EOS-04
EOS-04, also known as RISAT-1A, is an Earth observation satellite developed by the Indian Space Research Organisation (ISRO) as a follow-on to the RISAT-1 mission, equipped with a C-band Synthetic Aperture Radar (SAR) payload to provide high-resolution imaging under all weather and day-night conditions.[1][2][3] Launched on 14 February 2022 aboard the Polar Satellite Launch Vehicle (PSLV-C52) from the Satish Dhawan Space Centre in Sriharikota, India, EOS-04 operates in a sun-synchronous orbit at an altitude of approximately 529 km with a 97-degree inclination, enabling frequent revisits for monitoring dynamic Earth processes.[1][2][3] The satellite has a launch mass of 1,710 kg and generates 2,280 W of power, supporting its primary SAR instrument operating at 5.35 GHz frequency with multiple polarization modes (HH, VV, HV, VH) for versatile data collection.[2][3][4] The mission objectives focus on applications in agriculture, forestry, soil moisture assessment, hydrology, and disaster management, particularly flood mapping and surveillance, by delivering images with resolutions up to 1 meter in spotlight mode and swath widths ranging from 15 km to 223 km.[2][3][5] As part of ISRO's Earth Observation Satellite series, EOS-04 enhances India's remote sensing capabilities, providing data through X-band transmission to ground stations and offline access via the National Remote Sensing Centre (NRSC), with the satellite operational as of November 2025 and a mission life of 10 years (until approximately 2032).[2][5][4][6]Background and Development
Historical Context
The Radar Imaging Satellite (RISAT) series represents a pivotal advancement in India's all-weather Earth observation capabilities, initiated by the Indian Space Research Organisation (ISRO) to address limitations of optical satellites in cloud-covered or nighttime conditions. The program began with RISAT-2, an X-band synthetic aperture radar (SAR) satellite launched in April 2009 in collaboration with Israel Aerospace Industries, primarily for strategic surveillance. This was followed by RISAT-1, ISRO's first indigenous C-band SAR satellite, launched on April 26, 2012, aboard PSLV-C19, which enabled applications in agriculture, forestry, and disaster management through high-resolution imaging independent of weather.[7][8][9] Following RISAT-1's decommissioning on March 31, 2017, after a fragmentation event on September 30, 2016, that caused power system anomalies and rendered the satellite non-operational, ISRO identified the urgent need for follow-on missions to maintain continuity in SAR data acquisition. The incident highlighted vulnerabilities in satellite longevity and spurred enhancements in design reliability for subsequent vehicles. In response, ISRO accelerated development of a direct successor, initially designated RISAT-1A, to restore and expand C-band SAR coverage for national resource monitoring and security needs.[7][1] As part of ISRO's reorganization of its Earth Observation Satellite (EOS) series in the late 2010s, aimed at streamlining nomenclature across optical and radar platforms, RISAT-1A was renamed EOS-04 to align with the broader EOS framework, which integrates diverse remote sensing missions under a unified numbering system. The satellite was announced for development in 2019, with integration activities commencing in 2020 at ISRO's facilities in Bengaluru. Assembly, integration, and testing were conducted by a consortium led by Alpha Design Technologies Ltd. under ISRO oversight, marking increased private sector involvement in spacecraft realization. The total mission cost was approximately ₹490 crore (about US$65 million in 2023 values), reflecting efficient indigenous production.[10][11][12]Mission Objectives
The EOS-04 mission, operated by the Indian Space Research Organisation (ISRO), aims to deliver high-resolution synthetic aperture radar (SAR) imagery capable of penetrating clouds and operating continuously during day or night, enabling reliable Earth observation under all weather conditions. This core objective supports uninterrupted monitoring of terrestrial and oceanic features, addressing limitations of optical satellites in adverse environments.[4][5] Specific aims include facilitating applications in agriculture through crop assessment and monitoring, forestry via deforestation tracking and biomass estimation, soil and hydrology with moisture mapping and parameter retrieval for water resource management, and disaster management by detecting floods, cyclones, and landslides. The mission emphasizes the collection of hybrid polarimetric data—transmitting circularly polarized signals and receiving linearly polarized ones—to improve target discrimination and classification accuracy in these domains. Data from EOS-04 integrates into ISRO's broader Earth observation network, with processed products disseminated through the National Remote Sensing Centre's (NRSC) Bhoonidhi portal for user access and analysis.[5] Planned for a 10-year operational lifespan, EOS-04 builds on the heritage of the RISAT series by incorporating agile beam steering for expanded coverage and multi-mode SAR operations to enhance imaging flexibility and revisit frequency. These advancements ensure sustained data continuity and superior performance compared to predecessors like RISAT-1.[4][5]Spacecraft Design
Bus and Subsystems
The EOS-04 spacecraft utilizes a modular bus design derived from the RISAT-1 satellite, ensuring compatibility with the C-band synthetic aperture radar (SAR) payload while providing robust support for Earth observation missions. The overall configuration features a cylindrical structure approximately 3.4 m in height, with deployable solar panels that extend to generate power and maintain thermal balance. At launch, the spacecraft mass is 1,710 kg, optimized for integration with the PSLV launch vehicle and a projected mission life of 10 years.[13][5] The power subsystem delivers up to 2,280 watts through two deployable solar arrays and lithium-ion batteries, drawing on the proven architecture from the RISAT-1 bus to handle the high energy demands of radar operations. This unregulated 70 V bus supports all onboard systems, including peak loads during imaging passes, with end-of-life generation capacity around 2,298 W under summer solstice conditions. Attitude and orbit control is achieved via three-axis stabilization, employing four reaction wheels (50 Nms each), star sensors, inertial reference units, and hydrazine thrusters for precise pointing accuracy of 0.05° and drift rates below 3.0 × 10^{-4} °/s, essential for maintaining SAR beam alignment.[13][5] Communication subsystems facilitate high-rate data transfer using an X-band downlink capable of up to 320 Mbps per polarization channel (totaling 640 Mbps for dual channels), supported by a 1.4 TB solid-state recorder and a dual-gimbal antenna for real-time payload data transmission to ground stations. Telemetry and command operations occur via S-band links. The propulsion system employs a monopropellant hydrazine setup in blow-down mode, with eight 11 N thrusters for orbit maintenance and one central 11 N thruster for major maneuvers, ensuring long-term stability in sun-synchronous orbit. Thermal management relies on passive techniques, including optical solar reflectors (OSR), multilayer insulation (MLI), specialized paints, heat pipes, and electric heaters to regulate temperatures for the radar electronics and other components amid varying solar and Earth radiation inputs.[5][7]Payload Specifications
The payload of EOS-04 is an active C-band Synthetic Aperture Radar (SAR) operating at a center frequency of 5.35 GHz, designed for all-weather, day-and-night imaging with hybrid polarimetric capabilities that include transmit circular and receive linear polarizations, enabling configurations such as HH+HV or VV+VH.[2][5] This hybrid polarimetry supports single, dual, circular, and full polarimetric modes across all imaging operations.[14] The SAR instrument operates in multiple imaging modes to provide flexibility in resolution and coverage: spotlight mode achieves 1 m azimuth resolution over a 10 km swath, suitable for high-detail observations; stripmap modes (fine resolution FRS-1 and FRS-2) deliver 3 m resolution across 20–25 km swaths; and ScanSAR modes (medium resolution MRS and coarse resolution CRS) offer 33 m and 50 m resolutions with swaths up to 160 km and 223 km, respectively.[14][7] Key features of the payload include an agile antenna with electronic beam steering, supporting incidence angles from 12° to 55° and a right-looking geometry for efficient data acquisition.[5][15] It also accommodates circular polarization transmission to enhance scattering characterization. The onboard data handling subsystem features a solid-state recorder with 1.4 Tb capacity and supports real-time X-band downlink at up to 640 Mbps, with radiometric accuracy within ±1 dB.[5][15]| Imaging Mode | Resolution (Azimuth x Range) | Swath Width |
|---|---|---|
| Spotlight (HRS) | 1 m x 2 m | 10 km |
| Stripmap FRS-1 | 3 m x 2 m | 25 km |
| Stripmap FRS-2 | 3 m x 4 m | 20–25 km |
| ScanSAR MRS | 33 m x 8 m | 160 km |
| ScanSAR CRS | 50 m x 8 m | 223 km |