As noted in previous articles in this series, the JWST is parked at the Lagrange point L2. This is the point of gravitational equilibrium, located about 1.5 million kilometers from the Earth in a straight line between the planet and the Sun. This is the ideal location for the JWST to observe the universe without obstruction and with minimal orbital adjustments.

However, being so far from Earth means the data must travel farther to return safe and sound. It also means that the communications subsystem must be reliable, because the prospect of sending a repair mission to fix the problem, at least in the near future, is highly unlikely. “Given the cost and time,” says Michael Menzel, JWST systems engineer, “I wouldn’t encourage a rendezvous and maintenance mission unless something goes wrong.”

According to Menzel, who has been working on JWST for over 20 years, the plan has always been to use the well-understood K
_a-band frequencies for volume transmissions of scientific data. Specifically, JWST transmits data back to Earth over a 25.9 gigahertz link at up to 28 megabits per second. To_a-band is part of the wider K-band (another part, Ku-band, was also considered).

Lagrange points are equilibrium points at which competing gravitational forces on an object are reduced to zero. JWST is one of three vehicles currently occupying L2 (shown here at an exaggerated distance from Earth). IEEE Spectrum

Both the data acquisition and transmission rate of the JWST dwarf those of the older Hubble Space Telescope. Compared to Hubble, which is still active and generates 1 to 2 gigabytes of data daily, JWST can produce up to 57 GB each day (although this amount depends on planned observations).

Menzel says he first saw JWST frequency proposals around 2000 while working at Northrop Grumman. He became a mission systems engineer in 2004. “I knew what the risks were in this mission. And I wanted to make sure we didn’t have any new risks,” he says.

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In addition, K
_a-band frequencies can transmit more data than the X band (7 to 11.2 GHz) or S band (2 to 4 GHz) commonly chosen for deep space ships. A high data rate is required for the scientific work that JWST will be doing. In addition, according to Carl Hansen, a flight systems engineer at the Space Telescope Science Institute (JWST Science Operations Center), a comparable X-band antenna would be so large that the spacecraft would have trouble maintaining stability when shooting.

Although 25.9-GHz K
_a-band is the working channel of the telescope, it also uses two channels in the S-band. One of them is the 2.09 GHz uplink, which transmits future transmissions and schedules of scientific observations to the telescope at 16 kilobits per second. The other is a 2.27 GHz 40 kbps downlink, over which the telescope transmits technical data, including its operating status, system status, and other information related to the telescope’s day-to-day operations.

Any scientific data that JWST collects during its existence will need to be stored on board because the spacecraft does not maintain 24/7 communication with Earth. Data collected from its science instruments is stored on the spacecraft’s 68GB SSD after collection (3 percent reserved for engineering and telemetry data). Alex Hunter, also a flight systems engineer at the Space Telescope Science Institute, says that by the end of the JWST’s 10-year lifespan, they expect to reduce memory to about 60GB due to deep space radiation and wear and tear.

The built-in storage is enough to collect data for approximately 24 hours before it runs out. Long before this becomes a problem, JWST will have a planned capability to relay this priceless data back to Earth.

JWST will remain connected through the Deep Space Network (DSN), a resource it shares with the Parker Solar Probe, the transit satellite for exoplanet exploration, the Voyager probes, and the entire rover and orbiter ensemble, to name but a few. heavyweights. The DSN includes three antenna complexes: Canberra, Australia; Madrid, Spain; and Barstow, California. JWST needs to share antenna end time with a variety of other deep space missions, each with unique communication needs and schedules.

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Sandy Kwan, DSN systems engineer, says spacecraft contact windows are scheduled 12 to 20 weeks ahead. JWST had more planned contact windows during the commissioning phase as tools were commissioned, tested and calibrated. Much of this process required real-time communication with the Earth.

All communication channels use the Reed-Solomon Error Correction Protocol, the same error correction standard used in DVDs and Blu-ray discs, as well as QR codes. The lower data rate S-band channels use binary phase key modulation, which includes a phase shift of the signal’s carrier wave. The K-band channel, however, uses quadrature phase key modulation. Quadrature phase-shift keying can double the data rate of a channel at the expense of more sophisticated transmitters and receivers.

JWST’s communication with Earth includes an acknowledgment protocol – only after JWST receives confirmation that the file has been successfully received will it delete its copy of the data to clear space.

The communications subsystem was assembled with the rest of the spacecraft bus by Northrop Grumman using off-the-shelf components sourced from several manufacturers.

The development of JWST has been long and often delayed, but its communication system has always been the basis for the rest of the project. Maintaining the reliability of at least one system means one less problem. Menzel remembers, for example, the ideas of laser optical systems, which were invariably rejected. “I can count at least two times I’ve been approached by people who wanted to experiment with optical communications,” says Menzel. “Every time they came to me, I sent them off with the old “Thanks, but I don’t need this.” And I don’t want that.”