G2.13

G2.13   Missing microwave standards maintained by National/International Measurement Institutes

Gap 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 metrology 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., 2014). This gap shall be addressed by national/international metrology institutes, and thus cannot be addressed within GAIA-CLIM.

Activities within GAIA-CLIM related to this gap

The role of GAIA-CLIM is to follow and report the technological developments at national/international metrology institutes and to inform MWR users and manufactures about these developments.

Gap remedy(s)

Remedy #1

Specific remedy proposed

Metrology applicable to microwave remote sensing radiometry is currently under development at national/international measurement institutes (e.g. National Institute for Standards and Technology, USA). These efforts include the development of a standard radiometer and standard high-emissivity black body (BB) targets. It is expected that SI-traceable calibration for BB targets and transfer standards in the form of calibrated BB targets will be available at NIST in the next few years. The current status is presented in a conference paper (Houtz et al., 2014). Typical achievable uncertainties for the standard radiometer developed at NIST are on the order of 1 K in the frequency range 10 to 50 GHz. A standard BB target is also under development. The uncertainty in the BB Tb is around 0.1 K (1-sigma), covering the frequency range from 10 to 200 GHz. NIST plans to be able to calibrate other BB targets against their standards, which could then be used as transfer standards.

Measurable outcome of success

The successful outcome is to make MWR users and manufacturers aware of the above developments. The effective characterization of existing and/or new MWR against microwave standards would be an additional measure of success, which is subject to the availability of the transfer standards before the end of GAIA-CLIM.

Achievable outcomes

Technological / organizational viability: medium.  The technological development is ongoing. The transfer to existing MWR instruments brings organizational challenges.

Indicative cost estimate: high (>5 million).

Relevance

The remedy will make microwave standards available at least at one metrology institute (NIST), and thus it should be able to address the gap entirely. The availability of microwave standards will also help in addressing another identified gaps concerning MWR, specifically G2.16. In fact, the effective use of microwave standards may provide a solid benchmark for characterizing calibration and instrument uncertainties of different MWR instruments.

Timebound

2 to 5 years.

Gap risks to non-resolution