tidal forces induced by its close approach to the Earth. This information is also crucial to plan any interaction of a spacecraft with Apophis and other similar asteroids, especially for Planetary Defense purposes. Direct observations of asteroid subsurfaces in general are also required to better model the dynamics of granular materials in low gravity, and to determine material composition and mineralogy, while space weathering and thermal cycling alter surface properties as observed by optical remote sensing. DROID mission concept: Radar observation of Apophis from a spacecraft is the most mature technique capable of achieving these objectives, by providing a direct measurement of its interior. This is the goal of DROID – (Distributed Radar Observations of Interior Distributions), a mission concept developed in collaboration between NASA JPL and CNES [2] and discussed in more detail in the accompanying presentation [3]. The DROID mothership will release two CubeSats each carrying a low-frequency radar. The radar will be a version of JuRa (60 MHz) [4], modified to operate in a bistatic mode and using an inter-satellite link as a synchronization channel. The mothership and the two CubeSats (daughtercraft) will also have cameras for both science and navigation. Radar observation: Each daughtercraft radar can operate in a monostatic mode, or in a bistatic mode using the two platforms to measure the signal transmitted throughout Apophis, as CONSERT did onboard Rosetta orbiter and Philae lander [1,5,6,7]. Monostatic radar. A radar at 60 MHz offers a larger penetration (up to 100 meters or more) with a limited resolution (≈5 m). It corresponds to the instrument under implementation for the Juventas Cubesat on the Hera/ESA mission [4]. Furthermore, multi-pass processing allows us to build a 3D tomographic image of the interior to identify internal structure like layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution and heterogeneity at different scales (from sub-metric to global). Initial dynamics modeling of the two Cubesats orbiting Apophis at 3 body radii indicates that 20% full Doppler coverage is possible in 40 days [2,8]. Shallow subsurface characterization and radar images to support the shape modeling are also possible in this configuration, but with degraded performance due to a limited resolution. Bistatic radar. The bistatic radar will firstly measure the signal in transmission, allowing us to achieve a direct measurement of the dielectric permittivity, which is related to composition and microporosity [6]. This objective is less demanding in terms of data volume and operation compared to full bistatic coverage. Partial transmission coverage will provide slices of the body with average characterization and its special variability. With dense coverage, benefiting from a larger diversity of observation angles, the bistatic mode will allow a complete 3D tomography [8,9]. In general, multi-angular acquisition allows for a better decorrelation of the size effect and permittivity contrast in the return power. Ground-to-space: In addition to radar observation at close proximity, there is the possibility for joint ground-to-space radar observations at the epoch of the Apophis close approach. [10]. Such measurements would make use of high-power transmitters or sensitive radio astronomy observatories on Earth [11]. Ground-to-space configurations would be used to collect echoes in unique bistatic configurations or to collect echoes during spacecraft maneuvers at close approach (e.g., required spacecraft stand-off). Acknowledgement: The research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). References: [1] A. Herique et al. (2018) ASR 62, 2141‑2162., [2] R. Amini et al. (2022) Apophis T-7, #2012, [3] C. Raymond et al. (2022) this meeting., [4] A. Herique et al. (2022) JuRA radar, EPSC, [5] W. Kofman et al. (2015) Science 349, aab0639.,[6] Herique et al. (2016) MNRAS 462, S516‑S532., [7] A. Herique et al. (2019) A&A 630, A6., [8] M. Haynes et al. (2021) ASR 68 (9), [9] M. Haynes et al. (2021) LPSC #1295, [10] A. Herique et al. (2019) Apophis T-9 #2029, [11] M. Haynes et al (2022) Apophis T-7 #2020

Science rationale: Our knowledge of the internal structure of asteroids entirely relies on inferences from remote sensing observations of the surface combined with theoretical modeling [1]. Is Apophis a rubble-pile, as expected, or a monolithic rock, and how high is the porosity? What is the typical size of the constituent blocks? Are these blocks homogeneous or heterogeneous? If Apophis is bilobed, how does the material differ between each lobe? After many asteroid rendezvous and fly-by missions from different nations, these crucial and yet basic questions remain open. Direct measurements of the deep interior structure and composition are needed to better understand the accretion and dynamical evolution of asteroids in general. These measurements at Apophis in particular will directly improve our ability to understand and predict stability conditions as well as to interpret the response of Apophis to the tidal forces induced by its close approach to the Earth. This information is also crucial to plan any interaction of a spacecraft with Apophis and other similar asteroids, especially for Planetary Defense purposes. Direct observations of asteroid subsurfaces in general are also required to better model the dynamics of granular materials in low gravity, and to determine material composition and mineralogy, while space weathering and thermal cycling alter surface properties as observed by optical remote sensing. DROID mission concept: Radar observation of Apophis from a spacecraft is the most mature technique capable of achieving these objectives, by providing a direct measurement of its interior. This is the goal of DROID – (Distributed Radar Observations of Interior Distributions), a mission concept developed in collaboration between NASA JPL and CNES [2] and discussed in more detail in the accompanying presentation [3]. The DROID mothership will release two CubeSats each carrying a low-frequency radar. The radar will be a version of JuRa (60 MHz) [4], modified to operate in a bistatic mode and using an inter-satellite link as a synchronization channel. The mothership and the two CubeSats (daughtercraft) will also have cameras for both science and navigation. Radar observation: Each daughtercraft radar can operate in a monostatic mode, or in a bistatic mode using the two platforms to measure the signal transmitted throughout Apophis, as CONSERT did onboard Rosetta orbiter and Philae lander [1,5,6,7]. Monostatic radar. A radar at 60 MHz offers a larger penetration (up to 100 meters or more) with a limited resolution (≈5 m). It corresponds to the instrument under implementation for the Juventas Cubesat on the Hera/ESA mission [4]. Furthermore, multi-pass processing allows us to build a 3D tomographic image of the interior to identify internal structure like layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution and heterogeneity at different scales (from sub-metric to global). Initial dynamics modeling of the two Cubesats orbiting Apophis at 3 body radii indicates that 20% full Doppler coverage is possible in 40 days [2,8]. Shallow subsurface characterization and radar images to support the shape modeling are also possible in this configuration, but with degraded performance due to a limited resolution. Bistatic radar. The bistatic radar will firstly measure the signal in transmission, allowing us to achieve a direct measurement of the dielectric permittivity, which is related to composition and microporosity [6]. This objective is less demanding in terms of data volume and operation compared to full bistatic coverage. Partial transmission coverage will provide slices of the body with average characterization and its special variability. With dense coverage, benefiting from a larger diversity of observation angles, the bistatic mode will allow a complete 3D tomography [8,9]. In general, multi-angular acquisition allows for a better decorrelation of the size effect and permittivity contrast in the return power. Ground-to-space: In addition to radar observation at close proximity, there is the possibility for joint ground-to-space radar observations at the epoch of the Apophis close approach. [10]. Such measurements would make use of high-power transmitters or sensitive radio astronomy observatories on Earth [11]. Ground-to-space configurations would be used to collect echoes in unique bistatic configurations or to collect echoes during spacecraft maneuvers at close approach (e.g., required spacecraft stand-off). Acknowledgement: The research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). References: [1] A. Herique et al. (2018) ASR 62, 2141‑2162., [2] R. Amini et al. (2022) Apophis T-7, #2012, [3] C. Raymond et al. (2022) this meeting., [4] A. Herique et al. (2022) JuRA radar, EPSC, [5] W. Kofman et al. (2015) Science 349, aab0639.,[6] Herique et al. (2016) MNRAS 462, S516‑S532., [7] A. Herique et al. (2019) A&A 630, A6., [8] M. Haynes et al. (2021) ASR 68 (9), [9] M. Haynes et al. (2021) LPSC #1295, [10] A. Herique et al. (2019) Apophis T-9 #2029, [11] M. Haynes et al (2022) Apophis T-7 #2020

An automatic fully-digital pulse frequency modulation (PFM) to pulse width modulation (PWM) mode transition scheme for digitally controlled DC-DC buck converter is proposed in this paper. The meta-stability issues of time-based analog-to-digital converter (ADC) which result in incorrect loop control and output voltage artifacts are also addressed in the proposed design. The digital controller is designed and fabricated on a 0.18 μm 6 layer-metal CMOS technology with an active area of 1.61 mm 2 . The converter operates at a switching frequency of 1 MHz and supports a regulated output voltage ranging from 0.9 V to 3.3 V for an input voltage range between 3.3 V to 5 V. The converter can supply a maximum load current of 7.5 A, and has the peak efficiency of 95.56%.

The impacts of transistor-level total ionizing dose on system-level parameters of a CubeSat computing board are simulated. The temperature control loop simulation and uncertainty quantification shows TID-induced changes as probability distributions of key system performance parameters versus mission time.The impact of total ionizing dose (TID) effects on semiconductor components is well understood and often characterized by probability distributions of parametric changes. However, the propagation of such probabilistic changes to system-level performance parameters is needed. This paper presents an approach based on uncertainty quantification and system-level functional simulation that addresses this need and produces probabilistic predictions for performance degradation as functions of TID and thus mission time.

A simplified approach to estimating radiation-induced Si-SiO2 interface trap densities, based on steady-state populations of relevant mobile species, is presented. The dose-rate sensitivity arises from bimolecular reaction terms. Molecular hydrogen increases the effects of ionizing radiation. Exciton contributions explain effects at high dose-rates. Calculations are consistent with known trends in dose, dose rate, hydrogen content and temperature.

Analog-to-digital converters (ADCs) with different topologies respond differently to total ionizing dose (TID). A flexible behavioral modeling approach is proposed for system-level simulation of TID effects in successive-approximation-register (SAR) ADCs. The radiation-enabled approach can be adapted for a wide range of ADCs of various resolutions, clock speeds, and manufacturers. The empirical model is calibrated and validated, pre-rad and post-rad, for a particular ADC. Pre-rad calibration is performed by introducing distributions of static parameters corresponding to datasheet specifications. The post-rad calibration is accomplished by introducing error sources associated with the comparator and digital-to-analog converter (DAC), corresponding to experimental data. The calibrated model is used to examine the dynamic performance and to estimate the probability of parametric failure during the mission lifetime.

Temperature variation on board can have a significant impact on electronic circuit parameters. In this paper, we investigate and model how both irradiated NPN and PNP-Bipolar Junction Transistors (BJTs) at room temperature respond electrically to temperature variation. A temperature-dependent analytical model for total-ionizing-dose-induced excess base current in BJTs is proposed. Our model captures base current dependence on temperature on irradiated parts. To do so, the devices under test are irradiated at room temperature with all terminals grounded. After irradiation, base currents are obtained, and the concentrations of oxide defects (i.e., oxide trapped charge and interface traps) created during irradiation are calculated. At the end, results from our analytical model and experimental data are compared to SPICE simulations. Both base current and defect densities resulting from room temperature irradiations are used as inputs to SPICE simulations and the analytical model. Experimental data obtained from measurements at both low and high temperatures on parts irradiated at room temperature are shown to compare well to the simulation results and analytical model over a range of temperatures. The modeling work shows that SPICE simulations can support qualification for commercial-off-the-shelf (COTS) linear bipolar components with temperature for space applications.

Total ionizing dose (TID) functional failure analysis on positive and negative low-dropout linear voltage regulators is performed. Two parts have been chosen for this article: the LP2953 positive regulator and the LT1175 negative regulator. Different failure mechanisms are observed, which can be modeled with radiation-enabled SPICE simulations. The simulation results are shown to compare well with experimental data. Both voltage regulators contain three blocks, essential for accurate modeling: a bandgap reference, a power pass transistor, and an error amplifier. In this article, simulations were performed on each block independently to analyze the trends in parametric degradation after radiation exposure and describe the failure mechanisms. This article identifies the bandgap circuitry as the primary cause of failure for the negative regulator while degradations in all blocks contribute to the TID response of circuits.

A simulation paradigm is proposed to examine the effects of transistor-level degradation produced by total ionizing dose (TID) on top-level system performance parameters. The approach is demonstrated on a command and data handling (C&DH) board for deep-space CubeSats. Simulation and postirradiation measurements of a temperature control loop show that TID degradation changes temperature regulation significantly.

SEE test results are presented for the Snapdragon 801, 835 and 845 processors. The focus is on the performance of the processors under a normal work environment. Testing was performed to get an understanding of the crash and bit error performance. In the case of the 801, SEL data were also collected.

A total dose testing methodology for qualifying bipolar analog circuits for the Europa Clipper (EC) mission is presented. The method leverages the unique mission dose rate profile to bound device performance and reduces qualification test time from 1 year to 80 days. In addition, a new testing approach that simulates the mission dose profile revealed interesting information about the nature of radiation-induced defects in these “rad-hard” bipolar process lines. Results show that unlike bipolar parts from older generations (very sensitive to enhanced low dose rate sensitivity), these new processes seem to be dominated by oxide traps (N ot ) and not interface traps (N it ). This appears to be consistent with the mitigation used by vendors for reducing hydrogen (H 2 ) contamination in their processing steps.

A radiation-tolerant digital multiphase current-mode hysteretic point-of-load regulator fabricated on a commercial 180-nm CMOS process is presented. Experiments and simulations are used to demonstrate its single-event immunity and its total-dose tolerance over 100 krad(Si). Key electrical performance parameters are: 5-V input, 0.8-3.3 V output, 10-A load current, 93% peak efficiency, four-phase hysteretic quasi-current-mode buck converter with +/- 1.5% frequency synchronization, +/- 3.6% current sharing error, and 1% ripple.

A multiscale modeling platform that supports the "virtual" qualification of commercial-off-the-shelf parts is presented. The multiscale approach is divided into two modules. The first module generates information related to the bipolar junction transistor gain degradation that is a function of fabrication process, operational, and environmental inputs. The second uses this information as inputs for radiation-enabled circuit simulations. The prototype platform described in this paper estimates the total ionizing dose and dose rate responses of linear bipolar integrated circuits for different families of components. The simulation and experimental results show good correlation and suggest this platform to be a complementary tool within the radiation-hardness assurance flow. The platform may reduce some of the costly reliance on testing for all systems.

An improved total ionizing dose model for lateral p-n-p bipolar junction transistors is described. The model captures the impact of charged defects on radiation-induced excess base current. Failure to incorporate this mechanism in model underestimates gain degradation.

A four-phase, quasi-current-mode hysteretic buck converter with digital frequency synchronization, online comparator offset-calibration and, digital current-sharing control is presented. The switching frequency of the hysteretic converter is digitally synchronized to the input clock reference with less than ±1.5% error in the switching frequency range of 3–9.5 MHz. The online offset calibration cancels the input-referred offset of the hysteretic comparator and enables ±1.1% voltage regulation accuracy. Maximum current-sharing error of ±3.6% is achieved by a duty-cycle-calibrated delay line-based pulsewidth modulation generator, without affecting the phase synchronization timing sequence. In light-load conditions, individual converter phases can be disabled, and the final stage power converter output stage is segmented for high efficiency. The dc–dc converter achieves 93% peak efficiency for $V_{i}= 2$ V and $V_{o}= 1.6$ V.

Single event effect susceptibility of a 1-Mbit commercial MRAM was experimentally evaluated. The memory exhibited SEFIs when operated in a dynamic mode with an LET threshold of 2.29 MeV.cm 2 /mg and a saturated cross section of 2.2×10 -4 cm 2 /device. The memory was not sensitive to SEL, SEU or MBUs.