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.     

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.     

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.     

Oxidant enhancement in martian dust devils and storms: implications for life and habitability

We investigate a new mechanism for producing oxidants, especially hydrogen peroxide (H2O2), on Mars. Large-scale electrostatic fields generated by charged sand and dust in the martian dust devils and storms, as well as during normal saltation, can induce chemical changes near and above the surface of Mars. The most dramatic effect is found in the production of H2O2 whose atmospheric abundance in the "vapor" phase can exceed 200 times that produced by photochemistry alone. With large electric fields, H2O2 abundance gets large enough for condensation to occur, followed by precipitation out of the atmosphere. Large quantities of H2O2 would then be adsorbed into the regolith, either as solid H2O2 "dust" or as re-evaporated vapor if the solid does not survive as it diffuses from its production region close to the surface. We suggest that this H2O2, or another superoxide processed from it in the surface, may be responsible for scavenging organic material from Mars. The presence of H2O2 in the surface could also accelerate the loss of methane from the atmosphere, thus requiring a larger source for maintaining a steady-state abundance of methane on Mars. The surface oxidants, together with storm electric fields and the harmful ultraviolet radiation that readily passes through the thin martian atmosphere, are likely to render the surface of Mars inhospitable to life as we know it.     

Feasibility of using statellites in non-equatorial 24-hour circular orbits for communications

The early sixties witnessed the debate among competing candidate orbits that led to the emergence of perfect geostationary systems as virtually the sole “instruments” for satellite communication. The subsequent problem of overcrowding of geostationary ring on one hand and explosive growth in demand on communication capacity on the other led comsat experts to focus on the alternate routes through various near-earth and medium attitude satellite constellations later proposed for uninterrupted communication. However, the opportunities thrown up by quasi-stationary orbits for augmentation of the space communication capacity have gone abegging. This paper attempts to draw attention of communication satellite designers/planners to the immense potential for utilization of the non-equatorial, 24-hour circular orbits for communication. For the proposed quasi-stationary orbits, the change and/or control of the inclination of the plane is not envisaged in the launch and/or operational phase. The resulting significant payload weight advantage is associated with the problem of periodic as well as secular apparent angular satellite drift relative to the ground terminal. However, the problem may be largely overcome through controlled satellite tilting using solar radiation pressure or through the use of tethered auxiliary mass attachment. Alternatively, it may be possible to overcome the attitude control problem by the use of systems such as on-bard electronically steerable phased array antenna capable of following the line-of-sight to the co-operative ground station.     

Observation of aphelion cloud belt over Martian tropics,its evolution,and associated dust distribution from MCS data

The present study uses five Martian years of observations from Mars Climate Sounder onboard Mars Reconnaissance Orbiter for investigating the Aphelion Cloud Belt (ACB) over the tropics. Analysis of zonal mean water ice column opacity suggests that the spatial extension of the ACB is mainly confined over the tropics and mid-latitudes (-20 – 40°N) during L_S ~ 45 – 135° (L_S = 0° signifies northern spring equinox). The ACB is seen primarily in the nighttime only due to the truncation of the daytime profile observations at significantly higher altitudes (at ~30 km). Zonal mean ice extinction profiles show ACB’s altitudinal range within ~10 – 40 km, and the existence of a thin cloud band in the absence of a thick ACB during aphelion season. Three phases of the ACB could be identified as the formation phase during L_S = 45 – 75° (phase 1), the peak phase during L_S = 76 – 105° (phase 2), and the decaying phase during L_S = 106 – 135° (phase 3). Observation of the cloud latitude belt shows a northward movement starting from phase 2, prominent over regions nearby Lunae Planum and Xanthe Terra. During this phase, the top level of thick clouds within the ACB decreases to ~20 km in the southern hemisphere, while it increases a little over the northern hemisphere (NH). The decreasing tendency continues in phase 3 over the entire region ?10 – 10°N, and the thick cloud base moves higher over the NH, though the vertical depth of it becomes narrower than phase 2. Temperature profiles do not show any noticeable influence on the northward evolution of the ACB. However, the study at a regional level indicates a possible association of upper tropospheric dustiness with the ACB’s evolution. The mechanism is evident in the correlation analysis mostly at an altitude range of ~18 – 35 km. The migrating semidiurnal tide (SMD) as a proxy of dust or water ice forcing, and the calculated upper tropospheric dust radiative heating, shows an apparent northward movement of their peak amplitude within the three phases of the ACB. This match between the spatiotemporal variations of the SMD and the water ice was not observed previously. However, the correlating behavior seems to be prominent in the areas nearby Lunae Planum and Xanthe Terra and the upper-tropospheric region of the atmosphere.     

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.     

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.