Combine two observing branches

Japan’s Global Change Observation Mission, GCOM, was designed for sustained observations of water circulation and climate. Its two branches are GCOM-W and GCOM-C. The program’s long-term observing ambition describes a coordinated record, not a guarantee that every individual spacecraft will function for the entire program duration.

Shizuku, or GCOM-W, launched on May 18, 2012. Its AMSR2 instrument receives microwaves emitted naturally by the surface and atmosphere. This is passive sensing: unlike an active radar, it does not illuminate the target with its own transmitted pulse. A receiving antenna can be large and mechanically active while the measurement remains passive.

Shikisai, or GCOM-C, launched on December 23, 2017. Its SGLI instrument measures across wavelengths extending from near ultraviolet to thermal infrared, supporting studies of clouds, aerosols, vegetation, ocean color, snow and ice. Optical and thermal information complements the microwave branch rather than duplicating it.

In an original observation-planning exercise, a team has a broad microwave product and a more spatially detailed optical view of the same region. Before comparing them, the team checks their dates, footprints and measured quantities. A disagreement can reflect different observation conditions or scales. It does not automatically establish that one whole mission is wrong.

The launch photograph records H-IIA Flight 21 carrying Shizuku from Tanegashima. The spacecraft is enclosed in the fairing; neither its antenna nor its measured microwave signals are visible. The map later in this course shows a different stage of the work: observations transformed into a public information product.

Imagine comparing a launch checklist with a data-quality checklist. The first might establish successful transport and separation; the second examines instrument calibration, processing and validation. Both matter, but passing the first does not close every item on the second. GCOM’s scientific contribution comes from a sustained chain beyond launch.