There has been much talk about the rise in use of small satellites in different orbits. This is hardly surprising noting that the cost of building and launching satellites is coming down across the space sector and new technology has provided ways to deliver new capabilities using smaller technology.
However, sometimes the advent of new ways of doing things appears to some to be the panacea to all problems and glosses over an old adage we all know – it’s still horses for courses.
So with this in mind I thought it’s worth considering three areas that have different impacts on the way to design a space based satellite communications system, or satcom.
- Capacity per Platform including Capacity Flexibility
- Return path
So taking these in turn, let us talk about spectrum first.
Radio frequency spectrum
I’ve made the comment in the past that, in the 21st century, radio frequency spectrum is one of the most valuable resources on the planet. When you see people carrying two or more mobile devices and how much governments gain in terms of licensing fees for each spectrum auction you realise the world can’t get enough of the resource we call radio frequency spectrum.
In Inmarsat’s case we have substantial radio spectrum resources licensed for use all around the world allowing us to ensure we maintain all of the critical safety services we provide, whilst also supporting connectivity for everything from small telemetry terminals right through to ships and airlines with many passengers using one connection on the platform.
To be able to provide all of these connections in volume, when and where people need them and reliably a substantial amount of the radio spectrum is needed.
In recent times there have been some who have claimed that they can provide larger data connections to various markets but you can only provide as many as you have the spectrum to support.
If your radio spectrum holdings are not large then you will not be able to support very many high data connections at all. If you have many small flying satellites you need to split the use of this spectrum between them, further reducing how many higher data connections a single small satellite can provide.
When you combine access to a large amount of radio spectrum with a large, single geostationary satellite you have far more ability to increase the number of higher data rate connections you can deliver simultaneously.
Governments often have to move at short notice to solve a problem in a particular location and need to be assured their access to a connection in this location is available, capable and reliable.
If you only have limited spectrum this availability, capability and reliability requirements can be a real problem.
If you’re used to supplying lots of small, low data rate connections for things like telemetry then you don’t normally notice the potential for a radio spectrum or channel shortage, but once you need to provide higher data rate connections per customer this problem can quickly become apparent.
This feeds well into the second point – Capacity per Platform including Capacity Flexibility.
A large geostationary satellite can provide a large amount of capacity across up to a third of the earth’s surface and a network can cover the earth with only three satellites. In Inmarsat’s case this is enhanced because we have more than three satellites for each of our L-band and Global Xpress Ka-band networks, allowing us to layer this capacity.
The satellites also have the ability to move resources within themselves to suit the current situation. If a significant event happens, resulting in greater customer connectivity needs in a given location, we can move resources on the satellite from less busy beams into the beam or beams covering the event.
For governments this ensures the network can deliver greater capacity when it’s required, ensuring that governments can achieve availability and reliability even when moving to a new situation or location at short notice.
However, when you use much smaller satellites located much closer to earth, each satellite is only able to carry a fraction of the resources of a larger satellite and it’s not able to reshape the resource it can deliver.
A typical low orbit satellite is only overhead at any given location on earth for around seven minutes and can only provide the given resource available on that satellite in that one location for seven minutes before having to hand over to one of its fellow satellites in the constellation.
If too many users are in that one location they could quickly exceed the number of higher data rate channels available from any of the small satellites and any additional users will not be able to get a connection.
If you’re a government relying on gaining connectivity in the location you’ve had to deploy to, you can’t be dependent on ‘hoping’ too many others won’t be there so that you can access a channel.
Lastly let’s talk about return path. What we mean by that is the entire connection required to get you from your site and application via the satellite back to ground and your operations centre or some other data location. This connection can incorporate various ground based or terrestrial links as well as the satellite ground station.
Various companies offering lower earth orbit satellite networks point out that their latency e.g. the time taken for a signal to get from the ground to one of their satellites and back is much less than the time taken to reach a much larger satellite in geostationary orbit. This is true given it is about the distance the signal has to travel and the constant we call the speed of light.
The problem is that this explanation is overly simplistic and leaves out of the equation the various other parts required to provide a complete, working connection included in the return path.
A large geostationary satellite is able to deliver its signals back to a ground station anywhere within its footprint, which is typically a third of the earth. We then route the data from that ground site to a data centre and on to the customer’s network. In Inmarsat’s case we have this ‘layering’ with multiple satellites covering the earth and multiple earth stations or gateways. Thus we’re further able to optimise the end to end connection to suit the particular government’s requirements.
In the case of the smaller, lower orbit satellite operators they either need to install gateways all over the earth to pick up individual satellites connections, which adds significant cost and complexity, or they use technology like inter-satellite links.
In the case of inter-satellite links, these move the customer’s data across the various satellites until they reach a suitable gateway and then transit the customer data via a data centre back to the customer network.
Apart from the fact that the inter satellite links add time and distance to the connection they also add additional complexity and can result in lower quality data connectivity. This is because there is a lot of ‘switching’ going on to get the customer’s data back to them.
If your lower orbit satellites are swapping each other at say every seven minutes then that is one place where switching occurs, if you then need to keep re-orienting where your inter-satellite links go that is another place when switching occurs. All of this can add delay to the connection including the additional travel time to jump though the various satellites all the way back to the ground station.
If you’re running small, low data rate satellite terminals in fixed locations these switching and distance issues are much less likely to causes issues but once you start trying to use larger channel sizes the switching issue becomes a much more involved problem to resolve and manage.
There are many innovations occurring in the satellite communications area at the moment from much smaller and lighter satellites to much more capable satellite terminals and associated applications. However, there are also some basics that still need to be considered depending on what you are trying to achieve and ignoring those can be to your detriment.
In our experience working with governments for the past 40 years, we know they need certainty, they need to know services will be reliable and available wherever they need to move to as well as being available on the move.
As a result we think carefully about how we provide that certainty and things like spectrum resources, an ability to scale capable spacecraft and well-designed ground networks play into that thinking.
We have a number of innovation initiatives in the multiple new satellites we currently have in build so perhaps we can talk about what’s happening with these commitments next time.
About the author
Todd McDonell joined Inmarsat in May 2013 as the Vice-President of Global Government solutions and became President of Inmarsat’s Global Government business in July 2018. An early stint running operational support for the Royal Australian Navy’s submarine communications systems provided initial education into both military communications systems and satellite communications systems. Following his naval career Todd and some other former Australian Defence personnel created a company called TC Communications focused on the design, delivery and support of satellite communications systems. Todd became chief executive of the TC business in 2000 and grew the business into one of the preeminent Inmarsat partners. Inmarsat acquired the TC business in May 2013 and integrated it into the Inmarsat Global Government business unit.