Coordinate Conversion and Tracking for Very Long Range Radars

The problem of tracking with very long range radars is studied in this paper. First, the measurement conversion from a radar's r-u-v coordinate system to the Cartesian coordinate system is discussed. Although the nonlinearity of this coordinate transformation appears insignificant based on the evaluation of the bias of the converted measurements, it is shown that this nonlinearity can cause significant covariance inconsistency in the conventionally converted measurements (CM1). Since data association depends critically on filter consistency, this issue is very important. Following this, it is shown that a suitably corrected conversion (CM2) eliminates the inconsistency. Then, initialized with the converted measurements (using CM2), four Cartesian filters are evaluated. It is shown that, among these filters, the converted measurement Kalman filter with second order Taylor expansion (CM2KF) is the only one that is consistent for very long range tracking scenarios. Another two approaches, the range-direction-cosine extended Kalman filter (ruvEKF) and the unscented Kalman filter (UKF) are also evaluated and shown to suffer from consistency problems. However, the CM2KF has the disadvantage of reduced accuracy in the range direction. To fix this problem, a consistency-based modification for the standard extended Kalman filter (E1KF) is proposed. This leads to a new filtering approach, designated as measurement covariance adaptive extended Kalman filter (MCAEKF). For very long range tracking scenarios, the MCAEKF is shown to produce consistent filtering results and be able to avoid the loss of accuracy in the range direction. It is also shown that the MCAEKF meets the posterior Carmer-Rao lower bound for the scenarios considered.     

Simulation of unsteady combustion in a LOX-GH2 fueled rocket engine

This paper presents results from an investigation of unsteady combustion inside a small-scale, multi-injector liquid rocket engine. A time-accurate approach in an axisymmetric geometry is employed to capture the unsteady flow features, as well as the unsteady heat transfer to the walls of the combustion chamber. Both thermally perfect gas (TPG) and real gas (RG) formulations are evaluated for this LOX-GH₂ system. The Peng–Robinson cubic equation of state (EoS) is used to account for real gas effects associated with the injection of oxygen. Realistic transport properties are computed but simplified chemistry is used in order to achieve a reasonable turnaround time. Results show the importance of the unsteady dynamics of the flow, especially the interaction between the different injectors. The RG EoS, despite a limited zone of influence, is shown to govern the overall chamber behavior. The sensitivity of the results to changes in the system parameters is studied and some general trends are discussed. Although several features of the simulations agree well with past experimental observations, prediction of heat flux using a simplified flux boundary condition is not completely satisfactory. Reasons for this discrepancy are discussed in the context of the current axisymmetric approach.     

Modified unscented Kalman filtering and its application in autonomous satellite navigation

In this paper, a modified unscented Kalman filter (UKF) for nonlinear stochastic systems is proposed, and it is applied to autonomous orbit determination for Earth satellites. Based on some standard results about the boundedness of stochastic processes and a new formulation of the unscented transformation (UT), it is demonstrated that the design of the noise covariance matrix plays an important role in enhancing the filter stability. Furthermore, a particular design of the noise covariance matrix is proposed as a modification of the UKF. The modified UKF is less sensitive to the initial error than the usual one. High performance of the modified UKF is illustrated in comparison with the usual one by using the real data obtained from an Earth sensor.     

A surface thermal model and exospheric ballistic transport code of planet Mercury

The atmosphere of Mercury is of an exospheric nature. Its formation is due to several physical mechanisms including meteoroid impact, surface sputtering by solar wind ions and photon sputtering by solar UV radiation. The molecules and atoms emitted from the surface materials of Mercury include H, He, O, Ar, and S, etc. It is important to study their spatial distributions across the planetary surface via ballistic random walk. We have developed a surface thermal model coupled with Hodges-type Monte Carlo calculations to simulate the exosphere of Mercury, which will be a major scientific target of the BepiColombo mission of ESA and JAXA.     

Plasma Flow and Related Phenomena in Planetary Aeronomy

Understanding the processes involved in the interaction of solar system bodies with plasma flows is fundamental to the entire field of space physics. The features of the interaction can be very different, depending upon the properties of the incident plasma as well as the nature of the obstacle. The properties of the atmosphere/ionosphere associated with the obstacle are of particular importance into understanding the plasma interaction process, especially for non-magnetized obstacle. This paper discusses in detail the roles of the atmosphere and ionosphere systems of plasma interaction around Venus, Mars, comets and some particular satellites. The coupling between magnetosphere and ionosphere is also discussed for Earth and Giant planets.     

Coronal Holes and Open Magnetic Flux

Coronal holes are low-density regions of the corona which appear dark in X-rays and which contain “open” magnetic flux, along which plasma escapes into the heliosphere. Like the rest of the Sun’s large-scale field, the open flux originates in active regions but is subsequently redistributed over the solar surface by transport processes, eventually forming the polar coronal holes. The total open flux and radial interplanetary field component vary roughly as the Sun’s total dipole strength, which tends to peak a few years after sunspot maximum. An inverse correlation exists between the rate of flux-tube expansion in coronal holes and the solar wind speed at 1 AU. In the rapidly diverging fields present at the polar hole boundaries and near active regions, the bulk of the heating occurs at low heights, leading to an increase in the mass flux density at the Sun and a decrease in the asymptotic wind speed. The quasi-rigid rotation of coronal holes is maintained by continual footpoint exchanges between open and closed field lines, with the reconnection taking place at the streamer cusps. At much lower heights within the hole interiors, “interchange reconnection” between small bipoles and the overlying open flux also gives rise to coronal jets and polar plumes.     

Observations of HF propagation on a path aligned along the mid-latitude trough

Observations of the direction of arrival and time of flight of HF signals propagating on a 1400 km path oriented along the mid-latitude trough are presented. At night, the signal commonly arrives from directions offset from the great circle bearing by up to 80° and these events have been categorised into five main types. Statistics indicating how often these categories of propagation were observed in the period August 2006 to September 2007 are presented. The physical mechanisms which result in the off great circle propagation are also discussed.     

Plants survive rapid decompression: Implications for bioregenerative life support

Radish (Raphanus sativus), lettuce (Latuca sativa), and wheat (Triticum aestivum) plants were grown at either 98 kPa (ambient) or 33 kPa atmospheric pressure with constant 21 kPa oxygen and 0.12 kPa carbon dioxide in atmospherically closed pressure chambers. All plants were grown rockwool using recirculating hydroponics with a complete nutrient solution. At 20 days after planting, chamber pressures were pumped down as rapidly as possible, reaching 5 kPa after about 5 min and ∼1.5 kPa after about 10 min. The plants were held at 1.5 kPa for 30 min and then pressures were restored to their original settings. Temperature (22 °C) and humidity (65% RH) controls were engaged throughout the depressurization, although temperatures dropped to near 16 °C for a brief period. CO₂ and O₂ were not detectable at the low pressure, suggesting that most of the 1.5 kPa atmosphere consisted of water vapor. Following re-pressurization, plants were grown for another 7 days at the original pressures and then harvested. The lettuce, radish, and wheat plants showed no visible effects from the rapid decompression, and there were no differences in fresh or dry mass when compared to control plants maintained continuously at 33 or 98 kPa. But radish storage root fresh mass and lettuce head fresh and dry masses were less at 33 kPa compared to 98 kPa for both the controls and decompression treatment. The results suggest that plants are extremely resilient to rapid decompression, provided they do not freeze (from evaporative cooling) or desiccate. The water of the hydroponic system was below the boiling pressure during these tests and this may have protected the plants by preventing pressures from dropping below 1.5 kPa and maintaining humidity near 1.5 kPa. Further testing is needed to determine how long plants can withstand such low pressure, but the results suggest there are at least 30 min to respond to catastrophic pressure losses in a plant production chamber that might be used for life support in space.     

A Review of Density Holes Upstream of Earth's Bow Shock

Larmor size transient structures with density depletions as large as 99% of ambient solar wind density levels occur commonly upstream of Earth＇s collisionless bow shock. These ＂density holes＂ have a me...