Radiometer Calibration Using Colocated GPS Radio Occultation Measurements

We present a new high-fidelity method of calibrating a cross-track scanning microwave radiometer using Global Positioning System (GPS) radio occultation (GPSRO) measurements. The radiometer and GPSRO receiver periodically observe the same volume of atmosphere near the Earth's limb, and these overlapping measurements are used to calibrate the radiometer. Performance analyses show that absolute calibration accuracy better than 0.25 K is achievable for temperature sounding channels in the 50-60-GHz band for a total-power radiometer using a weakly coupled noise diode for frequent calibration and proximal GPSRO measurements for infrequent (approximately daily) calibration. The method requires GPSRO penetration depth only down to the stratosphere, thus permitting the use of a relatively small GPS antenna. Furthermore, only coarse spacecraft angular knowledge (approximately one degree rms) is required for the technique, as more precise angular knowledge can be retrieved directly from the combined radiometer and GPSRO data, assuming that the radiometer angular sampling is uniform. These features make the technique particularly well suited for implementation on a low-cost CubeSat hosting both radiometer and GPSRO receiver systems on the same spacecraft. We describe a validation platform for this calibration method, the Microwave Radiometer Technology Acceleration (MiRaTA) CubeSat, currently in development for the National Aeronautics and Space Administration (NASA) Earth Science Technology Office. MiRaTA will fly a multiband radiometer and the Compact TEC/Atmosphere GPS Sensor in 2015.United States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (United States. Air Force Contract FA8721-05-C-0002

The development and validation of a scoring tool to predict the operative duration of elective laparoscopic cholecystectomy

Background: The ability to accurately predict operative duration has the potential to optimise theatre efficiency and utilisation, thus reducing costs and increasing staff and patient satisfaction. With laparoscopic cholecystectomy being one of the most commonly performed procedures worldwide, a tool to predict operative duration could be extremely beneficial to healthcare organisations. Methods: Data collected from the CholeS study on patients undergoing cholecystectomy in UK and Irish hospitals between 04/2014 and 05/2014 were used to study operative duration. A multivariable binary logistic regression model was produced in order to identify significant independent predictors of long (> 90 min) operations. The resulting model was converted to a risk score, which was subsequently validated on second cohort of patients using ROC curves. Results: After exclusions, data were available for 7227 patients in the derivation (CholeS) cohort. The median operative duration was 60 min (interquartile range 45–85), with 17.7% of operations lasting longer than 90 min. Ten factors were found to be significant independent predictors of operative durations > 90 min, including ASA, age, previous surgical admissions, BMI, gallbladder wall thickness and CBD diameter. A risk score was then produced from these factors, and applied to a cohort of 2405 patients from a tertiary centre for external validation. This returned an area under the ROC curve of 0.708 (SE = 0.013, p  90 min increasing more than eightfold from 5.1 to 41.8% in the extremes of the score. Conclusion: The scoring tool produced in this study was found to be significantly predictive of long operative durations on validation in an external cohort. As such, the tool may have the potential to enable organisations to better organise theatre lists and deliver greater efficiencies in care

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Autonomous navigation of distributed spacecraft using intersatellite laser communications

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D. in Space Systems, Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2020Cataloged from the PDF of thesis.Includes bibliographical references (pages 149-157).Autonomous navigation refers to satellites performing on-board, real-time navigation without external input. As satellite systems evolve into more distributed architectures, autonomous navigation can help mitigate challenges in ground operations, such as determining and disseminating orbit solutions. Several autonomous navigation methods have been previously studied, using some combination of on-board sensors that can measure relative range or bearing to known bodies, such as horizon and star sensors (Hicks and Wiesel, 1992) or magnetometers and sun sensors (Psiaki, 1999), however these methods are typically limited to low Earth orbit (LEO) altitudes or other specific orbit cases. Another autonomous navigation method uses intersatellite data, or direct observations of the relative position vector from one satellite to another, to estimate the orbital positions of both spacecraft simultaneously.The seminal study of the intersatellite method assumes the use of radio time-of-flight measurements of intersatellite range, and a visual tracking camera system for measuring the inertial bearing from one satellite to another (Markley, 1984). Due to the limited range constraints of passively illuminated visual tracking systems, many of the previous studies restrict the separation between satellites to less than 1,000 kilometers (e.g., Psiaki, 2011), or simply drop the use of measuring intersatellite bearing and rely solely on obtaining a large distribution of intersatellite range measurements for state estimation (e.g., Xu et al., 2014). These assumptions have limited the assessment of the performance capability of autonomous navigation using intersatellite measurements for more general mission applications.In this thesis, we investigate the performance of using laser communication (lasercom) crosslinks in order to achieve all necessary intersatellite measurements for autonomous navigation. Lasercom systems are capable of measuring both range and bearing to a receiving terminal with greater precision than traditional methods, and can do so over larger separations between satellites. We develop a simulation framework to model the measurements of intersatellite range and bearing using lasercom crosslinks in distributed satellite systems, with consideration of varying orbital operating environments, constellation size and distribution, and network topologies. We implement two estimation algorithms: an extended Kalman filter (EKF) used with Monte Carlo sampling for performance analyses, and a Cram~r-Rao lower-bound (CRLB) computation for uncertainty analyses.We evaluate several case studies modeled off of existing and planned constellation missions in order to demonstrate the new capabilities of performing intersatellite navigation with lasercom links in both near-Earth and deep-space orbital applications. Performance targets are generated from the current state-of-the-art navigation capabilities demonstrated by Global Navigation Satellite Systems (GNSS) in Earth-orbit, and by radiometric tracking and orbit estimation using the Deep Space Network (DSN) in deep-space orbits. For Earth-orbiting applications, we simulate a relay satellite system in geosynchronous orbit (GEO) inspired by current optical communications missions in development by both ESA and NASA, and Walker constellations in LEO inspired by the upcoming mega-constellations for global broadband internet service, such as those proposed by SpaceX and Telesat.In both case studies, we demonstrate improved navigation performance over the current state-of-the-art in GNSS receiver technologies by using intersatellite measurements from lasercom crosslinks. Monte Carlo simulations show median total position errors better than 3 meters in LEO, 12 meters in GEO, and 45 meters in high-altitude or highly-eccentric orbits (HEO), showing promise as an alternative navigation method to GNSS in Earth-orbiting environments. We also simulate current and future Mars-orbiting missions as examples of deep-space applications. In one case study, we create an ad-hoc constellation comprised of low-altitude Mars exploration orbiters modeled off of current Mars-orbiting missions. In a second case study, we focus on a high-altitude constellation proposed for dedicated Earth-to-Mars networked communications.Again, in both case studies, we demonstrate improved navigation performance over the current state-of-the-art in DSN radiometric orbit solutions by using intersatellite measurements from lasercom crosslinks. Monte Carlo simulations show stable median total position errors better than 25 meters for Mars-orbit, which demonstrates a notable improvement both spatially and temporally versus DSN orbit estimation, mitigating the large cost and time constraints associated with DSN tracking. These results demonstrate the promise of using lasercom intersatellite links for autonomous navigation, offering enhanced performance over current state-of-the-art capabilities, and a greater applicability to missions both near Earth and beyond.by Pratik K. Dave.Ph. D. in Space SystemsPh.D.inSpaceSystems Massachusetts Institute of Technology, Department of Aeronautics and Astronautic

Seeker-based adaptive guidance via reinforcement meta-learning applied to asteroid close proximity operations

Current practice for asteroid close proximity maneuvers requires extremely accurate characterization of the environmental dynamics and precise spacecraft positioning prior to the maneuver. This creates a delay of several months between the spacecraft’s arrival and the ability to safely complete close proximity maneuvers. In this work we develop an adaptive integrated guidance, navigation, and control system that can complete these maneuvers in environments with unknown dynamics, with initial conditions spanning a large deployment region, and without a shape model of the asteroid. The system is implemented as a policy optimized using reinforcement meta-learning. The spacecraft is equipped with an optical seeker that locks to either a terrain feature, reflected light from a targeting laser, or an active beacon, and the policy maps observations consisting of seeker angles and LIDAR range readings directly to engine thrust commands. The policy implements a recurrent network layer that allows the deployed policy to adapt real time to both environmental forces acting on the agent and internal disturbances such as actuator failure and center of mass variation. We validate the guidance system through simulated landing maneuvers in a six degrees-of-freedom simulator. The simulator randomizes the asteroid’s characteristics such as solar radiation pressure, density, spin rate, and nutation angle, requiring the guidance and control system to adapt to the environment. We also demonstrate robustness to actuator failure, sensor bias, and changes in the spacecraft’s center of mass and inertia tensor. Finally, we suggest a concept of operations for asteroid close proximity maneuvers that is compatible with the guidance system