An algorithm for estimation and separation of ephemeris and clock errors in SBAS

The estimation and separation of ephemeris and clock errors is an integral part of a SBAS (Space Based Augmentation System). Generally, the global solution is based on the full state approach for satellite errors (ephemeris and clock) and station errors, using a large least square estimator; or the other way is to sequentially estimate the ephemeris and clock through a Kalman filter, using a complex model of the satellite dynamics. In this paper, the estimation and separation of ephemeris and clock errors is addressed through a unique approach of combining both the methods. The algorithm employs measurements, which are pre-processed for various errors and known biases. A single difference technique is used to separately estimate the ephemeris and clock components. The ephemeris Kalman filter uses a priori information of ephemeris errors along with measurements through a minimum variance estimator to provide ephemeris error estimate. A similar approach is adopted in the clock error estimation process, to provide clock and clock rate estimates. The algorithm results are presented using simulated data for known errors in ephemeris/clock and subsequent retrieval. This algorithm estimates these errors as corrections to the broadcast Global Positioning System (GPS) navigation data, required by a SBAS user for accuracy improvement.     

Precursory signature of several major earthquakes studied using 40 kHz low frequency signal

To investigate the precursory signature of earthquakes on low frequency (LF) signal propagation, six earthquakes, having magnitude greater than equal to 6.5 and depth less than equal to 30 km, are being studied. The base line level of 40 kHz signal, transmitted from JJY station, Japan, is analysed with respect to V_d statistical parameter. Results show that the V_d parameter values starts fluctuating from its ambient levels before and during the days of the earthquakes, with significant variation starting 1–3 days prior to the earthquake concerned. This present study is an approach for identifying the precursory signatures of earthquakes on LF signal propagation using a new methodology with V_d parameter.     

Seismogenic ionospheric anomalies associated with the strong Indonesian earthquake occurred on 11 April 2012 (M = 8.5)

Ionospheric perturbations in possible association with a major earthquake (EQ) (M?=?8.5) which occurred in India-Oceania region are investigated by monitoring subionospheric propagation of VLF signals transmitted from the NWC transmitter (F?=?19.8?kHz), Australia to a receiving station at Varanasi (geographic lat. 25.3°N, long 82.99°E), India. The EQ occurred on 11 April 2012 at 08:38:35?h UT (magnitude?≈?8.5, depth?=?10?km, and lat.?=?2.3°N, long.?=?93.0°E). A significant increase of few days before the EQ has been observed by using the VLF nighttime amplitude fluctuation method (fixed frequency transmitter signal). The analysis of total electron contents (TEC) derived from the global positioning system (GPS) at three different stations namely, Hyderabad (latitude 17.38°N, longitude 78.48°E), Singapore (latitude 1.37°N, longitude 103.84°E) and Port Blair (latitude 11.62°N, longitude 92.72°E) due to this EQ has also been presented. Significant perturbation in TEC data (enhancements and depletion) is noted before and after the main shock of the EQ. The possible mechanisms behind these perturbations due to EQ have also been discussed.     

Equatorial secondary cosmic ray observatory to study space weather and terrestrial events

Recently, equatorial secondary cosmic ray observatory has been established at Equatorial Geophysical Research Laboratory (EGRL), Tirunelveli, (Geographic Coordinates: 8.71°N, 77.76°E), to study secondary cosmic rays (SCR) produced due to the interaction of primary cosmic rays with the Earth’s atmosphere. EGRL is a regional center of Indian Institute of Geomagnetism (IIG), located near the equator in the Southern part of India. Two NaI(Tl) scintillation detectors are installed inside the temperature controlled environment. One detector is cylindrical in shape of size 7.62?cm?×?7.62?cm and another one is rectangular cuboid of 10.16?cm?×?10.16?cm?×?40.64?cm size. Besides NaI(Tl) detectors, various other research facilities such as the Geomagnetic observatory, Medium Frequency Radar System, Digital Ionosonde, All-sky airglow imager, Atmospheric electricity laboratory to measure the near-Earth atmospheric electric fields are also available at EGRL. With the accessibility of multi- instrument facilities, the objective is set to understand the relationship between SCR and various atmospheric and ionospheric processes, during space weather and terrestrial events.For gamma-ray spectroscopy, it is important to test the performance of the NaI(Tl) scintillation detectors and to calibrate the gamma-ray spectrum in terms of energy. The present article describes the details of the experimental setup installed near the equator to study cosmic rays, along with the performance testing and calibration of the detectors under various conditions. A systematic shift in the gain is observed with varying temperature of the detector system. It is found that the detector’s response to the variations in the temperature is not just linear or non-linear type, but it depends on the history of the variation, indicating temperature hysteresis effects on NaI detector and PMT system. This signifies the importance of isothermal environment while studying SCR flux using NaI(Tl) detectors, especially for the experiments conducted during daytime such as solar eclipses etc.     

Robust attitude stabilization of spacecraft subject to actuator failures

A robust nonlinear control scheme is developed to stabilize the 3-axis attitude of the spacecraft for cases where there is no control available on either roll or yaw axis. The stability conditions for robustness against unmatched uncertainties and disturbances are derived to establish the regions of asymptotic 3-axis attitude stabilization. The properties of the proposed sliding surface are investigated to obtain the domains of sliding mode for the closed-loop system. Several numerical simulations are presented to demonstrate the efficacy of the proposed controller and validate the theoretical results. The control algorithm is shown to compensate for time-varying external disturbances including solar radiation pressure, aerodynamic forces, and magnetic disturbances; and uncertainties in the spacecraft inertia parameters. The numerical results also establish the robustness of the proposed control scheme to negate disturbances caused by orbit eccentricity.     

A multiple-rendezvous,sample-return mission to two near-Earth asteroids

We propose a dual-rendezvous mission, targeting near-Earth asteroids, including sample-return. The mission, Asteroid Sampling Mission (ASM), consists of two parts: (i) flyby and remote sensing of a Q-type asteroid, and (ii) sampling of a V-type asteroid. The targeted undifferentiated Q-type are found mainly in the near-Earth space, and to this date have not been the target of a space mission. We have chosen, for our sampling target, an asteroid from the basaltic class (V-type), as asteroids in this class exhibit spectral signatures that resemble those of the well-studied Howardite–Eucrite–Diogenite (HED) meteorite suite. With this mission, we expect to answer specific questions about the links between differentiated meteorites and asteroids, as well as gain further insight into the broader issues of early Solar System (SS) evolution and the formation of terrestrial planets. To achieve the mission, we designed a spacecraft with a dry mass of less than 3 tonnes that uses electric propulsion with a solar-electric power supply of 15 kW at 1 Astronomical Unit (AU). The mission includes a series of remote sensing instruments, envisages landing of the whole spacecraft on the sampling target, and employs an innovative sampling mechanism. Launch is foreseen to occur in 2018, as the designed timetable, and the mission would last about 10 years, bringing back a 150 g subsurface sample within a small re-entry capsule. This paper is a work presented at the 2008 Summer School Alpbach,“Sample return from the Moon, asteroids and comets” organized by the Aeronautics and Space Agency of the Austrian Research Promotion Agency. It is co-sponsored by ESA and the national space authorities of its Member and Co-operating States, with the support of the International Space Science Institute and Austrospace.     

Variable structure control for satellite attitude stabilization in elliptic orbits using solar radiation pressure

The paper proposes the use of solar radiation pressure for satellite attitude control in elliptic orbits based on variable structure control. The system comprises of a satellite with two-oppositely placed solar flaps. Sliding mode control and terminal sliding mode control techniques have been adopted to develop nonlinear control laws for suitably rotating the control solar flaps to neutralize the adverse effect of eccentricity normally responsible for a considerable deterioration in the attitude control performance. The detailed numerical simulation of the governing nonlinear equation of the motion including the effects of various system parameters on the controller performance, establishes the feasibility of the proposed control strategy. The proposed controller is found to be robust against parameter uncertainties and external disturbances and thereby, the control strategy presented in the paper may be applicable to future satellite missions.     

Microbiological characterization of a regenerative life support system.

A Variable Pressure Plant Growth Chamber (VPGC), at the Johnson Space Center's (JSC) ground-based Regenerative Life Support Systems (RLSS) test bed, was used to produce crops of soil-grown lettuce. The crops and chamber were analyzed for microbiological diversity during lettuce growth and after harvest. Bacterial counts for the rhizosphere, spent nutrient medium, heat exchanger condensate, and atmosphere were approximately 10(11) Colony Forming Units (CFU) g-1 10(5) CFU ml-1, 10(5) CFU ml-1, and 600 CFU m-3, respectively. Pseudomonas was the predominant bacterial genus. Numbers of fungi were about 10(5) CFU g-1 in the rhizosphere, 4-200 CFU ml-1 in the spent nutrient medium, 110 CFU ml-1 in the heat exchanger condensate, and 3 CFU m-3 in the atmosphere. Fusarium and Trichoderma were the predominant fungal genera.     

A PC based test system for an on-board computer

This paper presents a robust method of testing a processor based on-board computer used in the Indian remote sensing satellite (IRS-1C). The novelty of this scheme lies in a judicious mix of software and hardware approaches adopted to realise this PC-based system. All the electrical interfaces (like telemetry, Telecommand, timer and star sensor) are simulated. They are fabricated as add-on cards to PC-AT (ISAbus). The on-board computer controls two star sensors apart from handling three telemetry streams and accepting more than one hundred Telecommands     

Orbital and Attitude Prediction Accuracy Requirements for Satellites Engaged in Remote Sensing and Scientific Missions

The criteria for selecting the orbital and attitude prediction accuracy requirements for communications satellites have been investigated in a previous paper by the author. The present paper extends the analysis to the satellites engaged in remote sensing and scientific missions. A unified approach has been developed to examine the problem of the prediction accuracy requirements for the two missions.