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Data rate

As stated in Johns et al. (2008), JWST is the first L2 mission to be defined as a high data rate mission. JWST is also the first mission that pushes the spectrum allocation group to design a new spectra band in the 26 GHz Ka-band to meet data rates of more than 8 Mbps. JWST pushes for enhancements to the DSN capabilities that previously were limited to 5 Mbps. JWST requirements are to downlink 270 Gb science and engineering data every day. One of the main challenges for missions beyond the moon (300,000 km+) is in the spacecraft to earth communications.

The scheme of the communications between the space-craft and the NASA ground stations is illustrated here 

Communications

Geostationary satellites have straight forward satellite to earth communications since they are stationary over the same spot of the earth, Low Earth Orbiting (LEO) satellites have many alternatives with ground stations and the for NASA missions the Tracking and Data Relay Satellite System (TDRSS), but the challenges for the JWST mission are that:

Due to the earth rotation each viewing period of from a given ground station is between 8 – 14 hours a day. Communication coverage can vary greatly, based on the satellite’s ground track and latitude of ground stations.

Ranging is required for JWST, using alternate ground stations in the southern and northern hemisphere. For LEO and L2 missions the accuracy of the ranging is dependent on the tracking of the spacecraft across the sky. For the JWSTs L2 orbit, 21 days of tracking equals about 15 minutes of tracking for a LEO spacecraft.

The JWST original concept was to have a daily 8-hour contact using X-band with an 8 Mbps downlink rate. 8 Mbps required an allocation of a 20 MHz X-band frequency. The NASA Spectrum office objected to provide more than the 10 MHz band in X-band range and suggested using Ka-band. JWST project decided to move to K-band and have one (1) 4-hour contact per day for communication and ranging. Furthermore, data will be transmitted to Earth in an uncompressed format.

 

It is foreseen that MIRI will observe 14% of the time during the first 650 days, which means 91 days of observations (8640000 sec.). In fast reading mode (slow mode will be used only in very special cases), the frame time is 2.7 sec for 4Mb per frame, which yield to 13 Tb. Considering side product data (including housekeeping) we estimate that 26 Tb of MIRI data will arrive to Earth during the first two years of the mission. MICE envisage therefore acquiring a 100 Tb disk for data archiving.

 

The main reason for archiving the MIRI data in MICE is for being able to reprocess them as the pipe line developed at STScI evolves. MICE envisages therefore using a hardware similar to that used by STScI. That is 32 to 54 Gb of RAM for running the pipeline, and a Linux Based Big multinode server array (quite a powerful machine but not a “super-computer”!).

 

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