By John Oncea, Editor
Free space optical communication is a transformative technology that will enable unprecedented data capabilities for space missions, allowing them to transmit more data than ever before while reducing size, weight, and power requirements.
Free space optical communications (FSO) – also known as satellite laser communication, laser communication in space, or lasercom – is a technology that uses infrared lasers to transmit data between satellites or between a satellite and a ground station. It offers several advantages over traditional radio frequency (RF) communication systems, including higher data rates, lower power consumption, and increased security.
FSO has been demonstrated on various small satellite and CubeSat missions, such as SOTA (10 Mbps) by Japan’s NICT, OSIRISv2 (1 Gbps) by DLR in Germany, and OCSD (first high-speed laser downlink from a CubeSat) by NASA. These missions have shown the viability of lasercom for small satellites and the rapid advancement of the technology which works by transmitting modulated laser beams carrying digital data encoded in its intensity or phase variations between communicating entities.
Nixon Calls The Moon
On July 20, 1969, President Richard Nixon, Neil Armstrong, and Edwin Aldrin had an interplanetary conversation – Nixon from the comfort of the Oval Office, Armstrong and Aldrin while standing on the moon. “This certainly has to be the most historic telephone call ever made from the White House,” Nixon told them. “Because of what you have done, the heavens have become a part of man’s world, and as you talk to us from the Sea of Tranquility, it inspires us to redouble our efforts to bring peace and tranquility to earth.”
The call “was routed from Washington to Mission Control in Houston, and from there, it bounced to Manned Space Flight Network dish antennas scattered around Earth, traveled 238,000 miles to the Apollo Lunar Module, and finally hopped to its ultimate destination: antennas attached to the backpacks carried by Armstrong and Aldrin. Armstrong’s response was then transmitted back to the Oval Office.”
The call utilized an S-Band Transponder developed by General Dynamics Corp. that enabled communications between Apollo 11 and Mission Control. This transponder had to be specially designed to withstand extreme conditions like cold, heat, and radiation.
The technology was developed by the U.S. Air Force, U.S. Geological Survey, and corporate contractors like BellCom (a Bell System subsidiary formed at NASA’s request). BellCom and Bell Labs scientists played a crucial role in developing the communications system for the space program.
So, while Nixon used a regular green telephone in the Oval Office, the call went through multiple stages involving advanced transponders, dish antennas, and specialized equipment to facilitate long-distance communication between Earth and the Moon.
To say things have changed since then would be an understatement. According to General Dynamics Mission Systems:
- Bandwidth and data rates have increased exponentially, allowing for much higher-quality video and data transmission from spacecraft. The Apollo missions could only transmit low-resolution video and limited telemetry data.
- Modern communications rely heavily on digital technology and computer networks, whereas Apollo was entirely analog. This allows for more efficient data encoding and transmission.
- Satellite communications networks provide global coverage today, whereas Apollo had to rely on a limited number of ground stations to communicate as the spacecraft moved.
- Modern spacecraft use higher frequency bands like Ku-band and Ka-band which have more bandwidth compared to the S-band and VHF systems used for Apollo.
Advances in electronics have made communications systems smaller, lower power, and more robust compared to the bulky equipment of the 1960s. Private companies like SpaceX now develop their own advanced communications systems for spacecraft, supplementing NASA’s networks. The internet and modern networking allow real-time communications and data sharing between ground stations, engineers, and controllers in a way that was impossible during Apollo when communications were point-to-point.
While the core principles are the same, today’s space communications are orders of magnitude more capable, efficient, and globally interconnected compared to the systems that first brought humans to the lunar surface. Will FSO move the need the same way?
The Benefits Of FSO
“Although RF systems are typically used for low-rate space communication,” writes NASA, “recent developments in FSO communications have made it a compelling alternative to RF systems, particularly for high-rate communication.”
FSO systems are made up of a transmitting terminal and a receiving terminal. To transmit information, the data is modulated onto electromagnetic waves at optical frequencies and sent to the receiving system over a channel. FSO links operate at much higher frequencies than RF links, usually at near-infrared bands like 1064 nm or 1550 nm.
Visible light is not often used due to safety concerns for technicians at the terminals. The use of higher frequencies and wider bandwidths can support higher data rates, but the shorter wavelengths also result in narrower beam widths that require more accurate and precise pointing toward the communication terminal. Other advantages offered by FSO include:
- High Data Rates: FSO systems can achieve much higher data rates than traditional RF systems. This enables faster transmission of large amounts of data, which is particularly useful for applications such as Earth observation, scientific research, and high-definition video transmission.
- Low Power Consumption: FSO systems typically require less power than RF systems, making them suitable for satellites with limited power resources.
- Narrow Beams: Laser beams are highly focused and can be precisely directed, resulting in reduced interference and better security compared to RF communication.
- Less Susceptible to Interference: Laser communication is less susceptible to interference from electromagnetic radiation and can operate in environments where RF communication may be impractical or ineffective.
FSO can provide 10 to 100 times more effective bandwidth than RF systems, writes NASA. This allows missions to transmit significantly more data, such as high-definition video and imagery, back to Earth. FSO equipment can be more compact and lightweight compared to RF systems, making it attractive for space missions where weight and volume are critical considerations while freeing up space and resources for science instruments45. For example, despite enabling unprecedented data capabilities, NASA’s Laser Communications Relay Demonstration (LCRD) payload is only the size of a standard king-sized mattress.
Finally, the narrow beamwidth of lasers and the receiver’s small field of view make lasercom links difficult to detect, intercept, or interfere with, as an adversary must be physically located within the beam.
All of this, in addition to FSO operating in the optical spectrum thereby avoiding the crowded RF bands like S-band and X-band that have been used for decades, make the technology an attractive alternative.
The Challenges Of Using FSO
Challenges for FSO include the precise pointing, acquisition, and tracking (PAT) required for inter-satellite links, especially when dealing with high relative velocities between satellites in different orbits, NASA writes. Atmospheric effects also can impact space-to-ground links, necessitating the use of adaptive optics and other mitigation techniques. Other challenges include:
- Atmospheric Attenuation: Laser beams can be affected by atmospheric attenuation, especially during adverse weather conditions such as rain, fog, or clouds. However, advancements in adaptive optics and error correction techniques help mitigate this issue.
- Pointing, Acquisition, and Tracking (PAT): FTO requires precise pointing, acquisition, and tracking of the laser beams between the communicating entities. This necessitates sophisticated mechanisms and algorithms to maintain alignment, especially for mobile or agile platforms.
- Space Debris and Safety: Concerns about space debris and the safety of laser beams necessitate careful planning and coordination to avoid collisions and ensure compliance with international regulations.
- Initial Setup Cost: While laser communication offers numerous advantages, the initial setup cost can be higher compared to traditional RF systems. However, with advancements in technology and increased adoption, the cost is expected to decrease over time.
Despite these challenges, lasercom is being rapidly adopted by both commercial and government space missions. NASA’s LCRD, launched in 2021, is demonstrating the agency’s first two-way laser relay communications system. The Space Development Agency (SDA) is incorporating lasercom into its future satellite architectures, with each satellite having 3-5 laser links to communicate with other satellites, aircraft, ships, and ground stations.
Where Things Currently Stand
FSO technology has been demonstrated in various space missions, including experiments conducted by space agencies such as NASA, ESA, and others. Ongoing research and development efforts are focused on improving the efficiency, reliability, and scalability of laser communication systems.
Commercial entities are increasingly investing in laser communication technology for applications such as satellite internet constellations, Earth observation, and space exploration. Future advancements may involve the integration of laser communication with other emerging technologies such as quantum communication for enhanced security and performance.
FSO offers significant advantages over traditional RF systems, including higher data rates, lower power consumption, and increased security. While challenges such as atmospheric attenuation and precise beam alignment need to be addressed, ongoing research and development efforts are driving the widespread adoption of this technology in various space applications.
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