We’ve had a good time discussing satellites on and off for a while now. India’s satellite program in general. And GSAT 16 in particular. This was prompted by the recent news that the 2nd raising manoeuvre of GSAT 16 took place on Dec-9.Here are some notes.
India’s satellite program
GSAT-16 was launched as part of the INSAT program. INSAT stands for Indian National Satellite System.
INSAT is a series of geostationary satellites that that ushered in, and fulfilled the demand of the telecommunications, TV and radio broadcasting, and meteorological sectors.This is different from India’s other major satellite program – IRNSS – Indian Regional Navigation Satellite System.
INSAT is launched by ISRO. It is a joint venture of the Department of Space, Department of Telecommunications, India Meteorological Department, All India Radio and Doordarshan.
It was commissioned in 1983.
GSAT 16 is the 25th satellite launched by INSAT,and the 11th functional one.
Cheat sheet for GSAT 16
Launch date: 6-Dec-2014
2nd raising manoeuvre: 9-Dec-2014
Geo Stationary Orbit: 12-Dec-2014
Mission duration : 12 years
Launch mass: 3,100 kilograms
Power: 5.6 kilowatts from solar array
Rocket: Ariane 5 ECA
Launch site: Kourou ELA-3, Guiana Space Centre, French Guiana
Orbital parameters: Geocentric, Geostationary, 55° East Longitude
Insurance Amount: Rs. 865 crore
We discussed why geostationary satellites are placed at 36000 km in space from earth.
The Launch site
The Guiana Space Centre or, more commonly, Centre spatial guyanais (CSG) is a French and European spaceport near Kourou in French Guiana, at the north of South America. Operational since 1968, it is particularly suitable as a location for a spaceport as it fulfills the two major geographical requirements of such a site:
- it is quite close to the equator, so that the spinning earth can impart some extra velocity to the rockets for free when launched eastward, and
- it has uninhabited territory (in this case, open sea) to the east, so that lower stages of rockets and debris from launch failures cannot fall on human habitations.
The European Space Agency (ESA), the French space agency CNES (National Centre for Space Studies), and the commercial Arianespace company conduct launches from Kourou.This is the spaceport used by the ESA to send supplies to the International Space Station using the Automated Transfer Vehicle.
The location was selected in 1964 to become the spaceport of France. In 1975, France offered to share Kourou with ESA. Commercial launches are bought also by non-European companies. ESA pays two thirds of the spaceport’s annual budget and has also financed the upgrades made during the development of the Ariane launchers.
The near-equatorial launch location provides an advantage for launches to low-inclination (or geostationary) Earth orbits compared to launches from spaceports at higher latitude. For example, the eastward boost provided by the Earth’s rotation is about 463 m/s (1,035 miles per hour) at the Guiana Space Centre versus about 406 m/s (908 miles per hour) at the United States east coast Cape Canaveral and Kennedy Space Center spaceports which are at 28°27’N latitude in Florida. The proximity to the equator also makes maneuvering satellites for geosynchronous orbits simpler and less costly.
|The payload for any communication satellite is transponders.
GSAT 16 has 42 transponders as follows:
Each has 36 megahertz bandwidth.
What is a transponder ?
A transponder in a communications satellite is a communications channel between the receiving and the transmitting antennas.
A transponder is typically composed of:
- An input band limiting device (a band pass filter)
- An input low-noise amplifier (LNA), designed to amplify the (normally very weak, because of the large distances involved) signals received from the earth station
- A frequency translator (normally composed of an oscillator and a frequency mixer) used to convert the frequency of the received signal to the frequency required for the transmitted signal
- An output band pass filter
- A power amplifier (this can be a traveling-wave tube or a solid state amplifier)
Most communication satellites are radio relay stations in orbit, and carry dozens of transponders, each with a bandwidth of tens of megahertz.
Most transponders operate on a “bent pipe” principle, sending back to earth of what goes into the conduit with only amplification and a shift from uplink to downlink frequency.
With data compression and multiplexing, several digital video and audio channels may travel through a single transponder on a single wideband carrier.
|Original analog video only had one channel per transponder.
Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder, or using landlines to send it to an earth station for multiplexing with other stations.
NASA distinguishes between a “transponder” and a “transceiver”, where the latter is simply an independent transmitter and receiver packaged in the same unit, and the former derives the transmit carrier frequency from the received signal.
IEEE Waveguide bands
e.g. – C band – This corresponds to roughly between 4 and 8 GHz. (microwave)
The C band is used for long-distance radio telecommunications, satellite communications transmissions, and some weather radar systems.
For satellite communications, the microwave frequencies of the C-band perform better under adverse weather conditions in comparison with Ku band (11.2 GHz to 14.5 GHz) microwave frequencies. The adverse weather conditions, collectively referred to as rain fade, all have to do with moisture in the air, including rain and snow.
The IEEE C-band is a portion of the electromagnetic spectrum in the microwave range of frequencies ranging from 4.0 to 8.0 gigahertz (GHz);however, this definition is the one followed by radar manufacturers and users, not necessarily by microwave radio telecommunications users.
Nearly all C-band communication satellites use the band of frequencies from 3.7 to 4.2 GHz for their downlinks, and the band of frequencies from 5.925 to 6.425 GHz for their uplinks.
The C-band communication satellites typically have 24 radio transponders spaced 20 MHz apart, but with the adjacent transponders on opposite polarizations.  Hence, the transponders on the same polarization are always 40 MHz apart. Of this 40 MHz, each transponder utilizes about 36 MHz. (The unused 4.0 MHz between the pairs of transponders acts as “guard bands” for the likely case of imperfections in the microwave electronics.)
The C-band is primarily used for open satellite communications, whether for full-time satellite TV networks or raw satellite feeds.
The satellite communications portion of the C-band is highly associated with television receive-only satellite reception systems, commonly called “big dish” systems, since small receiving antennas are not optimal for C-band systems. Typical antenna sizes on C-band capable systems ranges from 7.5 to 12 feet (2.5 to 3.5 meters) on consumer satellite dishes, although larger ones also can be used.
The C-band frequencies of 5.4 GHz band [5.15 to 5.35 GHz, 5.47 to 5.725 GHz, or 5.725 to 5.875 GHz, depending on the region of the world] are used for IEEE 802.11a Wi-Fi and cordless telephone applications, leading to occasional interference with some weather radars that are also allocated to the C-band.
Slight variations in the assignments of C-band frequencies have been approved for use in various parts of the world, depending on their locations in the three International Telecommunications Union radio regions. Note that one region includes all of the Americas; a second includes all of Europe and Africa, plus all of Russia, and the third region includes all of Asia outside of Russia, plus Australia and New Zealand. This latter region is the most populous one, since it includes the People’s Republic of China, India, Pakistan, Japan, and Southeast Asia.
C-Band Variations Around The World
|INSAT / Super-ExtendedC-Band||6.725-7.025||4.500-4.800|
GSAT is based on ISRO’s I-3K bus design.
A communication satellite is a consumer item in the realm of spacecraft. They are then made based on template designs.
A satellite bus or spacecraft bus is the general model on which multiple-production satellite spacecraft are often based. The bus is the infrastructure of a spacecraft, usually providing locations for the payload (typically space experiments or instruments).
A bus-derived satellite would be used as opposed to a specially produced satellite. Bus-derived satellites are usually customized to customer requirements, for example with specialized sensors or transponders, in order to achieve a specific mission
A bus typically consists of the following subsystems:
- Command and Data Handling (C&DH) System
- Communications system and antennas
- Electrical Power System (EPS)
- Thermal control
- Attitude Control System (ACS)
- Guidance, Navigation and Control (GNC) System
- Structures and trusses
- Life support (for crewed missions).
References / Further Information
Newspaper articles, ISRO news
Wikipedia page for GSAT-16, Guiana Space Centre, INSAT, C Band, Transponder (satellite communications), Satellite bus, IRNSS,