Deep Space Optical Communications (DSOC)

NASA’s Deep Space Optical Communications (DSOC) experiment is the agency’s first demonstration of laser, or optical, communications from deep space, covering Earth-Mars distances. DSOC is a technology demonstration, which means it will test key technologies that may be used in future missions. While the Psyche spacecraft will provide power to the DSOC flight laser transceiver and help it point at Earth, the experiment is not intended to relay Psyche mission data. DSOC operations are planned for about two years, beginning roughly 20 days after launch.

How DSOC Works

The DSOC system is composed of three elements, all of which incorporate new advanced technologies:

• A near-infrared laser transceiver, attached to the Psyche spacecraft, transmits and receives data through an 8.6-inch (22-centimeter) aperture telescope. The transceiver will transmit high-rate data to Earth using its 4-watt, near-infrared laser and receive low-rate data from Earth using an attached photon-counting camera.
• A high-power (5-kilowatt) ground-based laser transmitter operated from the Optical Communications Telescope Laboratory (OCTL) at JPL’s Table Mountain facility near Wrightwood, California, will deliver a beacon and low-rate uplink data to the flight laser transceiver.
• The 200-inch (5.1-meter) Hale Telescope at Caltech’s Palomar Observatory in San Diego County, California, will receive the downlinked high-rate data from the DSOC flight laser transceiver during the technology demonstration in the first two years of Psyche’s deep space journey.

Laser communication systems come with unique advantages and challenges. Both radio and near-infrared laser communications use electromagnetic waves to transmit data, but near-infrared light packs the data into significantly tighter waves, enabling ground stations to receive more data at once. Using this kind of narrower, more concentrated laser beam from space requires incredibly accurate pointing and tracking to transfer data efficiently to a ground station.

Also, as the distance between the laser transmitter and receiver increases, the laser signal becomes weaker, requiring highly sensitive sensors to detect and record the diminished laser light. Because the laser signal is so weak over large distances, background “noise” – in the form of stray sunlight and scattered light in Earth’s atmosphere – can overwhelm the data carried by the few laser photons that arrive at the detector.

To counter these challenges and demonstrate the efficacy of deep space laser communications, DSOC’s flight laser transceiver is mounted on an assembly of struts and actuators that stabilize the optics despite spacecraft vibrations. This essentially “decouples” DSOC’s flight hardware from the spacecraft, preventing these tiny motions from nudging the pointing of the downlink laser off target. The transceiver’s telescope is also fitted with a long cylindrical sunshade (to block stray light from hitting the receiver) that protrudes from the side of the spacecraft body, making it one of Psyche’s easily identifiable features.

During operations, the spacecraft will assist the initial coarse pointing of the DSOC flight transceiver by rotating to point the flight transceiver in the general direction of the ground-transmitted beacon at Table Mountain. The DSOC transceiver has the ability to search for and then lock onto the beacon, stabilize its line of sight, and transmit the narrow high-rate data downlink beam to Palomar’s 200-inch (5.1-meter) Hale Telescope.

The large-aperture Hale Telescope then collects the faint signal and guides it to a superconducting nanowire photon-counting detector that can precisely measure and process the time of arrival of the photons. Through this back-end signal processing, the data that is modulated and encoded into the laser beam in deep space can be decoded and converted to information on the ground.

Timeline After Launch

The dates below could shift depending on how the Psyche spacecraft’s initial checkouts proceed.

• Roughly 20 days after launch: DSOC calibration and commissioning phase is expected to begin, preparing the technology demonstration for operation.
• Roughly 50 days after launch: First expected contact opportunity between DSOC ground systems and the flight transceiver aboard Psyche.
• June 2024: First phase of this technology demonstration ends.
• January 2025: Second phase of the tech demo begins.
• October 2025: DSOC tech demo ends.
  

The History and Future of Laser Communications

DSOC is a natural extension of the experiments in laser communications that have come before it. In 2013, NASA’s Lunar Laser Communications Demonstration tested record-breaking uplink and downlink data rates between Earth and the Moon. In 2021, NASA’s Laser Communications Relay Demonstration launched to test high-bandwidth optical communications from geostationary orbit and to demonstrate relay capabilities so that spacecraft don’t need to maintain a direct line of sight with Earth to communicate. In 2022, NASA’s TeraByte InfraRed Delivery system downlinked the highest-ever data rate from a satellite in low-Earth orbit to a ground-based receiver.

These demonstrations form the foundations for NASA’s operational use of laser communications. The agency’s Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal (ILLUMA-T) will launch in 2023 to the International Space Station, empowering the astronauts living and working there with new communications capabilities. The Orion Artemis II Optical Communications System will enable high-speed communications during the next human mission to the Moon.

But to test how optical communications may be possible beyond the Moon, new technologies need to be demonstrated. With its launch set for October 2023, DSOC will take optical communications into deep space for the first time. This will set the foundation for establishing higher data-rate returns from future robotic and human missions to Mars and beyond.

For more on DSOC, visit https://www.nasa.gov/mission_pages/tdm/dsoc/.