I want to welcome you to Chapter 24, Satellite Observations — Page 462, Lesson 434

I want to welcome you to Chapter 24, Satellite Observations. This is a topic that has revolutionised meteorology, especially for us in aviation. Let's start with the introduction. Meteorology has benefited considerably from the use of satellites in recent years. Apart from the obvious advantages of satellite communications over the old land-based systems—providing prompt and trouble-free communication of meteorological data—satellite photography has given us weather images that were impossible to produce in the past. Before satellites, those images were often merely 'artist's impressions' of the weather. Now we get the real picture. There are two fundamental types of satellite we need to know: the polar orbiting satellite and the geostationary satellite. And there are two methods of producing the weather picture: visual photography and infrared. We'll cover each of these in detail. Let's start with Polar Orbiting Satellites. These are sometimes called 'so-called' polar orbiting satellites, and they have been put up principally by Russia—their series is called Meteor—and by the USA—their series is called NOAA, which stands for the National Oceanic and Atmospheric Administration. Let me give you the specific orbital details for the NOAA satellite. Its orbit is inclined at an angle of 99° to the Equator. It takes 1 hour and 42 minutes to orbit the Earth. Its altitude is between 820 and 870 kilometres above the surface. And it covers a band that is 1500 nautical miles wide as it passes over. Now, here's an important operational characteristic. Each successive orbit is a little further west than the previous one. Because of this, there will be an overlap between adjacent swaths. That overlap is greatest at the poles and small near the Equator. Any spot on the globe will experience a southbound pass of the satellite in the morning and a northbound pass in the afternoon or evening. So you get two passes per day over any given location. Although the picture definition is good, polar orbiting satellites do not give a continuous view of the weather—you only get snapshots during those passes. Now let's move to Geostationary Satellites. These are put into orbit over the Equator. Because they take 24 hours to complete one orbit, they will appear to be stationary over a selected longitude. That's why we call them geostationary. In 1987, there were five geostationary satellites in orbit. Let me list them: Meteosat 2 over the Greenwich meridian (that's 0° longitude), GOES E over longitude 75° West, GOES W over longitude 135° West, GMS 2 over longitude 140° East, and INSAT over longitude 70° East. GOES stands for Geostationary Operational Environmental Satellite, and GMS stands for Geostationary Meteorological Satellite. These geostationary satellites are considerably higher than the polar orbiting satellites—they orbit at about 36,000 kilometres above the surface. Because they are so much farther away, the picture definition may not be as good as that from a polar orbiter. However, the advantage of a continuous picture outweighs this disadvantage. Because of the equatorial orbit, the picture becomes somewhat distorted towards the poles, but this distortion may be corrected by computer processing. Meteosat, for example, covers about—and the text cuts off there, but you get the idea: it covers a large portion of the Earth's disc continuously. So to summarise the key contrast: polar orbiting satellites give you high-definition snapshots twice a day, while geostationary satellites give you a continuous, lower-definition view of a fixed area. Both are essential tools for modern meteorology and for your flight planning.