GSICS Microwave Subgroup Meeting (February 16, 2021)

Theme: SI Traceable Microwave Instrument Characterization and Community FCDR Reference Standards

Agenda

1300-1500 (Central European Time), 0700-0900 (US EST), 2000-2200 (Beijing), 2100-2300 (Tokyo/Seoul),1200-1400 (UTC)

1. Traceable measurement technique and measurement capability for power on the microwave and millimeter frequency ranges (Xiaohai Cui from NIM (National Institute of Metrology, China))
2. Metrology of Antenna Radiation Pattern (Donglin Meng from NIM (National Institute of Metrology, China))
3. Space-based Microwave Reference Radiometer (Philip Rosenkranz from MIT)
4. Radio Frequency Interference: Updates of Interest to Passive Earth Sensing Users (Ed Kim from NASA)
5. Atmospheric Temperature FCDR from the MSU/AMSU/ATMS: Recent Progresses (Cheng-Zhi Zou from NOAA)

Meeting Summary

The GSICS Microwave Subgroup (MWS) meeting on SI-traceable microwave radiometer calibration began with two presentations from the China National Institute of Metrology (NIM). The first delved into traceable power measurement techniques and capabilities for microwave and millimeter frequency ranges. It offered the Subgroup glimpses onto the development of power sensors, micro-calorimeters and best-practices needed for state-of-art microwave radiometric measurement. The second briefing gave an introduction into antenna radiation pattern metrology. This presentation included an introduction into how antenna response is measured, as well as the merits of the NIM Spherical Near-Field (SNF) antenna characterization method and facility, compared to other far- and near-field methods and facilities. Both presentations provided analysis to determine measurement uncertainty.

The third briefing in this meeting, from MIT, offered a high-level view of a space-based microwave reference radiometer. It laid out the characteristics that are important for a reference radiometer – e.g., linearity, frequency stability, calibration stability, low antenna sidelobe levels, and stabilized orbit altitude. It also listed the following desirable features of an on-orbit microwave radiometer: Phase-locked local oscillator(s); 2- 3- or 4-point calibration (e.g. target, space, noise diode); Reduced sensitivity to gain fluctuations (e.g. Dicke switching); Low-sidelobe antenna; Dual polarization for comparisons to rotating-pol instruments like ATMS; and Temperature control of radiometer and target. The presentation also suggested the idea of using GNSS-RO refractivity as an SI-traceable measurement standard that would offer a stable reference for microwave radiometer measurements. The final presentation was given by NASA, and communicated issues associated with Radio Frequency Interference (RFI) in microwave instrument measurements. It revealed how the United Nations’ International Telecommunication Union (ITU) provides spectrum protection regulations through the World Radiocommunication Conference (WRC). The process of determining these regulations includes cost-benefit and power limit studies associated with releasing portions of the radio frequency (RF) spectrum for commercial use. In the recent WRC-19, relaxation of some RF protections could introduce terrestrial RF sources at on-orbit microwave sounder operating frequency bands. A discussion about methods to mitigate microwave radiometer RFI occurred at the conclusion of this presentation. Note that the final briefing on the MSU/AMSU/ATMS FCDR was intended for the January 21, 2021 meeting, and is summarized on that GSICS Wiki page.

Meeting minutes from this meeting are available from this link.

-- RobbiIacovazzi - 08 Feb 2021