The JWST communication subsystem provides 2-way communications with the observatory via the NASA Deep Space Network.

JWST's spacecraft bus communications subsystem provides 2-way communications between the Observatory and the Flight Operations Team at the Mission Operations Center in STScI.  This subsystem enables command and control of the Observatory, and provides telemetry and science data downlink. S-band frequencies are used for command uplink, low-rate telemetry downlink, and ranging. Ka-band frequencies are used for high rate downlink of science data and telemetry. All communications are routed through NASA's Deep Space Network of 3 ground stations located in Canberra (Australia), Madrid (Spain), and Goldstone (USA). There are limits on the onboard data volume and data accumulation rates that are based on the SSR storage capacity and the downlink capacity of the integrated system.

Onboard antennas

JWST has a 0.6 m Ka-band high-gain antenna (HGA) as well as a 0.2 m S-band medium-gain antenna (MGA). Both are mounted on a common articulated platform, generally referred to as the HGA platform. The HGA platform can be articulated to point at the Earth for any orientation of the observatory that conforms to an allowed field of view for the telescope. The broad beam pattern of the MGA ensures that 40 kbps real time S-band telemetry is available with any visible ground station. S- and Ka-band links can be operated simultaneously and support all communications for commissioning and normal operations.

The Ka-band downlink data rate has 3 selectable speeds: 0.875, 1.75, and 3.5 Mbytes/s. The highest speed is the default for JWST operations. The lower rates can be selected when needed, such as, for bad weather at the ground station.

High-gain antenna

Routine 2-way communications, including downlink of science data from the solid state recorder, can occur during science observations and during slews. As seen from Sun-Earth L2, the Ka-band downlink has a beam width about the same angular size as Earth. As such, the HGA pointing must be periodically adjusted to stay centered on the Earth. The HGA repointing maneuvers are expected to result in a small but measurable pointing disturbance, so they are planned not to occur during science integrations. In order to ensure communications, the HGA must be moved every 10,000 s, which sets a limit on the maximum nominal duration of a science integration. There are exceptions to this limit for certain observing modes requiring long uninterrupted integrations. In long duration exposures, the HGA will move as needed in order to maintain communications during a contact. This may result in short periods where the science data quality is impacted by the antenna movement disturbance.  Any science data files during which HGA movement has occurred are flagged to alert the observer.

Some observatory engineering activities can only take place during a real time communications contact and require the suspension of science observations.