SARAL
SARAL (Satellite with ARgos and ALtiKa) is a joint Indo-French satellite mission launched on February 25, 2013, by the Indian Space Research Organisation (ISRO) from Sriharikota using the PSLV-C20 rocket, in collaboration with the French space agency CNES, to conduct altimetric observations for oceanographic research.[1][2] The mission operates in a sun-synchronous orbit at approximately 800 km altitude with a 35-day repeat cycle, enabling precise, repetitive global measurements of sea surface height, significant wave heights, and wind speeds essential for monitoring ocean circulation, topography, and surface elevation.[3][4] The satellite's primary payload, AltiKa, is a Ka-band radar altimeter developed by CNES that provides higher resolution data compared to previous C-band or Ku-band instruments, while the ARGOS-3 system, also from CNES, facilitates the collection and location of environmental data from buoys, balloons, and other platforms worldwide.[2][3] Built on ISRO's Indian Mini Satellite-2 (IMS-2) bus with a launch mass of 407 kg and a designed lifespan of five years, SARAL has exceeded expectations and remains operational as of 2025, contributing to applications in operational oceanography, climate modeling, and coastal management.[4][5] Its data supports international efforts like the Global Ocean Observing System (GOOS) by improving understanding of phenomena such as El Niño, sea level rise, and marine weather forecasting.[3]Background and development
Mission objectives
The SARAL mission's primary objective is to deliver continuous, high-precision altimetric measurements of sea surface height using the Ka-band AltiKa altimeter, enabling detailed studies of ocean circulation, significant wave heights, wind speeds, and topography with enhanced resolution compared to previous C-band systems.[3][2] This focuses on advancing operational oceanography, including mesoscale variability (wavelengths of 50–500 km and periods of days to a year), coastal processes, and mean sea level rise tracking.[6][7] Secondary objectives include augmenting global environmental monitoring through the ARGOS system, which collects data from platforms worldwide for applications in meteorology, climate, and marine biodiversity, while the DORIS receiver and laser retroreflector array ensure precise orbit determination to support altimetry accuracy.[2][3] Specific aims encompass bridging the observational gap between the Envisat and Sentinel-3 missions by re-occupying their ground tracks, complementing Jason-2 data for improved mean sea level monitoring, and contributing to marine meteorology and ecosystem studies.[3][2] Technically, SARAL targets an along-track resolution of 7–8 km for altimetry measurements, facilitated by the Ka-band's smaller footprint, and operates in a sun-synchronous orbit to achieve coverage over more than 80% of Earth's ice-free oceans with a 35-day repeat cycle.[8][6] This joint effort between ISRO and CNES underscores collaborative Earth observation goals.[2]International cooperation
The SARAL mission represents a key bilateral collaboration between the Indian Space Research Organisation (ISRO) and the French Centre National d'Études Spatiales (CNES), formalized through a Memorandum of Understanding (MOU) signed on February 23, 2007. This agreement outlined the joint development and operation of the satellite to advance ocean altimetry observations, leveraging complementary expertise from both agencies.[2][9] Under the MOU, responsibilities were clearly divided to optimize resource allocation. ISRO provided the satellite platform using its minisatellite bus based on the IMS-2 design, handled the launch via the Polar Satellite Launch Vehicle (PSLV), and managed the Indian ground segment for satellite operations, telemetry, tracking, command, platform data processing, archiving, and distribution. CNES, in turn, developed and supplied all payloads—including the ALtiKa altimeter, ARGOS-3 system, DORIS receiver, and Laser Retroreflector Array (LRA)—while overseeing the French ground segment for payload data processing, archiving, and international distribution, along with providing scientific expertise and shared project management.[10][2] This partnership built on a longstanding history of Indo-French space cooperation, which dates back to the 1960s but gained momentum in the early 2000s through missions like Megha-Tropiques, an Indo-French satellite launched in 2011 for tropical atmospheric studies. SARAL emerged as a dedicated, cost-effective altimetry initiative, proposed by ISRO in 2002 initially as part of Oceansat-3 but evolved into an independent minisatellite by 2006 due to scheduling needs; it served as a critical gap-filler following the unexpected failure of the European Space Agency's Envisat mission in April 2012.[2][11][12] Development commenced in 2007 shortly after the MOU, culminating in the satellite's launch on February 25, 2013, with a planned nominal mission duration of five years to ensure sustained data collection for oceanographic research.[2][13]Spacecraft and orbit
Design specifications
SARAL is a mini-satellite utilizing the Indian Mini Satellite-2 (IMS-2) platform developed by the Indian Space Research Organisation (ISRO), featuring a three-axis stabilization system for precise attitude control.[1][2] The platform employs an aluminium honeycomb sandwich structure designed for satellites in the 400-450 kg class, providing modular integration for payloads with housekeeping functions such as propulsion, power management, and data handling.[2][14] The satellite has a launch mass of 407 kg, with a body configuration measuring approximately 1.62 m in length, 1.2 m in width, and 1.897 m in height, including deployable solar arrays for enhanced power generation.[1][15] The power subsystem consists of two deployable solar panels that generate an average of 906 W, supplemented by a 46.8 Ah lithium-ion battery pack to support operations during eclipse phases.[1][2] Key subsystems include an attitude and orbit control system (AOCS) equipped with star trackers, gyroscopes, reaction wheels, and mono-propellant thrusters for maintaining three-axis stabilization and performing orbit adjustments.[2][16] Telemetry, tracking, and command functions are handled via S-band links, while payload data is transmitted at 32 Mbps using X-band for high-rate downlink.[8][2] The platform's thermal control system and environmental hardening ensure reliability over a nominal 5-year mission life, withstanding launch vibrations up to the specifications of the PSLV vehicle and exposure to space radiation in low Earth orbit.[1][2] This design supports a sun-synchronous orbit configuration for consistent observational geometry.[14]Orbital parameters
The SARAL satellite operates in a sun-synchronous, near-circular, dawn-dusk orbit with a local time of ascending node (LTAN) at 6:00 AM, enabling consistent solar illumination conditions for its instruments during global ocean observations.[17][2] This orbit type ensures repeatable ground tracks, facilitating long-term monitoring of sea surface height, waves, and winds.[18] Key orbital parameters during the initial repetitive phase (February 2013 to July 2016) include a mean altitude of 800 km, an inclination of 98.55°, an eccentricity of 0.000165, and a nodal period of 100.59 minutes.[18] The orbit features a 35-day exact repeat cycle comprising 501 revolutions and 1002 passes, with a ground track separation of 75 km at the equator, providing complementary coverage to missions like Jason-2.[14][18] This configuration supports approximately 14 orbits per day, achieving over 99.5% coverage of ocean surfaces.[2][7]| Parameter | Value | Description/Source |
|---|---|---|
| Altitude (mean) | 800 km | Nominal repetitive phase[18] |
| Inclination | 98.55° | Sun-synchronous polar orbit[18] |
| Eccentricity | 0.000165 | Near-circular trajectory[18] |
| Nodal Period | 100.59 minutes | Time per orbit[18] |
| Repeat Cycle | 35 days (501 rev.) | Exact ground track repetition[14] |
| Ground Track Spacing | 75 km (equator) | Inter-track distance for coverage[18] |