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.     

Variation of ionospheric total electron content in Indian low latitude region of the equatorial anomaly during May 2007–April 2008

The ionospheric total electron content (TEC), derived by analyzing dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Varanasi (geomagnetic latitude 14°, 55′N, geomagnetic longitude 154°E) is studied. Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the solar minimum period from May 2007 to April 2008. It is found that the daily maximum TEC near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semiannual variation is seen with two maxima occurring in both spring and autumn. Statistical studies indicate that the variation of EIA crest in TEC is poorly correlated with Dst-index (r = −0.03) but correlated well with Kp-index (r = 0.82). The EIA crest in TEC is found to be more developed around 12:30 LT.     

Operations of a spaceflight experiment to investigate plant tropisms

Plants will be an important component in bioregenerative systems for long-term missions to the Moon and Mars. Since gravity is reduced both on the Moon and Mars, studies that identify the basic mechanisms of plant growth and development in altered gravity are required to ensure successful plant production on these space colonization missions. To address these issues, we have developed a project on the International Space Station (ISS) to study the interaction between gravitropism and phototropism in Arabidopsis thaliana. These experiments were termed TROPI (for tropisms) and were performed on the European Modular Cultivation System (EMCS) in 2006. In this paper, we provide an operational summary of TROPI and preliminary results on studies of tropistic curvature of seedlings grown in space. Seed germination in TROPI was lower compared to previous space experiments, and this was likely due to extended storage in hardware for up to 8 months. Video downlinks provided an important quality check on the automated experimental time line that also was monitored with telemetry. Good quality images of seedlings were obtained, but the use of analog video tapes resulted in delays in image processing and analysis procedures. Seedlings that germinated exhibited robust phototropic curvature. Frozen plant samples were returned on three space shuttle missions, and improvements in cold stowage and handing procedures in the second and third missions resulted in quality RNA extracted from the seedlings that was used in subsequent microarray analyses. While the TROPI experiment had technical and logistical difficulties, most of the procedures worked well due to refinement during the project.     

A multiobjective optimization approach to determine the parameters of stepped frequency pulse train

Frequency stepping techniques are commonly used in modern radar system to get high range resolution with the disadvantage that its autocorrelation function (ACF) yield undesirable “grating lobes”. Wider mainlobe deteriorates the range resolution capability of the waveform and higher peak sidelobe either hides the small targets or causes the false target detection. Several techniques have been used to choose the parameters of linear frequency modulated (LFM) pulse train to suppress the grating lobes without paying much attention to the mainlobe width and peak sidelobe level. In this paper a multiobjective optimization (Nondominated Sorting Genetic Algorithm-II (NSGA-II)) approach is proposed to optimize the parameters of the LFM pulse train to achieve reduced grating lobes, low peak sidelobe level and narrow mainlobe width. The optimization problem has been studied in two different ways: first one is associated with the reduction of grating lobes and the minimization of peak sidelobe level of the ACF with constraints and second one is related to the minimization of the peak sidelobe level and mainlobe width of the ACF with constraints. Simulation studies have been carried out to justify the potentiality of the proposed approach.     

Correlation of Binarized Images

Correlator technique has been used in pattern recognition applications. We introduce a correlation performance measure, peak to background ratio (PBR), defined as the ratio of the peak intensity to the average background variation in a single correlation output. Using this performance measure, we analyze the binary correlator and show that the binarization of the images can result in better performance. We consider a real-time implementation of the binary correlator, using VLSI technology. The analytical results are supported by computer simulations.     

An attitude control approach for compensation of satellite drift in elliptic synchronous orbits

This paper explores the possibility of developing a new attitude control method for satellites in elliptic, 24-hour orbits, in order to compensate for the effect of longitudinal periodic drift relative to the ground station. A simple solar attitude control technique has been proposed for achieving the fixed apparent satellite orientation with respect to the ground segment of the space mission. The proposed control approach appears to be quite attractive for various satellite applications as it can substantially overcome the problems created by the continual periodic angular drift as well as undesirable pitching excitation in the elliptic orbits. Generalizing the analytically developed open-loop control policy results in a significant improvement of the controller performance.     

Evaluation and utilization of CloudSat and CALIPSO data to analyze the impact of dust aerosol on the microphysical properties of cirrus over the Tibetan Plateau

The present study elucidates on the evaluation of two versions (V3 and V4.10) of vertical feature mask (VFM) and aerosol sub-types data derived from the Cloud-Aerosol LiDAR and Infrared Pathfinder Satellite Observations (CALIPSO), and its utilization to analyze the impact of dust aerosol on the microphysical properties of cirrus over the Tibetan Plateau (TP). In conjunction to the CALIPSO, we have also used the CloudSat data to study the same during the summer season for the years 2007–2010 over the study area 25–40°N and 75–100°E. Compared to V3 of CALIPSO, V4.10 was found to have undergone substantial changes in the code, algorithm, and data products. Intercomparison of both versions of data products in the selected grid between 30–31°N and 83–84°E within the study area during 2007–2017 revealed that the VFM and aerosol sub-types are in good agreement of ～95.27% and ～82.80%, respectively. Dusty cirrus is defined as the clouds mixed with dust aerosols or existing in dust aerosol conditions, while the pure cirrus is that in a dust-free environment. The obtained results illustrated that the various microphysical properties of cirrus, namely ice water content (IWC), ice water path (IWP), ice distribution width (IDW), ice effective radius (IER), and ice number concentration (INC) noticed a decrease of 17%, 18%, 4%, 19%, and 10%, respectively due to the existence of dust aerosol, consistent with the classical “Twomey effect” for liquid clouds. Moreover, the aerosol optical depth (AOD) showed moderate negative correlations between ?0.4 and ?0.6 with the microphysical characteristics of cirrus. As our future studies, in addition to the present work undertaken, we planned to gain knowledge and interested to explore the impact of a variety of aerosols apart from the dust aerosol on the microphysical properties of cirrus in different regions of China.     

Mechanical properties of ANTRIX balloon film and fabrication of single cap large volume balloons

The zero pressure plastic balloons used for high altitude studies are generally made from polyethylene material. Tensile properties of the thin film polymer are the key parameters for material selection due to extremely low temperature of −90 °C encountered by the balloons in the tropopause region during the ascent at equatorial latitudes. The physical and structural properties of the material determine the uniformity of the stress distribution over the entire shell. Load stresses from the suspended load propagate via load tapes heat sealed along with the gore seals as per the balloon design. A balance between this heat seal strength and the film strength is a desirable property of the basic resin in terms of the bubble strength, gauge uniformity, and long-term storage properties. In addition, the design of the top shell of the balloon and its stress distribution play an important role since only a fraction of the balloon is deployed during the filling operation and the ascent. In this paper we describe the mechanical properties of the ‘ANTRIX’ film developed by us and the optimized design of single cap balloons, which have been successfully used in our experiments over the past 5 years.     

Spatial asymmetry in topside ion density and vertical E × B plasma drift velocity within 75°E–95°E

The ion density measured by the Ionospheric Plasma and Electrodynamics Instrument (IPEI) on board the ROCSAT -1 over the 75°E and 95°E meridian at 600km altitude has been utilized to examine the latitudinal and longitudinal distribution within the Indian sector, in particular, the north-south and east-west asymmetries of the equatorial ionization anomaly (EIA). A longitudinal gradient in ion density at 600?km higher towards 95°E develops during the noontime and afternoon hours when the EIA is at its peak. The density gradient persists till evening hours when pre-reversal enhancements occur. The vertical E?×?B plasma drift velocity measured simultaneously by ROCSAT -1 for the same space-time configuration has also been studied. In addition to diurnal, seasonal and solar activity variations in E?×?B drift velocity, the longitudinal gradient is also observed. The EIA at the altitude of 600?km peaks at different latitudes and are mostly asymmetric about the magnetic equator. From midnight till 0800 LT, the ion density across the equator is nearly uniform in the equinoxes. But in the solstices, the density exhibits a north-south gradient. In the June solstice, density is higher in the northern hemisphere and decreases gradually towards south. The gradient in density reverses in December solstice. Normally, the EIA peaks within 1200 LT and 1600 LT while around 2000 LT, pre-reversal enhancement of ionization occurs affecting the EIA evening structure. The strength of the EIA also exhibits seasonal, year-to-year and hemispheric variations. The longitudinal asymmetry of drift velocity along 75°E and 95°E longitude sectors is the contributing factor behind the observed longitudinal asymmetry in ion density. Significant positive correlation between the strength of the EIA and E?×?B drift is observed in both longitudes.     

Seasonal variations in vertical distribution of meteor decay time as observed from meteor radars at 8.5°N and 80°N

The decay times of meteor radar echoes have been used for decades to investigate characteristics of the mesosphere and lower thermosphere (MLT) region. As the meteor echo decay time depends on background atmospheric parameters, in the present communication, we examine the seasonal variation of the vertical distributions of underdense meteor echo decay times with respect to echo strength. Observations from two similar radars located at two distinct geographical locations, Thumba (8.5°N, 77°E) and Eureka (80°N, 85.8°W) were used for the present study. Here, the radar received signal power is categorized into strong and weak echoes and vertical profiles of their decay times are constructed. It has been noticed that the monthly mean decay time vertical profile turning altitude (i.e., inflection point) varies in the range of 80–87?km of altitude depending on latitude. The turning altitude is observed at relatively lower heights in the winter than in summer at both the latitudes. The present analysis shows that the meteor decay time below the mean turning altitude follows a decreasing trend with decreasing altitude, which is quite distinct to the behaviour of ambipolar diffusion. It is also observed that there is a difference in mean decay time of strong and weak echoes below 90?km of altitude, which is very prominently seen at lower altitudes. This difference shows a seasonal pattern at high latitude, but does not show any seasonal variation at low latitude. The present results are discussed in light of current understanding of the meteor decay time.