Very-small-aperture terminal

Very-small-aperture terminal
A 2.5 m parabolic dish antenna for bidirectional satellite Internet access

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 kbps up to 4 Mbps. VSATs access satellites in geosynchronous orbit to relay data from small remote earth stations (terminals) to other terminals (in mesh configurations) or master earth station "hubs" (in star configurations).

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 (utilising phased array antennas) or mobile maritime communications.



The concept of the geostationary orbit was originated by Russian theorist Konstantin Tsiolkovsky, who wrote articles on space travel at the turn of the century. In the 1920s, Hermann Oberth and Herman Potocnik, aka Herman Noordung described an orbit at an altitude of 35,900 kilometers whose period exactly matched the Earth's rotational period, making it appear to hover over a fixed point on the Earth's equator.[1]

Arthur C. Clarke contributed to the understanding of satellites through an article published in Wireless World in October 1945 titled "Extra-Terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?". In this article, Clarke not only determines the orbital characteristics necessary for a geostationary orbit, but also discusses the frequencies and power needed for communications.

Live satellite communication was developed in the sixties by NASA,[2] named Syncom 1-3. It transmitted live coverage of the 1964 Olympics in Japan to viewers in the US and Europe. Soon after, on April 6, 1965 the first commercial satellite was launched into space, Intelsat I, nicknamed 'Early Bird'[3]

The first commercial VSATs were C band (6 GHz) receive-only systems by Equatorial Communications using spread spectrum technology. More than 30,000 60 cm antenna systems were sold in the early 1980s. Equatorial later developed a C band (4/6 GHz) 2 way system using 1 m x 0.5 m antennas and sold about 10,000 units in 1984-85. In 1985, Schlumberger Oilfield Research co-developed the world's first Ku band (12–14 GHz) VSATs with Hughes Aerospace to provide portable network connectivity for oil field drilling and exploration units. Ku Band VSATs make up the vast majority of sites in use today for data or telephony applications. The largest VSAT network (more than 12,000 sites) was deployed by Spacenet and MCI for the US Postal Service.


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.

Future applications

Advances in technology have dramatically improved the price/performance equation of FSS (Fixed Service Satellite) over the past five years. New VSAT systems are coming online using Ka band technology that promise higher bandwidth rates for lower costs.

FSS satellite systems currently in orbit have a huge capacity with a relatively low price structure. FSS satellite systems provide various applications for subscribers, including: telephony, fax, television, high speed data communication services, Internet access, Satellite News Gathering (SNG), Digital Audio Broadcasting (DAB) and others. These systems are applicable for providing various high-quality services because they create efficient communication systems, both for residential and business users.

Constituent parts of a VSAT configuration

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.

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, so that the antenna is constantly pointing at the satellite it uses to transmit and receive signals.


Initially the use of VSAT antennas at sea was for transmission of television signals. One of the first companies to manufacture stabilized VSAT antennas was SeaTel of Concord, California which launched their first stabilized antenna in 1978. Sea Tel dominates the supply of two-way VSAT stabilised antenna systems to the marine market with almost 72 percent of the market in 2007 compared with Orbit’s 17.6 per cent.[4] Initially maritime VSAT was using Single Channel Per Carrier - SCPC technology - which suited large volume users like oil drilling rigs and oil platforms and large fleets of ships from one shipowner sailing within one or few satellite footprints. This changed when the company iDirect launched its IP-based Time Division Multiple Access (TDMA) technology that dynamically allocated bandwidth to each ship for shared bandwidth, lowering the entry level cost for getting maritime VSAT installed, which turned out to be of key importance to small-to mid-sized fleets, and thus to the market acceptance of VSAT.


According to the Maritime VSAT report [5] issued by the Comsys Group their research shows that stabilised maritime VSAT services (not including oil & gas rigs) reached more than $400 million in 2007. In 2010 the COMSYS group released the "2nd Maritime VSAT Report" where the market estimate had increased to $590 million in 2009 with predictions for 2010 at $850 m. The estimated size of the market in terms of vessels eligible to get VSAT was in this report set to in excess of 42.000 with just over 34.000 to go. The major companies market share in terms of number of vessels in service were in 2009 (2007 in parenthesis) according to these reports: Vizada: 17,6% (26.0%), Ship Equip: 11.0% (10.7%), Cap Rock 2.8% (2.9%), MTN 7.5% (6.4%), Stratos - % (3.6%), KVH 5.4% (- %) Elektrikom 4.9% (3.2%), Intelsat 3.4% (- %), Eutelsat 3.1%, NSSL 3.1%, Radio Holland 3.0%, Telemar 3.0%, DTS 2.6% and others accounted for 32.6% (27.7%). Many of the major providers have branded their maritime VSAT offering such that Vizada offers its service through the Marlink division, and the SeaLink and WaveCall products, while Ship Equip calls its offering Sevsat.[6]


Modern VSAT systems are a prime example of convergence, and hence require skills from both the RF and IP domains. VSAT specific training includes:


External links

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