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Extended Orbital Flight of a Cubesat in the Lower Thermosophere with Active Attitude Control
No full textA wide variety of scientifically interesting missions could be enabled by orbital flight altitudes of 150 – 250 km. For the present work, this range of altitudes is defined as extremely Low Earth Orbit (eLEO). The use of low-cost nanosatellites (mass < 10 kg) has reduced the cost barrier to orbital flight over the last decade and the present study investigates the feasibility of using primarily commercial, off-the-shelf (COTS) hardware to build a nanosat specifically to allow extended mission times in eLEO.
CubeSats flying in the lower thermosphere have the potential to enable close monitoring of the Earth’s surface for scientific, commercial, and defense-related missions. The results of this research show that the proper selection of primary and attitude control thrusters combined with precise control techniques result in significant extension of the orbital life of a CubeSat in eLEO, thus allowing detailed explorations of the atmosphere. In this study, the orbit maintenance controller is designed to maintain a mission-averaged, mean altitude of 244 km. An estimate is made of the primary disturbance torque due to aerodynamic drag using a high-fidelity calculation of the rarefied gas drag based on a Direct Simulation, Monte-Carlo simulation.
The primary propulsion system consists of a pair of electrospray thrusters providing a combined thrust of 0.12 mN at 1 W. Results of a trade study to select the best attitude control option indicate pulsed plasma thrusters operating at 1 W are preferable to reaction wheels or mangetorquers at the selected altitude. An extended Kalman filter is used for orbital position and spacecraft attitude estimations. The attitude determination system consists of sun sensors, magnetometers, gyroscopes serving as attitude sensors.
The mission consists of two phases. In Phase I, a 4U CubeSat is deployed from a 414 km orbit and uses the primary propulsion system to deorbit to an initial altitude within the targeted range of 244 +/- 10 km. Phase I lasts 12.73 days with the propulsion system consuming 5.6 g of propellant to deliver a ∆V of 28.12 m/s. In Phase II the mission is maintained until the remaining 25.2 g of propellant is consumed. Phase II lasts for 30.27 days, corresponding to a ∆V of 57.22 m/s with a mean altitude of 244 km. The mean altitude for an individual orbit over the entire mission was found to vary from a maximum of 252 km to a minimum of 236 km. Using this approach, a primary mission life of 30.27 days could be achieved, compared with 3.1 days without primary propulsion.<br /
Direct Radiative Effects of Aerosols Over South Asia From Observations and Modeling
No full textQuantitative assessment of the seasonal variations in the direct radiative effect (DRE) of composite aerosols as well as the constituent species over the Indian sub continent has been carried out using a synergy of observations from a dense network of ground based aerosol observatories and modeling based on chemical transport model simulations. Seasonal variation of aerosol constituents depict significant influence of anthropogenic aerosol sources in winter and the dominance of natural sources in spring, even though the aerosol optical depth doesn't change significantly between these two seasons. A significant increase in the surface cooling and atmospheric warming has been observed as season changes from winter DRE(sub SUR) = 28 +/- 12 W m(exp 2) and DRE(sub ATM) = +19.6 +/- 9 W m(exp 2) to spring DRE(sub SUR) = 33.7 +/- 12 W m(exp 2) and DRE(sub ATM) = +27 +/- 9 W m(exp2). Interestingly, springtime aerosols are more absorptive in nature compared to winter and consequently the aerosol induced diabatic heating of the atmosphere goes as high as approximately 1 K day(exp -1) during spring, especially over eastern India. The atmospheric DRE due to dust aerosols (+14 +/- 7 W m(exp 2) during spring overwhelms that of black carbon DRE (+11.8 +/- 6 W m(exp -2) during winter. The DRE at the top of the atmosphere is mostly governed by the anthropogenic aerosols during all the seasons. The columnar aerosol loading, its anthropogenic fraction and radiative effects shows a steady increase with latitude across Indian mainland leading to a larger aerosol-induced atmospheric warming during spring than in winter
Effects of changes in atmospheric water vapor content on physical properties of atmospheric aerosols at a coastal station
No full textObservational results are presented on the effects of changes in the atmospheric water vapor content (W g cm-2) on spectral optical depth (τpλ ) and the retrieved columnar size distributions of atmospheric aerosols over a natural, coastal environment obtained from simultaneous estimates of these parameters (τpλ and W) using a ground based multiwavelength solar radiometer. It is found that, during meteorologically calm periods (i.e. in the absence of any significant air mass types), the aerosol optical depths increase with increase in W, with the shorter wavelengths being more sensitive to the changes in W, compared to the near infrared (IR) wavelengths. The aerosol (columnar) size distributions, retrieved from τpλ as a function of W, are found to change from bimodal for very low values of W to unimodal for higher values of W. The mean radius and mode radii of the retrieved size distributions also increase with increase in W
Temporal variation of aerosol optical depth and associated shortwave radiative forcing over a coastal site along the west coast of India
No full textOptical characterization of aerosol was performed by assessing the columnar aerosol optical depth (AOD) and angstrom wavelength exponent (α) using data from the Microtops II Sunphotometer. The data were collected on cloud free days over Goa, a coastal site along the west coast of India, from January to December 2008. Along with the composite aerosol, the black carbon (BC) mass concentration from the Aethalometer was also analyzed. The AOD0.500 μm and angstrom wavelength exponent (α) were in the range of 0.26 to 0.7 and 0.52 to 1.33, respectively, indicative of a significant seasonal shift in aerosol characteristics during the study period. The monthly mean AOD0.500 μm exhibited a bi-modal distribution, with a primary peak in April (0.7) and a secondary peak in October (0.54), whereas the minimum of 0.26 was observed in May. The monthly mean BC mass concentration varied between 0.31 μg/m3 and 4.5 μg/m3, and the single scattering albedo (SSA), estimated using the OPAC model, ranged from 0.87 to 0.97. Modeled aerosol optical properties were used to estimate the direct aerosol shortwave radiative forcing (DASRF) in the wavelength range 0.25 μm4.0 μm. The monthly mean forcing at the surface, at the top of the atmosphere (TOA) and in the atmosphere varied between − 14.1 W m−2 and − 35.6 W m− 2, − 6.7 W m−2 and − 13.4 W m−2 and 5.5 W m−2 to 22.5 W m−2, respectively. These results indicate that the annual SSA cycle in the atmosphere is regulated by BC (absorbing aerosol), resulting in a positive forcing; however, the surface forcing was governed by the natural aerosol scattering, which yielded a negative forcing. These two conditions neutralized, resulting in a negative forcing at the TOA that remains nearly constant throughout the year
Spatial variation of aerosol spectral optical depth and columnar water vapour content over the Arabian Sea and Indian Ocean during the IFP of INDOEX
No full textExtensive solar spectral extinction measurements
are made over the Arabian Sea and Indian Ocean
(between 60°E and 78°E longitude and 15°N and 20°S
latitude) using a ten-channel multi wavelength solar
radiometer (MWR) and a 4-channel EKO Sun photometer
on-board the cruise #141 of ORV Sagar Kanya
during the Intense Field Phase (IFP) of the Indian
Ocean Experiment (INDOEX) from 20 January to 12
March 1999. From these measurements, the columnar
aerosol optical depths and columnar content of water
vapour are estimated; and their spatial variations
examined. The results show occurrence of fairly high values of aerosol optical depths along the West Coast of India.
The optical depths decrease gradually as one moves
down south, with an e–1 scaling distance of ~ 1200 to
1300 km. Extremely low (near zero) values are encountered
in mid Indian Ocean due south of 10° latitude
(i.e. on the south of the ITCZ) followed by a weak
increase close to Mauritius. Presence of an extensive
region of enhanced aerosol optical depths (which we
call the West Asian High) is seen in the mid-Arabian
Sea between 5° and 10°N westward of 65°E longitude
which is stronger and wider than those seen along the
coastal regions of India. This effect is seen at the longest
wavelength (1025 nm) also. Variation of columnar
water vapour content (W g cm–2) shows highest values(> 3 g cm–2) around the equator (~ 3°N to 5°S extendingover the entire longitude region) possibly associatedwith the ITCZ. Moderately high values are
encountered along the coastal regions of India and
also close to Mauritius. Compared to these, the northwest
Arabian Sea (between 5 to 15°N and 60° to 70°E)
shows rather dry atmosphere with W < 2 g cm–2, indicatingprevalence of drier continental conditions. Theimplications of the findings are discussed
Aerosol characteristics and radiative impacts over the Arabian Sea during the intermonsoon season: Results from ARMEX field campaign
No full textDuring the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II), extensive measurements of spectral aerosol optical depth, mass concentration, and mass size distribution of ambient aerosols as well as mass concentration of aerosol black carbon (BC) were made onboard a research vessel during the intermonsoon period (i.e., when the monsoon winds are in transition from northeasterlies to westerlies/ southwesterlies) over the Arabian Sea (AS) adjoining the Indian Peninsula. Simultaneous measurements of spectral aerosol optical depths (AODs) were made at different regions over the adjoining Indian landmass. Mean AODs (at 500-nm wavelength) over the ocean (similar to0.44) were comparable to those over the coastal land (similar to0.47), but were lower than the values observed over the plateau regions of central Indian Peninsula (similar to0.61). The aerosol properties were found to respond distinctly with respect to change in the trajectories, with higher optical depths and flatter AOD spectra associated with trajectories indicating advection from west Asia, and northwest and west-coastal India. On average, BC constituted only similar to2.2% to total aerosol mass compared to the climatological values of similar to6% over the coastal land during the same season.
These data are used to characterize the physical properties of aerosols and to assess the resulting short-wave direct aerosol forcing. The mean values were similar to27 W m(-2) at the surface and -12 W m(-2) at the top of the atmosphere (TOA), resulting in a net atmospheric forcing of +15 W m(-2). The forcing also depended on the region from where the advection predominates. The surface and atmospheric forcing were in the range -40 to -57 W m(-2) and +27 to +39 W m(-2), respectively, corresponding to advection from the west Asian and western coastal India where they were as low as -19 and +10 W m(-2), respectively, when the advection was mainly from the Bay of Bengal and from central/peninsular India. In all these cases, the net atmospheric forcing (heating) efficiency was lower than the values reported for northern Indian Ocean during northern winter, which is attributed to the reduced BC mass fraction
Optical, radiative, and source characteristics of aerosols at Minicoy, a remote island in the southern Arabian Sea
No full textExtensive measurements of aerosol radiative and microphysical properties were made at an island location, Minicoy (8.3°N, 73.04°E) in the southern Arabian Sea. A large variability in aerosol characteristics associated with changes in air mass and precipitation characteristics was observed. Six distinct transport pathways were identified on the basis of cluster analysis. The Indo-Gangetic Plain, along with the northern Arabian Sea and west Asia (NWA), was identified to be the region having the highest potential for aerosol mass loading at the island. This estimate is based on the concentration weighted trajectory as well as cluster analysis. Dust transport from the NWA region was found to make a substantial contribution to the supermicron mass fraction. The black carbon mass mixing ratios observed were the lowest compared to previous measurements over this region. Consequently, the atmospheric radiative forcing efficiency was low and was in the range 10–28 W m−2
Optical, radiative, and source characteristics of aerosols at Minicoy, a remote island in the southern Arabian Sea
No full textExtensive measurements of aerosol radiative and microphysical properties were made at an island location, Minicoy (8.3 degrees N, 73.04 degrees E) in the southern Arabian Sea. A large variability in aerosol characteristics associated with changes in air mass and precipitation characteristics was observed. Six distinct transport pathways were identified on the basis of cluster analysis. The Indo-Gangetic Plain, along with the northern Arabian Sea and west Asia (NWA), was identified to be the region having the highest potential for aerosol mass loading at the island. This estimate is based on the concentration weighted trajectory as well as cluster analysis. Dust transport from the NWA region was found to make a substantial contribution to the supermicron mass fraction. The black carbon mass mixing ratios observed were the lowest compared to previous measurements over this region. Consequently, the atmospheric radiative forcing efficiency was low and was in the range 10-28 W m(-2)
Aerosol black carbon over Arabian sea during intermonsoon and summer monsoon seasons
No full textExtensive, collocated measurements of the mass concentrations of composite and black carbon (BC) aerosols were made over coastal Arabian Sea, adjoining Indian Peninsula, for the first time during the inter-monsoon and summer monsoon periods, of 2003, as part of Arabian Sea Monsoon Experiment (ARMEX). Results showed that the diurnal variations are weak in March, and vanish completely by May/June, associated with the change in the synoptic circulations. The concentration of BC (and its share to total aerosol mass) decreases continuously, from ~700 ng m-3 (2.5%) in March to ~104 ng m-3 (0.5%) by June. Consequently, the net atmospheric forcing (heating) efficiency decreases from ~70 W m-2 (for reported winter conditions) to ~30 W m-2 for inter-monsoon and to ~15 W m-2 for summer monsoon seasons. These will have implications on regional climate forcing
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