Read channels and scan coverage

The FY-4A instrument description lists AGRI, a 14-channel imager; GIIRS, an infrared sounder; and LMI, a lightning mapper, alongside space-environment instruments. An imager maps radiation across a scene. A sounder’s spectral measurements help estimate atmospheric structure, while a lightning mapper detects brief optical events. These are different measurement roles on the same platform.

The museum photograph shows a Fengyun-4 model body. It helps make the spacecraft tangible, but an exhibit model cannot establish a channel’s sensitivity or an instrument’s performance. For those questions, use the specification and validation evidence for the identified flight instrument. Here the specification example is Feng Yun 4A, not every member of FY-4.

NSMC gives FY-4A AGRI examples of a 15-minute full-disk scan and a one-minute scan of a 1,000-by-1,000-kilometer sector. The smaller region allows more frequent updates within that region. Do not turn the sector specification into a claim that the entire visible Earth is updated once a minute.

Imagine a study follows two storm regions, only one inside a selected rapid sector. Its observation table should keep the regions’ actual acquisition times separate. Also distinguish acquisition cadence from delivery delay: a scan may be complete before a processed product reaches a forecaster. Useful timing includes both how often a measurement is made and how old it is when used.

An infrared image relates measured radiation to a brightness temperature. Depending on the channel and scene, the signal may come from cloud tops, the surface and the intervening atmosphere. A cold, opaque cloud top is not a reading of the air temperature for someone standing underneath it. Low clouds and a similarly cold surface can also be hard to separate.

In an invented sequence, a tracked cloud feature has brightness temperatures of 250, 244 and 238 kelvin at fifteen-minute intervals. It cooled by twelve kelvin over thirty minutes, or 0.4 kelvin per minute. That calculation describes the observed sequence. It does not by itself prove a rainfall amount or a future storm outcome; interpretation needs the channel, feature tracking, other observations and a suitable method.