Why polar-orbiting?
A polar-orbiting satellite can provide an observational platform for the entire planet surface, while geostationary counterparts are limited to approximately 60 degrees of latitude at a fixed point over the earth. Polar-orbiting satellites are able to circle the globe approximately once every 100 minutes. Relatively low orbit allows detection and collection of data, by instruments aboard a polar-orbiting satellite, at a higher spatial resolution than from a geostationary satellite. The high resolution data combined with global coverage allows polar-orbiting systems to provide near real-time environmental information for initializing Global Forecast Models and improving output accuracy.
The NPOESS satellites are inserted into a sun-synchronous polar orbit. Sun-synchronous means that they pass over their target on the Earth at roughly the same local time. An early morning satellite will make its ascending pass over the equator in the early morning, independent of Earth's west to east rotation. For example, if a morning satellite flies over Washington, D.C. at 0600 Eastern time, then roughly three hours later it will fly over California at 0600 Pacific time. And later that day it will fly over Tokyo at 0600 Japan time.
The label applied to a polar-orbiting satellite is determined by the local time as it crosses the equator. The crossing from north to south is labeled as its descending node time; from south to north is labeled as its ascending node time. The NPOESS satellite will be flying ascending node times of 1330 and 0530 i.e., they will cross the equator, from south to north, at 1330 and 0530 respectively. The METOP satellite will be flying an ascending node time of 0930 i.e., it will cross the equator, from south to north, at 0930.
