JWST Solid State Recorder
JWST can store at least 58.8 Gbytes of science data. Science data downlinks occur in two 4-hr contacts per day where each contact can transmit at least 28.6 Gbytes of recorded science data.
JWST downlinks science data in two 4-hr contacts per day; each contact can transmit at least 28.6 Gbytes of recorded science data to the ground.
Limits on data rates and data volume
APT calculates the expected data rate for observations and warns users if planned observations may exceed the data volume limits. When constructing the weekly observation plan, the JWST planning system verifies that the data rates within each visit are acceptable, and that the data volume to be accumulated between contacts will not exceed the downlink capacity or the SSR capacity.
Sending science data to Earth
During normal science operations, JWST will downlink data in 4-hour contacts, nominally occurring twice per day, approximately 12 hours apart. In one contact, JWST can transmit at least 28.6 Gbytes of recorded science data. If a contact is missed, science observations can continue without filling the recorder, and the ground can catch up on the next contact.
Data rate limits within ISIM
The rate at which science data can be written to the SSR is regulated by the ISIM Command and Data Handling subsystem (ICDH). The maximum ICDH sustained data rate is about 48 Mbits per second, including data packetization overheads. This corresponds to about six 2048 × 2048 full frame image files every 10.5 s. The actual data rate depends on the number of detectors simultaneously in use, their exposure parameters, and the precise timing of when their exposure readouts arrive in the ICDH for processing. The number of detectors in use at any one time could be as large as 14. For example, observations with both NIRCam modules (10 detectors), along with parallel NIRSpec observations (two detectors), and the FGS for guiding would be sending data from 13 detectors to the ICDH. The relative timing of the arrival of data packets is unpredictable, and this uncertainty is factored into the 48 Mbps limit.
To prevent the loss or corruption of packets, the APT templates set the number of detectors in use and the rate at which data is generated. For example, in the NIRCam rapid readout mode, only one NIRCam module (five 2K × 2K detectors) can be used with Ngroups = 1. To use both modules (ten 2K × 2K detectors) in rapid readout mode requires Ngroups = 2. Combinations using multiple instruments must stay within the 48 Mbps limit.
This article uses the S.I. definitions of gigabyte and megabyte: 1 Gbyte = 109 bytes, and 1 Mbyte = 106 bytes.