What is VSAT (Very Small Aperture Terminal)?

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- everything RF

Apr 6, 2019

Very Small Aperture Terminal (VSAT) is a ground station with dish antennas that transmits and receives data from satellites. The dish antenna we see in houses for D2H or at the top of tall buildings is a simple example of a VSAT terminal. These terminals use dish antennas with an aperture of less than 3.8 meters (most VSAT antennas have an aperture from 0.75 to 1.25 m). The size of antenna depends on the frequency band of operation and the link budget. The link budget can be calculated based on the power requirements, antenna size, bandwidth requirements and other factors. 

VSATs usually operate in the C, Ku, and Ka frequency bands with data rates between 4 Kbps to 16 Mbps.

A VSAT System consists of a dish antenna, Block Upconverter (BUC), low noise block downconverter (LNB), satellite modem and IP router(s). Thy system has two parts: an outdoor unit and an indoor unit. The outdoor unit consists of a dish antenna, block upconverter (BUC) and low noise block downconverter (LNB). The indoor unit consists of the satellite modem and an IP router which connects to the end user devices.

During transmission, the indoor unit sends the signal to BUC which upconverts and amplifies it and then sends it out to the satellite via the dish antenna.

When receiving signals, the VSAT Antenna receives a signal, sends it to a Low Noise Block (LNB) which filters, amplifies and down-converts the signal and then sends it to the Satellite modem for further processing. It is to be noted that this Up/Down conversion is required because signals need to be transmitted at high frequencies through air while the indoor unit process signals at considerably lower frequencies.

VSATs are highly versatile and allow both narrowband and broadband data communication with satellites in geostationary and geosynchronous orbits. Due to their small size, they can be used for transportable, on-the-move and mobile communications purposes apart from being used in stationary locations.

Dish antennas placed on rooftops for domestic use

The ability of VSATs to be used for both fixed and mobile applications make them suitable for commercial as well as military applications. An everyday example of VSAT terminals are the dish antennas used for D2H (Direct to Home) satellite television broadcast. They are also widely used by businesses for enterprise resource management and media networks to provide live coverage. VSATs are often used by the military and law enforcement authorities.

Maritime VSATs are utilized on ships at sea. They require a stabilizing system (generally gyroscopically stabilized) which ensures that the antenna is accurately pointed towards the satellite because the ships are constantly in motion in all the three axes. A robust stabilizing system is necessary to reduce losses and error along with avoiding communication failure. They are housed in special radome to prevent the precise aiming of the antenna from getting disturbed by the surrounding environmental conditions.

A maritime VSAT terminal in a spherical radome

VSAT networks have various real-world advantages. They require minimal infrastructure as they communicate directly with satellites which makes it suitable to be used in rural/remote areas where a wired connection or cellular network is unavailable. They can be installed and set-up in a short period of time and are economical while requiring little maintenance.

However, one of the biggest disadvantages of VSAT terminals is the high latency that they operate with, as the signal is first transmitted to a satellite and then transmitted again to the destination. Another disadvantage is that the signal can be affected by weather conditions.