A broad climatology of very high latitude substorms

Magnetic data from a newly commissioned Indian Antarctic station Bharati (corrected geomagnetic (CGM) coordinates 74.7°S, 97.2°E) and closely-spaced IMAGE chain observatories (∼100° magnetic meridian in Northern hemisphere) has been analyzed to study the climatology of substorms which were localized poleward of the standard auroral oval. We considered four austral summers (year 2007–2010) when data from Bharati was available. Several very high latitude substorms were observed in this duration when the solar activity remained unexpectedly low for a long time. Various features of very high latitude substorms, e.g., local time dependence, interplanetary state, hemispherical asymmetry and their nightside low latitude signatures are examined. Events studied here, suggested the following properties of substorms occurring at very high latitudes: (1) maximum occurrence was observed near magnetic midnight (21:00–02:00 MLT). (2) In contradiction to earlier reports, many substorms were observed even during negative IMF Bz condition. In addition, majority of substorms occurred during low or moderate solar wind streams. (3) Magnetic signatures were often pronounced in the winter hemisphere. (4) Even if widely used standard AE indices fail to monitor very high latitude substorms, their low latitude signatures are often evident.     

Autonomous fault-tolerant attitude reference system using DTGs insymmetrically skewed configuration

A novel symmetrically skewed configuration for an attitude reference system (ARS) using three dynamically tuned gyros (DTGs) is developed. Simple schemes for autonomous detection and identification of a faulty DTG in real time and subsequent reconfiguration of the attitude estimation algorithm are proposed. The performance of the present configuration is shown to be better than that of configurations proposed earlier, and it is shown to have better features. It tolerates all types of failures of DTG failures, requires very simple computations, and gives less error in attitude estimate than the other configurations     

Ion composition behaviour in low and mid-latitudes during high solar activity

In the present analysis, the mass spectrometer data from the ISS-b satellite, available in the form of contour plots at an average height of 1100 km for every alternate hour, is used. This analysis showed some interesting results in terms of the diurnal variation of the transition heights; at times dominance of He⁺ over the other ions, and the seasonal variations of different constituents. In the development of ion composition models, it is suggested that this type of result should be taken into consideration.     

The role of hardware in-loop motion simulation for Indiansatellites

Testing of hardware packages of sensors, actuators, and control electronics in the control loop for the IRS and SROSS satellites using motion simulators is reported. Simulation exercises and test results are described. Both air-bearing platforms and servo tables were used. The problems observed varied from interface incompatibilities and logic-level mistakes to onboard software malfunctions. unit-to-unit differences, and controller gain parameters. The importance of testing with hardware in a dynamic environment, is demonstrated     

Incoherent-scatter measurements of E-F valley and comparisons with theoretical and empirical models

High resolution electron density measurements from Arecibo incoherent scatter (i.s.) radar are used to make a detailed study of the E-F valley. Features of the important valley parameters like height, width and depth are examined. These features are then compared with the available theoretical and empirical models. The depth of the valley obtained from the empirical models agrees with i.s. measurements for near-noon periods, but disagrees with these measurements for pre-noon and post-noon periods. Further, the i.s. measurements indicate that E-F valley is rather small during daytime as compared to models which give larger width. During the night, the valley is quite wide and deep but the presence of sporadic-E (Es) contaminates the Ne-h profiles observed with the i.s. radar. As a result the valley parameters cannot be determined unambiguously during the night.     

Modeling the benefits of an artificial gravity countermeasure coupled with exercise and vibration

The current, system-specific countermeasures to space deconditioning have limited success with the musculoskeletal system in long duration missions. Artificial gravity (AG) that is produced by short radius centrifugation has been hypothesized as an effective countermeasure because it reintroduces an acceleration field in space; however, AG alone might not be enough stimuli to preserve the musculoskeletal system. A novel combination of AG coupled with one-legged squats on a vibrating platform may preserve muscle and bone in the lower limbs to a greater extent than the current exercise paradigm. The benefits of the proposed countermeasure have been analyzed through the development of a simulation platform. Ground reaction force data and motion data were collected using a motion capture system while performing one-legged and two-legged squats in 1-G. The motion was modeled in OpenSim, an open-source software, and inverse dynamics were applied in order to determine the muscle and reaction forces of lower limb joints. Vibration stimulus was modeled by adding a 20 Hz sinusoidal force of 0.5 body weight to the force plate data. From the numerical model in a 1-G acceleration field, muscle forces for quadriceps femoris, plantar flexors and glutei increased substantially for one-legged squats with vibration compared to one- or two-legged squats without vibration. Additionally, joint reaction forces for one-legged squats with vibration also increased significantly compared to two-legged squats with or without vibration. Higher muscle forces and joint reaction forces might help to stimulate muscle activation and bone modeling and thus might reduce musculoskeletal deconditioning. These results indicate that the proposed countermeasure might surpass the performance of the current space countermeasures and should be further studied as a method of mitigating musculoskeletal deconditioning.     

Presolar grains in micro-inclusions of iron meteorites

In residues of several iron meteorites, obtained by dissolution of the metal in dilute H₂SO₄, the isotopic ratio of ¹⁹⁰Os/¹⁸⁴Os has been measured by radiochemical neutron activation analysis and is found to be anomalous in several cases. The largest deviation so far seen is a 40% enrichment of ¹⁸⁴Os in a mineral of Sikhote Alin. A lack of homogenization of the two isotopes that are produced in different nucleosynthetic processes suggests that iron meteorites are more primitive than any other matter available to us so far. Isotopic composition of Xe is found to be planetary in general. However the two light isotopes ¹²⁴Xe and ¹²⁶Xe are strongly depleted in some non-magnetic residues. Correlation diagrams of isotopic ratio pairs indicate that a two component mixture model is not adequate. EPR studies of the non-magnetic fractions of the residues show a large concentration of stabilized free radicals which could be ascribed to an irradiation of micro-inclusions prior to compaction into the metal matrix.     

Onboard Attitude Error Computation for Automatic Spin Axis Control of Bhaskara

The Indian experimental remote sensing satellite, Bhaskara, uses two horizon crossing sensors mounted at 45° and 135° with respect to spin axis for onboard roll error computation and automatic control of the spin axis orientation within 3° of the orbit normal. A new method for roll error computation and control and digital hardware for the same are described. The system has flexibility to vary the threshold, to adapt to altitude variations and various mission requirements, and to work with a single horizon sensor. Flight performance of the system is satisfactory.     

Auto Reconfiguration of Reaction Wheels in IRS

The Indian remote sensing satellite (IRS) employs four reaction wheels for attitude control. The fourth, a skew wheel, provides redundancy to the failure of any of the orthogonal wheels. To avoid attitude loss before the redundant skew wheel takes over due to failure of any of the orthogonal wheels, an automatic reconfiguration scheme has been developed and implemented through a microprocessor. The details of reconfiguration logic and the computer simulation results are presented.