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Pole position

MetOp’s polar orbit

Before the advent of MetOp, European weather-forecasting satellites (the Meteosat series) had all been positioned in geostationary orbit, at around 36,000 kilometres above the equator, such that they circle the planet at the same rotational speed as the Earth itself. This means that, from the perspective on someone on the ground, these weather satellites seem to remain stationary, fixed above one point over the Equator.

Geostationary orbits have great benefits: from so high up, a large part of the Earth’s surface is in view and the same satellite always has the same geographical zone under surveillance. This vantage point gives the big picture, enabling continuous monitoring of atmospheric conditions and the evolution of meteorological events for short-term weather prediction. However, there are disadvantages of this orbit. Any given geostationary satellite can only ever see one, always the same, side of the Earth, and some areas of the Earth are outside the scope of view of a satellite that is placed over the Earth’s equator (only the areas between latitudes of about 81° north and south are visible); notably, the northern and southern polar regions cannot be seen from geostationary orbit. Full Earth observation is impossible to do without researching and monitoring the poles – in the truest sense, they are our Earth’s climatic chambers: the ice cover, temperature, precipitation and wind activity there influences the whole planet, and particularly the regions at high latitudes like Europe and a large part of North America. More detailed information than can be supplied by the geostationary satellites alone is required for weather forecasts, and in particular to feed the very complex Numerical Weather Prediction models used by the meteorologists for the short-term forecast.

Images captured by MetOp-A (© Eumetsat)

Images captured by MetOp-A (© Eumetsat)

Polar precision

Satellites such as MetOp which operate from a lower-altitude (around 800 km as opposed to the geostationary 36,000 km) ‘polar’ orbit, provide necessary additional data. At this altitude, so much closer to the phenomena they are tracking than their geostationary counterparts, their observations can be much more detailed, although the area of view is less wide. (Also, the lower orbit allows microwave radiometers to be used, which require relatively large antennas in order to achieve ground resolutions fine enough to be useful. The advantage of microwave radiometers is their ability to measure through clouds to sense precipitation, temperature in different layers of the atmosphere, and surface characteristics such as ocean surface winds.)

Sun-synchronous orbit

And, crucially, they scan the entire globe. They can do this because, unlike geostationary satellites, their orbit does not follow the equator around the Earth but dissects it as they travel around the poles: thus the Earth is constantly turning underneath the path of the satellite, making for a different geographical view with each orbit. MetOp circles the Earth at a mean altitude of 830 kilometres in a very-nearly polar orbit (inclined at 98.7° to the equator), taking about 100 minutes to complete one orbit, during which time the Earth has rotated some 25°, and passing over the poles 14 times every 24 hours. As the orbit shifts continuously due to this rotation, MetOp, with its suite of measuring instruments, passes over each point on the Earth’s surface in a five-day cycle. However, thanks to the large swath of most of its instruments, a global picture can be built up in a couple of days.

Good morning, MetOp!

MetOp flies in the ‘morning orbit’: that is, MetOp always crosses the equator at 9.30 a.m. in a north–south direction on the sunlit side of the Earth. The satellite’s orbit is so designed that it always passes over its target areas at a constant local time – when MetOp overflies a particular point on the Earth’s surface it always does so at exactly the same time.

This is because MetOp follows what is called a sun-synchronous trajectory, whereby the plane of the satellite’s orbit remains at a fixed angle with respect to the sun as the Earth revolves under the satellite. (The orbital plane of a sun-synchronous orbit also rotates by approximately one degree each day, to keep pace with the Earth’s revolution around the sun.) In other words, from MetOp’s point of view, the sun is always in exactly the same place for a given point of the satellite orbit, and that means that it is always the same time.

MetOp follows a sun-synchronous trajectory

MetOp follows a sun-synchronous trajectory

One benefit of always overflying a spot at the same local time is that the surface illumination angle will be nearly the same every time the satellite takes observations of a given spot. This consistent lighting is a very useful characteristic for weather and other remote-sensing satellites, for monitoring the evolution of meteorological events in a given area over time.

This orbit was selected so that MetOp can work in conjunction with the ‘afternoon satellite’ of the US National Oceanic and Atmospheric Administration (NOAA), which, flying in a complementary low Earth orbit, crosses the equator (sunlit side) at 2:30 p.m local time. Together, these two orbits maximise the coverage area over which the observations are made. The European and American systems have a collaboration agreement (the Initial Joint Polar System) and share a number of common instruments on-board all satellites. All the data from both the MetOp and the NOAA weather satellite systems are constantly exchanged and redistributed to hundreds of users around the world in only 2 hours 15 minutes of the observations being made in space.

MetOp and the NOAA satellite: complementary orbits

MetOp and the NOAA satellite: complementary orbits

MetOp