Development of CubeSat Spacecraft-to-Spacecraft Optical Link Detection Chain for the CLICK B/C Mission

The growing interest in and expanding applications of small satellite constellation networks necessitates effective and reliable high-bandwidth communication between spacecraft. The applications of these constellations (such as navigation or imaging) rely on the precise measurement of timing offset between the spacecraft in the constellation. The CubeSat Laser Infrared CrosslinK (CLICK) mission is being developed by the Massachusetts Institute of Technology (MIT), the University of Florida (UF), and NASA Ames Research Center. The second phase of the mission (CLICK-B/C) will demonstrate a crosslink between two CubeSats (B and C) that each host a \u3c 2U laser communication payload. The terminals will demonstrate full-duplex spacecraft-to-spacecraft communications and ranging capability using commercial components. As part of the mission, CLICK will demonstrate two-way time-transfer for clock synchronization and data transfer at a minimum rate of 20 Mbps over separation distances ranging from 25 km to 580 km. The payloads of CLICK B and C include a receiver chain with a custom photodetector board, a Time-to-Digital Converter (TDC), a Microchip Chip-Scale Atomic Clock (CSAC), and a field-programmable gate array (FPGA). The payloads can measure internal propagation delays of the transmitter and the receiver, and cancel environmental effects impacting timing accuracy. The photodetector board is 2.5 cm x 2.5 cm and includes an avalanche photodiode (APD) and variable-gain amplifiers through which the detected signal is conditioned for the TDC to be time-stamped. This design has been developed from the UF and NASA Ames CubeSat Handling Of Multisystem Precision Time Transfer (CHOMPTT) project and associated MOCT (Miniature Optical Communication Transceiver) demonstration. The TDC samples the signal at four points: twice on the rising edge at set thresholds, and twice at the falling edge at those same thresholds. These four time-offset samples are sent to the FPGA, which combines the measurements for a reported timestamp of the detected laser pulse. These timestamps can then be used in a pulse-position modulation (PPM) demodulation scheme to receive data at up to 50 Mbps, to calculate range down to 10 cm, and for precision time-transfer with \u3c 200 ps resolution. In this paper, we will discuss the designed capabilities and noise performance of the CLICK TDC-based optical receiver chain

Development and Results of a Lasercom Testbed for the CLICK B/C CubeSats and Future Missions

The expansion of interest in small satellite constellation networks underscores the need for precise timing synchronization and reliable high-bandwidth communication between spacecraft. The CubeSat Laser Infrared CrosslinK (CLICK) mission is being developed by the Massachusetts Institute of Technology, the University of Florida, and NASA Ames Research Center. The first phase of the mission, CLICK A, was launched on July 14, 2022, aboard SpaceX’s CRS-25 and put into orbit from the International Space Station, where it successfully demonstrated the downlink to Earth. The second phase of the mission (CLICK B/C) will additionally demonstrate a crosslink between two 3U CubeSats (B and C) that each host a 1.5 U laser communication payload. The terminals will demonstrate full-duplex spacecraft-to-spacecraft communications and ranging capability using commercial-off-the-shelf components at low size, weight and power (SWaP). As part of the mission, CLICK will demonstrate two-way time transfer for chip-scale atomic clock (CSAC) synchronization and data transfer. This data transfer will use pulse-position modulation (PPM) at rates between 20 Mbps and 50 Mbps over separation distances ranging from 25 km to 580 km. A time-transfer precision of \u3c 200 ps between the spacecraft is targeted. CLICK B/C is scheduled to launch in 2025. The University of Florida hosts a testbed to support CLICK developments. Its goal is to enable testing of the optical data- and timing-transfer chain on ground. This encompasses the vital components of the CLICK hardware for both TX (transmission) and RX (receiving). For TX, the electronics and laser system to generate optical pulses are included, with the latter consisting of a micro-integrable tunable laser assembly as seed laser and a semiconductor optical amplifier as shutter. In turn, the RX side consists of an avalanche photodetector (APD) to capture the pulses, electronics to condition and convert the analog signal into the digital domain (time-to-digital and analog-to-digital), and a field-programmable gate array as DSP (digital signal processing) platform. The DSP implements the algorithm to decode the PPM scheme and extract timing information. In between the optical TX and RX, an electrical variable optical gain amplifier is placed to simulate varying distances between satellites and the associated change in received power. The final setup is envisioned to use separate hardware platforms for TX and RX to test the timing transfer between independent CSACs. Here we present the status of the testbed and the associated development of CLICK hardware and DSP, in particular the APD and PPM decoder, along with results of the lasercom testing, showing initial tracking of test data

ADM canonical formalism for gravitating spinning objects

In general relativity, systems of spinning classical particles are implemented into the canonical formalism of Arnowitt, Deser, and Misner [1]. The implementation is made with the aid of a symmetric stress-energy tensor and not a 4-dimensional covariant action functional. The formalism is valid to terms linear in the single spin variables and up to and including the next-to-leading order approximation in the gravitational spin-interaction part. The field-source terms for the spinning particles occurring in the Hamiltonian are obtained from their expressions in Minkowski space with canonical variables through 3-dimensional covariant generalizations as well as from a suitable shift of projections of the curved spacetime stress-energy tensor originally given within covariant spin supplementary conditions. The applied coordinate conditions are the generalized isotropic ones introduced by Arnowitt, Deser, and Misner. As applications, the Hamiltonian of two spinning compact bodies with next-to-leading order gravitational spin-orbit coupling, recently obtained by Damour, Jaranowski, and Schaefer [2], is rederived and the derivation of the next-to-leading order gravitational spin(1)-spin(2) Hamiltonian, shown for the first time in [3], is presented.Comment: REVTeX4, 18 pages. v1: published version. v2: corrected misprints in (8.4) and (9.3), updated reference

Long working hours and risk of 50 health conditions and mortality outcomes : a multicohort study in four European countries

Background: Studies on the association between long working hours and health have captured only a narrow range of outcomes (mainly cardiometabolic diseases and depression) and no outcome-wide studies on this topic are available. To achieve wider scope of potential harm, we examined long working hours as a risk factor for a wide range of disease and mortality endpoints. Methods: The data of this multicohort study were from two population cohorts from Finland (primary analysis, n=59 599) and nine cohorts (replication analysis, n=44 262) from Sweden, Denmark, and the UK, all part of the Individual-participant Meta-analysis in Working Populations (IPD-Work) consortium. Baseline assessed long working hours (>55 hours per week) were compared to standard working hours (35-40 h). Outcome measures with follow-up until age 65 years were 46 diseases that required hospital treatment or continuous pharmacotherapy, all-cause, and three cause-specific mortality endpoints, ascertained via linkage to national health and mortality registers. Findings: 2747 (4.6%) participants in the primary cohorts and 3027 (6.8%) in the replication cohorts worked long hours. After adjustment for age, sex, and socioeconomic status, working long hours was associated with increased risk of cardiovascular death (hazard ratio 1.68; 95% confidence interval 1.08-2.61 in primary analysis and 1.52; 0.90-2.58 in replication analysis), infections (1.37; 1.13-1.67 and 1.45; 1.13-1.87), diabetes (1.18; 1.01-1.38 and 1.41; 0.98-2.02), injuries (1.22; 1.00-1.50 and 1.18; 0.98-1.18) and musculoskeletal disorders (1.15; 1.06-1.26 and 1.13; 1.00-1.27). Working long hours was not associated with all-cause mortality. Interpretation: Follow-up of 50 health outcomes in four European countries suggests that working long hours is associated with an elevated risk of early cardiovascular death and hospital-treated infections before age 65. Associations, albeit weak, were also observed with diabetes, musculoskeletal disorders and injuries. In these data working long hours was not related to elevated overall mortality. (C) 2021 The Authors. Published by Elsevier Ltd.Peer reviewe

Job Strain as a Risk Factor for Peripheral Artery Disease : A Multi-Cohort Study

Background Job strain is implicated in many atherosclerotic diseases, but its role in peripheral artery disease (PAD) is unclear. We investigated the association of job strain with hospital records of PAD, using individual-level data from 11 prospective cohort studies from Finland, Sweden, Denmark, and the United Kingdom. Methods and Results Job strain (high demands and low control at work) was self-reported at baseline (1985-2008). PAD records were ascertained from national hospitalization data. We used Cox regression to examine the associations of job strain with PAD in each study, and combined the study-specific estimates in random effects meta-analyses. We used tau(2), I-2, and subgroup analyses to examine heterogeneity. Of the 139 132 participants with no previous hospitalization with PAD, 32 489 (23.4%) reported job strain at baseline. During 1 718 132 person-years at risk (mean follow-up 12.8 years), 667 individuals had a hospital record of PAD (3.88 per 10 000 person-years). Job strain was associated with a 1.41-fold (95% CI, 1.11-1.80) increased average risk of hospitalization with PAD. The study-specific estimates were moderately heterogeneous (tau(2)=0.0427, I-2: 26.9%). Despite variation in their magnitude, the estimates were consistent in both sexes, across the socioeconomic hierarchy and by baseline smoking status. Additional adjustment for baseline diabetes mellitus did not change the direction or magnitude of the observed associations. Conclusions Job strain was associated with small but consistent increase in the risk of hospitalization with PAD, with the relative risks on par with those for coronary heart disease and ischemic stroke.Peer reviewe

Differential limit on the extremely-high-energy cosmic neutrino flux in the presence of astrophysical background from nine years of IceCube data

We report a quasi-differential upper limit on the extremely-high-energy (EHE) neutrino flux above $5\times 10^{6}$ GeV based on an analysis of nine years of IceCube data. The astrophysical neutrino flux measured by IceCube extends to PeV energies, and it is a background flux when searching for an independent signal flux at higher energies, such as the cosmogenic neutrino signal. We have developed a new method to place robust limits on the EHE neutrino flux in the presence of an astrophysical background, whose spectrum has yet to be understood with high precision at PeV energies. A distinct event with a deposited energy above $10^{6}$ GeV was found in the new two-year sample, in addition to the one event previously found in the seven-year EHE neutrino search. These two events represent a neutrino flux that is incompatible with predictions for a cosmogenic neutrino flux and are considered to be an astrophysical background in the current study. The obtained limit is the most stringent to date in the energy range between $5 \times 10^{6}$ and $5 \times 10^{10}$ GeV. This result constrains neutrino models predicting a three-flavor neutrino flux of $E_\nu^2\phi_{\nu_e+\nu_\mu+\nu_\tau}\simeq2\times 10^{-8}\ {\rm GeV}/{\rm cm}^2\ \sec\ {\rm sr}$at$10^9\ {\rm GeV}$. A significant part of the parameter-space for EHE neutrino production scenarios assuming a proton-dominated composition of ultra-high-energy cosmic rays is excluded.Comment: The version accepted for publication in Physical Review

Development of a Symmetric Echo-Planar Spectroscopy Imaging Framework for Hyperpolarized 13C Imaging in a Clinical PET/MR Scanner

Here, we developed a symmetric echo-planar spectroscopic imaging (EPSI) sequence for hyperpolarized 13C imaging on a clinical hybrid positron emission tomography/magnetic resonance imaging system. The pulse sequence uses parallel reconstruction pipelines to separately reconstruct data from odd-and-even gradient echoes to reduce artifacts from gradient imbalances. The ramp-sampled data in the spatiotemporal frequency space are regridded to compensate for the chemical-shift displacements. Unaliasing of nonoverlapping peaks outside of the sampled spectral width was performed to double the effective spectral width. The sequence was compared with conventional phase-encoded chemical-shift imaging (CSI) in phantoms, and it was evaluated in a canine cancer patient with ameloblastoma after injection of hyperpolarized [1-13C]pyruvate. The relative signal-to-noise ratio of EPSI with respect to CSI was 0.88, which is consistent with the decrease in sampling efficiency due to ramp sampling. Data regridding in the spatiotemporal frequency space significantly reduced spatial blurring compared with direct fast Fourier transform. EPSI captured the spatial distributions of both metabolites and their temporal dynamics in vivo with an in-plane spatial resolution of 5 × 9 mm2 and a temporal resolution of 3 seconds. Significantly higher spatial and temporal resolution for delineating anatomical structures in vivo was achieved for EPSI metabolic maps than for CSI maps, which suffered spatiotemporal blurring. The EPSI sequence showed promising results in terms of short acquisition time and sufficient spectral bandwidth of 500 Hz, allowing to adjust the trade-off between signal-to-noise ratio and encoding speed

Searching for a Stochastic Background of Gravitational Waves with LIGO

The Laser Interferometer Gravitational-wave Observatory (LIGO) has performed the fourth science run, S4, with significantly improved interferometer sensitivities with respect to previous runs. Using data acquired during this science run, we place a limit on the amplitude of a stochastic background of gravitational waves. For a frequency independent spectrum, the new limit is $\Omega_{\rm GW} < 6.5 \times 10^{-5}$. This is currently the most sensitive result in the frequency range 51-150 Hz, with a factor of 13 improvement over the previous LIGO result. We discuss complementarity of the new result with other constraints on a stochastic background of gravitational waves, and we investigate implications of the new result for different models of this background.Comment: 37 pages, 16 figure

Quantum state preparation and macroscopic entanglement in gravitational-wave detectors

Long-baseline laser-interferometer gravitational-wave detectors are operating at a factor of 10 (in amplitude) above the standard quantum limit (SQL) within a broad frequency band. Such a low classical noise budget has already allowed the creation of a controlled 2.7 kg macroscopic oscillator with an effective eigenfrequency of 150 Hz and an occupation number of 200. This result, along with the prospect for further improvements, heralds the new possibility of experimentally probing macroscopic quantum mechanics (MQM) - quantum mechanical behavior of objects in the realm of everyday experience - using gravitational-wave detectors. In this paper, we provide the mathematical foundation for the first step of a MQM experiment: the preparation of a macroscopic test mass into a nearly minimum-Heisenberg-limited Gaussian quantum state, which is possible if the interferometer's classical noise beats the SQL in a broad frequency band. Our formalism, based on Wiener filtering, allows a straightforward conversion from the classical noise budget of a laser interferometer, in terms of noise spectra, into the strategy for quantum state preparation, and the quality of the prepared state. Using this formalism, we consider how Gaussian entanglement can be built among two macroscopic test masses, and the performance of the planned Advanced LIGO interferometers in quantum-state preparation