Professional Pilot, January 2016

differences as IR have allowed us to discriminate between clouds and a snow covered landscape as well as to peer inside clouds such as the cumulonimbus of tropical cyclones to help meteorologists better understand their internal structure and behavior Radar systems aboard some satellites also give us the ability to see and track rainfall over much of the globe especially over regions that are not covered by ground based radar stations such as over the oceans Together this wealth of imagery has greatly improved not just weather forecasting but also enables pilots to track and avoid adverse weather Orbits Meteorological satellites fall into 2 basic categories Geosynchronous or geostationary and polar orbiting Geosynchronous satellites are those that are parked over a specific location and provide a continual view of the same piece of real estate They remain in position by matching their orbital path and speed to Earths rotation in order to remain directly above a fixed location To maintain a matched orbit with Earth geosynchronous satellites must orbit at 22236 miles 35786 km above the ground The altitude at which these satellites orbit has benefits and limitations A benefit is that the sensors gain a wide field of view so that they can 56 PROFESSIONAL PILOT January 2016 often provide coverage over much of a continent Another benefit is that it can take frequent images of the same area allowing meteorologists to watch as weather systems develop and move Most geosynchronous satellites are set to make and transmit an image every few minutes and are normally disseminated either on a half hourly or hourly interval Some high resolution sensors are capable of transmitting data continuously However there are 2 primary limitations of geosynchronous satellites Since such wide coverage means any given pixel on the sensor must cover a large area the resolution of the image tends to be relatively coarse And much of the image is not viewed from directly overhead which greatly distorts the pixels at the edge so those pixels must cover more area than the pixels viewing the center of the area Of course modern sensors can pack many more pixels into every square inch increasing the resolution but many meteorological satellites are more than a decade old and do not benefit from recent advances in sensor technology Polar orbiting meteorological satellites receive their name because they are in an orbit that crosses over the poles In such an orbit the satellite travels over the same spots in space while the Earth rotates beneath it This results in a continual scan of the entire planet but one in which any given spot on Earth is visited only 4 or 5 times during a day depending on the orbital speed and field of view of the sensor Most polar orbiting satellites orbit at much lower altitudes than their geosynchronous counterparts as low as 3300 miles or 1000 km which is an optimal altitude at which to revisit any given spot at the same time of day on every orbital pass sun synchronous The lower altitudes of polar orbiters give these satellites the advantage of much higher resolution and less edge distortion but they do so at the expense of a wide field of view and the ability to continuously monitor a given geographic area However an additional benefit of the polar orbits and one that is very relevant to transcontinental air travel is that these images are available for any location including the polar circle routes Getting satellite imagery Meteorological satellites are maintained by a number of nations including the United States the European Community Japan and India However access to current imagery from many of these satellites is restricted to hydrometeorological agencies researchers and paying customers Fortunately many international flightplanning services maintain access and provide it to their subscribers If you subscribe to a flightplanning service check with them regarding access to non US satellite imagery Imagery from the constellation of United States weather satellites is made freely available to the public These images also are generally provided by flightplanning companies to their subscribers but also can be viewed on any number of weather related websites as well as directly from agencies such as the US National Weather Service at http adds aviationweather gov or NASA at http weather msfc nasa gov GOES A nice listing of pages that serve up meteorological satellite imagery can be found at http weather msfc nasa gov GOES satlinks html Visible imagery Although the optical sensors aboard satellites capture all wavelengths of visible light most visible satellite images are displayed in gray scale This is done simply because at least visually there is little information to be gained by viewing a full color image of white or gray clouds Plus in gray scale the contrast between shades becomes more evident allowing the viewer to differentiate surface snow cover types Infrared water vapor image from the GOES 12 GOES East satellite The photo more clearly identifies a strong cold front that has pushed south into Florida generating storms off the mid Atlantic coast Such images can help pilots find a route that places them in drier air away from frontal storms Image courtesy NOAA and CIMSS