Typically the first thing one is told about any new Ku or Ka-band satellite is that bandwidth capacity won’t be a problem and it’s going to be much cheaper than other connectivity solutions on the market.
Let’s put the ‘thorny’ issue of cost to one side and just focus on the topic of capacity and the service level an airline or more importantly an airline passenger can reasonably expect to receive.
All current in-service Ku and Ka-band satellites are hard configured at build with a fixed number of transponders (a device comprising both a transmitter and a receiver in the same housing) that provide a beam (spot or wide area) of radio frequency covering specific geographic areas. Dependent on the expected throughput capacity required some of these geographic areas might have multiple transponders allocated to them.
Once that satellite is up in geostationary orbit 35,786 kilometres (22,236 mi) above the Earth’s equator you can’t redirect bandwidth capacity between geographical areas – remember – those transponders were hard configured at build. You therefore need to know where you want the capacity before you launch and as most satellites have an expected service life of 10-15 years you need to be pretty damn sure of your future demand analysis.
Another important consideration is that not all of this capacity is available to airline passengers – a significant proportion of the capacity is allocated to direct-to-home TV/video broadcast – and also means that the significant majority of transponders are focused over land masses and not oceans.
To add a little context a typical Ku-band satellite transponder has a bandwidth of 36 MHz (some have 72 MHz), which translates into a theoretical maximum throughput bit rate of about 10-12 Mbps (megabit per second) per transponder. As an example the Intelsat IS-10 satellite has 24 x 36 MHz transponders providing direct-to-home video channels to Asia, Africa, Middle East, Europe.
The actual effective throughout will be lower due to overheads (losses) arising from the transmission protocol used; data packet retransmissions caused by line noise, weather interference and also congested intermediate network nodes.
Due to capacity constraints many satellite connectivity service providers are launching (or have launched) Ka-band satellites. As an example the all Ka-band satellite ViaSat-1 which started commercial service on January 16th 2012 has a total throughput bit rate of 140 Gbps – making it the highest capacity satellite in the world.
ViaSat-1 has 56 Ka-band transponders covering a footprint that according to ViaSat makes it available to 70 per cent of the US population, including the most populated areas of Alaska and Hawaii. Assuming that the total throughput rate is evenly split across those 56 transponders each one has a theoretical maximum (download) throughput of about 2.5 Gbps.
Sounds great – but do the math. Each transponder can ‘only’ simultaneously serve a maximum of 208 homes or aircraft with ViaSat’s Exede 12 Mbps download service (offered to approximately 1 million US subscribers).
To be clear, all current in-service Ku and Ka-band satellites share their capacity in a similar manner, therefore ViaSat-1, as the highest capacity satellite in the world, is the least capacity constrained.