Viasat is leading a new wave of communications satellite innovation. This is a by-product of our belief that there is always a better way and not being satisfied with the “state-of-the-art” industry capabilities. It demands we think differently as we develop next generation technologies to enable satellite systems that are demonstratively better than have been offered in the past. Our unique industry position allows us to optimize the entire system, from the ground network to space payloads to user equipment. For high-throughput satellite payloads like ViaSat-1, ViaSat-2 and ViaSat-3, new technologies will need to be developed that get the industry beyond the status-quo and allow for orders of magnitude improvement in capacity and coverage area. We’ll discuss how new solid-state integrated circuit technologies are a tool that can be used to improve critical dimensions of performance for new satellite payloads.
An area where solid-state technology is changing the satellite industry is with high power amplifiers (HPA). These amplifiers are the critical part of satellite payloads that amplify the communications signal as it leaves the satellite so that, once the signal is received on earth, it is strong enough for ground equipment to detect and decode. For our GEO stationary satellites, that is a 35,700 kilometer journey. Solid-state high power amplifiers (SSPAs) are one of the areas that can make significant improvements in dimensions critical to satellite payloads.
For over half a century, Traveling Wave Tube Amplifiers (TWTAs) have been the workhorse of the satellite industry. The technology was first invented in WWII for high power RADAR transmitters and was capable of amplifying signals to very high output power levels (100s or 1000s of watts). This technology was eventually adapted for satellite communications. In more recent years, SSPA technologies, such as Gallium Arsenide and Gallium Nitride (GaN), have evolved to a point where they can more efficiently generate these types of power levels and have some benefits over the traditional TWTA solution. Our Viasat team has decades of experience developing these technologies for space, ground and airborne applications.
Most of the power on a communications satellite is often devoted to the power amplifier transmitting a desired signal back down to earth. The better a HPA is at performing this task, the better the system can perform. Because of this, there are several figures-of-merit that are important when we architect a satellite payload. In all these dimensions, SSPAs can be as good as or significantly better than a TWTA solution.
1. Size, weight and power (SWaP)
4. Output distribution losses
6. RF output power
Size, weight (or mass since we are discussing a space application) and power are precious resources on our spacecraft and, because a typical high-throughput satellite will have many dozens of power amplifiers on a single satellite, there is a large multiplier effect. The larger each power amplifier is, the fewer of them can fit within the spacecraft. The more mass an power amplifier has, the fewer of them that the spacecraft structure can carry and\or the larger the rocket needs to be to break it free from earth’s gravity. The more power an amplifier consumes, the larger the solar arrays need to be to supply the power. In general, the smaller, lighter and less power a power amplifier uses to do the same job, the better. TWTAs are relatively large vacuum tube structures that require significant size and mass when compared to a few millimeter square solid-state integrated circuit that can be packaged in a module 10 to 100 times smaller. Additionally there are the practicalities of interconnecting TWTAs or SSPAs to their antenna feeds. These interconnects can have significant size and mass impacts as well as loss that degrades efficiency.
Linearity is a measure of how much an amplifier distorts the input signal as it increases it. All power amplifiers induce some level of distortion and the closer they operate to their max output power limit, the more distortion they add. Because the advanced modulation techniques that are used in today’s high speed communication links require low distortion, there are a few options. Oversizing the power amplifier allows it to operate with less distortion at the needed power level but it incurs a size and power penalty for the larger amplifier. Another other option is to correct the distorted signal with a linearizer device. For the case of TWTAs, a separate solid-state linearizer is used which adds more size and mass to the TWTA solution while an SSPA linearizer can be accomplished within the SSPA itself.
Efficiency is a measure of how much of the DC power an HPA consumes is converted to usable signal power that is sent to the ground vs power converted to waste heat that the spacecraft thermal system has to manage. This is one of the most critical performance parameters for a power amplifier and, prior to improvements in solid-state technology, this was the primary reason that TWTAs were not replaced with SSPAs. Previously, SSPAs based on Gallium Arsenide devices could not compete with TWTAs for efficiency or power density but newer GaN processes have changed the game. Our design teams continue to leverage the decades of experience we have building SSPAs to optimize their performance. GaN based SSPAs are now very competitive with TWTA efficiencies and future technologies such as Graphene promise even higher efficiencies and power densities.
When evaluating the impact to efficiency, we consider much more than just the TWTA or SSPA efficiency. Practical implementation details such as signal distribution after the power amplifier output are also a critical part of the overall system efficiency. These losses manifest themselves as precious signal power that is wasted as heat. In order to manage the TWTA high output power, size and mass is sacrificed to use low loss metal waveguide transmission lines to keep these distribution losses low as the amplifier is connected to the antenna feed. This low-loss solution drives more complexity as the routing becomes a nightmare of interleaved rigid waveguides necessary for the dozens and dozens of TWTAs within the system. Imagine hundreds of rigid waveguides criss-crossing within the payload as the signals are distributed from where the TWTAs have room to fit to where the antennas need to be on the outside of the spacecraft. By contrast, SSPAs have the enormous benefit of being small and compact enough that they can be placed out at the antenna feed and avoid distribution loss altogether.
Reliability is a key design requirement for our spacecraft payload designs since a significant investment is being made in an asset that needs to work for 15+ years with no opportunity for repair. TWTAs represent a single point failure opportunity that can cause an entire coverage area of the system to disappear that is unrecoverable. To overcome the single point failure concern, a redundant spare TWTA would be required. Large, lossy waveguide switch or combiner networks are implemented to allow the spare TWTA to be used in-case of a failure. While not being used, the spare is not contributing anything to the performance of the system but its size and mass have to be included in-lieu-of other components that can improve system performance. On the other hand, SSPAs use power combining schemes that are inherently redundant by the fact that they combine many smaller amplifiers together to achieve the overall desired output. They exhibit graceful degradation as failures occur resulting in only slightly degraded performance rather than full outages.
RF output power is a measure of how large the radio signal can be increased by a power amplifier. Payload system designs can vary significantly in HPA output power size requirements and depending on the frequency and power level, SSPAs can be perform as well if not better than TWTAs. SSPAs use power combining techniques to add the power of many smaller power transistors together, often on the integrated circuit itself. These power combining techniques have good and bad aspects. All power combining methods do have inherent losses so they will degrade the system efficiency. For this reason, very high power applications are still often performed using TWTAs since the SSPA power combining losses become overwhelming. That being said, newer high efficiency solid-state technologies such as GaN allow for higher power transistor building blocks so fewer need to be combined for a given power level. This results in reduced output losses and higher system efficiency.
Although SSPAs have a lot of promise in changing the way we build and design satellites, they come with their own challenges. One of the primary concerns is thermal management. A key benefit of working with newer solid-state power amplifier technologies is the increasing power density but this benefit comes with significant thermal challenges. The reliability of solid-state devices are directly related to their transistor device temperature so a good thermal management design is critical. When you consider that all the power lost due to inefficiencies is concentrated in just a few square millimeters, good thermal management directly under the SSPA die is essential. Also, taking advantage of the ability to place the SSPA directly at the antenna feed complicates this further since the thermal design has to extend out to the feed and maintain a safe and reliable operating temperature at the SSPA.
Solid-state technology has been changing our world for over 50 years. Why should the satellite industry be any different? Viasat continues to “raise the bar” of what customers should expect broadband satellite communications to be and solid-state technologies will be a key tool to meeting those expectations.