Assessing Calibration Stability Using the Global Precipitation Measurement (GPM) Microwave Imager (GMI) Noise Diodes

Abstract

With rising demand for smaller, lower mass microwave instruments, internal calibration using noise diodes is becoming increasingly more attractive for space-borne radiometer applications. Since noise diodes can exhibit on-orbit excess temperature drift, internally calibrated systems typically require vicarious on-orbit recharacterization. The GMI is the first instrument of its kind to include both internal (noise diodes) and external (hot load/cold sky) calibration systems. The dual-calibration system provides the unprecedented capability to directly measure transient behaviors in the hot load, cold sky view, and receiver nonlinearity. Furthermore, the behavior of the noise diodes can be directly evaluated, which may shed light on improvements to internal calibration for future missions. This paper directly examines the behavior of the GMI noise diodes using the hot load and cold sky views for the first 6 months of operations. Two of the seven channels with noise diodes have exhibited on-orbit noise diode excess temperature drift of about 1 K. The other noise diodes have remained exceptionally stable. The noise diodes are used to evaluate transient behaviors in the GMI hot load, cold sky view, and nonlinearity. The hot-load brightness temperature variation due to gradients is re-evaluated and shown to be smaller at the lower frequencies than at preflight calibration. Radio frequency interference (RFI) in the cold view is evaluated using the noise diode backup calibration. The on-orbit nonlinearity is trended over the first 6 months and shown to be stable over that time period.