L/Ku/Ka-band satellites – what does it all mean?

Gogo’s “2Ku” dual Ku-band antenna.

Originally published three years ago, this feature on the differences between L-, Ku-and Ka-band inflight connectivity systems has been one of our most popular. It has now been updated with the latest information on the options available.

The terms L-band, Ku and Ka satellites are bandied around quite freely. But do you really know want they mean and the differences between them?

The “band” in use refers to the radio frequencies used to and from the satellite:

• L-band uses frequencies in the 1 to 2GHz range
• Ku-band utilises approximately 12-18GHz, and
• Ka-band services uses the 26.5-40GHz segment of the electromagnetic spectrum.

And in case you were wondering “Ku” stands for “Kurz unten” – German for the band just underneath the “short” or K-band. Not surprisingly “Ka” stands for “Kurz above”. This is because Ku is the lower part of the original NATO K band, which was split into three bands (Ku, K, and Ka) because of the presence of the atmospheric water vapour resonance peak at 22.24 GHz, (1.35 cm), which made the centre unusable for long-range transmission.

So what you cry? Generally, the higher the frequency the more bandwidth you can squeeze out of the system. The difference is just like an FM radio broadcast being compared with medium wave. The higher frequency VHF radio (100MHz) band gives you greater bandwidth than medium wave/AM (1MHz) and the sound quality is better.

Scale this up to the satellite’s microwave frequencies and Ka-band should give you more digital bandwidth than Ku, which in turn should give greater bandwidth than L-band.

But it is only half the story.

Physicist and mathematician Claude Shannon developed what became known as “Shannon’s Theorem” in 1948. This still holds true today and is a student essential to understanding satellite throughputs. We’ll ignore the maths, but essentially it says:

• The higher the bandwidth, the more data can be transferred
• The higher the frequency the more bandwidth is available
• A high signal-to-noise ratio is better
• An increase in the transmit power level can give an increase in the communication link throughput.

So it isn’t just about the frequency – you have to take into account the power density available, and satellite spot beams generally provide a higher level, be it on Ku- or Ka-band.

And the headline bandwidth figure usually refers to the transponder bandwidth from the satellites. Now we need to share that out among the many users.

You also have to consider a whole host of other factors, such as how big is your antenna? What is the elevation of the satellite above the horizon? How many receivers are sharing the satellite’s spot beam at this time and even, what is the weather like?

Yes, weather. Both Ku and Ka can suffer from rain fade (Ka more than Ku) – this is not usually a problem at 35,000 feet, but high levels of humidity in tropical areas can also affect signals.

Jen Marts with Cobham’s SB200 antenna for L-band.

Taking L-band first. Inmarsat offers its IP-based 432kbps SwiftBroadband (SBB), but its lightweight 200kbps SB200 service, with equipment that can easily be fitted into a bizjet, is also popular.

A maximum of four channels per aircraft can also be “bonded” and used for streaming IP services at any one time.

A newer High Data Rate (HDR) waveform, can also provide increased data throughput on conventional aircraft – up to 700kbps using a full 200kHz bearer – but, again, only in streaming mode.

To put a spanner in the works, the satellite thats backs up the new air-to-ground European Aviation Network is S-band – around 2.4GHz. Its speeds are currently unknown, but expected to be in the 1-3Mbps range.

But if Inmarsat’s L-band SwiftBroadBand service is not quick enough for you, how about Inmarsat’s Global Xpress (GX Aviation) Ka-band service?

The higher frequencies mean data throughputs in the region of 30-50 megabits per second (Mbps) are possible, compared with 432kbps with the current L-band SwiftBroadband – up to a 100x speed increase.

ViaSat also offers its Ka-band in-flight Exede system, which can deliver up to 12 Mbps to each passenger.

There are alternatives, including Ku-band services from Panasonic, Gogo, Global Eagle and ViaSat Yonder.

Ku-band typically offers connection speeds of around 1–12 Mbps, although it can be higher.

Also available is spot-beam Ku, using new High Throughput Satellites (HTS). For example, Intelsat’s EpicNG promises up to 80 Mbps per aircraft and 200 Mbps per spot beam. Each spot beam has a higher power density, hence the higher bandwidths available.

Honeywell’s fuselage-mounted Ka-band MCS-8200 antenna for Inmarsat’s GX Aviation service.

Both Ka and Ku are also benefitting from new modem designs that promise to boost data throughputs even further.

In fact, both Ku and Ka providers talk about speeds of up to 100Mbps to the aircraft. The reality is, airlines want good, consistent, reliable connectivity first and foremost. The Ku or Ka argument is becoming secondary.

“Get Connected” has tested both Gogo’s 2Ku product and GX Aviation’s Ka-band and both delivered around 10Mbps in the tests.

So there you have it. Each band – L, Ku and Ka – works on a different set of frequencies. In general, the higher the frequency the higher the throughput.

However, three key parameters can be manipulated in order to optimise the capacity of a communications link – bandwidth, signal power and channel noise.

Communication channel providers develop their technologies in order to achieve the optimal link capacity based on their market needs.

An increase in the transmit power level results in an increase of the communication link throughput, likewise a decrease in power will result in the opposite effect reducing the throughput.

Another way to improve the link throughput would be to increase the size of the receiving antenna in order to have a higher level of energy received at the aircraft.

But this is where operational constraints become apparent, as, this would lead to an unfeasible installation for a commercial or business aircraft.