Very Small Aperture Terminals VSATs

VSAT networks are specialized satellite networks principally used for data traffic. In Chapter 8, Figure 8.1, we saw that a VSAT network most commonly consists of a hub and a series of small outstation satellite terminals, which we call VSATs.

Small VSATs have parabolic dish antennas with diameters ranging from 0.5 m for a very modest installation to up to 2.4 m for a much higher capacity installation. Hubs are much larger facilities to compensate somewhat for the small disadvantaged VSATs. One hub may serve more than 2000 VSAT outstations. Hub antennas range from 3 to 10 m in diameter. Their HPAs range from 20 to 200 watts of output power from 6/4-GHz operation and from 3 to 100 watts for 14/12-GHz operation.

VSAT outstations are optimized for cost because there are so many, in one system over 2000. Thus, in that system, the cost multiplier is 2000. For example, a variance in cost of $1000 multiplied by 2000 would become an increment in cost of $2 million.

Both a VSAT and a hub each consist of an indoor unit and an outdoor unit. A block diagram of a typical hub is shown in Figure 15.41 and of a typical VSAT in Figure 15.42. For the VSAT, the outdoor unit is miniaturized and jammed up into the parabolic reflector shell to make waveguide runs as short as possible, 1 or 2 inches. This reduces loss and noise due to ohmic noise; it also can save money.

Block Diaram Typicalvsat Terminal
Figure 15.41. Hub unit, part of VSAT network.
Vsat Block Diagram
Figure 15.42. Atypical VSAT block diagram.

Hubs with larger diameter antennas will require tracking. Step-tracking is a cost-effective alternative. The tracking requirement will depend largely on the stationkeeping capabilities of the satellite accessed.

Figure 15.41 shows a hub configured for a very widely used application, that is, to interexchange data messages with VSAT outstations. The outdoor unit will probably be located in the antenna pedestal, with the indoor unit in a standard rack. Both Figure 15.41 and 15.42 show a configuration for the outbound link as a single thread high rate TDM data stream. The inbound link may use any one of several access protocols, including DAMA, as described in Chapter 8 (Ref. 17).

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    What is step tracking in vsat communication?
    2 years ago

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