With the big prize for Low Earth Orbiting satellite providers being able to offer comms as well as positioning services, there are still technical challenges relating to the significant data rates they are required to process without the luxury of base stations.
Early satellite communication systems such as Inmarsat were thought up in the 1970s. They were primarily focussed on voice - with little to no data communication. These were generally geostationary satellites or high orbit satellites. They also had limited bandwidth and could only support a very limited number of simultaneous users. This made the satellite communication services both functionally limited and expensive to use. In order to compete with ground-based networks, new ways of providing space-based communications was needed.
Entrepreneurs such as Jeff Bezos and Elon Musk have stepped in to develop their own, flexible, low cost, low-earth communication systems, with the dream of providing connectivity ‘wherever you can see the sky’. The idea is to move away from fragmented, outdated geo-stationary systems, replacing them with high volumes of smaller, smarter mobile satellites.
Technical challenges
However, there are two main challenges facing the mobile satellite sector. The first is significantly reducing the cost of production, whilst maintaining robust reliability in space. Previously, the design and manufacture of orbiting satellites, which were high reliability and/or military, had the luxury of low volumes and significant, government-backed budgets. Conversely, today’s new generation of satellites need to be manufactured in high volumes, with good reliability and obsolescence built in.
The second challenge relates to the technical aspects of receiving high volumes of a data without a satellite dish to focus the signal. This is where the next generation of mobile antenna could play a crucial role.
Traditionally, the cost of production for space proof hardware was extremely high. When we had a limited number of space-based satellites, they commanded high costs to ensure good, long-term reliability. Qualification for low volumes made them very expensive. In our experience, specialised FPGAs alone could cost up to £50k for a space qualified part. This made sense for satellites that were going to last for 25 years. Now we’re being asked to design units that will be replaced within five years. The game is changing!
In today’s high-volume, low-cost market, we need to innovate to square this circle. How can OEMs manufacture high performance, robust satellites in big numbers, while staying within tight budgetary constraints?
Whereas established GPS satellites use MEO (Middle Earth Orbit), the new generation of satellites are much closer to the planet, using LEO (Low Earth Orbits). On the face of it, this change in distance from the Earth to the satellite is a mere detail, however, in practice, this shift in position requires a substantially different type of satellite.
The new generation of satellites use complex modulation schemes that require a good signal to noise ratio. Terrestrial networks normally have a maximum distance of around 35km from base station to phone. In a big city, the use of micro cells can mean this distance can drop to just 100s metres. LEO satellites can be up to 2,000km high, so lots of distance and atmosphere to go through. This makes for much higher path loss, making the satellite to ground terminal a much more challenging problem than terrestrial mobile. We want high data rates but at significantly increased distances.
Whatever you do, the physical distance here is always significantly more that would be the case with traditional, ground-based mobile systems. So, maintaining signal generally means achieving much higher performance. This means achieving higher antenna gains, to combat an environment where the path loss is much more significant.
Ditch the dish?
Traditionally, we would say that satellite dishes are ideal for focusing signal, working like a torch combining light rays to create a single, strong beam. The problem for the new generation of LEO satellites is that associated dishes are large, expensive and frankly clunky. Who wants to drive around with a rotating dish on the top their car or even on their phone?
Thankfully, there is an alternative. There is a technology called phased array that allows you to use a flat panel, which is effectively a highly condensed array of transmitters and receivers. Traditionally, these have been individually controlled, with each element receiving its own signal. Using complex mathematics, we can organise these elements so that in one direction, all of the beams add up with one another – mimicking the performance of the satellite dish by controlling the phase and gain of each element.
This is what’s known as constructive interference. Crucially, changing the phase of the individual signals can point the beam wherever it is needed.
The key advantage of the phased array is its flat, mechanical structure, whilst allowing radio beams to be focused in a specific direction. This technique gives comparable performance to conventional satellite antenna and importantly can be electronically not mechanically steered.
However, phased arrays generally require a large number of components, contributing to a significant bill of materials, which makes them expensive and power hungry. To address this issue, we have been working to adapt this well-established technology into a novel hybrid electronic / mechanical design.
To achieve standard internet speeds, you typically need a narrow beam to achieve the necessary antenna gain. You can have a flat panel with a wide beam, but the gain of the antenna will be quite low, which means although you will be able to achieve communications, you won’t be able to achieve very high speeds.
Switching speeds
These arrays are electronically steered, which means they move at the speed of electronics. Hence their switching speeds are quite good, so you can move your beam very quickly – within micro-seconds. The problem with this type of high gain antenna is that they need a lot of elements; and each element will require a receiver, a power amplifier and phase shifters to move the beam. So, with a 128 by 128 array, for example, you will require 4,096 elements - as these things tend to be square.
All told, this technology works, but it is expensive and power hungry. Some companies use LCD panels to achieve the phase shifting and reduce component counts. This works quite well in static environments, but for more dynamic applications, they encounter challenges of performance because LCD panels switch slowly. If your acceptable beam width is 30 degrees, this is fine, but if it’s down to one to two degrees, this becomes an issue due to the switching speeds being unable to keep up with the motion of satellites combined with vehicular motion.
Low energy and lightweight
As part of our research in this area, we have developed a solution that provides good operational delivery, while achieving a significant weight and power advantage. By way of comparison, high performance arrays’ power consumption can be measured in kilowatts. In tests, our technology can get down to around 90 watts. This opens up the possibility of using this technology on UAVs and vehicles, which would normally not be able to realistically provide this amount of power on an ongoing basis.
These complex technical challenges are now becoming a significant potential gateway to the brave new world of multi-use LEO orbiting satellites.
A number of our technologies, creating hybrid arrays and the novel application of materials and coatings, are already showing huge potential in this emerging area of antenna development. Smaller, lighter and lower power arrays are definitely a key part of the future of satellites – we have the technology, but we still need to make it commercially viable before we all end up watching Netflix via mobile satellites.
Author details: David Tanner, Director, Novocomms
