Browsing by Author "Kesaraju, Saiveena"
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Item Open Access Ground-Based Observational Techniques for Meteoroid Lunar Impact Generated Electromagnetic Pulses and Lunar Sub-Surface Structure Detection(The Pennsylvania State University, 2017-08) Kesaraju, Saiveena; Mathews, John DavidThe lunar surface is pockmarked with large and small craters mostly formed due to meteoroid impacts on the Moon. Most of the craters formed are not erased with time due to lack of "weathering" processes such as no atmosphere and little erosion. The main focus of this research is to develop ground-based observational techniques to search for ongoing hypervelocity meteoroid impacts on the lunar surface. Additionally, to design radar observational techniques to detect and map sub-surface structures that have been buried by the lunar regolith. It is hypothesized that the developing, optically-dense hot ejecta cloud associated with the hypervelocity meteoroid impacts produce an associated complex plasma component that rapidly evolves resulting in a highly-transient Electromagnetic pulse (EMP) in the VHF/UHF spectral region. An observational EMP search was conducted in May 2014 for about 5 hours using an overlapping-band (425-445 MHz) at the Arecibo (AO; Puerto Rico) and Haystack-(HO, Massachusetts, USA) observatories simultaneously to track the common visible lunar surface from two different locations on the Earth. Observations from two locations is helpful in eliminating the false impacts. Interleaved radar observations were used to calibrate the timing and synchronize both the AO and HO systems. As the AO/HO UHF EMP search was interference dominated, an alternative search mechanism using the Arecibo L-band ALFA Array that consists of seven beams arranged in the hexagonal manner was conducted in February 2016. During these observations, at any given time few of the receive-beams were on-Moon and few off-Moon thus allowing discrimination against local interference that might resemble the expected EMP signals. While still encountering local out-of-band radar interference, this observational paradigm did yield a few likely lunar impact EMPs. Additionally, to detect the sub-surface lunar structures, high power large aperture - Jicamarca Radio Observatory (JRO) 50 MHz radar located near Lima, Peru was used to map the lunar surface and subsurface features. This was accomplished by developing or refining various calibration and imaging procedures. This radar provides the ability to map the lunar sub-surface because the 6-meter wavelength radar signal penetrates the low-loss regolith and scatters from larger sub-surface structures allowing study of these structures. This analysis further depends on the (de)polarization of the return signals. Interpretation of lunar radar signal polarization is greatly complicated by the double traverse of the ionosphere at or near wavevector near to perpendicular to the geomagnetic field geometry as described. Preliminary radar observations were conducted in October 2015 by transmitting a circular polarized coded pulse during the lunar transit over JRO. The detected lunar echoes of the duration of 13 minutes were then processed to generate the lunar Range-Doppler maps and identify the (sub)surface features. Preliminary science results from the observations are given. Each of the three observational set-up's along with the signal processing paradigms such as Inverse Synthetic Aperture Radar (ISAR) mapping to form the lunar maps and the time-frequency technique to process the collected observational data are explained. Implications of the observed transient EMP events, processed lunar surface maps, characterization of the observed satellite radar echoes and the difficult radio-frequency interference environment (terrestrial-origin, Moon-bounce signals) surrounding these observations are discussed.Item Restricted Radar images of the Moon at 6-meter wavelength(Elsevier, 2017-11) Vierinen, Juha; Tveito, Torbjørn; Gustavsson, Björn; Kesaraju, Saiveena; Milla, MarcoWe present new range-Doppler images of the Moon using 6-mwavelength. The radar images were obtained using the Jicamarca Radio Observatory 49.92 MHz radar. The observations were performed using circular polarization on transmit and two orthogonal linear polarizations on receive, allowing scattering images to be obtained with the polarization matched to the transmitted wave (polarized), and at a polarization orthogonal to the transmitted wave (depolarized). Due to the long wavelength that penetrates efficiently into the subsurface of the Moon, the radar images are especially useful for studies of subsurface composition. Two antenna interferometry on receive was used to remove the Doppler north-south ambiguity. The images have approximately 10 km resolution in range 20 km resolution in Doppler, allowing many large scale features, including maria, terrae, and impact craters to be identified. Strong depolarized return is observed from relatively new larger impact craters with large breccia and shallow regolith. Terrae regions with less lossy surface material also appear brighter in both depolarized and polarized images. A large region in the area near the Mare Orientale impact basin has overall higher than mean radar backscatter in both polarized and depolaried returns, indicating higher than average presence of relatively newly formed large breccia in this region. Mare regions are characterized by lower polarized and depolarized return, indicating that there is higher loss of the radio wave in the subsurface, reducing the echo. We also report unexpected low polarized and depolarized backscatter from an old impact basin in the Schiller–Schickard region, as well as from the region poleward from Mare Imbrium.