scope:

Description of Driving Event:

The NOAA-15 (K) satellite was launched on May 13 1998. The satellite was the first in the new series of Polar Operational Environmental Satellites (POES) that had design heritage back to Tiros-N launched in 1978. NOAA-15, however, had three S-band antennas operating in the 1700 MHz frequency range that were new to this mission.

Approximately 18 days after launch, data quality degradation was noted during tape recorder playbacks on the S-band #2 link. In October science data from the AMSU-B instrument provided by the UK was observed to have contamination toward one side of the scan field of view. The degradation was consistently evident or not depending upon the satellite orbital location. An Anomaly Review Board was established and investigation revealed that the contamination appeared to rotate around the globe as the sun moved from north to south during the annual season change.

Further investigation revealed a thermal correlation to the link degradation and science data contamination as the S-band antennas experienced large thermal gradients throughout the orbit. A detailed thermal stress analysis performed for the antenna confirmed stress levels within internal connecting elements that were sufficient to cause breakage. Once broken, changes in antenna gain, axial ratio, and SWR would significantly degrade link performance and antenna patterns. The failure mode was confirmed in a thermal cycling test of a flight antenna.