Sunday, April 11, 2010

ISRO GSAT-4 : The Satellite details

GSAT-4 is the nineteenth geostationary satellite of India built by ISRO and fourth in the GSAT series. Its three GSAT predecessors were launched by GSLV during 2001, 2003 and 2004 respectively. After its commissioning, GSAT-4 will join the group of India’s eleven operational geostationary satellites.
Some of the new Technologies being tested in GSAT-4 include:
• Electric Propulsion System
• Bus Management Unit
• 1553 Bus for Data Communication
• Miniaturised Dynamically Tuned Gyros
• 36 AH Lithium Ion Battery
• 70 V Bus for Ka band TWTAs
Besides, the Technology Experiments carried onboard GSAT-4 are:
On-board Structural Dynamics Experiment to monitor on-orbit structural dynamic behavior of the satellite during various phases of the mission corresponding to various flight/mission and satellite configurations
Velocity Measurement Package to measure the incremental velocity imparted to GSAT-4 during LAM firings and station keeping manoeuvres Thermal Control Coating Experiment to study the degradation characteristics of thermal control materials in space environment with time
The cuboid shaped GSAT-4 has a lift-off weight of 2220 kg of which propellants weigh 1155 kg and the dry mass of the satellite is 1063 kg. GSAT-4 structure is based on ISRO’s standard I-2000 bus. The two solar arrays (each with two panels) of GSAT-4 together generate about 2800 W of power.
GSAT-4 is the first geostationary satellite of ISRO to employ integrated Bus Management Unit (BMU) which combines the functions of Telemetry, Telecommand, Sensor Electronics and Control Electronics. BMU acts as the brain of GSAT-4.
Like its INSAT and GSAT predecessors, GSAT-4 has a conventional chemical propulsion system for orbit raising and station keeping manoeuvres. Besides, GSAT-4 is the first ISRO satellite having Electric Propulsion Sytem (EPS) to perform North South Station Keeping. The satellite will demonstrate the capabilities and advantages (very high Isp, meaning efficiency) of EPS employing state-of-the-art stationary plasma thrusters.
GSAT-4 at a glance:
Structure : I-2000
Overall Size (m) : 2.4 X 1.6 X 1.5
Liftoff mass (kg) : 2220
Generated Power (W) : 2760
Payload Power (W) : 1785
Propulsion (Chemical) : MMH as fuel and MON-3 as Oxidiser
Propulsion (Electric) : Xenon based stationary plasma thrusters (four)
Mission Life : > 7 years
Orbital Location : 82 deg E longitude in GSO
GSAT-4 Payloads:
GSAT-4 carries communication as well as navigation payloads. They are:
• Ka – band bent pipe and regenerative transponder
• GAGAN payload operating in C, L1 and L5 bands
Of these, Ka-band Transponder operates on 30 GHz uplink and 20 GHz downlink. This payload provides 8 spot beams covering entire India. Spot
beams allow frequency reuse through geographical separation. The payload also comprises beacon transmitters in 30 GHz and 20 GHz to facilitate propagation studies. Ka band payload also has the facility of RF tracking and antenna pointing.
New technologies incorporated in Ka-Band Payload include Multiple Spot Beams (eight) with Frequency Reuse, Double Frequency Conversion, Very High Stability Local Oscillator and Onboard Base band Processing and Switching.
The advantages of using a regenerative transponder are many. It allows the use of smaller ground terminals at the user end by incorporating efficient processing on-board the satellite. Regenerative transponder also increases system flexibility by facilitating network interconnection on-board satellite without the use of a hub, which in turn results in increased capacity, reduced errors and greater throughput.
Each of the 8 beams will have 8 narrow band channels of 64 Kbps and one wide band channel of 2048 Kbps. Interconnectivity between the narrow band channels within the same beam or with any of the other beams is possible.
Similarly, interconnectivity is possible with wide band channels between any of the beams or all beams can be used together in broadcast mode. Another objective of this payload is to develop advanced Digital Signal Processor based subsystems, implement various interface protocols and verify interconnectivity of terminals between multiple beams.
The intended applications for Ka band include Wide band Multimedia Services, Mobile Information System, SPACE LAN, e-Commerce and High Bandwidth Internet.

The second payload carried by GSAT-4 is GAGAN, which is a navigational payload operating in C, L1 and L5 bands. Essentially, the GAGAN payload of GSAT-4 forms the space segment of GAGAN Satellite Based Augmentation System (SBAS) developed by India. GAGAN stands for GPS Aided Geo Augmented Navigation. Through SBAS, the positional information from the GPS satellites is improved by a network of ground based receivers and the same is made available to any user through geostationary satellites.
GAGAN is a Wide Area Differential Global Positioning System (WADGPS) employing a geostationary satellite overlay system. It was conceived to provide a position accuracy of better than 7.6 metre needed for the precision landing of civilian aircraft. The GAGAN system consists of the Space Segment, the Ground Segment and the User Segment. The GPS
and Geostationary overlay system form the Space Segment while the Ground Segment comprises Indian Reference Stations (INRES), Indian Master Control Centre (INMCC) and Indian Land Uplink Stations (INLUS). The User Segment consists of SBAS receivers capable of receiving GPS signals and corrections from the Geostationary satellite.
In the GAGAN architecture, Data from INRES is transmitted to INMCC. This data is processed by INMCC and sent to INLUS. INLUS transmits the corrected GPS information and time synchronisation signal to a geostationary satellite. The satellite then transmits a GPS like signal on L-band frequency. Accuracy of the order of 3 meter horizontal and 4 meter vertical is feasible in such a system.
Thus, GAGAN navigation payload of GSAT-4 receives the correction signals sent by Indian Land Uplink Stations in C-band and translates these into GPS L1 and L5 band signals and transmits these navigation signals. These signals can be received by GPS SBAS receivers, thus enabling them to get a highly accurate and reliable navigational fix.
The Technology Demonstration Phase(TDS) of GAGAN was successfully completed in August 2007. As part of the TDS, eight Indian Reference Stations (INRES) were installed at eight Indian airports. They are linked to the Indian Master Control Centre (INMCC) located at Kundanhalli near Bangalore. In June 2009, the final operational phase (FOP) of GAGAN was initiated.

Geostationary Satellites of India : Ushering in a Revolution
GSAT-4 is the nineteenth Indian geostationary satellite built by ISRO. In the past two and a half decades, India’s geostationary satellites have revolutionised the country’s telecommunications, TV broadcasting and Weather Monitoring sectors. More recently, ISRO’s INSAT and GSAT series of satellites circling the Earth in the 36,000 km high geostationary orbit have brought in a revolution in India’s healthcare and educational sectors. Besides, they have been instrumental in taking the benefits of space technology directly to the doorsteps of rural India through Village Resource Centres (VRCs). Today, geostationary satellites are an integral part of India’s national infrastructure. The country has about 200 communication transponders in geostationary orbit that operate in S, C, Extended C and Ku bands.

INSAT system has become a major catalyst for the expansion of television coverage in India. Satellite television (DTH) now covers 100% area and 100% population. The terrestrial coverage is over 65 percent of the Indian landmass and over 90 percent of the population. Around
30 million of TV Receive Only (TVRO) terminals were distributed and operational all over India by various DTH service providers.
Similarly, a total of about 650 Earth stations and nearly 120,000 VSAT terminals are operating in INSAT telecommunications network providing 9600 two-way speech circuits. Besides, Mobile Communication Services are also offered by the INSAT system.

Two of India’s geostationary satellites – INSAT-3A and KALPANA-1 – are also providing meteorlogical services by sending weather imagery and relaying meteorological data collected by automatic Data Collection Platforms established in various parts of the country. At the same time, the Search and Rescue Transponder onboard INSAT-3A has picked up many distress signals and thus has enabled the saving of many lives through timely search and rescue operations.

India has a dedicated geostationary communication satellite called EDUSAT for educational field. Currently, about 52,000 classrooms from primary to university level as well as those in the non formal educational sector are in the EDUSAT network facilitating the extension of quality education to students in semi urban and rural areas.
Additionally, India’s geostationary satellites have facilitated the extension of quality healthcare services to rural India. Presently, 306 remote/rural/district/medical college hospitals and 16 Mobile Telemedicine units are connected to 60 specialty hospitals in the ISRO telemedicine network.
This apart, as per the relatively recent Village Resource Centre (VRC) initiative, India’s geostationary satellites have been instrumental in taking the benefits of space technology directly to the Indian villages by providing the much needed connectivity.

In this context, coupled with the growing demand for geostationary communication transponders, the launch of GSAT-4, which is a technology demonstrator for advanced satellite communications, acquires added significance.

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