Satellite Communication

Mobile satellite communications is now well established and works almost ocean-wide just like a terrestrial mobile phone. Most systems allow phone calls and send emails and offer fast mobile data and network services at speeds between 64 Kbps to 512 Kbps, which allow for fast satellite Internet access.
Although mobile satellite communications enable telephone calls from a yacht, it must be considered that it may not be possible to communicate with other vessels or rescue services on the high seas. Therefore, mobile satellite communications are no replacement for conventional marine GMDSS communication systems.

Satellite communication is a relatively new alternative for long-distance communication. It features many advantages over conventional point-to-point radio. Instead of transmitting an analogue signal directly from the vessel to a shore station, a digital signal is transmitted upward to a satellite. The satellite then relays this signal to another satellite or to a receiver elsewhere on the surface of the Earth.
Satellite communication is private. Communication over conventional SSB or VHF can be monitored by everyone with a appropriate receiver in range. Signals transmitted for satellite communication, however, are highly directional, making it much more difficult to be pick up. In the case of digital signals, data can also be encrypted, making it even more secure against eavesdropping.

Satellite communication allows direct access to the global communication infrastructure (telephone and computer networks). SSB or VHF, on the other hand, both require an intermediary - such as a ship-to-shore operator - to make the appropriate connections ashore.

Satellite communication is not greatly affected by atmospheric or meteorological conditions. The signal does not have to bounce since it only needs to reach an overhead satellite, which will always be in line-of-sight. Because the signal is being transmitted primarily upward, it passes through a relatively thin layer of the Earth's atmosphere.
SSB and VHF transmissions, however, must push their way through a great quantity of distortion-producing atmosphere as they travel across the surface of the Earth.

Despite the many advantages, satellite communication is expensive. This is because the satellite infrastructure on itself is complex and expensive:

• Life times - Satellites have a limited life time, dependent on fuel and orbit. Satellites burn fuel to keep in the correct orbit. Low orbiting satellites are being pulled back toward the earth by gravity and hence have a short life time, 10 years. Higher orbiting geostationary satellites last about 15 years before running out of fuel and drift out into space.
• Number of calls - The satellite is basically a telephone exchange, it can only handle a limited number of calls. Low orbiting satellites handle between 1700 to 2500 calls per satellite. Higher orbiting satellites can see more of the earth's surface and hence designed to handle a high number of calls, between 5000 to 25000 calls per satellite.
• Coverage - Depends on technology deployed. Bent pipe technology is the most common due to low cost and small size. This is when all calls are passed to a ground station for processing. On low orbiting satellites, if the satellite cannot see a ground station then it cannot connect calls.
  Interlink satellite technology uses larger and more expensive satellites. This is when satellite can transfer calls between themselves without going via a ground station.
• Mobility - You do need an antenna above deck to receive and make calls. The low transmit power from your satellite phone needs to travel a long way. Low orbiting satellites (LEO) are in the region of 800 km above your head and move quickly across the sky. Geostationary satellites (GEO) are some 36000 km above the equator and you need a clear path between you and the satellite for your signal to reach it. do expect to lose/drop calls.
• Systems cost - It is very expensive to put satellites up there and maintain them. Satellites are designed to be reliable, you cannot send someone up in the sky to fix them when they go wrong. Launch costs are high and each payload carries typically only between 3 to 6 communication satellites.

Satellite Systems

Most communications satellites are in geosynchronous, or geostationary, orbits (GEO satellites). This means that each satellite is positioned over the Equator at an altitude of about 36 000 km, such that its speed around the earth matches the earth's rotation. Both the satellite and the surface of the Earth are rotating around the Earth's axis, but since they are rotating at the same rate, the satellite appears to stay in one place over the Equator. This simplifies signal transmission since once an antenna on the ground is directed toward the satellite, it does not have to be readjusted. A vessel as sea, however, does not stay in one place. Even if it did, though, it would tend to provide an insufficiently stable surface for precise antenna alignment. So at sea, only systems that work with omni-directional antennas are feasible. As a consequence, only low data rates are achievable with geosynchronous satellites.
Another drawback of geosynchronous satellite systems is that since they must be located above the Equator, they cannot cover the polar regions above 70° Latitude.

For truly global covearge a satellite system consisting of "low earth-orbit satellites" (LEO satellites) can be established. LEO satellites orbit the Earth in high speed, low altitude orbits (600-1200km) with a typical orbital time of 90 minutes. This way they provide a coverage area of about 2800 km in radius. Since the satellites are not geostationary, they move fast with respect to the ground. At least one satellite must have line-of-sight to every coverage area at all times to guarantee coverage. Depending on the positions of satellite and terminal, a usable connection time of an individual LEO satellite will typically last 4-15 minutes on average. So the network management system must be able to continously reconnect the earthbound terminal with the appropriate satellite that is currently "in sight" without interrupting the active terminal data trafic (similar to the terestrial mobile network).

Available Service Providers

• Inmarsat - The oldest satellite phone operator. Currently operating eleven GEO satellites. Good ocean region coverage. Reliable but expensive call and mobile unit cost. In recent years the call cost has dropped to a competetive level. Services include BGAN (Broadband Global Area Network).
• Thuraya - European, North- and East Africa, Middle East, Asia and Australia coverage through three GEO satellites. Services include GMPRS mobile data service at 60 kbps (uplink) / 15 kbps (downlink) and a high-speed 144 kbps data transfer via "ThurayaDSL" modem.

• Iridium - Truly global, offering special broadband satellite voice and data communication for maritime vessels ("OpenPort"). This enables internet connectivity with up to 128 kbps.
• Globalstar - Poor global ocean region coverage. Services include voice, email and data up to 9.6kbps and medium-speed 9.6 kbps data transfer through the "Globalstar Duplex Satellite Modem".

Inmarsat

Inmarsat inherited the operational assets of the International Maritime Satellite Organisation (IMSO), which was etablished 1979 a an international partnership of government and private entities representing 75 countries. Headquartered in London, its charter is to provide mobile satellite communication services to the world. Aside from its commercial services, Inmarsat Ltd. provides GMDSS services to ships and aircrafts as a public service. Currently, Inmarsat uses four of its own satellites and leases maritime communication capacity on several other satellites. All of the units in the Inmarsat system are in geosynchronous orbits.

As part of the GMDSS requirements for commercial vessels, Inmarsat-C is to stay for some time to come. Inmarsat-C is a data-only terminal with store-and-forward capability that has been around for a number of years. It offers simple reliable operation, and valuable safety features. The transceiver is compact and the antenna is about the size of a medium soft-drink cup. Included in the service is the reception of Notices to Mariners and other safety related information such as daily weather updates at no charge. Receipt of a message to the terminal can be acknowledged with a report of the vessels position obtained from the built in GPS receiver.

The new "IsatPhone" satellite cell phone provides voice and data over the I4 satellite network. This is the newest satellite phone on the market, now providing some competition with Iridium. The IsatPhone Pro currently provides coverage over the entire planet, except the polar regions.
The Isatphone Pro is an affordable satellite cell phone option for people who work, live, or travel to areas where communication may be non-existent or emergency back up communications is needed. The Isatphone Pro has a built-in GPS receiver. An emergency button allows to send the current positions by text or email. IsatPhone provides voice service at 4.8 kbps and low-speed data services at 2.4 kbps.

Iridium

Iridium is back in commercial service via the Iridium satellite network, which was re-introduced in 2001. Iridium came back on line after receiving a government contract for service, so it looks like it should be around for the foreseeable future.
Iridium's sixty-six satellite network is fully operational, providing truly global service. Calls go from phone up to the satellite, which passes the call satellite to satellite until it downlinks at the Hub station in Tempe, Arizona. From Tempe, all calls are routed via terrestrial lines to their final destination. Calls from one terminal to another go up to the satellite network and directly down to the called handset, bypassing the terrestrial phone network.

The new "9575 Extreme" handheld phone is priced at around $1300. It can be connected to a Wi-Fi router called "Iridium AccessPoint" priced $150, which allows to set up a local wireless local network connecting to the internet through the Iridium satellite network. With Iridium AccessPoint, smartphones, tablets and laptops can be used the same way as with any terestrial WLAN. However, at a limited data rate!

Globalstar

Globalstar is the other service available with a handheld phone. The signal goes from the phone, up to the satellites and down to a ground station. The reason satellite is plural is that the system is sort of like cellular telephone except that the cellular base stations are moving (real fast) and the terminals are relatively stationary. If multiple satellites are in view, the one that has the best reception handles the call.

Unlike Iridium, if the satellite is out of view of a ground station, it doesn't relay the signal to another satellite that is in view, then down. This means that what Globalstar calls world-wide coverage doesn't necessarily mean mid-ocean coverage, and it will never have this mid-ocean coverage.

Globalstar offers high-end handheld phones (GSP-1700) priced at around $500 featuring voice and data modes at 9.6 kbps. Also special medium-speed modems and Wi-Fi terminals are avaialble for data rates of up to 9.6 Kbps.