How is free space laser communication used with satellites and moon bases?

Free-space laser communication, also called satellite laser communication or lasercom, is a technology that uses lasers to transmit data between satellites and ground stations or from satellite to satellite. It’s used in space to send high-resolution images, videos, and data from spacecraft, including deep space missions and the International Space Station (ISS), to Earth. It’s also expected to support a “LunaNet” and provide high-bandwidth connectivity and other services for a future moon base.

Inter-satellite free-space laser communication is implemented with an optical inter-satellite link (OISL) architecture that replaces radios with infrared (IR) lasers. OISL supports higher bandwidth and more secure communication, and the system is lighter and more flexible than conventional radio communications.

OISL networking will be integrated into NASA’s Space Communications and Navigation (SCaN) system. SCaN is a comprehensive program that manages and directs the ground-based facilities and services for the Deep Space Network (DSN), Near Earth Network (NEN), Space Network (SN), and the Tracking and Data Relay Satellites (TDRS).

A major benefit of the OISL network is the higher data rates, which enable data to be downloaded using shorter contact times. This results in less spacecraft power, potentially extending mission durations, and requires fewer relay terminals and ground stations.

The laser communications relay demonstration (LCRD) platform was launched into orbit in 2022, about 22,000 miles from Earth, to test the capabilities of laser communications. LCRD was NASA’s first technology demonstration of a two-way laser relay system. The LCRD is now integrated with the Low-Earth Orbit User Modem and Amplifier Terminal (ILLUMA-T).

The integrated system sends data at 1.2 Gbps and was designed to use laser communications to enhance the ISS’s data capabilities. The ILLUMA-T sends data to the LCRD, relaying it to Hawaii or California ground stations (Figure 1).

Figure 1. The LCRD (top right) acts as a data communication relay station connecting the ISS with ground stations. (Image: *NASA*)

Depending on the mission requirements, free-space laser communication networks can use a variety of optical modulation techniques. Some of the common ones include:

On-off keying (OOK) switch keying is the simplest implementation of amplitude-shift keying (ASK) modulation. It has a large average transmission power but is more spectrally efficient than frequency-shift keying.

Digital pulse interval modulation (DPIM) uses intensity modulation. DPIM has a higher transmission capacity than asynchronous pulse-position modulation (PPM) but a slightly higher error probability.

Level pulse-position modulation (LPPM) uses single pulse positions. LPPM can achieve a high data transmission rate for deep-space optical communication with a small laser pulse repetition frequency.

Differential pulse position modulation (DPPM) is a very power-efficient modulation technique that can support high data rates, does not require synchronization, and limits the propagation of errors.

Quadrature amplitude modulation (QAM) transmits digital information by combining two carrier waves that are 90° out of phase. This technique is used in lasers to generate complex modulation formats and can support high-speed transmissions over long distances.

LunaNet

LunaNet is a joint project between NASA and the European Space Agency (ESA). It’s sometimes called a Lunar Internet and will provide several services for cis-lunar spacecraft and installations between the Earth and moon and on the moon.

It will implement a delay/disruption-tolerant network (DTN) architecture that can store and forward data, eliminating the need to reschedule data transfers and maximizing network bandwidth utilization. DTN also improves network robustness. If there’s a disruption in connectivity between nodes, DTN stores the data until the network comes back online. It can be implemented with several topologies (Figure 2).

Figure 2. Examples of LunaNet connectivity options. (Image: *UpGreat*)

LunaNet will support operational independence from terrestrial-based data processing for high-precision lunar navigation. The network elements will provide necessary measurements for onboard orbit determination and guidance for satellites orbiting the moon or lunar surface.

LunaNet will provide greater situational awareness for astronauts, rovers, and other activities on the lunar surface. It will monitor space weather independently of Earth-based systems and provide timely alerts to potentially dangerous solar activity.

Summary

Free-space laser communication combines higher data rates with lighter-weight systems to enhance mission performance and extend satellite lifetimes. It will also serve as the backbone technology for LunaNet, which will use a DTN architecture to deliver additional services like improved navigation and space weather forecasting for cis-lunar satellites and lunar base operations.