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Optical Intersatellite Links for Low Earth Orbit Constellations

Parent Category: 2021 HFE

2021 08 HFE LEOs fg01

By Jeff Behlendorf and Kent Queensland

There has been a great deal of excitement around the next generation of satellite communication systems deployed in Low Earth Orbit (LEO). They can potentially bring high-speed internet services to every inch of the planet in a way never before seen. Imagine all the possibilities that arise through this globally connected scenario, from achieving life-changing goals, like providing full access to information and resources to remote countries that desperately need it, to being able to share a social media status update from the middle of the Sahara Desert. What seemed like a dream just years ago is now closer to reality than ever.

The race to assemble an operational space network started a few years ago. The top contenders are SpaceX, Telesat, OneWeb, and others. These companies are in the process of building out these satellite constellations today, many of which are even orbiting around our planet as we speak. SpaceX has already launched 1,500 satellites into orbit (out of 42,000 planned in total); Telesat has almost 300; and OneWeb has 180.

These satellite constellations will grow up to tens of thousands of units and serve as nodes in an orbiting network on a global scale. Every passing month, satellite internet traffic is growing immensely. More so, the number of ground stations is growing at a faster rate. One of the most critical objectives of these satellite constellations is providing high-speed internet services to everyone. This means every potential user on Earth could become a ground station for each satellite network. New and better technologies are needed to guarantee seamless and higher-data-rate communication between each satellite and its ground stations.

In the conventional form of satellite communication networks, two ground stations use a satellite as a relay point to talk to each other. If we think about one-way communication, the first ground station sends a data stream up to a satellite. Said satellite acts as a relay point and redirects the data stream to the second ground station, the receiver. This requires that the satellite is able to see both the first and the second ground station at the same time. If one of the stations is out of sight, the data stream won’t reach the satellite, or the forwarded stream won’t reach its receiver.

In LEO constellations, the geographic area that any given satellite can cover over the Earth is small; ergo, you need multiple satellites in orbit for a more complete coverage. Imagine someone trying to send a data file from New Zealand all the way to Spain. The two countries are on opposite sides of the planet. It is impossible to have a single satellite function as a relay between both parties. The first solution would be to build access points throughout the globe to function as intermediaries. Besides being an expensive solution, you would need to set up some access points over water, since two-thirds of the planet is covered in water. The answer is to place the entire the network in orbit and have the satellites communicate directly with each other, relaying the data time and time again in the orbiting network until it reaches its destination.

A vital part for this to work is the concept of an Intersatellite Link (ISL). The central role of ISLs is to interconnect all the satellites operating within a constellation and to provide point-to-point data transfers between any two points. Due to the high speeds required, the ideal ISL is known as the Optical Inter Satellite Link (OISL). The OISL features very high data rates and excellent performance. It utilizes a narrow beam of light that minimizes interference with other communication systems and offers robust security against jamming and eavesdropping.

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Diagram 1.

To set up these OISLs, satellites need to be equipped with a Fast or Fine Steering Mirror (FSM) system. As the name implies, the system is composed of a very smooth and flat mirror mounted on top of a dual-axis positioning system that can be steered through the use of actuators placed below it. The FSM is tasked with redirecting the optical data stream to the desired destination. It does so by tilting the mirror a precise number of degree minutes in both the X and Y-axis. The key factor to consider is that the target the satellite needs to hit is usually thousands of miles away. An error of less than a fraction of a degree in one axis may lead to the data stream missing its intended destination. The difficulty in providing a seamless optical data connection is exacerbated by the fact that satellites in LEO move at velocities more than 15,000 miles per hour.

Finally, compensating adjustments to the position of the mirror need to be made hundreds of times per second as the satellites are moving in different orbital planes around the Earth. Each mirror movement must be as ultra-precise to guarantee that the optical data stream stays precisely pointed at its target destination.

The FSM is a fundamental part of the entire satellite constellation system. Its job may seem fairly simple when analyzed from the outside. It must tilt towards the right direction to redirect a light stream. However, multiple backstage processes must occur before achieving this.

To better explain the process, please refer to diagram 1. The system is composed of the mirror, the axis shaft, an actuator, a controlling driver, and the most critical piece: a pair of positioning sensors per axis. When a light stream is directed towards a satellite, the sensors detect the exact position of the mirror. The information is sent towards the controlling driver, which determines if the position is correct. If not, the controller emits a new signal into the actuators, which, in turn, rotate the mirror into the needed position.

There are several types of electromechanical actuators available to use to position the mirror within the different kinds of FSMs. These include piezoelectric, voice coil, and variable reluctance actuators. Overall, their functionality and performance are very similar. For these positioning systems to operate correctly, it is imperative to always to know the exact position of the mirror, and the mirror’s movements need to be as precise as possible. Often, mirror movements are within nanometer-scales due to the great distances the light must travel. If your FSM system is unable to detect the position of the mirror at nanometer scales, reliable data transmission will not be possible because of the inability of the FSM to keep the optical data stream exactly on target. The sensors that determine the exact position of the mirror must be capable of doing so quickly and precisely. Small errors are greatly magnified, crippling the system’s ability to stay locked to the target and reliably transfer data, and when the information being transferred is critical, there is zero-tolerance for lost data.

Companies like Lion Precision, part of Carlisle Interconnect Technologies, provide sensors that accomplish these tasks with a pair of non-contact inductive displacement sensors per axis. The FSM must be capable of precisely targeting laser signals between satellites moving at speed or directly at stationary ground stations, requiring mirror repositioning commands at hundreds of times per second.

Lion Precision’s EDA400 sensor system is a high-speed and ultra-precise solution developed specifically for this purpose. Using a pair of inductive sensors on each axis of the mirror (X and Y), the system can determine the exact mirror position as feedback to the Optical Intersatellite Link (OISL) control system.

The differential system is designed to minimize the effects of the large temperature fluctuations and the higher-radiation environment in space. When the mirror is in the centered (or null) position, the two sensors for that axis are at the same distance from the mirror and return an equivalent measurement. The information is then analyzed by the sensor driver electronics, which compares data between the two sensors, generating a net output of zero. As the mirror tilts, the two sensors are moving farther away or getting closer to the mirror in exactly the same magnitude, which, after comparing the two sensors, generates an output proportional to the tilt of the mirror. This provides a failsafe solution that ensures that the measurements are correct, as long as the net comparison between both sensors is zero. When subjecting the system to extreme temperatures, the pair of sensors is affected on the same scale, so any potential error is mitigated by netting the changes between both sensors.

Testing throughout the years has proven that as long as an FSM sensor is able to provide precise position feedback to its driver, the FSM can effectively relay optical signals over long distances. This translates into always requiring the best sensor equipment available when you aim to achieve worldwide data communication.

Along with precision, the FSM system must also possess other critical characteristics to be installed in a satellite. Mass and power consumption have become essential criteria now more than ever. Given the number of satellites that must be launched for an entire constellation, any savings on the weight directly translate to launch cost savings. The EDA-400 system is specifically designed with these needs in mind.

Space communications networks are changing rapidly, and as launches continue for these LEO communication networks, the ability to implement new technologies to address new challenges is essential to success. Once the rocket is launched, there are no more opportunities to provide maintenance to the satellite itself. Any damages sustained or bugs that may be detected later will remain as such. You need reliable equipment from a company that has a proven track record working with multiple aerospace customers.

Looking into the future at the thousands of satellites that will be launched, companies may struggle with system shortages. A single satellite needs, on average, two FSMs for it to perform as expected and be profitable. If you multiply said amount by the thousands of satellites being deployed, the total amount of FSMs needed is considerably high. In a world affected by supply chain challenges, where commodities to computer chip shortages have become a way of life, it is best to always have reliable options.

When choosing the correct partner, make sure you join someone with heritage and experience in space products. The conditions in which satellites operate are harsh, and if the manufacturing company isn’t experienced, the risk of getting a system unfit for space operation is a risk too far. When deploying a global network, as expensive as this is, preventing errors is the best bet. Always remember to conduct as much research as possible, as a crucial mistake can have dramatic impacts on your budget and timeline.

The future looks promising with the new generation of satellite communication technologies. Interconnecting the entire globe is now only a few years into the future. The internet has become a first-degree necessity and building the tools that allow every person worldwide to access it is the right step forward.

About the Authors

Jeff Behlendorf is the director of product management, Integrated Products at Carlisle Interconnect Technologies, and Kent Queensland is a senior application engineer at Lion Precision, a Carlisle company.

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