The numerical and alphabetical designation of the satellites within 

 a series denotes their launch status Prior to its launch^ a satellite 

 is given a letter designation for plaimlng purposes. After launch and 

 once it has attained orbit, it is reclassified with a numeral to 

 designate its sequence in the satellite series. 



Nimbus is pleasantly different in being named after the Latin 

 word for rain clouds rather than being an acronym. It is aptly named 

 as it was developed to study clouds with APT and AVCS television 

 systems during the day and with HRIR and MRIR sensors at night. 



Nimbus was originally designed to overcome basic limitations in 

 the TIROS satellites and to become the first in a series of operational 

 satellites. However, for various reasons. Nimbus was redesignated a 

 research and development satellite and ESSA 2 replaced it as the first 

 operational satellite. 



The Applications Technology Satellite (ATS) series includes research 

 satellites in geo- synchronous and equatorial orbits. There are two 

 ATS satellites currently in operation: ATS 1 and 3« 



49 



