Mission: science – meteorological data collection
Launch date: 2006
Type of orbit: low-Earth orbit, non-Sun-synchronous
Period: 100 minutes
Perigee: 496 km
Apogee: 540 km
Dr Adrian McDonald
The satellite that I use mostly to measure temperature is something called COSMIC/FORMOSAT-3. It’s not actually one satellite, it’s a constellation of six low-Earth orbiting satellites so they’re about 500 kilometres away from the Earth’s surface. And what those satellites do is look at signals from the GPS satellites. Those signals can be used to make measurements of temperature, and that’s associated with the fact that, if we look from our low-Earth orbiting satellite forward towards our GPS satellite, rather than that path being perfectly straight, it’s slightly bent, so it’s refracted.
And that small bending angle, you can measure that because you know the GPS satellite’s got a very accurate clock on it, and that very accurate clock allows you to time the measurement from the GPS satellite to the low-Earth orbiting COSMIC/FORMOSAT satellite. And by knowing the time, and we know that the fact of the speed of light is a constant, we can work out the distance, and therefore our distance is a curved path rather than a straight path, and the level of curvature of the path tells us about the temperature, because refraction is controlled by the temperature of the atmosphere.