TROPICS MICROWAVE RADIOMETER CALIBRATION ANOMALY IDENTIFICATION AND CORRECTION FOR THE NASA GLOBAL PRECIPITATION MISSION

P. De La Llana¹, W. L. Jones¹

¹University of Central Florida, Corresponding email: Patrick.delallana@ucf.edu, Central FL Remote Sensing Lab, University of Central Florida, Orlando, FL

Global Precipitation Mission (GPM) has assembled a constellation of cooperative Earth Observing satellite to enable global measurements of precipitation on a near real-time temporal scale. An integral part of this process is the intersatellite radiometric calibration (XCAL) between microwave radiometers on sun-synchronous satellites, with the GPM satellite observatory high quality active and passive microwave remote sensors. While this process has been highly successful for several decades, future GPM constellations will include a new class of passive microwave remote sensors of unknown quality and potentially shorter lifetimes. As such, these systems provide XCAL challenges that are outside of the traditional XCAL approach, and in this case the for the TROPICS (Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats) Pathfinder. The TROPICS G-band radiometer had a random brightness temperature measurement anomaly, whose root cause was found to be caused by the Noise Diode.

Fortunately, the TROPICS sensor payload has two independent radiometers, only one of which exhibited this random anomaly, so we developed a theoretical electromagnetic model with empirically derived coefficients, which was used this to identify the root cause and thereby provide a suitable correction for the affected radiometer. This analysis presents the details of this investigation that includes the derivation of the radiometer models from observations of deep-space cosmetic background radiances. These radiometric models have non-linear, time-dependent parameters that are ambient temperature dependent and are highly correlated orbit phase and the seasonal solar cycle. Further, for a one-year time series of collocated TROPICS and GPM ocean brightness observations, GPM XCAL comparisons are performed, and results demonstrate that the previous observed G-band brightness temperature anomalies have been satisfactorily corrected, with the resulting correction algorithm errors being objectively quantified.

We then present Intersatellite radiometric calibration between TROPICS Pathfinder and GPM Microwave Imager (GMI), using our double difference (DD) technique, whereby GMI is the radiometric standard and TROPICS is the target MWR to be calibrated. The following is a flowchart with a description of the XCAL procedure used in our submission:

The GPM XCAL working group radiative transfer model (RTM) is run to calculate the theoretical brightness temperature (Tbmod). The single-difference bias (BSD) is calculated as follows: BSD = Tbobs – Tbmod where Tbobs is measured data. Tbmod for both reference and target has the same common mode RTM biases which remove instrument effects when doing the DD. The DD is thus the differences of the single differences. The following is a flowchart with a description of the DD method used in our procedure:

After generating these GMI/TROPICS DD biases, the results will be carefully examined to discover any systematic changes that may result. For example, the DDs are cross-correlated as a function of orbital position to discover if time-varying calibration (over an orbit period) exists.

This analysis is a necessary step in developing XCAL procedures for future constellations of small-Sat MWRs with varying radiometric quality. The results and lessons learned from creating this procedure is directly applicable for further research for new small satellite missions of varying MWR quality and orbital parameters.