Earth station equipment

The usual design aim is to minimise the total lifetime cost of the satellite link. It is often possible to make trade-offs between the money invested in equipment on the ground versus the satellite capacity utilised.

One of the most prominent items is the antenna, its function is to concentrate the signal to and from the satellite. This is almost always achieved using a "dish" antenna with a curved reflecting surface, rather like the reflector in a torch. Energy from the satellite is concentrated at the focal point of the reflector where it is collected by the antenna feed system. In a two-way system energy from the feed is also collected into a narrow beam in the direction of the satellite. There are a number of different geometrical designs for the antenna and feed arrangement.

To secure the antenna in place and allow it to be accurately pointed at the satellite, a mount is required. The mount is either specially fabricated for an installation, or chosen from one of the standard type shown below.

King post mount. This is simply a pole, set in concrete at its base.

Non-penetrating roof mount or "pallet" mount. This is an open frame weighted down with blocks of "ballast". A variation on this style of mount is the penetrating-roof mount which is fixed to a horizontal surface using expansion bolts or chemical fixings (bolts that are permanently "glued" into holes made in the surface using epoxy resin).

Wall mounts. These are plates or assemblies of struts designed to be fixed on the face of a wall, or on the inside corner of two walls.

Whichever form of mount is selected, the antenna is connected to the mount using an "azel" head. This is a moveable joint designed to allow the antenna to be moved in both azimuth (side to side motion) and elevation (up and down motion) and then locked firmly in position once pointed at the satellite.

More complex arrangements are possible for use with antenna tracking systems that use a motor and control system to alter the pointing of the antenna. Tracking systems are used both to keep an antenna on station and, where appropriate, to re-position the antenna to an alternative satellite. In general, a small antenna will not require a tracking system.

When antennas are deployed in cold, damp climates; snow and ice can accumulate and degrade their performance. If this is to be avoided a heating system can be employed to keep the reflector and feed above freezing to prevent ice forming or snow settling. These are usually powered using a separate supply on the roof.

Getting signals into and out of the antenna

Associated with the antenna there are two amplifiers, one to boost the received signal once it is collected from the antenna, and another to boost the signal to be transmitted prior to feeding it into the antenna. In order to prevent the high-powered transmitted signal from interfering with the very low power received signal, they are differentiated in two respects: they are at different frequencies; they are in opposite polarisations.

The transmit and receive signals are combined in an ortho-mode transducer (OMT), which is coupled to a small horn antenna directed at the reflecting surface or sub-reflector of the antenna. The horn antenna is known as the feed horn, and is precision manufactured as it has a profound influence on the performance of the antenna. In some systems the feed is adjustable, however such an adjustment would normally take place in a well-equipped laboratory.

The receive amplifier, or LNA, must be mounted as close to the feed as possible, in a VSAT this will be a small box, about four times the size of a few matchboxes, bolted directly to the OMT. The transmit amplifier may be located on, or close to ,the feed, or may be located away from the antenna, perhaps indoors. In VSAT systems the OMT, feed horn, transmit and receive amplifiers may all be integrated into a single unit.

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Linking equipment

A few modern VSAT systems include all of the equipment in the outdoor unit, with one or more data connections to the customer's equipment. However, for most systems an Inter-Facility Link (IFL) is required to connect the outdoor portion of the equipment to that indoors.

The IFL carries: the signal to be transmitted, either as input to an external SSPA or as the output from an internally housed amplifier to be input to the OMT; the signal received, as output from the receive amplifier, or LNA; a supply of power to the outdoor equipment.

The IFL may combine all three of these functions into a single cable or transport them separately. If the signal to be transmitted is to be input to an external amplifier it will normally be carried in a cable. If the signal is the output from an indoor amplifier, a tubular waveguide is normally employed.

A waveguide is a metal tube, normally rectangular in cross section, perhaps a few centimetres wide and one centimetre high. Microwaves travel along the interior of the tube, reflecting off of the conducting walls. The waveguide is normally rigid, but flexible sections are available that can be formed around gentle bends. The principle advantage of a waveguide is that it exhibits a very low loss at microwave frequencies compared with a cable.

In systems employing a single co-axial cable, the cable used is normally one or two centimetres in diameter and quite stiff. Variants of the IFL cable are available to cater for low cost, low loss, and to meet stringent fire regulations. For each type of cable there will be a maximum length of cable run, dictated by the loss of the cable and its resistance to carrying power to the outdoor unit.

Many new VSAT systems employ different cables to carry each signal and the power. This allows low cost cables that are more flexible and easy to handle to be used. Furthermore, if the length of cable run needs to be extended a simple amplifier can be installed.

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The receive amplifier

The receive amplifier goes a long way towards determining the performance of the VSAT and, correspondingly, the cost of the satellite capacity. The key performance parameter is the noise temperature of this component, the lower the better. A typical Ku band receive amplifier will have a noise temperature of about 140K, whilst the noise temperature of a C band system may be 40 to 60K.

These noise temperature figures should be as small as possible because they form the largest part of the noise temperature of the receiver. Hence, the device is often called a Low Noise Amplifier (LNA). The LNA also converts the received signal to a lower frequency, more suited for transmission along the IFL. Because of this conversion function, another common name for this device is the Low Noise Converter (LNC). The output from the LNA/LNC is usually L band, a range of frequencies from 0.5 to 1.5GHz.

Translation of one frequency to another is known as mixing, the signal received from the satellite is mixed with a reference signal derived from an oscillator in the LNC. There are two principal methods to derive this reference frequency: a Dielectric Resonant Oscillator (DRO) or a Phase-Locked Loop Oscillator (PLL). The PLL system is more frequency stable and higher quality, but more costly to implement.

Some modern systems forego the accuracy of the PLL system to take advantage of the low cost and simplicity of the DRO. Such systems are specially designed to overcome the technical limitations of the DRO. DRO LNCs are also commonly used in TV reception, which is relatively insensitive to the type of noise they introduce.

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The transmit amplifier

The role of the transmit amplifier is to provide a sufficiently powerful signal that will traverse the satellite and arrive back on earth to be reliably recovered by the receiver. The transmit amplifier is often the most expensive component in a small installation, so correct sizing is important. For a VSAT, the most common choice is a solid state power amplifier (SSPA) rather than its more powerful thermionic counterpart, the Travelling Wave Tube (TWT).

A SSPA will typically be sealed against the elements and mounted outside, on the antenna. A small unit may be mounted on the feed support strut, larger units are often mounted behind the antenna reflector. Typical output power levels available from a SSPA include: Ku band: 0.5W, 1W, 2W, 5W and 10W; C band: 5W, 10W and 20W.

The additional power from the C band units is partly because it is easier to manufacture a powerful device at these frequencies, and partly because C band systems require more power and so there is more market demand for the larger amplifiers.

If a terminal is only transmitting a single signal, or carrier, and other conditions are satisfied, it may operate at its maximum power, i.e. in saturation. Otherwise, it must be operated below its maximum power. The difference between the maximum power available from the amplifier and the actual amount used is known as "backoff".

If the power from the amplifier were radiated in all directions equally, the isotropic radiated power would be the same as the amount of power emitted by the amplifier. However, the antenna does not radiate power equally in all directions, it is concentrated in the direction of the satellite. This has the effect of boosting the signal in this one direction so that, at that point, it is equivalent to a much higher power amplifier operating into an isotropic antenna. This is known as the Equivalent Isotropic Radiated Power (EIRP). the EIRP is the product of the output power of the amplifier and the transmit gain of the antenna. For example, if the gain of the antenna is 48dBi and and the power amplifier provides two watts of power, or 3dBW, the EIRP will be 48 + 3 = 51.6 dBW.

The calculation of how powerful the signal must be, the EIRP, to achieve reliable communication is called the link budget, and is one of the fundamental calculations that must be performed in the design of any satellite system.

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The indoor equipment

The indoor equipment comprises a modem and, optionally, some baseband equipment.

The role of the modem is to convert a serial data stream into a form that will be transmitted over the satellite. This is done by using ether (?) data to modify a high-frequency signal, called a carrier. The carrier will be modified in such a way that the modem in the receiver can reconstruct the received signal into the original data stream.

The word "modem" is derived from MODulator/DEModulator, because the modification of the carrier is called modulation, and most links require a two-way device and so the functionality to reverse the process is provided in the same unit.

The output from the modem enters the IFL link. For a VSAT the usual carrer frequency at this stage is in the L band, between 500MHz and 1.5GHz. The outdoor equipment translates the L band signal into the final uplink frequency and transmits it to the satellite at a high power.

Alongside the modem, often integrated into the same housing, there may be some baseband equipment. The role of the baseband equipment is to marshal information ready for sending to the modem. For example, the baseband equipment might include:

A voice or video codec; which takes audio or video signals, compresses them, and converts them to a serial bitstream

A packet switch or FRAD; which combines data from several sources again into a single bitstream. The devices are usually aware of several protocols and can convert between them. Modern FRADS can often include voice codecs and routers as well.

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Routers

Management agents; where provided these will allow almost every aspect of the operation of the VSAT installation to be controlled and managed

Where remote management is implemented, in order to allow the VSAT to be managed there must be some bandwidth made available for the control signal. This might be a separate control channel shared by several VSATs, as in an SCPC DAMA system, a method of mixing the control data with the user’s data on the same link as provided in frame relay and TDM.TDMA systems, or an overhead added to each channel by the modem to provide a clear channel in addition tot the user's communication path. These options are discussed more fully in the next section.

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