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Study into Phased Arrays in NGSO Earth Station Antennas |
Currently, almost all satellites which provide broadcast, point-to-point and point-to-multipoint networks to business and domestic customers are located in GeoStationary Orbit (GSO). Over the next decade, it is anticipated that a number of new satellite systems will be launched, providing a wide range of broadband services using Non-GeoStationary Orbits (NGSO). In the former case, the satellite appears at a nominally fixed point in the sky and it is possible to maintain communications with the satellite using a static ground terminal antenna; only a single satellite is used for the communications link at all times. The key features which differentiate NGSO systems from GSO networks are that the ground terminal must track relatively fast moving satellites as they traverse the visible sky and communications must be handed from one satellite to another as one leaves this visible region and another appears above the horizon.
The proposed systems, because they use Non-GeoStationary orbits, must employ a large number of satellites if they are to provide continuous coverage over the entire globe. For such systems to be commercially viable, it is essential that they attract a large customer base so that the substantial investment in the satellite infrastructure can be recouped through access charges. Ultimately, the rate of take-up of the systems and the total market volume will depend on the cost of ground terminal equipment and access charges; based on the submissions of the key satellite providers, it is believed that the cost of terminals must be in the region $1000-$2000 if customer acceptance is to be achieved.
Most potential satellite network operators and system suppliers claim that the targets set for the terminal costs are achievable and many propose low profile phased array systems as the antenna design solution. At this time, no such phased array systems have been designed which meet the cost targets. This study provides a review of the state of the art in phased array technology and gives an appreciation of the use of phased array antennas for the high volume ground terminal application.
Phased arrays can provide complete two dimensional scanning or single plane beam movement. For a number of the NGSO scenarios, it appears that the latter is sufficient for a given location on the earth; however, the antenna would need to be set to a given cross track angle to enable this to be achieved. This has a major impact on the cost of phased array technology, since it offers the potential to reduce the number of active modules used in the arrays significantly.
At present, there are no commercially available phased arrays available at any frequency. Such antennas are, however, used up to Ku-Band for a number of military and earth observation applications and Ka-Band units have been produced for use with US MILSTAR satellites. A number of existing commercial applications, notably for TVRO satellite reception, could beneficially make use of phased arrays to switch between satellites such as Astra and Eutelsat, for example; however, but units have not been put on the market due to the continued high cost of such antennas.
The approximate cost of a T/R module for a military Ku-Band phased array is about $100. If this is extrapolated to a Ka-Band communications application, where separation of the transmit and receive functions is mandatory due to the frequency separation and need for high isolation between the transmit and receive functions, it is reasonable to assume that each Transmit or Receive module might also cost $100. On this basis, the active modules for a typical dual band phased array with 100 elements for both transmit and receive would cost $20,000. It is therefore concluded that, in order for phased arrays to become commercially viable, a cost reduction for the active modules of at least two orders of magnitude is required.
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