The basic operational concept of this series is to maintain two satellites in polar orbit at all times. One maintains an orbit so that it will pass traveling from north to south (descending node) during the morning. The second satellite passes from south to north (ascending node) during the afternoon. Each of these satellites also passes over approximately 12 hours later traveling in the opposite direction. Their orbits are sun- synchronous and circular at a mean 850-km altitude.
POES carry an Advanced Very High Resolution Radiometer (AVHRR) as well as a TIROS Operational Vertical Sounder (TOVS), which consists of three separate sensors, the High Resolution Infrared Radiation Sounder/2 (HIRS/2), the Microwave Sounding Unit (MSU), and the Stratospheric Sounding Unit (SSU). More information on these instruments is available in references listed in the bibliography. This report will offer more detail only on the AVHRR data, since the detail is needed to understand the different types of transmission from the POES and also the different types of AVHRR data which are available.