Inter-satellite Optical Links: A New Frontier for Communications Technology
With an eye on widespread internet access and defense, many space agencies and companies have zeroed in on laser-based systems for satellite-to-satellite communication.
Innovations in optical and laser technology have ushered in an era of explosive growth for low earth orbit (LEO) satellite communication with applications ranging from broadband internet to ballistic missile detection.
Since the late 1990s, the global demand for data has increased at an extraordinary rate. From online gaming to high-resolution infotainment systems, consumer demand for low-latency, high-throughput connectivity has been insatiable.
But it’s not just consumers that are driving this demand for data; it’s also governments and defense agencies. In today’s rapidly-evolving geopolitical situation, militaries seek quick and precise threat-detection capabilities. To keep up with this demand, low earth orbit (LEO) satellite constellations are playing an increasingly important role in connecting the world today.
LEO Constellations Cut Latency
Although satellites have played a critical role in communications infrastructure for decades, there has been significant investment in LEO “constellation” communication systems in recent years. An LEO constellation—such as SpaceX’s Starlink—consists of dozens or even hundreds of satellites that communicate with each other and transmit information to the ground.
Low earth orbit is generally defined as orbits less than 1,000 kilometers above the Earth’s surface. The key advantage of LEO systems is their proximity to the Earth compared to geostationary earth orbit (GEO) and medium earth orbit (MEO) satellite systems. This proximity reduces the propagation delay of signals to the ground.
A comparison of LEO, MEO, and GEO satellite positions relative to the earth. Image used courtesy of Satellite Today
Additionally, satellites in LEO systems are typically closer together than satellites in GEO or MEO systems. This means the hardware needed to transmit optical data between satellites in the LEO can be smaller and consume less power. In addition to SpaceX, UK-based firm OneWeb Technologies has also launched a few hundred LEO satellites. Both Starlink and OneWeb’s constellation are designed to provide broadband internet access across the globe.
Optical Comms Boosts Transmission Throughput
While radiofrequency (RF) technology has historically been used for inter-satellite communication, modern satellites are increasingly using laser-based (optical) technology to talk to each other.
There are two key benefits of laser-based technology. First, since infrared laser waves are higher frequency (meaning shorter wavelength) than radio waves, they can pack more information into a single transmission. Second, lasers suffer from less dispersion over long distances compared to radio waves. This means they are more difficult to intercept, increasing the security of the data transmission.
Comparison of radio wave and infrared laser wavelength. Image used courtesy of NASA
The Anatomy of Inter-satellite Optical Communication
Optical systems for inter-satellite communication consist of three key components: a receiver (RX), a transmitter (TX), and a telescopic module for the magnification and direction of the outgoing and incoming laser beams.
In the transmitter, an electrical data signal is converted to the optical domain by means of an optical reference and modulator. In the receiver, a telescope is used to focus the incoming beam and turn it into an electrical signal by combining it with a pseudo-random noise sequence fed to a local oscillator. A feedback loop is used to ensure the local oscillator remains at the same phase and frequency as the incoming data transmission. This is similar in concept to a phase-locked loop (PLL), which is a fundamental component of many electronic circuits.
Block diagram of optical transceiver system for space communications. Image used courtesy of Advances in Space Research
Since laser-based communication is point to point, it is also essential to have a tracking mechanism in a satellite constellation. This way, a transmitting satellite can locate a receiving satellite. This is known as an acquisition and tracking system.
The coarse positioning system (CPS) positions the transmission/reception optical system to roughly where it needs to be to properly transmit and receive optical signals. The fine positioning system (FPS) positions the system, so it can establish and maintain a laser communications link.
Flow chart showing acquisition, coarse pointing, and fine pointing. Image used courtesy of NASA (Downloads as PDF)
In conjunction, the optical transmission system and tracking system work together to establish communication links between satellites.
Photonic ICs for Satellites Gain Traction
As LEO satellite systems have gained traction, so have miniature photonic integrated circuit (PIC)-based optical communication solutions. In 2018, Tesat announced one of the world’s smallest laser communications terminals for CubeSats, called CubeL. A few years later in 2021, Tesat announced a successful test of ConLCT80, a miniature optical communications terminal with applications in U.S. governmental projects.
Tesat ConLCT80, a miniature laser communications terminal used for optical satellite communication. Image used courtesy of Tesat
Additionally, ongoing research is being done into PIC-based solutions to solve inter-satellite optical communications challenges. A PIC is a circuit that operates in the optical domain. According to researchers at UCSB, PIC-based lasers offer reduced cost, size, weight, and power compared to conventional systems. Using compact Erbium-doped fiber amplifiers (EDFA), PIC-based optical laser systems can be as small as 6 mm.
PIC-based optical communication system using compact EDFA. Image used courtesy of UCSB
Due to the extreme environmental conditions in space, these PICs must be radiation tolerant just like their electrical counterparts. NeoPhotonics Corporation, for instance, recently announced a radiation-tolerant, tunable laser for use in LEO satellite systems.
The Future of Inter-satellite Communication
With laser-based inter-satellite communications systems already deployed in LEO constellations, it is clear the technology is here to stay. In addition to providing internet broadband access through Starlink and OneWeb Technologies’ constellation, governments and defense contractors are also keen to deploy their own optical LEO satellite networks.
The United States’ Space Development Agency plans to deploy its own LEO constellation equipped with sensors for a variety of military uses. Additionally, the agency plans to work with L3Harris Technologies and SpaceX to use an LEO constellation for advanced missile tracking capabilities.