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Non-satellite instrument techniques involved
Microwave Radiometer
Gap remedies
Related gaps
G2.36 Lack of traceable uncertainties in MWR measurements and retrievals
Detailed description

The traceability of the microwave radiometer (MWR) estimates and their uncertainty requires the traceability of MWR calibration to SI standards. This implies the use of certified black-body (BB) targets and temperature sensors (measuring the target physical temperature). Commercial BB targets have reached a mature state, but their characterization is usually limited. Despite this, many realizations of microwave brightness temperature standards exist in the form of heated or cooled calibration targets, although none are currently maintained as a standard by a national/international measurement institute (Walker, 2011). Thus, despite the efforts for fully characterizing the MWR absolute calibration, the traceability of any ECVs from MWR to national/international standards is currently not feasible. However, the development is ongoing (Houtz et al., 2015; 2016; 2017). This gap shall be addressed by national/international measurement institutes, and cannot be addressed within GAIA-CLIM.

Operational space missions or space instruments impacted
Other, please specify:

Microwave and Infrared temperature and humidity sounders

Validation aspects addressed
Radiance (Level 1 product)
Geophysical product (Level 2 product)
Gridded product (Level 3)
Assimilated product (Level 4)
Time series and trends
Calibration (relative, absolute)
Spectroscopy
Gap status after GAIA-CLIM
GAIA-CLIM has partly closed this gap

This gap will be considered closed when MW standards are available in at least one national/international measurement institute for calibrating secondary standards to be used for MWR calibration. The role of GAIA-CLIM is to follow and report the technological developments at national/international measurement institutes (e.g. NIST) and to inform MWR users and manufacturers about these developments.

Dependencies

G2.13 should be addressed together with G2.36

The remedy of G2.13, i.e. the development of MW standards maintained at national/international measurement institutes and the availability of transfer standards, will set the basis for SI-traceability of MWR observations and retrievals. However, tools for evaluating the MWR total uncertainty budget can be developed independently of the solution of G2.13.

References
  • Houtz D. A., D. K. Walker and D. Gu, Simulations to characterize a passive microwave blackbody design, 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Milan, pp. 3485-3488, DOI: 10.1109/IGARSS.2015.7326571, 2015.
  • Houtz D. A., D. K. Walker, D. Gu (2016), Cryogenic Design and Uncertainty Analysis of the NIST Microwave Blackbody, 14th Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment (MicroRad), Espoo, Finland, April 11-14, 2016.
  • Houtz D. A., W. Emery, D. Gu, K. Jacob, A. Murk, D. K. Walker, and R. J. Wylde, Electromagnetic Design and Performance of a Conical Microwave Blackbody Target for Radiometer Calibration, IEEE Transactions on Geoscience and Remote Sensing, vol. 55, no. 8, pp. 4586-4596, doi: 10.1109/TGRS.2017.2694319, Aug. 2017.
  •  Walker D. K., Microwave radiometric standards development at US NIST, IEEE GRSS Newsletter, 161, 2011.

The traceability of ground-based microwave radiometer (MWR) estimates and their uncertainty requires the traceability of MWR calibration to SI standards. Currently, no SI standard is available for MWR at any national/international measurement institute. Thus, full SI-traceability of ECVs from MWR is currently not feasible. However, at least one national measurement institute is currently developing SI standards for MWR. It is expected that SI-traceable standards for MWR will be available in the next few years. This will then allow the availability of transfer standards to MWR manufacturer and user communities.