TECHNOLOGY

VSAT

A very small aperture terminal (VSAT), is a two-way satellite ground station or a stabilized maritime Vsat antenna with a dish antenna that is smaller than 3 meters. The majority of VSAT antennas range from 75 cm to 1.2 m. Data rates typically range from 56 kbit/s up to 4 Mbit/s. VSATs access satellite(s) in geosynchronous orbit to relay data from small remote earth stations (terminals) to other terminals (in mesh topology) or master earth station “hubs” (in star topology).
VSATs are most commonly used to transmit narrowband data (point of sale transactions such as credit card, polling or RFID data; or SCADA), or broadband data (for the provision of satellite Internet access to remote locations, VoIP or video). VSATs are also used for transportable, on-the-move (utilizing phased array antennas) or mobile maritime communications.

Configurations

Most VSAT networks are configured in one of these topologies:

• A star topology, using a central uplink site, such as a network operations center (NOC), to transport data back and forth to each VSAT terminal via satellite,
• A mesh topology, where each VSAT terminal relays data via satellite to another terminal by acting as a hub, minimizing the need for a centralized uplink site,
• A combination of both star and mesh topologies. Some VSAT networks are configured by having several centralized uplink sites (and VSAT terminals stemming from it) connected in a multi-star topology with each star (and each terminal in each star) connected to each other in a mesh topology. Others configured in only a single star topology sometimes will have each terminal connected to each other as well, resulting in each terminal acting as a central hub. These configurations are utilized to minimize the overall cost of the network, and to alleviate the amount of data that has to be relayed through a central uplink site (or sites) of a star or multi-star network.

Constituent parts of a VSAT configuration

• Antenna
• Block upconverter (BUC)
• Low-noise block downconverter (LNB)
• Orthomode transducer (OMT)
• Interfacility link cable (IFL)
• Indoor unit (IDU)

All the outdoor parts on the dish are collectively called the ODU (Outdoor Unit), i.e. OMT to split signal between BUC and LNB. The IDU is effectively a Modem, usually with ethernet port and 2 x F-connectors for the coax to BUC (Transmit) and from LNB (Receive). The Astra2Connect has an all-in-one OMT/BUC/LNA that looks like a QUAD LNB in shape and size which mounts on a regular TV sat mount. As a consequence it is only 500 mW compared with the normal 2W, thus is poorer in rain. Skylogic’s Tooway system also uses an integrated OMT/BUC/LNB assembly named TRIA (Transmit/Receive Integrated Assembly) which is 3W.

Maritime VSAT

Maritime VSAT is the use of satellite communication through a VSAT terminal on a ship at sea. Since a ship at sea moves with the water, the antenna needs to be stabilized with reference to the horizon and True North. The antenna is constantly pointing at the satellite it uses to transmit and receive signals.

DVB (DIGITAL VIDEO BROADCASTING)

Digital Video Broadcasting (DVB) is a suite of internationally accepted open standards for digital television. DVB systems distribute data using a variety of approaches, including:

• Satellite: DVB-S, DVB-S2 and DVB-SHCable: DVB-C, DVB-C2
• DVB-SMATV for distribution via SMATV
• Terrestrial television: DVB-T, DVB-T2Microwave: using DTT (DVB-MT), the MMDS (DVB-MC), and/or MVDS standards (DVB-MS)
• Digital terrestrial television for handhelds: DVB-H, DVB- SH

These standards define the physical layer and data link layer of the distribution system. Devices interact with the physical layer via a synchronous parallel interface (SPI), synchronous serial interface (SSI), or asynchronous serial interface (ASI). All data is transmitted in MPEG transport streams with some additional constraints (DVB-MPEG).

These distribution systems differ mainly in the modulation schemes used and error correcting codes used, due to the different technical constraints. DVB-S (SHF) uses QPSK, 8PSK or 16-QAM. DVB-S2 uses QPSK, 8PSK, 16APSK or 32APSK, at the broadcasters decision. QPSK and 8PSK are the only versions regularly used. DVB-C (VHF/UHF) uses QAM: 16-QAM, 32-QAM, 64-QAM, 128-QAM or 256-QAM. Lastly, DVB-T (VHF/UHF) uses 16-QAM or 64-QAM (or QPSK) in combination with (C) OFDM and can support hierarchical modulation.
Modes and features of latest DVB-x2 system standards in comparison:

SCPC & MCPC

SCPC

Single channel per carrier (SCPC) refers to using a single signal at a given frequency and bandwidth. Most often, this is used on broadcast satellites to indicate that radio stations are not multiplexed as subcarriers onto a single video carrier, but instead independently share a transponder. It may also be used on other communications satellites, or occasionally on non-satellite transmissions.
In an SCPC system, satellite bandwidth is dedicated to a single source. This makes sense if it is being used for something like satellite radio, which broadcasts continuously. Another very common application is voice, where a small amount of fixed bandwidth is required.

Advantages:

• Simple and reliable technology
• Low-cost equipment
• Any bandwidth (up to a full transponder)easy to add additional receive sites (earth stations)
• Usually 64 kbit/s to 50 Mbit/s

MCPC

With multiple channels per carrier (MCPC), several subcarriers are combined or multiplexed into a single bit stream before being modulated onto a carrier transmitted from a single location to one or more remote sites. This uses time-division multiplexing (TDM) as well as frequency-division multiplexing. In digital radio and digital television, an ensemble or other multiplex or multichannel stations can be considered MCPC, though the term is generally only applied to satellites. The major disadvantage of MCPC is that all of the signals must be sent to a single place first, then combined for retransmission — a major reason for using SCPC instead.

Geostationary Satellites

A geostationary satellite is a satellite revolving approximately 22,000 (36,000 km) miles above the earth’s surface in the plane of the equator, with an orbital period equal to one sidereal day. Satellites positioned in a GEO orbit appear to an observer on the surface of the earth as stationary objects in the sky.
Geostationary satellites appear to be fixed over one spot above the equator. Receiving and transmitting antennas on the earth do not need to track such a satellite. These antennas can be fixed in place and are much less expensive than tracking antennas. These satellites have revolutionized global communications, television broadcasting and weather forecasting, and have a number of important defense and intelligence applications.

Inclined Orbit Satellites

A satellite in an inclined orbit is the same as a satellite in a geostationary orbit, with one important difference: Once a satellite has been placed into a geostationary orbit it gradually starts to drift into a figure-eight orbit, due to the gravitational influence of the sun and moon. Keeping a satellite in its geostationary orbit (in order to maintain contact with a fixed antenna) requires activating thrusters powered by large supplies of rocket fuel, activated in bursts every few weeks. Inclined-orbit technology employs a motorized antenna technology to track (i.e. maintain contact with) the satellite’s natural figure-eight orbit. Allowing the satellite to oscillate with the natural gravitational pull dramatically saves on fuel, thus both extending the satellite lifespan and reducing its operational costs.