Satellite Communications –

See more articles

Satcom for vessels of all sizes

By Staff

August 25, 2000

Satellite Communications

Access to a reliable, easy-to-use communications link can not only make any voyage safer and more enjoyable, but it allows one to operate a business office while at sea. Today, this is quite possible, as satellite communications systems, global and regional, are now available for vessels of all sizes. Satellite systems (Satcoms) designed for the marine environment have seen significant advances since their inception 20 years ago. They now provide voice, telex, fax, and data services to and from a vessel as easily as from a home or office telephone.

There are several maritime satcom standards for equipment and service that are available and in use for various classes of vessels. Systems for large vessels are available with up to six assigned channels; some models provide for operation of four simultaneous calls and data rates up to 64Kbit/s. The smaller systems generally provide for one or two voice, a fax, and a data channel. Communications software is available to interconnect to Internet e-mail, as well as to transfer still video images taken on a digital camera.

International Systems (Global Coverage)

The International Maritime Satellite Organization

(INMARSAT), the global communications satellite consortium of member nations (now 81 countries), provides communications around the clock for vessels of all sizes voyaging in all oceans of the globe. These satellites, two for the Atlantic Ocean region, one for the Pacific Ocean region, and one for the Indian Ocean region, provide for complete global coverage with the exception of the far polar regions above about 76 degrees north and below 76 degrees south latitude.

The Inmarsat satellites are in geosynchronous orbit (GEO) over the equator; to anyone on earth, they appear as if they were standing still in space. Calls are set up and connected via the international switched telephony network using telephone/telex/data numbers established for this purpose. This is similar in call setup to that used when making any public international terrestrial call. Each satellite providing coverage for an ocean region has a number assigned to it — the same as now exists for a country. (See Fig. 1.)

For example, to make a call to a vessel in the Atlantic from a telephone in the U.S., you would dial 011 (international call), 871 (Atlantic East Satellite), and the nine-digit Inmarsat telephone number assigned to the satcom. To call from the vessel to a shore number in the U.S. (Thomas Reed Publications, for example), you’d dial 001 (U.S. country code), 617 (Boston area code), 248-0084 #. The # symbol transmits the number string similar to the send key on a cellular phone.

Inmarsat-A, introduced in 1982, was the first satcom system. It’s based on FM-modulated analog technology using very large antennae of a size and weight that makes them suitable only for large ships such as cruise liners, oil tankers, and large cargo vessels. Voice, fax, and data services are available for this system, including multiple channel operation and switched data up to 64Kbit/s. However, the Inmarsat-A system has been made more or less obsolete by the newer digital modulated systems. Although Inmarsat will continue to support the existing “A” systems as long as there is a requirement and demand, they will no longer approve new systems for manufacture.

Inmarsat-B is the digital successor to Inmarsat-A and conserves satellite bandwidth and power that results in additional capacity and lower charges for the second generation of Inmarsat satellites. The antennae size and weight stayed approximately the same, thereby making the system suitable for the same class of vessels as A, and for moving large data files for seismologic vessels and offshore oil platforms.

A range of specialized video services also became available at the 64 Kbit/s data rate using data compression techniques. It is possible to send high-quality photographs and compressed video to and from an on-board satcom. Ships can send and receive visual medical information during emergencies and disaster operations, and send and receive instant visual information on the repair of engines to an onshore agent who can then pinpoint the defect and quickly suggest methods of repair.

Inmarsat-C was developed as a low-cost satcom suitable for fitting on a vessel of any size, large or small. The system’s size – it’s slightly larger than a car radio with a small omnidirectional antenna — and weight made it especially suitable for yachts and fishing vessels that were too small for the large Inmarsat-A or, as of late, B, installation. This system does not provide voice communications, but it does include a way to send text messages (fax/telex/e-mail) in a very short time. Inmarsat-B requires the user (at either end) to prepare the message/data on his or her end terminal (PC) and transmit it via the Inmarsat-C system. After a delay of a few minutes, the message may be viewed, printed, and/or stored. Any person familiar with electronic mail systems, including Internet e-mail, will readily understand how Inmarsat-C functions to deliver and receive messages.

Shore-based service providers offer Internet e-mail connection and regularly updated weather reports, stock market information, medical information, and news. They also make possible a range of services such as data reporting, position reporting, and polling. Vessel operators can program their C satcom to transmit regular data reports, including GPS coordinates, on the condition of their vessel and other operational information to shore-based offices. Alternatively, shore-based managers and families of the boat operator can “poll,” or interrogate, the vessel automatically or manually, receiving the same type of information. (See Fig. 2.)

Coupled with C’s Enhanced Group Call (EGC) capabilities, vessels are able to automatically receive a category of messages known as SafetyNET. This service is directed to vessels in or approaching specific geographic regions such as an area around a storm warning, a hazard to navigation, or a ship in distress. The vessel only receives these safety at sea messages for the area the vessel is operating in. In addition, Inmarsat C’s automatic distress calling feature, with fast and reliable alert calling to the Rescue Coordination Centers (RCC) in closest proximity to the vessel in distress, is unparalleled by any other marine communications system. For this reason, large-vessel operators who are required to meet the mandatory GMDSS requirements of the IMO are installing Inmarsat-C in lieu of the larger and more expensive Inmarsat-A and B.

Inmarsat-M was introduced to complement the existing global services with a vessel satcom that would provided voice as well as fax and data, albeit at 2.4 Kbit/s for fax and data, versus 9.6 Kbit/s for Inmarsat-A and B. The marine M versions can be obtained with antennae one-third the weight and size of the A and B and at an equipment cost reduced by approximately half. This system is suitable for vessels approximately 70 feet and longer and is used on many large vessels complemented by an Inmarsat-C to meet their GMDSS requirement.

Inmarsat Mini-M is the latest and smallest marine system; it was introduced in late summer 1997. It provides the capability for voice, fax, and data service at 2.4Kbit/s at reduced service cost and is suitable for vessels as small as are practical for blue-water cruising. The smallest of these systems, the Mini-M has a fully stabilized 5-pound tracking antenna (9.5 inches high and 8.5 inches in diameter); the below-decks equipment is the size of a notebook laptop PC. While this system can be used on vessels of any size, it fills the gap for vessels less than 70 foot nicely. This system was made possible with the launch of the Inmarsat third generation satellites in 1996-97, which have advanced, multiple-spot beam technology supplementing the traditional global beams. The spot beams concentrate the satellite’s transmit signal energy to specific points of focus on the globe’s surface, increasing signal strength and allowing for smaller antenna operation. Each of the four operational satellites provides overlapping spot beam coverage to 95 percent of the world’s continents and a significant part of the world’s oceans. The illustration on the previous page depicts the Inmarsat global and spot beam coverage. It should be noted that less than complete coverage is available in midocean southern hemisphere, Atlantic, Indian, and Pacific Oceans for Mini-M. Vessels cruising in these latitudes should supplement their voice communications installation with Inmarsat-C.

To summarize, there have been three generations of Inmarsat satellites since Inmarsat was established in 1979. The first generation comprised a combination of satellites and packages on existing international satellites that were already in orbit when Inmarsat was formed. These were all analog FM-modulated systems and provided three ocean global beams for the Inmarsat network (now known as Inmarsat A since the newer systems came on line).

In 1990, the second generation satellites, totally owned by Inmarsat, were launched. This second generation was still global beam only, but now could accommodate digital carriers which allowed the development of first Inmarsat-C, followed by M and then B, while still allowing capacity for the FM-modulated A service. Then, in 1996 and 1997, the third generation was launched which now has the high-powered spot beam technology onboard, as well capacity for digital and FM services on the global beams. This system now allows for the small Mini-M plus all of the earlier technologies/standards. With each generation more and more capacity has been added to accommodate the growing number of users in the maritime, land mobile, and aeronautical fields.

All standards – A, C, M, B, and Mini-M (listed in order of activation) – are in place and available. Inmarsat still supports A but the economics (double the cost for service) discourage its use and many large vessels are converting to B, M/C, or Mini-M/C. This is because of the 1999 GDMSS requirement by the IMO. Inmarsat A, B, and C qualify for GDMSS, but M and Mini-M do not.

Low elliptical orbiting (LEO) satellite systems will be introduced in 1998 and will offer small, handheld voice and messaging communicators with global coverage. The first of these, a handheld messaging communicator for use on the ORBCOMM Satellite Network, will be followed by the IRIDIUM voice plus data system. These devices may offer boaters very cost- effective offshore communications for safety and business. When they become commercially available, Reed’s Almanacs will discuss them fully.

Domestic Systems (Regional Coverage)

American Mobile Satellite Corporation (AMSC) is a U.S. public company with the principal shareholders being Hughes Communications, Singapore Telecom, and AT&T Wireless Services.

The AMSC satellite, like Inmarsat, is GEO over the equator and uses similar overlapping spot-beam technology focused on North and Central America, with a separate beam for coverage of Hawaii. The AMSC satellite system is backed up by a separate companion system, owned by TMI, Canada.

Launched in 1995, maritime coverage for the AMSC system is limited to the coastal regions of Central and North America from the northern borders of Colombia and Venezuela, covering the whole of the Caribbean, 200 miles off the West Coast, as far east as Bermuda, and ranging north to the Arctic Circle. A special spot beam is focused to provide 200-mile surrounding coverage for Hawaii (see illustration). AMSC service and equipment is similar to Inmarsat-M in size and capability (voice, fax and data), with slightly higher data rates because of the more powerful spot beams. Until late 1996, the smallest marine stabilized antenna was 19 inches in diameter and weighed 30 pounds, making it suitable for vessels down to about 70 feet. In November 1996, a nonstabilized, broad- beam marine tracking antenna was introduced that was the size of a child’s bicycle helmet and weighed in under three pounds, making it very suitable for vessels as small as 30 feet.

Similar regional satellite systems are in operation by OPTUS Communications, Sidney, for offshore coverage around Australia, and by Telécomunicaciónes de Mexico, using the same satcom equipment available to North and Central American users of the AMSC/TMI systems. (See Fig. 3.)

Satcom Considerations for Small Vessels

Even with all of the advanced capabilities indicated above, satcom systems are easy to operate and don’t require the protocols required for VHF, MF, and HF(SSB) radio. If you can operate a telephone, cellular phone, fax, or PC, you can operate any of the Inmarsat and AMSC Satcoms. Several Satcom models are available for each Inmarsat standard and for AMSC operation. Essentially, these consist of an above-the-deck stabilized auto tracking antenna (omnidirectional for C) encased in a small weatherproof dome. Below decks is the electronic and peripheral equipment such as a telephone, fax, and computer.

For global cruising, you should consider satcoms both for safety at sea and business communications. They should not be considered as a replacement for VHF, MHF, and HF (SSB) inter-ship and ship-to-shore hailing or navigation channel communications. Radio communications are not only required by regulation for this purpose, they are economically the most efficient when within range of other vessels and coastal facilities that are serviced by radio telephone. However, for consistent global communications and distress calling not affected by atmospherics and radio propagation effects, satcoms cannot be equaled by any other communications means.

Once you buy a satcom, you must register the equipment and obtain a satcom phone number. This process requires selecting an Inmarsat Service Provider or Authorized Accounting Authority and signing a service contract. This is similar to selecting a cellular service provider and arranging for service, but is somewhat more complicated because you must work with an international organization that requires strict IMO policy adherence. The service provider then submits the commissioning and registration forms to Inmarsat through the Inmarsat Routing Organization (RO) of the country of the vessel’s registry. For example, if the vessel is registered in the U.S., the paperwork is submitted to the U.S. RO, Comsat. In Canada, paperwork is handled at Stratos Mobile Networks; in the U.K., it goes to British Telecom; in France, to French Telecom, etc.

Most routing organizations are also service providers, but you aren’t obligated to use your nation’s RO as your service provider. Inmarsat service providers represent a number of land stations that interconnect your call to the public-switched network. They compete internationally for your business and rates can vary as much as 25 percent. Service providers also have a host of value-added services such as prepaid and credit card arrangements and technical and medical assistance. Shopping for the best value makes sense.

Most satcoms are shipped with the necessary paperwork and instructions. If you are commencing a global voyage and purchasing Inmarsat equipment, don’t wait until the last minute and expect to have the satcom installed and be on your way. When the paperwork is complete, in order, and the service agreement arranged, it can be as fast as a 12- to 24-hour turnaround time to become operational. However, if your paperwork is not in order, your service provider not arranged, and your vessel has foreign registry, it can take up to a week or more. Some vessel owners, for economic reasons, have their vessels registered (flagged) in countries different than in which they reside, which can add confusion to the commissioning process.

Generally, the dealer or distributor from whom you purchased the satcom will assist in the commissioning and registration process after purchase. Be careful, though, because some dealers are not fully versed in the commissioning requirements. Commissioning for an AMSC system is much simpler than for Inmarsat. However, there are more than one manufacturers of Satcom equipment for this system, and several distributors of both the satcoms and service. Because there are economic choices to be made in selecting equipment and a service provider, a specialized satcom consultant’s advice may be helpful.

Additional Satcom Information

Because the Inmarsat and AMSC equipment and service providers are many and varied, the following references will assist you in finding out more about equipment and service in your part of the world.

  • For additional information or general Inmarsat system questions, contact Inmarsat Customer Relations at: +44 1 71 728 1777.

  • In addition, you will find an excellent “Tool Box” section where you can download information on fax and data set-up for the Inmarsat satcoms, as well as obtain a handbook on SafetyNET and other system information.

  • U.S. Coast Guard information on SafetyNET can be found on the USCG Internet page:

  • Free consultation to prospective satcom owners about selecting equipment and service providers, as well as help in completing the commissioning paperwork, can be obtained from Quest Telecom International, (207) 664-0122


Emergency Position Indicating Radio Beacons (EPIRBs) are designed to alert rescue authorities in emergencies. EPIRBs are made in six different types:

Class A and B EPIRBs are the most common aboard cruising boats; they are the least expensive. These units transmit a simple characteristic signal on 121.5 and 243 MHz, similar to Emergency Locator Transmitters (ELTs) used on aircraft. Class A units are designed to float free from a sinking vessel and automatically activate. Class B units, which are less expensive, must be manually activated. Unfortunately, their detection range is limited due to several factors. They are intended to be received either by passing aircraft or by certain orbiting satellites. A passing aircraft must be listening on one of these frequencies and a satellite must be in line of sight of both the EPIRB and a ground terminal for detection to occur. These devices are subject to a high false alarm rate from satellites (over 99 percent). Consequently, confirmation will be required before search and rescue forces are dispatched. Caution: EPIRBs manufactured before October 1988 may have design or construction problems, or may not be detectable by satellite.

Class S EPIRBs are essentially Class B models that either float or are an integral part of a survival craft (lifeboats and life rafts).

Class C EPIRBs are now obsolete. They transmitted a distinctive signal on VHF Channels 16 and 15 and were intended for coastal waters. They were, however, relatively ineffective.

Category I and II epirbs transmit on both 406MHz and 121.5 MHz; the two categories correspond to Class AS and B. The 406 MHz signal is received by a satellite transponder. The information, which includes the vessel’s identity, and may also include her position if the EPIRB is interfaced with a GPS or loran set, is immediately relayed to a ground station. If there is no ground station visible to the satellite, the information is stored and relayed as soon as one comes in view. The 121.5 MHz signal allows aircraft and rescue vessels to home in on the scene of distress.

A 406 MHz EPIRB does not require a FCC license, but as each has its own identification number — a 15-digit number built in by the manufacturer — it must be registered with NOAA so that the source of the signal can be identified. (Forms for registration are supplied with each EPIRB or may be obtained on the Internet.) Registration includes information on the vessel and its owner. If the boatowner’s address or telephone number changes, the EPIRB must be re-registered; if the boat or EPIRB is sold, it must be re-registered by the new owner. Failure to register will greatly lessen the value of the EPIRB in an emergency — lives have been lost from failure to register.

The Coast Guard must certify Category I and II EPIRBs for sale in the U. S. Many commercial vessels are required to carry a Category I unit, including fishing boats.

Testing EPIRBs. The Coast Guard urges epirb owners to periodically examine them for watertightness, battery expiration date, and signal presence. FCC rules allow class a, B, and S epirbs to be turned on briefly (for three audio sweeps, or one second only) during the first five minutes of each hour. Signal presence can be detected by an FM radio tuned to 99.5 MHz or an AM radio tuned to any vacant frequency and located close to the epirb. Category I and II epirbs can be tested through their own self-test function.

The Coast Guard has up-to-date information on EPIRBs at: