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Satellite Frequency Bands

Due to lower frequencies, L-Band is easiest to implement for marine satellite stabilised systems. There is not much L-Band bandwidth available. The higher you go in frequency, the more bandwidth is available, but the equipment needs to be more sophisticated.  
L C Ku Ka
If one could equate the cost and availability of L-Band space segment to say, city real estate, C-Band might be the suburbs, Ku band the countryside, and Ka-Band the prairies of the wild west. Maybe a little more difficult to get to, but a lot of it available at a reasonable price.
L-Band (1-2 GHz) | C-Band (4-8 GHz) | Ku-Band (12-18 GHz) | Ka-Band (26.5-40 GHz) |
Being a relatively low frequency, L-band is easier to process, requiring less sophisticated and less expensive RF equipment, and due to a wider beam width, the pointing accuracy of the antenna does not have to be as accurate as the higher bands.

Only a small portion (1.3-1.7GHz) of L-Band is allocated to satellite communications on Inmarsat.  Inmarsat uses L-band for their Fleet Broadband, Inmarsat-B and C.

The older Inmarsat A and B antennas were typically 1 meter  in diameter, but, with the launch of more powerful satellites and the use of steerable spot beams, the new Fleet broadband antennas are down to less than 30cm (12 inches).

L-Band is also used for low earth orbit satellites, military satellites, and terrestrial wireless connections like GSM mobile phones. It is also used as an intermediate frequency for satellite TV where the Ku or Ka band signals are down-converted to L-Band at the antenna LNB, to make it easier to transport from the antenna to the below deck, or indoor equipment.

Since there is not much bandwidth available in L-band, it is a costly commodity.
Satellite C-band usually transmits around 6 GHz and receives around 4 GHz. It uses large (2.4- 3.7 meter) antennas. These are the large white domes that you see on top of the cruise ships and commercial vessels.

C-band is typically used by large ships that traverse the oceans on a regular basis and require uninterrupted, dedicated, always on connectivity as they move from region to region.  The shipping lines usually lease segment of satellite bandwidth that is provided to the ships on a full time basis, providing connections to the Internet, the public telephone networks, and data back-hauls to their head office.

C-band is also used for terrestrial microwave links,  which can present a problem when vessels come into port and interfere with critical terrestrial links. This has resulted in serious restrictions within 300Km of the coast, requiring terminals to be turned off when coming close to land.
Ku-Band (pronounced Kay-You) refers to the lower portion of the K-Band. The "u" comes from a German term referring to "under" whereas  the "a" in Ka- Band  refers to "above" or the top part of K-Band. The middle portion of K-Band is a bit of a mystery. Other than a mention of K-Band radar there are few references on the Internet to it's use. 

Ku-Band is most commonly used for satellite TV and is used for most VSAT systems on yachts and ships today. There is much more bandwidth available in Ku -Band and it is therefore less expensive that C or L-band.

The main disadvantage of Ku-Band is rain fade. The wavelength of rain drops coincides with the wavelength of Ku-Band causing the signal to be attenuated during rain showers. This can be overcome by transmitting extra power but this of course comes with a cost as well.

The pointing accuracy of the antennas need to be much tighter than L-Band Inmarsat terminals, due to narrower beam widths, and consequently the terminals need to be more precise and more expensive.

Ku band coverage is generally by regional spot beams, covering major land areas with TV reception. VSAT Vessels moving from region to region need to change satellite beams, sometimes with no coverage in between beams. Lately this process has been improved with a transatlantic beam on Telstar 11n, and the satellite terminals and modems being programmed to automatically switch beams.

VSAT Antenna sizes typically range from the standard 1 meter, like the SeaTel 4009, to 1.5 meters for operation in fringe areas and, more recently, as low as 60cm for spread spectrum operation.
Ka-Band (pronounced Kay-A) is an extremely high frequency requiring great pointing accuracy and sophisticated RF equipment. Like Ku-band it is susceptible to rain fade. It is commonly used for high definition satellite TV.  It is also used today for terrestrial VSAT services from companies like Hughes Networks.

Ka-Band bandwidth is plentiful and once implemented should be quite inexpensive compared to Ku-Band .

In 2010 there is news that Inmarsat will be providing a global Ka-Band VSAT service beginning in 2014. As more Ka-Band bandwidth becomes available, there will be several other satellite providers offering Ka-Band VSAT on a more regional basis.

The advantage of the  Inmarsat solution is that it will be  global, seamless, much smaller antennas, and should be much cheaper that Ku-Band services today. If priced correctly, this could revolutionize the marine VSAT industry. It will be difficult for smaller companies to compete with the expected $1.4M investment that Inmarsat is making in it's I5 satellites.