GSAT-9 Satellite Overview

Photo: ISRO/AVF/ISAC

GSAT-9, also known as South Asia Sat, is a Geostationary Communications Satellite built by the Indian Space Research Organization and operated by Insat to deliver a range of communications services to India and members of the South Asian Association for Regional Cooperation (SAARC). The satellite targets launch in the spring of 2017 atop India’s Geosynchronous Satellite Launch Vehicle Mk.II, conducting its ninth mission.

The 2,230-Kilogram satellite is based on ISRO’s I-2K platform and hosts 12 Ku-Band transponders and a GPS Aided GEO Augmented Navigation (GAGAN) payload. The spacecraft’s all-Ku payload delivers a dedicated India beam and coverage zones over the SAARC states to deliver different communications services including Direct-to-Home Television distribution.

Image: ISRO

GAGAN is a regional satellite-based augmentation system to improve the accuracy of a Navigation Satellite System by providing reference signals that correct for slight variations in GPS signals caused by external influences. The system has been primarily envisioned to enhance air traffic navigation, allowing aircraft to navigate the Indian airspace with an accuracy of three meters and helping with landings in tough weather or challenging terrain.

The project was created in 2008 by the Airport Authority of India in cooperation with ISRO to mark the first step in the transition of India’s airspace to modern communication, navigation and traffic management systems. In architecture, the system is similar to the Wide Area Augmentation System (WAAS) developed for the U.S. by the Federal Aviation Administration and deployed for operation in the early 2000s. India obtained WAAS codes for the L1 and L5 GPS frequency bands from the U.S. Air Force and the Department of Defense soon after the American system went on line.

WAAS Architecture – GAGAN similar – Image: FAA

The system operates by using a series of reference stations on the ground that constantly monitor small variations in the GPS signals caused by ionospheric and atmospheric properties. Data from the reference stations is processed in near real time and then sent to the geostationary GAGAN terminals via dedicated uplink stations. The satellites then broadcast the information needed for GPS signal correction with an overall turnaround time of under five seconds. Per the technical requirements of the program, GAGAN aims for a position accuracy of 7.6 meters, however, application of the system has shown that the typical accuracy will be three meters or better.

GAGAN comprises 15 reference ground stations across the Indian subcontinent, three Navigation Uplink Stations and three Mission Control Centers. The first component of the GAGAN space segment was installed on the GSAT-4 satellite that launched in April 2010 on a GSLV rocket but failed to reach orbit due to a malfunction of the Cryogenic Upper Stage of the vehicle. The first GAGAN payload reached orbit in 2011 aboard the GSAT-8 satellite, followed by GSAT-10 in 2012 and GSAT-15 in 2015 – completing the fully operational constellation.

Photo: ISRO

GSAT-9 will reinforce India’s GAGAN system and add redundancy in case one of the other space terminals encounters technical problems. To date, the system has been used in a variety of areas beyond aviation such as mapping of India’s forest lands at very high accuracy and GAGAN also finds various military applications including missile guidance.

GSAT-9 has been designed for a 12-year service life and will operate from 48 degrees East in Geostationary Orbit from where it can cover the entire Indian and SAARC territory.

GSAT-9 is based on ISRO’s I-2K satellite bus for satellites in the 2,000-Kilogram mass range, suitable for different applications including communications and meteorology. The satellite has a dry mass of 976 Kilograms and a bus size of 2.4 by 1.6 by 1.5 meters. Fueled for launch, the spacecraft weighs 2,230 Kilograms including propellants for its climb to Geostationary Orbit and stationkeeping once arriving in its operational slot. The spacecraft structure makes use of lightweight honeycomb panels with aluminum/CFRP face-sheets connected to a central cylinder that acts as the load-carrying structure, running the entire length of the vehicle and facilitating the propellant tanks.

GSAT-9 hosts a pair of deployable solar arrays covered with Advanced Triple Junction solar cells that generate 3,500 Watts of electrical power. A micro-stepping device is employed as a Solar Array Drive Assembly, pointing the solar arrays toward the sun. The satellite’s central power bus is regulated by a central EPS unit.

GSAT-9 Stowed View – Image: ISRO

GSAT-9 employs a hybrid propulsion system comprising a chemical main propulsion system and an electric propulsion system that will be used for stationkeeping maneuvers in Geostationary Orbit. The chemical propulsion system features two spherical propellant tanks facilitated within the central cylinder of the spacecraft.

GSAT-9 uses Unsymmetrical Dimethylhydrazine as fuel and Mixed Oxides of Nitrogen [MON-3: Nitrogen Tetroxide with 3% Nitric Oxide] as oxidizer that is fed to the engines via propellant lines facilitating pressure regulators. Tank pressurization is accomplished with high-pressure Helium stored at a pressure of 23.5Mpa that is regulated down to under 2MPa for tank pressurization.

The Main Propulsion System is centered around the Liquid Apogee Motor providing 440 Newtons of thrust, operating at a mixture ratio (O/F) of 1.65. It has a nozzle ratio of 160 providing a specific impulse of 3,041N*sec/kg. The engine’s injector is a co-axial swirl element made of titanium while the thrust chamber is constructed of Columbium alloy that is radiatively cooled. Electron welding technique is used to mate the injector to the combustion chamber.

Photo: ISRO

LAM is a robust engine that can tolerate injection pressures of 0.9 to 2.0 MPa, propellant temperatures of 0 to 65°C, mixture ratios of 1.2 to 2.0 and bus voltages of 28 to 42 Volts. The engine is certified for long firings of up to 3,000 seconds and a cumulative firing time of >23,542 seconds.

The Attitude Determination System makes use of Star Trackers and Earth Sensors as the primary source of pointing information to achieve a very stable Earth-pointed orientation. Actuation employs reaction wheels assisted by the 22-Newton thruster when needed for wheel unloading.

The use of electric propulsion on GSAT-9 is considered experimental. The satellite uses a four 18-millinewton Stationary Plasma Thrusters from an 80kg supply of Xenon which will enable the satellite to keep on station in GEO for over a decade.