Communications

Satellite antennae


Malcolm Latarche
Malcolm Latarche
ShipInsight

10 July 2019

Satellite antennae

Besides, the highly compact cylindrical Iridium antenna, the smallest and least powerful satellite antenna for use on board vessels is the usually conical shaped antenna of the Inmarsat C system. Both are omnidirectional and therefore require no moving parts inside the protective cover. The low power of the Inmarsat C antenna is one of the reasons why the system has to operate on a store and forward basis.

The next step up to Inmarsat Fleet requires a dish antenna that can move on its horizontal and vertical axes to stay aligned with the satellite. Inmarsat Fleet has three sizes of antenna with diameters of 33cm, 55cm and 77cm the particular size corresponding to the system installed. These antennae like all other marine versions are protected by a radome that can either be dome shaped or spherical.

VSAT antennae are more sophisticated still and also are generally much larger than the Inmarsat Fleet versions. They are 3-axis stabilised systems and the dish which can be 3m or more in diameters (although the trend is to smaller 1m size dishes) can move rapidly in any direction to maintain connectivity with the satellite. Most antennae are designed for use with a single band only but as ships are beginning to subscribe to more than one service and more antennae are needed, some manufacturers are looking to combine bands in a single hybrid antenna. As far as hybrid antennas are concerned, there are several major manufacturers which have developed antennas that are capable of instantly switching between Ku-band and Ka-band networks.

Communication cost management systems

As ship operators began to get a taste for more communication options, some ships were fitted with a variety of different systems. There might be the standard GMDSS set up, an Iridium phone or data terminal and even possible a broadband or VSAT system as well. Spurred by fierce competition in the sector, service providers had, in the 2000s, begun to offer a number of different tariffs and special offers to attract extra traffic.

These included ‘super quiet time’ reduced rates for ships sending data during periods of the day when traffic was normally low (this would of course vary by region as it was time based) and special days around public holidays when crew calling costs were discounted. As the rates varied between different networks, determining the least expensive options became rather complicated for the ships with multiple options. At the same time, a number of third party specialists had begun developing devices that could compress data transmissions beyond what was possible using the equipment provided by service providers.

The way was clearly open for innovative companies to develop systems for controlling and managing the situation and these came to be known as ‘middleware’. The equipment sits between the ship’s LAN and the various communication systems with a second identical device located in the shore office.

At scheduled times or whenever a user wishes to transfer data, the devices create a link between the vessel network and the network in the shipowner’s office. The link and the transfer are optimised by compressing the data and choosing the best carrier available. The choice made is based on user-programmed information on cost structures of the different services and calculations made by the middleware on the time or data size needed for transmission. Such systems determine the most cost-effective method of transmitting data and can switch between communication systems as appropriate once initial data has been input. Within a very short time, the service providers themselves began offering similar devices having either developed their own product or by way of acquiring one of the third party manufacturers.

The improved take up in VSAT services where users pay a monthly subscription and are allocated much larger data allowances has made cost management a little easier for ships with VSAT.

One of the claimed advantages of VSAT is that with leased equipment, any technological advances will be made available by service providers upgrading equipment. However, as demand grows some service providers may offer outdated equipment at reduced rates and some shipowners may be keen to avoid ongoing lease costs by negotiating to buy rather than lease equipment.

This may be a good strategy but there is also the chance that as technology advances are adopted, some of the older equipment will either become obsolete or unable to take advantage of new opportunities. For example, anyone that has purchased a Ku-band antenna outright will not be able to make use of the switchable services and redundancy offered by them.

Another factor that needs to be considered by shipowners is the terms of leasing contracts that may be offered. These will be very different from the service contracts offered for older L-band connectivity and should be scrutinised before committing in order to avoid unpleasant surprises further down the line.