Weather Satellite Product

Overview

A geostationary weather satellite stares. Parked at 35,786 km over a fixed longitude, it sees the same third of the planet continuously, which is what distinguishes it from polar orbiters that revisit a scene twice a day. The staring geometry is what makes hurricane loops, rapid-scan severe-storm monitoring, and continuous lightning-era imagery possible: the Imaging Radiometer delivers a full-disc image every 10 minutes and can interleave 30-second mesoscale sectors over an active storm.

The price of the high perch is resolution and signal: a 0.5 km visible pixel at that range subtends 14 microradians, so the whole spacecraft is built around optical stability — a Instrument Optical Bench of graphite composite, Reaction Wheel sets selected for low jitter, and pointing knowledge at the 25 µrad level from the Attitude Control Subsystem.

The imager

The Imaging Radiometer is a scanning radiometer. Two Scan Mirror Assembly assemblies sweep the line of sight: one scans east-west swaths at constant rate, the other steps north-south between swaths, with 21-bit Encoder feedback on each axis — angle knowledge fine enough that each pixel's ground location is known to a fraction of its own width. The Imager Telescope feeds dichroics and Spectral Band Filter stacks that split the beam into 16 bands: visible and near-IR channels on silicon arrays, and longwave channels such as the 6.2 µm water-vapour and 10.3 µm window bands on HgCdTe IR Detector arrays held at 60 K by the Pulse-Tube Cryocooler.

Infrared meteorology is absolute radiometry: a cloud-top temperature is only as good as the calibration, so every scan cycle the mirror glances at the Onboard Blackbody blackbody and at cold space, bracketing each pixel between two known radiances. That keeps brightness temperatures traceable to about 0.1 K across 15 years.

The Atmospheric Sounder complements the imager with vertical profiles. Its Michelson Interferometer Fourier-transform spectrometer resolves thousands of narrow channels across the CO2 and water vapour bands; because each channel's weighting function peaks at a different altitude, temperature and humidity can be retrieved layer by layer — the profiles that feed numerical weather prediction directly.

Spacecraft engineering around the instruments

The instruments dictate the layout. The single Solar Wing flies on the south side so the north panel never sees the Sun; that face hosts the Cryogenic Radiator the cryocooler rejects into. The Solar Array Drive turns the wing one revolution per day, passing power through its Array Slip Ring, and the Li-ion Battery carries the full observing load through the 72-minute equinox eclipses — forecasting does not pause at satellite midnight.

Getting to GEO consumed more than half the launch mass: the Apogee Engine burned roughly 2,800 kg of propellant from the Propellant Tank pair over several apogee passes to circularize the transfer orbit. On station, lunisolar perturbations tilt the orbit about 0.85° per year, countered by north-south firings of the Stationkeeping Thruster set during planned windows when imaging can tolerate the disturbance.

Data paths

Raw instrument data — about 120 Mbit/s — streams continuously through the X-band Downlink Transmitter to the ground processing centre, where images are navigated, calibrated, and remapped. The processed products come straight back up and out through the L-band Rebroadcast Transponder transponder, so any field terminal under the footprint receives finished imagery with no terrestrial network. Two further relays ride along: the Data Collection Transponder collects bursts from thousands of buoys and river gauges, and the Search & Rescue Transponder repeats 406 MHz distress beacons into the COSPAS-SARSAT rescue system.

The Space Weather Instrument Suite suite watches the other direction. The Solar X-ray Imager monitors solar flares, the Energetic Particle Sensor pair counts the energetic protons that precede radiation storms, and the Fluxgate Magnetometer on its 8 m Magnetometer Boom resolves nanotesla signatures of geomagnetic storms — warnings that protect power grids, aviation, and other satellites, issued from the same platform that tracks the hurricanes below.