The electric solar wind sail (E-sail) is a space propulsion concept that uses the natural solar wind dynamic pressure for producing spacecraft thrust. In its baseline form, the E-sail consists of a number of long, thin, conducting, and centrifugally stretched tethers, which are kept in a high positive potential by an onboard electron gun. The concept gains its efficiency from the fact that the effective sail area, i.e., the potential structure of the tethers, can be millions of times larger than the physical area of the thin tethers wires, which offsets the fact that the dynamic pressure of the solar wind is very weak. Indeed, according to the most recent published estimates, an E-sail of 1 N thrust and 100 kg mass could be built in the rather near future, providing a revolutionary level of propulsive performance (specific acceleration) for travel in the solar system. Here we give a review of the ongoing technical development work of the E-sail, covering tether construction, overall mechanical design alternatives, guidance and navigation strategies, and dynamical and orbital simulations.
This article reports the first comprehensive results obtained from a fully functional, recently established infrared spectral-emissivity measurement facility at the National Institute of Standards and Technology (NIST). First, sample surface temperatures are obtained with a radiometer using actual emittance values from a newly designed sphere reflectometer and a comparison between the radiometer temperatures and contact thermometry results is presented. Spectral emissivity measurements are made by comparison of the sample spectral radiance to that of a reference blackbody at a similar (but not identical) temperature. Initial materials selected for measurement are potential candidates for use as spectral emissivity standards or are of particular technical interest. Temperature-resolved measurements of the spectral directional emissivity of SiC and Pt-10Rh are performed in the spectral range of 2-20 µm, over a temperature range from 300 to 900 • C at normal incidence.Further, a careful study of the uncertainty components of this measurement is presented.
This study evaluates the potential of X-band interferometry for monitoring of agricultural grasslands. Time series of HH-polarization COSMO-SkyMed 1-day repeat-pass interferometric SAR (InSAR) pairs is analyzed in regard to detecting mowing events, and assessing vegetation height and biomass on grasslands. The time series of four InSAR pairs was analyzed in regard to the ground reference data collected during an extensive campaign covering 11 agricultural grasslands. The calculated temporal interferometric coherence was found to be inversely correlated to the vegetation height and wet above-ground biomass. It was found that grass removal increases the coherence magnitude indicating a potential use of this parameter for the detection of mowing. However, precipitation and farming activity between the acquisitions interfere with this effect. Temporal coherence was expressed as a function of the vegetation height through the random motion of scatterers in the vegetation layer. For vegetation height limited to the range between 0 and 1 m, a very strong correlation between the grass height and the linearised temporal coherence was found, with a coefficient r = 0.81. No significant correlation was found between the backscattering coefficient and the wet above-ground biomass as well as the height of grass. However, a strong negative correlation was found between the backscattering coefficient and the measured soil moisture.
Results of filter radiometer characterization with a wavelength-tunable Ti : sapphire laser in the wavelength band around 900 nm are presented. The effect of interference between the reflections from filter surfaces in the case of coherent laser light was studied and reduced with a special filter design with antireflection coatings. Measuring the responsivity as a function of wavelength over a very narrow band was used to reveal the remaining interference effects. Uncertainty analysis and test results indicate that filter radiometers can be characterized with a relative standard uncertainty of 9×10−4 using the scanning method. The results agree well with more conventional monochromator-based measurements.
This paper presents the design, implementation, and pre-launch test results of the Command and Data Handling Subsystem (CDHS) for ESTCube-1. ESTCube-1 is a one-unit CubeSat, which will perform an electric solar wind sail experiment. The development process of the CDHS for ESTCube-1 was focused on robustness and fault tolerance. A combination of hot and cold hardware redundancy was implemented. Software, including a custom-written internal communications protocol, was designed to increase the system's fault tolerance further by providing fault detection and fall-back procedures. Tests were carried out to validate the implementation's performance and physical endurance. The final CDHS design is operational within the set requirements. Tests that verify fault tolerance of the system in orbit are suggested.
This paper presents the design, development, and pre-launch characterization of the ESTCube-1 Attitude Determination and Control System (ADCS). The design driver for the ADCS has been the mission requirement to spin up the satellite to 360 deg·s −1 with controlled orientation of the spin axis and to acquire the angular velocity and the attitude during the scientific experiment. ESTCube-1 is a one-unit CubeSat launched on 7 May 2013, 2:06 UTC on board the Vega VV02 rocket. Its primary mission is to measure the Coulomb drag force exerted by a natural plasma stream on a charged tether and, therefore, to perform the basic proof of concept measurement and technology demonstration of electric solar wind sail technology. The attitude determination system uses three-axis magnetometers, three-axis gyroscopic sensors, and two-axis Sun sensors, a Sun sensor on each side of the satellite. While commercial off-the-shelf components are used for magnetometers and gyroscopic sensors, Sun sensors are custombuilt based on analogue one-dimensional position sensitive detectors. The attitude of the satellite is estimated on board using an Unscented Kalman Filter. An ARM 32-bit processor is used for ADCS calculations. Three electromagnetic coils are used for attitude control. The system is characterized through tests and simulations. Results include mass and power budgets, estimated uncertainties as well as attitude determination and control performance. The system fulfils all mission requirements.
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