This paper presents an all-digital FPGA-based architecture for the determination of residual amplitude modulation (AM) in phase-modulated satellite transmitter output. A quadrature demodulation-based technique is implemented for the residual AM demodulation. The algorithm measures the RMS, Max, and Min of the AM index. The hardware implementation of the system is carried out with an FPGA and an RF sampling transceiver to demonstrate the proposed algorithm. The system is capable of measuring residual AM index with a mean absolute deviation of 0.16 % as compared with the modulation analyzer instrument. The measurements provide the same accuracy up to signal strengths as low as -18 dBFS.

RF Rotary Joints (RFRJ) are of profound importance in mechanically steered antennas to ensure seamless energy transfer from the stator side (transponder) to the rotor side (antenna). A high power and low insertion loss dual-channel RFRJ has been designed and developed at X-band to double the throughput by frequency reuse. The design is based on septum polarizer in a circular waveguide to generate two orthogonal circularly polarized TE11 modes. Each of these modes support one communication channel. Additive manufacturing technique by 3D printing has been adopted to realize the hardware in just two pieces - one each for the stator and the rotor. The thermal management of the high power RFRJ has been carried out efficiently. The design has ensured the performance for complete 360 degrees rotation angles. The measured results, insertion loss less than 0.3dB and channel isolation better than 30dB, indicate that the performance is satisfactory for a Radar or satellite based mechanical beam steering applications.

The Indian Solar Observation Satellite, developed by the Indian Space Research Organisation (ISRO), is a critical mission designed to enhance our understanding of solar phenomena and their effects on space weather. Positioned in a halo orbit around the Earth-Sun Lagrangian point L1, the satellite has advanced instruments to observe the Sun’s outer atmosphere, solar flares, sunspots, and coronal mass ejections. Operating in a high-radiation environment with abundant high energy charged particles, the satellite’s electronics must be highly radiation-tolerant to ensure accurate data collection with minimal errors. Due to the vast distance between the satellite and the ground station, the modulator in the data transmitter is designed for reconfigurability, enabling it to maintain a stable link without requiring changes to the power amplifier or antenna configuration. The system has been specifically engineered to be immune to Single Event Upsets (SEU) and Single Event Functional Interrupts (SEFI), ensuring reliable space performance. Comprehensive RF-level testing and evaluation were performed before the integration of the transmitter into the satellite. Post launch, the data transmission system demonstrated exceptional performance, effectively supporting the satellite’s mission objectives and ensuring high-quality data downlink from the solar observatory.

In this paper, we propose a dual-band Frequency Selective Surface (FSS) array designed for X-band transmission and Ka-band reflection. The proposed planar FSS will be used as a preliminary design for hyperbolic dichroic sub-reflector, in a dual-reflector antenna system meant for deep space application. This design eliminates the need for separate antennas for each frequency, thereby significantly reducing the overall size and mass, which is a critical resource in a satellite application. Towards this, a unit cell, consisting of two printed dielectric layers stacked together to form the FSS, is designed. This design allows X-band signals (7.1-8.5 GHz) to pass through it while reflecting Ka-band (31.8-32.3 GHz) signals. The design is simulated and optimized using full-wave EM solver and validated through measurements. Simulated and experimental results are discussed in the paper proving the effectiveness of the design.

The Indian Remote Sensing (IRS) satellites, operated by the Indian Space Research Organisation (ISRO), serve various purposes, including resource monitoring, disaster prediction & management and strategic applications. Development of Earth Observation satellite (EOS), dedicated to ocean monitoring is essential to support data for regional and global customers. Due to international and multi-user applications, the satellite requires a high-power amplifier for effective payload data downlink. A 24W GaAsFET-based power amplifier was designed to meet the link requirement for the satellite. Comprehensive electrical, power, thermal, and structural analyses were performed to ensure that the design of the amplifier system met the necessary specifications. The system has undergone the complete RF level Test and Evaluation (T&E) before integrating it into the satellite. Following the launch of the oceanographic Earth Observation Satellite (EOS) into orbit, the amplifier system has demonstrated exceptional performance.

With growing number of Geo Communication Satellites, the limited frequency band of 15 MHz allotted in C-Band for main frame data links has become highly crowded. In order to improve the co-location capacity and bandwidth efficiency, direct sequence spread spectrum based transmitters were developed. These transmitters can cause interference to the existing phase modulation based downlinks of other satellites especially during transfer orbit and degrade the link margin. To mitigate this issue, a dual modulation based transmitter has been developed which can be operated in both phase modulated and spread spectrum modes as per mission requirements. Added with additional feature of onboard frequency programmability, these transmitters provide redundancy of modulation scheme and increase overall system level reliability. These transmitters will be flown in all future ISRO satellites. This paper includes the details of link estimation, block level design of dual modulation transmitters, integrated performance of the hardware and challenges resolved during development.

Design of dual band (X/Ka) dual circularly polarized Reflector antenna system with common phase center for deep space satellite TT&C and data Transmission applications is presented. The antenna consists of a 2 meter Axially Displaced Ellipsoidal (ADE) Reflector with a coaxially fed horn antenna as feed. This antenna is planned for Telemetry, Tracking and Telecommand (TT&C) as well as payload Data transmission application from deep space orbiting spacecraft to ground communication. The performance of simultaneous excitation of X and Ka band antenna feed assembly with ADE reflector is simulated and results show that a very good performance can be achieved in both the bands.

An U-shaped resonator based bandstop filter (BSF) has been reported in this paper to reject the high power unwanted signal or interfere near to the desired signal. The BSF uses mixed coupling technique - electrical and magnetic - to realize its performance. The U-shaped resonators which are open on both sides are coupled to the feed-line or main line using a coupling gap. The resonators are approximately half-wavelength at the desired frequency of the BSF. As the RF signal passes through the filter the U -shaped resonator selectively captures the particular frequency and allows other frequencies to appear at the output. The mode analysis has been performed to understand the behavior of the circuit. To demonstrate the concept, a bandstop filter rejecting two undesired signals at 1.30 GHz and 1.35 GHz has been designed and fabricated. The filter allows to pass L1 and L5 frequencies with lower insertion loss.

This article describes design technique of Low Noise Amplifier (LNA) at C-band (5.7GHz-7.2GHz) based on self-bias with low current approach. Design carried out using GaAs based 130nm MMIC technology. Simulated results show noise figure of better than 1.22dB with a minimum gain of 27.9dB in the given frequency band. Input return loss is better than 13dB and output return loss is better than 15dB.The die operates at 5V,36.7mA. A comparison study with existing hardware and an MMIC design that uses conventional biasing approach has also been presented. 55% efficient in terms of power consumption and 50% reduction in size of the receiver front end can be achieved using this LNA. MMIC die is designed with a chip size of 2.9mm x1.8mm.

This research comprehensively investigates a free space optical communication link established between a sun-synchronous LEO (low earth orbit) satellite and a GEO (geostationary) satellite positioned at a 55-degree east sub-satellite point. Unlike previous models limited to LEO-to-ground connections, this study illuminates the superiority of establishing an optical LEO-to-GEO link, unveiling unprecedented advantages in data transmission efficiency, latency reduction, and reliability enhancement, thus setting a new standard in satellite communication protocols. The research outlines the intricate technical aspects and challenges of establishing an inter-satellite optical communication link. It also focuses on variations in LEO access times, LEO to GEO and GEO to ground connections. Furthermore, the investigation delves into the precise determination of link margin that accounts for various factors affecting the transmitted signal's propagation in free space, including free space path attenuation, pointing losses and system noise losses. The findings and insights derived from this research are fundamental in advancing the understanding of free space optical communication technology for inter-satellite links. Enhanced data rates, low latency, and improved reliability are some of the improved deliverables this research provides us.

The paper focuses on design, development and implementation of a 45° phase shifter at Ka-band, 25.5 GHz to 27 GHz, for high data rate transmission for space applications. The phase shifter is designed using both reflection type and load line type topologies. Both the phase shifters are simulated, fabricated and tested. The load line type of phase shifter resulted in better performance compared to the reflection type. Achieved phase shift of 45° and amplitude imbalance of 0.2 dB, low insertion loss of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.5 \text{dB}\pm 0.2\text{dB}$</tex> in the required frequency band.

In this paper, a light weight, dual linearly polarized stacked microstrip reflectarray antenna for Ku-Band satellite on-orbit telemetry, tracking and commanding (TT&C) application that can be placed on Earth Viewing (EV) side of GEO satellite is presented. Unit element that will suit both downlink and uplink frequencies which are separated by ratio of 1.15 is chosen and reflectarray is generated based on optimal phase value compensation technique. Simulated peak gain of 26.7 dB is achieved for downlink frequency with cross polarization of 28 dB. At uplink frequency, simulated peak gain of 27.3 dB is achieved with cross polarization of 28 dB.

In this paper, a light weight, linearly polarized normal mode helix antenna for inter-satellite communication application in UHF band is discussed. Near isotropic radiation pattern except along the boresight axis makes it suitable for inter satellite communication at shorter distances consuming low power. It has return loss better than 15dB and minimum coverage gain of -5dBi over ±50° with respect to axis perpendicular to boresight axis.

Geo Imaging Satellite’s orientation is subjected to several biases for pointing on-board camera towards the region of interest. In-orbit telemetry of Geo Imaging Satellite is phase modulated on Ku-Band carrier and downlinked through Phased Array Antenna (PAA) to ensure that the coverage is limited to the Indian national region in spite of spacecraft biases. As PAA is not compatible with the phase modulation based tone ranging performed through Ku-Band TT&C Transponder in geo spacecrafts, a High Gain Antenna (HGA) is incorporated to support ranging operations in Ku-Band. Ku- Band downlink signal is to be switched to HGA during ranging operations by means of a commandable switching network.This paper describes the design, development and realization of Ku-Band attenuator network that is designed to route downlink signals to PAA or HGA based on the mission requirements while ensuring that there are no single point failures. It includes the details of frequent ranging requirement for Geo Imaging Satellite compared to other spacecrafts, need for HGA, system level architecture of Ku-Band attenuator network, circuit level design & simulation, hardware realization and performance of flight hardware.

Circularly polarized quadrifilar helix antennas are designed at X-Band for Low earth orbit Data transmission and TT&C applications. The proposed antennas provide a broader coverage on earth compared to that of circularly polarized patch antennas with return loss better than 15dB for around 10% bandwidth. For Data antenna, +/- 62 degrees coverage gain of 1dBi, minimum peak gain of 6dBi is achieved and for TT&C antenna, +/- 90 degrees coverage gain of 0dBi is achieved. The larger coverage area implies seamless data reception for longer periods of time. In this letter, design and experimental validation of impact on the radiation pattern with change in the take-off angle of the filars along with fabrication criticalities are discussed in detail.

Small steerable reflector antennas are of prime importance on scientific satellites to transmit the experimental data towards the ground station. However, achieving higher efficiency and having minimum dynamic envelop during the rotation is a challenging task. In this paper, design of a reflector with diameter 28λ at X-band in Axially Displaced Ellipsoid (ADE) geometry is illustrated. It is experimentally demonstrated that the antenna can provide an aperture efficiency of more than 70% which is significant especially considering the small diameter of the reflector. This efficiency has been achieved by proper choice of design parameters and material used. Thermo-plastic based struts are used to support sub-reflector from the feed.

Quantum Key Distribution (QKD) has emerged as a revolutionary technique for ensuring unconditional secure communication between two parties by exploiting the fundamental principles of quantum mechanics. At present, satellite based QKD is the only choice for distributing secure keys over larger distances, as fiber-based and terrestrial free-space systems have limited range. In this paper, the system engineering study and performance analysis for the satellite based QKD system is presented by carrying out the simulations for different orbits and orbital parameters. This work also presents the methodology to estimate the quantum keys generated for satellite to ground scenario across some of the suitable locations across Indian mainland. The orbital parameters of a LEO orbit are optimized to obtain a required accumulated key of more than 1 Mbits per day across all the ground stations. Two standard operating wavebands viz. 785 nm and 1550 nm, most suitable for quantum communication, are considered for both daylight and eclipse conditions and the difference in accesses, access gaps, access duration, key generation and key accumulation is obtained. A typical use-case scenario analysis for utilization of accumulated key over a year is also presented.

For satellite based optical communication network, acquisition, tracking and pointing (ATP) is inevitable as the laser beam is innately narrow. Performance of ATP system will ultimately be the yardstick of success of optical communication terminal. The ultimate target of this development is to acquire, point and track two remotely located optical communication terminal moving with a relative velocity or spacecraft moving against a stationary ground station. In this work we have demonstrate stare-scan approach of ATP. The transmitting beacon is stationary and receive station has spiral tracking provision for acquiring. Here we have developed an automated mechanism to acquire a terminal using quad detector and gimbal. The experimental detail and measurement at different modes are explained. The future scope of work is also defined.

Transmission of telemetry data to the ground station is critical for all spacecraft missions. The allocated spectrum for Telemetry downlinks of GEO spacecraft has been overcrowded in recent years due to a steady increase in the number of GEO missions. Implementation of Code Division Multiple Access (CDMA) systems in the same bandwidth is proposed as a solution to overcome this problem. To prove the compatibility of the proposed scheme with existing telemetry downlinks in the same bandwidth, a real-time data acquisition-based experiment has been carried out with hardware in radiated mode to simulate in-orbit conditions in an anechoic chamber. This paper explains the estimation of interference, test methodology & results and recommends the optimal configuration of the ground receiver for demodulation of the CDMA signal in the presence of interference from multiple narrow-band phase modulated signals. The proposed configuration has been proved successful in a recent GEO spacecraft mission.

In polarization-encoded satellite-to-ground quantum communication, the on-board transmitter and a receiver at the optical ground station encode and decode linearly polarized quantum bits for execution of quantum key distribution (QKD) protocol. Dynamic polarization tracking and compensation is a key technique in realizing such a space based QKD system. In this work, we report the design and development of motorized waveplate based active polarization compensation system. Integrated testing and validation with Decoy-state enabled BB84 QKD testbed over ~5m free space channel and quantum link polarization extinction ratio (PER) ≥ 100:1 has been carried out successfully. Satellite motion was emulated as a continuous 1°/s polarization basis rotation at the Quantum Tx output. When active polarization compensation system was enabled, sazimuth (Az.) deviation was measured with ±1.5° accuracy. During this experiment, overall signal QBER was consistently maintained at <3.5% with secure key generation rate of ~1350kbps, proving the suitability of this scheme for realistic satellite-to-ground QKD scenarios.