Intersatellite links and the impact on the ground
In theory, the use of intersatellite links reduces the number of required ground stations. Consequently, many satellite operators are looking to shift to the ISL model. However, while the ISL model does deliver benefits in certain situations, it definitely doesn’t spell the end for the ground infrastructure.
Joakim Espeland, CEO, Quadsat
The ground segment remains a massive part of enabling satellite communications, however LEO has added significant complexity, not only in terms of how ground equipment operates but also because more ground stations are required. In response to this growing complexity on the ground and as the industry shifts towards large constellations, there’s a growing focus on developing and deploying intersatellite links (ISLs or crosslinking as it’s also known). This approach allows satellites in a constellation to communicate and transmit data directly to each other, rather than only transmitting data to the ground.
This way, data can be transported in space between satellites then transmitted to the ground where it’s needed, rather than waiting for the original receiving satellite to be over a particular site for downlinking.
INTERSATELLITE LINKS IN ACTION
While there’s a definite buzz around satellite crosslinking at the moment, the technique has actually been around for decades. Commercial satcom provider Iridium has been using ISLs since the late 1990s when its LEO constellation was launched. The constellation has 66 active satellites, with each satellite cross-linked to up to four other satellites in the constellation. Crosslinking in this way enables Iridium to provide network optimization and redundancy because data can be transmitted from its source to where it needs to be by rerouting it in space via crosslinked satellites. This enables data to reach the destination gateway via the fastest possible route in space.
The technology used for crosslinking varies by operators, from RF, to microwave, to optical (laser) links. Iridium reportedly uses microwave crosslinks because although optical links could have supported a greater bandwidth, microwave crosslinks were deemed adequate to achieve the network’s desired capabilities, providing a
Joakim Espeland, CEO, Quadsat
LEO has added significant complexity, not only in terms of how ground equipment operates but also because more ground stations are required
Intersatellite Links and the Impact on the Ground
global mesh of coverage. Starlink on the other hand uses optical (laser) technology to crosslink between satellites and has around 9,000 lasers that work together to provide global coverage, with links able to support 100Gbps at a time.
Some see intersatellite links as being the solution to a major challenge facing the industry today, that being how to deliver global coverage and rapid data transfer that LEO constellations enable, without having to build and deploy a massive network of costly and complex ground stations.
It’s not just commercial applications that are adopting crosslinking; government and military organizations too are increasingly focusing on the technique. One example is the United States’ Proliferated Warfighter Space Architecture (PWSA) program which includes a mesh type architecture of satellites connected by optical intersatellite links.
POTENTIAL BENEFITS OF CROSSLINKING
Intersatellite linking can bring a number of benefits to satellite operators, from speeding up data transfer to improving redundancy and reliability. As mentioned, information can be passed from satellite to satellite until it reaches one positioned to downlink where it is needed. This makes for efficient and fast data transfer.
Network redundancy may also be enhanced because if a ground station experiences an outage due to technical failure, weather events, or other issues, satellites can reroute their traffic via crosslinks to reach alternative stations. This added redundancy strengthens service reliability for critical applications, making it highly attractive to government and military organizations.
There is also potential that ISLs may provide satellite operators with greater flexibility in data traffic management, enabling them to ensure that the needs of different customers and applications are met.
Alongside the current interest in ISLs, we’re also seeing a growing trend for ground stations with cloud centers attached. Using crosslinking, operators can reroute the data via linked satellites to bring it down and feed it straight into those cloud centers. Having local access points accelerates data ingestion into the cloud and could reduce overall networking costs.
RISKS OF RELIANCE ON INTERSATELLITE LINKING
On the flip side, there are significant concerns about overly relying on crosslinking technology.
Crosslinking capability adds to the complexity and cost of the satellite itself. The associated equipment, particularly optical terminals, increases the weight, power consumption, and manufacturing costs of each satellite. Additionally, maintaining precise alignment between satellites to ensure reliable laser links is technologically demanding and adds to operational risk. Interestingly, SpaceX announced last year that it plans to sell its optical crosslinking terminals developed for Starlink satellites. It will be interesting to see what impact this move has on the industry.
Bandwidth is another issue. Although optical crosslinks can handle high data rates, there are still limitations, and RF and microwave links have a more limited capacity. For
some applications requiring high-throughput communications, relying solely on crosslinking would not be feasible. Another potential issue that could arise from routing all traffic through satellite mesh networks is congestion in orbit. Without careful network design and data traffic management, bottlenecks could occur, particularly as constellation sizes scale up dramatically.
There is a possibility that in some cases, crosslinking could actually increase data transfer times rather than reduce them. For example, if data is generated over Europe but must be routed across several satellite hops before downlinking, it might introduce unnecessary latency compared to a direct ground link. Regulatory requirements are another consideration because certain types of communications may be required to be downlinked within specific areas. Defense and security-related data, for instance, often have strict rules about where and how data must be downlinked.
WHAT DOES THIS MEAN FOR GROUND TERMINALS AND EQUIPMENT?
There’s a common misconception that by using ISLs, an entire constellation could operate with just one or two ground stations. This is far from correct. There’s no doubt that intersatellite links are extremely useful, however, they cannot replace the need to communicate with the ground. This is especially true for those applications where you need to ensure low latency and high bandwidth. Latencysensitive applications such as video conferencing, financial trading systems and online gaming, require the lowest possible delay, something that only ground stations located close to the point of use can deliver. There’s no getting around the fact that mega constellations still need lots of ground stations, even when actively using intersatellite links.
Aside from the latency issue, another reason that ground stations are still required is because of the potential challenge that comes from bandwidth requirements, which could easily outstrip what ISLs alone can handle. Massive Earth observation files, live video streaming, and bulk data backhaul, all benefit from direct and immediate downlinks. If operators were to rely entirely on inter-satellite linking, this would undoubtedly strain available link capacity, create bottlenecks and slow delivery.
Another crucial factor to consider is redundancy. If operators rely on only a few ground stations, this will create critical points of failure. A distributed network of ground terminals ensures that service remains available even if one location experiences issues. There is also potential that if fewer ground stations are in use, then they will likely reach capacity sooner than would otherwise be the case.
Rather than viewing inter-satellite linking as a potential replacement for the ground segment, it makes much more sense to view it as a technology that can be used to enhance and complement the ground segment. When advanced and efficient ground stations are operated alongside effective intersatellite linking, operators stand to hit the sweet spot. As with all things satcom, there’s no one single solution that can fit all applications and meet all user needs. Instead, what is required is flexibility and greater control so that operators can optimize networks and achieve balance in terms of data going up and coming back down.
