Search

Increased electronic filing, or e-filing, of nonprofit data in the United States and abroad quickens public availability, eliminates dependence on third-party proprietary datasets, and broadens access to information offering insight into previously neglected topics. On the other hand, increasingly open data require researchers to independently ensure data quality, understand limitations, and consider how data availability shapes research questions. As electronic filing becomes mandatory in the United States and more widespread elsewhere, this research considers how expanded e-filer data impact nonprofit research approaches and opportunities while assessing the generalizability and limits of existing e-filed data in the United States. While the data suggest increasing compliance with e-filing, analyses show that older nonprofits with greater capacity and sophisticated financial characteristics are historically more likely to e-file. Further, particular subsectors are more likely to e-file than others. Therefore, using existing e-filed IRS Form 990s for panel data analysis presents some generalizability concerns, although this will improve over time. Moving forward, e-filing and machine-readable access requirements bode well for both quantitative and qualitative nonprofit research, while the United States' experience provides helpful lessons for other research settings. [ABSTRACT FROM AUTHOR]

Recent advances with space navigation technologies developed by NASA in space-based atomic clocks and pulsar X-ray navigation, combined with past successes in autonomous navigation using optical imaging, brings to the forefront the need to compare space navigation using optical, radiometric, and pulsar-based measurements using a common set of assumptions and techniques. This review article examines these navigation data types in two different ways. First, a simplified deep space orbit determination problem is posed that captures key features of the dynamics and geometry, and then each data type is characterized for its ability to solve for the orbit. The data types are compared and contrasted using a semi-analytical approach with geometric dilution of precision techniques. The results provide useful parametric insights into the strengths of each data type. In the second part of the paper, a high-fidelity, Monte Carlo simulation of a Mars cruise, approach, and entry navigation problem is studied. The results found complement the semi-analytic results in the first part, and illustrate specific issues such as each data type's quantitative impact on solution accuracy and their ability to support autonomous delivery to a planet. [ABSTRACT FROM AUTHOR]

With the advent of the Deep Space Atomic Clock, operationally accurate and reliable one-way radiometric data sent from a radio beacon (i.e., a DSN antenna or other spacecraft) and collected using a spacecraft's radio receiver enables the development and use of autonomous radio navigation. This work examines the fusion of radiometric data with optical data (i.e. OpNav) to yield robust and accurate trajectory solutions that include selected model reductions and computationally efficient navigation algorithms that can be readily adopted for onboard, autonomous navigation. The methodology is characterized using a representative high-fidelity simulation of deep space cruise, approach, and delivery to Mars. The results show that the combination of the two data types yields solutions that are almost an order of magnitude more accurate than those obtained using each data type by itself. Furthermore, the combined data solutions readily meet representative entry navigation requirements (in this case at Mars). [ABSTRACT FROM AUTHOR]

Mean element propagation of perturbed two body orbits has as its mathematical basis the averaging theory of nonlinear dynamical systems. Mean elements define an orbit's long-term evolution characteristics consisting of both secular and long-period effects. Using averaging theory, a near-identity transformation can be found that transforms between the mean elements and their osculating counterparts that augment the mean elements with short period effects. The ability to perform the conversion is necessary so that orbit design conducted in either mean elements or osculating can be effectively converted between each element type. In the present work, the near-identity transformation is found using the Fast Fourier Transform. An efficient method is found that is capable of recovering the mean or osculating elements to first-order. [ABSTRACT FROM AUTHOR]

Correction to: The Journal of the Astronautical Sciences (2021) 68:300-325 https://doi.org/10... Article was updated to include missing open access license text. Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. [Extracted from the article]

The long-term evolution characteristics and stability of an orbit are well characterized using a mean element propagation of the perturbed two body variational equations of motion. The averaging process eliminates short period terms leaving only secular and long period effects. In this study, a non-traditional approach is taken that averages the variational equations using adaptive numerical techniques and then numerically integrating the resulting equations of motion. Doing this avoids the Fourier series expansions and truncations required by the traditional analytic methods. The resultant numerical techniques can be easily adapted to propagations at most solar system bodies. [ABSTRACT FROM AUTHOR]

Prior results have developed a methodology for selecting a long-lived constellation of three satellites that provide persistent, stable coverage to either the North or South Pole with no requirement for stationkeeping under the influence of only gravitational perturbations. In the present study, the sensitivity of this coverage in the presence of nongravitational forces is determined, and a design strategy is formulated that minimizes any potential sensitivity to these accelerations. The class of orbits and methods are extended to the search for global coverage constellations of six satellites. Two constellations are designed that provide 99.999% global coverage for a ten-year period also without the need for any deterministic orbit maintenance. [ABSTRACT FROM AUTHOR]

Higher altitude orbits (in the 500 to 20,000 km range) at the Moon are dominated by Earth perturbations and result in motions that do not ascribe to the standard notions of orbits dominated by nonspherical gravity effects (such as from oblateness). This fact complicates orbit design of lunar orbiter constellations that require specific and persistent coverage over a selected lunar region. Using a combination of analytical theory and numerical simulation, a technique is developed for designing a lunar constellation of three spacecraft where two spacecraft are always in view from the lunar surface for the polar regions. [ABSTRACT FROM AUTHOR]