Indian Space Research Organization
  • Bengaluru, Gujarat, India
Recent publications
Aditya-L1 is India’s first observatory-class solar space mission to study the Sun from the Lagrange L1 point. The Solar Ultra-Violet Imaging Telescope (SUIT) is one of the payloads onboard Aditya-L1. SUIT is an off-axis Ritchey–Chrétien (RC) telescope, which images the Sun onto a 4k×4k CCD covering a field-of-view of 1.5 R⊙ with a plate scale of \(0.7''\) pixel−1. One of the primary objectives of SUIT is to study the early evolution of solar flares with high temporal cadence in the near-UV wavelengths (200 – 400 nm). The SUIT onboard intelligence was developed to achieve this objective. The complete intelligence algorithm is divided into several sub-modules, each working on a specific aspect of intelligence. These are: the HEL1OS flare-trigger module: generates flare trigger using HEL1OS hard X-ray data, the flare-localization module: locates the flare on the SUIT full-disc images, the Region of Interest (RoI) tracking module: accounts for the shift in RoI coordinates caused by rotation of the Sun, auto-exposure control module: adjusts the exposure time depending upon the flare intensity for better contrast. In this article, these onboard-intelligence modules are explained in detail. The working principles of these modules are tested using available data from various existing missions and also using synthetic data, and the obtained results are presented. The modules are implemented in hardware using an Actel RTAX 2000S FPGA and are tested using a laboratory setup. From the testing, it is found that flares are successfully localized in a mean time of 40 seconds from the GOES soft X-ray catalog start time. Also, a temporal cadence of under three seconds for a single-filter flare RoI image is achieved.
The International Liquid Mirror Telescope (ILMT) is a 4-m class survey telescope that has recently achieved first light and is expected to swing into full operations by January 1, 2023. It scans the sky in a fixed [Formula: see text] wide strip centered at the declination of [Formula: see text] and works in Time Delay Integration (TDI) mode. We present a full catalog of sources in the ILMT strip that can serve as astrometric calibrators. The characteristics of the sources for astrometric calibration are extracted from Gaia EDR3 as it provides a very precise measurement of astrometric properties such as RA ([Formula: see text]), Dec ([Formula: see text]), parallax ([Formula: see text]), and proper motions ([Formula: see text] & [Formula: see text]). We have crossmatched the Gaia EDR3 with SDSS DR17 and PanSTARRS-1 (PS1) and supplemented the catalog with apparent magnitudes of these sources in [Formula: see text], and [Formula: see text] filters. We also present a catalog of spectroscopically confirmed white dwarfs with Sloan Digital Sky Survey (SDSS) magnitudes that may serve as photometric calibrators. The catalogs generated are stored in an SQLite database for query-based access. We also report the offsets in equatorial positions compared to Gaia for an astrometrically calibrated TDI frame observed with the ILMT.
A miniaturized Y-shaped dual-band bandpass filter with multiple transmission zeros/poles is presented in this work. The proposed filter configuration comprises series coupled lines, shunt cascaded coupled line and parallel open stubs, and stepped impedance resonators. Analysis of the proposed filter is executed using a circuit simulator, and the magnitude and bandwidth variations have been studied. Equations for transmission zero frequencies have been derived and verified based on lossless transmission line theory and even-odd mode analysis to validate the proposed filter configuration. A prototype operating at 1.38 GHz (GPS) and 3.4 GHz (Wi-Max) applications is fabricated and experimented. The final prototype occupies a compact area of 0.33λg × 0.20λg, where λg is the guided wavelength at the first band (1.38GHz). The experimental results show that the roll-off rate for the first and second passbands is 212.5 and 188 dB/GHz, respectively, indicating high selectivity. The insertion loss is better than 20 dB indicating good isolation between the two passbands.
Accurate change detection from high-resolution satellite and aerial images is of great significance in remote sensing for precise comprehension of Land cover (LC) variations. The current methods compromise with the spatial context; hence, they fail to detect and delineate small change areas and are unable to capture the difference between features of the bi-temporal images. This paper proposes Remote Sensing Change Detection Network (RSCDNet) - a robust end-to-end deep learning architecture for pixel-wise change detection from bi-temporal high-resolution remote-sensing (HRRS) images. The proposed RSCDNet model is based on an encoder-decoder framework integrated with the Modified Self-Attention (MSA) andthe Gated Linear Atrous Spatial Pyramid Pooling (GL-ASPP) blocks; both efficient mechanisms to regulate the field-of-view while finding the most suitable trade-off between accurate localization and context assimilation. The paper documents the design and development of the proposed RSCDNet model and compares its qualitative and quantitative results with state-of-the-art HRRS change detection architectures. The above mentioned novelties in the proposed architecture resulted in an F1-score of 98%, 98%, 88%, and 75% on the four publicly available HRRS datasets namely, Staza-Tisadob, Onera, CD-LEVIR, and WHU. In addition to the improvement in the performance metrics, the strategic connections in the proposed GL-ASPP and MSA units significantly reduce the prediction time per image (PTPI) and provide robustness against perturbations. Experimental results yield that the proposed RSCDNet model outperforms the most recent change detection benchmark models on all four HRRS datasets.
Elevation information plays a significant role in Earth Sciences as it enables the interpretation of the data in three-dimensional (3D) space, mainly when the landscape of the study area contains rugged or irregular topography. Topography influences the Earth's surface processes, including geology, hydrology, climatology, and human settlement patterns. The Himalayas, a large mountain belt, are the result of topographic expression from the interplay between tectonic activities, climatic fluctuations, and Earth's surface processes. Termed the third pole, the Himalayan region hosts numerous glaciers, peaks, valleys, and rivers and has a profound role in the "water-ice-air-ecosystem-energy-human" interactions related to most parts of the South Asian countries. Specific Earth system processes of the Himalayan mountains act as a climate proxy; thus, long-term studies about the Himalayas provide patterns of climate change. Elevation information is a critical need to investigate a range of surface processes in the rugged terrains of the Himalayan region. The availability of the void-filled global digital elevation model products has paved a better way to integrate remote sensing methods with 3D models. Data from multi-view optical satellite images and Synthetic Aperture Radar (SAR) satellites have been commonly used to understand the elevation component for the study areas that contain rugged topography. However, research results show that certain limitations exist using these sensors while studying mountainous regions due to extensive sloped facies. Similarly, the accuracies of void-filled global digital elevation models differ significantly from the true heights. The recent Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) is the latest addition in space-borne laser altimetry missions launched by NASA and hosts a solo sensor, namely ATLAS, that produces height measurements from surface reflected photons for every 0.7 m along-track direction. The resultant data are conducive for quantifying the heights over various surfaces like glaciers, sea ice, water-bodies, land, and canopy. This chapter attempts to demonstrate the applications of photon data from the ICESat-2 laser altimeter for various strategic needs in the Himalayan region. The applications include retrieval of topographic profiles over rugged topography, hydrological profiles, water surface and canopy height detection, lake ice phenology, and refining the existing elevation models using the heights from surface reflected photons of ICESat-2.</
Axisymmetric shell structures are characterized by the presence of flexural modes which occur in degenerate pairs. However, since no practically realized structure is perfectly axisymmetric, the frequencies constituting a mode pair become unequal and causes what is commonly termed as “frequency split” between the modes. In applications such as vibratory gyroscopes, it is desired that the frequencies of the working pair of modes be equal. This work proposes an easily implementable (particularly in the industry) technique which reduces frequency split of a target mode pair. To address the frequency split trimming problem, this work investigates a model that describes the planar dynamics of a vibrating ring (which is prototypical to axisymmetric shell structures), with added mass segments. The effects of mass, as well as span, of an added segment on the ring’s natural frequencies and mode shapes are brought out. Novelties of the proposed trimming technique are that it employs added mass in the form of segments and that it has a negligible effect on the axes of the structure’s mode shapes. The technique is extended to general axisymmetric bodies, and it is validated experimentally for the case of a hemispherical resonator.
This brief presents the analysis and design of an ultra-wideband bandpass filter (UWBPF) with four transmission zeros and seven poles. The UWBPF model consists of series transformers, coupled lines, and a shunt-stepped impedance open stub. The even-odd mode analysis method and scattering parameters theory are used to extract the closed-form equations for transmission zeros and poles of the proposed UWBPF. For validation, a prototype with a center frequency of 2.49 GHz is designed, simulated, and tested. The measured 3 dB fractional bandwidth is 103.2%, and the fabricated prototype occupies an area of $0.165\lambda _{g}^{2}$ . The circuit, full-wave simulated, and experimental results are in a good match.
Wide bandgap GaN heterojunction field-effect transistors (HFETs) are capable to provide microwave output power density more than 10 W/mm gate width. This chapter presents GaN-based various HFET structures, e.g., AlGaN/GaNAlGaN/GaN and AlInN/GaN. Equivalent circuit model for GaN transistors along with the details of equivalent circuit elements is also discussed.
Output load impedance for a power amplifier is to ensure maximum allowable output voltage and current swings of amplifying device to maximize RF output power and efficiency, maintaining linearity. This chapter presents design and analysis of linear power amplifiers having required output load matching conditions for achieving predicted output power and efficiency. Output load requirement for different constant RF output power is discussed with device voltage and current waveforms. Concept of constant power contours with de-embedding of various types of circuit elements are discussed with examples using Smith chart plots. Stability issues and various techniques for achieving stable performance of microwave power amplifiers are also discussed.
This chapter presents design and analysis of Class-AClass-A microwave solid-state high power amplifiers. Mathematical expressions for maximum output power and power added efficiencyPower added efficiency for a given device parameters are derived for its linear and overdriven operating conditions. Design of broadband Class-A amplifiers using reactive element in addition to optimum resistive load impedance of GaN devices exploiting its higher breakdown voltage is also discussed.
Broadband performance of Class-BClass-B power amplifiers is limited due to requirement of short circuit impedances for all even harmonic frequencies, whereas Class-J and extended Class-J amplifiers provide predicted RF output power and efficiency removing the limitation of using short circuit impedances for even harmonic components with the penalty of higher drain voltage swing. This chapter presents design and analysis of high efficiency Class-JClass-J and extended Class-J microwave solid-state high power amplifiers. Class-J power amplifier uses output matching networkMatching network by pulling fundamental and only few harmonic components to provide high DC to RF efficiencyDC to RF efficiency over a broad frequency band maintaining linearity of amplifiers. Mathematical expressions are derived for maximum output power, power added efficiencyPower added efficiency and output impedance of Class-J and extended Class-J amplifiers for a given device.
This chapter presents design and analysis of Class-BClass-B microwave solid-state high power amplifiers. Maximum output power and power added efficiencyPower added efficiency for a given device are derived for its linear and overdriven operating condition. Broadband performance of Class-B power amplifiers is limited due to requirement of short circuit impedance for all even harmonic frequency components. Designs of Class-B amplifiers with resistive and tuned loads are also discussed.
Gallium nitrideGallium Nitride (GaN) is a wide bandgap semiconductor with high electron saturation velocity, high breakdown voltageBreakdown voltage and better thermal stability for high power microwave frequency applications. This chapter presents crystalline structure and associated electrical properties of GaN material. Spontaneous and piezoelectric polarization effects in GaN heterostructures are also discussed to explain generation of two-dimensional electron gas (2DEG) with very high sheet charge density required for achieving high power handling capability of GaN-based transistors.
This chapter presents design and analysis of high efficiency high power Class-FClass-F microwave solid-state amplifiers. Class-F power amplifier needs large number of resonators for harmonic impedance matching to produce non-overlapping device voltage and current waveforms for providing high DC to RF efficiencyDC to RF efficiency. Mathematical expressions for maximum output power and power added efficiencyPower added efficiency of Class-F amplifier for a given device are derived. Broadband performance of Class-F power amplifiers is limited due to requirement of short and open circuit impedances for large number of harmonic frequencies. Various types of Class-F amplifiers such as continuous Class-FContinuous Class-F and extended continuous Class-F amplifiers are discussed for achieving broadband performance with predicted compromise of output RF power and efficiency.
Two types of high-entropy alloys (HEAs) AlCrCoFeNiTi and FeCrCoNiW0.3 + 5 at.% C are fabricated using atmospheric plasma spray (APS) technique. Laser surface processing (LSP) is performed on the developed alloys using Nd:YAG pulsed laser. Post processing the surface roughness of the alloys are reduced by ~ 29%. The impact of laser surface processing reveals the presence of a single BCC phase and FCC phase with the evolution of more W-rich and Cr-rich carbides in AlCrCoFeNiTi and FeCoCrNiW0.3 + 5 at.% C coatings, respectively. The microstructural study exhibits the formation of lamellar microstructure with minimum pores and interlaminar cracks. Post laser processing the microhardness of both the APS coated alloys are increased by 5%, nanoindentation results reveal an increase in the average elastic modulus (Er) by 12%, and average nanohardness by 18%. The FeCoCrNiW0.3 + 5 at.% C coatings achieved maximum wear resistance of 39.71% among the two alloys, indicating the improvement achieved through laser processing. Also the observed improvements in surface morphology of both the alloys are reported.
Graphene has emerged as one of the most fascinating materials for the scientific community. The exceptional properties of graphene make it a better candidate than the existing materials. So far, the reduced form of graphene oxide (GO) is the best substitute for large quantity graphene. There are numerous ways of reducing GO via synthetic reducing agents that are not eco-friendly and cost-effective. This investigation aims to expand the scope for the reduction of GO using naturally existing reducing agents. In the present work, a comparative study of the extent of reduction using metal/acids and vitamin C containing green reductants is done. The X-ray diffraction peak for GO was detected at 2θ = 10.8° which gradually shifted to higher 2θ values (24-26°) after reduction, indicating that GO was reduced well using all the reductants. The C/O ratio of GO (calculated by XPS), gradually increased from 2.5 to 4–5 for all the reducing agents. Inspired by the better reduction in lemon reduced GO (LrGO), its electrochemical characterization was performed using cyclic voltammetry (CV). The increased supercapacitive value validates the improved electrochemical behavior of LrGO over GO. Finally, our findings conclude that green reductant (lemon juice) serves as a good, eco-friendly, and economic reducing agent for the synthesis of reduced graphene oxide with improved electrochemical properties.
CFRP sandwich structures have proved themselves as a good candidate for satellite antenna reflectors due to high bending stiffness with lower mass. Bare CFRP acts as a reflector for frequencies up to Ku-band beyond which reflective properties are insufficient. Studies are carried out by researchers to improve the reflectivity of CFRP sandwich structures and one of the widely adopted methods is metallisation of the reflective surface of CFRP. Glass transition temperature (Tg) of CFRP is much below the melting point of metals, which makes the process of metalising more challenging. This is overcome by a few approaches such as chemical vapour deposition, sputtering, and cold spray. In the present study, a unique concept of metalising the CFRP sandwich dish by co-curing it with copper-cladded polyimide film is presented. To facilitate the bond between CFRP and copper-cladded polyimide film, the resin available in CFRP prepreg itself is used during the co-curing process. The procedure was successfully adopted in a high-throughput satellite payload of ISRO missions.
Impedance matchingImpedance matching is an important requirement of any microwave circuit for achieving required optimum performance. Matching circuit of amplifiers determine its gain, noise figure, output power, efficiency, operating bandwidth, etc. This chapter presents various impedance matching techniques suitable for design of microwave high power amplifiers and other microwave circuits. Matching networks using series and shunt transmission line sections as well as using lumped capacitor and inductor components with various configurations are discussed. Broadband and multiband matching techniques are also discussed suitable for development of broadband and multiband power amplifiers.
Institution pages aggregate content on ResearchGate related to an institution. The members listed on this page have self-identified as being affiliated with this institution. Publications listed on this page were identified by our algorithms as relating to this institution. This page was not created or approved by the institution. If you represent an institution and have questions about these pages or wish to report inaccurate content, you can contact us here.
1,278 members
Sankaran Muthusamy
  • Communication and Power Area
Sethunadh R
  • Vikram Sarabhai Space Center (VSSC)
Vamsi Krishna Velidi
  • Communication Systems
Ishwari Datt Rai
  • Indian Institute of Remote Sensing (IIRS)
Babu Govindha Raj K
  • Department of Space and ISRO HQ
Information
Address
380015, Bengaluru, Gujarat, India