One more spacecraft to predict weather

British satellite manufacturer SSTL will build the follow-on spacecraft for an innovative weather forecasting system run by Taiwan and the US. The COSMIC constellation derives information about the atmosphere from the way it disturbs GPS signals. A clutch of spacecraft currently provide this service, and SSTL has been contracted to provide up to 12 more satellites. The intention is to launch a new batch in 2016.

“It’s a great win for the company, beating off international competition, and it’s also a very interesting project,” said Alex da Silva Curiel from SSTL. “It involves operational meteorology, good climate science and ionospheric science,” he said.

The Cosmic project is a joint initiative between NSPO, the National Space Organisation of Taiwan, and NOAA, the National Oceanic and Atmospheric Administration, an American body. It relies on a smart observation technique known as radio occultation. This recognises that a transmission from a global positioning satellite, which ordinarily is used to plot a time and a position, will be bent as it passes through air.

The degree to which this refraction occurs is dependent on the properties of the atmosphere at that moment, and can be exploited to pull out information about temperature, pressure and water vapour — key parameters required in weather forecasting. NSPO/NOAA’s Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) operates a series of small spacecraft in a low-earth orbit to make these observations. But this satellite series, referred to as FORMOSAT-3, is beginning to fail and needs replacing.

The satellite manufacturer has now been engaged to build the next generation – FORMOSAT-7. The multimillion-pound contract will see an initial six satellites launch in three years’ time. A further six could follow in 2018. Also, SSTL will assemble the chassis, or bus, of every spacecraft. The satellite manufacturer is basing the bus design on its “100 series” of earth observation spacecraft. These take the shape of a cube, but NSPO/NOAA’s desire to launch six satellites at the same time means the profile of the spacecraft will be more wedgelike to make them all fit on the one rocket. “We basically take the circular cross section of the launcher as seen from the top, and divide it by six. And that is the available shape for each spacecraft.

That makes for a long and thin shape,” explained da Silva Curiel. “We can still use the same electronics that we would normally use in a SSTL-100 series satellite, but the structure is a new structure.”
Each spacecraft will weigh just under 200 kg. The first batch would go into low-inclination orbits, to provide data of greatest value to Taiwan. The second set would be placed in high-inclination-angle orbits to retrieve information relevant to higher latitudes on the globe. FORMOSAT/COSMIC observations have proven very useful in medium-range forecasting – those projections that look several days ahead.
Jonathan Amos