315 Bearing Anomaly for the Sentinel 6 Supplemental Calibration System  

Gale Paulsen*, Dylan Van Dyne* , Fredrik Rehnmark *, Phil Chu*, and Ted Iskenderian**  

Abstract  

The Supplemental Calibration System (SCS) was designed to rotate an elliptical reflector for the Sentinel 6  Advanced Microwave Radiometer (AMR). By rotating the reflector, the system can perform calibrations by  comparing the reflection from space to the reflection from an onboard calibration target. During the  qualification testing of the SCS, a flaw was discovered that resulted in apparent brinelling of bearings that  support the rotating reflector. An extensive investigation ensued to determine the root cause of the problem.  Though the most likely root causes  of the presumed brinelling ended up being the result of undersized  bearing clamp rings and thermal mismatch, there were requirements that drove these design choices. This  paper describes the design of the bearing assembly, requirements that drove the design, and documents  a challenging investigation filled with ambiguous  test results.  

Introduction  and Assembly Overview  

Sentinel 6 is a spacecraft designed to continue the long- term continuity of satellite altimetry for sea surface  height. This mission is a multi -agency collaboration between ESA, EU, EUMETSAT, NASA -JPL, NOAA ,  and CNES  [1]. One of the upgrades to the Sentinel 6 microwave radiometer is a calibration system capable  of maintaining measurement accuracy to within a few centi meters  over the life of the mission [2]. To achieve  this accuracy, the microwave  radiometer is equipped wi th a  Supplemental C alibration  System  (SCS) . This system  helps correct signal drift by  directing the radiometer signal  to two different calibration  targets of known temperatures.  It is the design and testing of  this system, which is the focus  of this paper.  

The SCS is mounted to the  Reflector Support Assembly  (RSA). It is shown as a blue  assembly in Figure 1 . Elements  that comprise the SCS include   a rotati ng Secondary Mirror  (Figure 2); a stationary Space View Mirror to provide a “cold” calibration target;  a Warm Calibration Target  (WCT) to provide a “warm” calibration target; a structure to support the SCS  components and t o interface to the RSA; a Standard Dual Drive Actuator (SDDA) (Figure 3) to rotate the  Secondary Mirror; a Launch Lock Mechanism to support the mirror during exposure to the launch  environment; and a Feed Horn (not shown) to direct the microwaves to a wave guide assembly.  In the  standard operating mode, the system is positioned in its “science position” where it is collecting information  on sea surface he ight. In this case, the Secondary Mirror is directed at the larger reflector on the RSA. To  maintain signal accuracy, the system is required to perform a calibration approximately once every five 

* Honeybee Robotics Spacecraft Mechanisms Corporation,  Ensign Bickford Industries,  Altadena , CA  ** Jet Propulsion Laboratory, California Institute of Technology , Pasadena, CA  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020    Figure 1. SCS location on the Sentinel 6 Spacecraft