Circling the earth, from pole to pole, are three United States weather satellites, designated NOAA 12, 14 and 15. (Other countries have similar satellites, with the total being close to a dozen.) These low-orbiting satellites, on an altitude of approximately 500 miles, complete a pass around the earth every 100 minutes. While encircling, they scan the earth's surface, employing both infrared and visual sensors.
These scanned images are not stored in the satellite but are immediately sent back to earth as an FM signal (137.5 and 137.62 MHz) and received by a passive helix antennae (average size is 20" by 6" and weighing 3 lbs.). These signals are broadcast uncoded and, similar to the service provided by radiofax, are free to anyone with a receiver.
These scanned images of the earth's weather provide: real time weather, what you see is what is happening now; images are received day and night-24 hours a day with the infrared signal (IR) providing nighttime imagery as well; software programs with each system provide latitude and longitude gridding, land mass outline (geopolitical boundaries), color enhancement for water and cloud temperature, zooming cursor positioning, GPS input of your position, and much more. In essence, these systems provide users with their own weather channel where each satellite pass shows weather features within approximately 2,000 miles of its location. Resolution on polar satellite imagery is two miles, so all significant weather features are visible. Either used alone or in conjunction with weather charts, satellite imagery enables highly accurate analysis and forecasts. Imagery is an excellent weather tool, keeping a voyager current on formation and movement of lows, highs, fronts, thunderstorms, fog, ice, etc. Satellite weather technology has been with us since the 1960s but it is only the recent availability of laptop computers that can be used on board that satellite weather imagery has become a practical tool to sailors.
More than a dozen boats in this year's Newport-Bermuda Race, as well as boats in the Victoria-Maui Race and almost every boat in the recently completed Whitbread Round the World Race were equipped to receive weather satellite imagery. Some of the raceboats in the upcoming Around Alone Race boats will also be equipped with weather satellite systems. More and more cruising sailors are using these systems as well, especially by sailors venturing into areas where weather charts and forecasts are not easily available.
Information on polar orbiting weather satellite systems can be found on the at http://www.ocens.com.