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.
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 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.
Despite the many advantages, satellite communication is expensive. This is because the satellite infrastructure on itself is complex and expensive:
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.
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 ProvidersGEO-based:
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.
Inmarsat - C
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.
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.
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 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.
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.
An overview of the basic features of some handheld satellite phones can be found here:
- Inmarsat IsatPhone2
- Iridium Extreme.
|Cover << Sail Away << Marine Radio Communication << .||. >> NAVTEX||last updated: 22-Feb-2008|