Toward a Global Microwave Standard
February 6, 2012
Contact: David Walker
Much of what is known about decadal climate change – and much of what appears on the evening weather forecast as well – comes from satellite-based remote sensing of microwave radiation at different levels in the Earth's atmosphere. Microwave measurements are generally reported as the apparent temperature of the object being monitored. Yet, at present, there is no accepted brightness-temperature (radiance) standard for microwaves that can be used for authoritative calibration of microwave sensors, for resolving discrepancies between readings from different satellites, or for comparing one program's results with another's.
Weather and climate uses for microwave remote sensing measurements require that the observed temperature be accurate within 1 kelvin or less. But existing measurements cannot be made with that accuracy or reliability. "Right now," says David Walker, Project Leader for Microwave Remote Sensing in PML's Electromagnetics Division, "new data coming from nominally identical instruments can differ by as much as a couple of kelvin."
"People have gotten remarkably good results from satellite microwave monitoring," Walker says, "but it takes a tremendous amount of post-processing of data and some recalibration tricks. And there's still much more uncertainty than we would like." For example, 30 year datasets from three separate, reputable sources show temperature trends in the middle troposphere that differ by a factor of three in magnitude, due in part to differences in the input data from various sensors. (See Figure 1, below) A significant contributor to these differences is the lack of reliable calibration for the initial measurements.
To solve general measurement problems such as this, the international scientific community has adopted the International System of Units (SI). By providing a robust way to ensure that measurements can be traced back to the SI, it is possible to ensure quantitative equivalence of measurements made by different instruments, in different places, and at different times. To attack that microwave measurement problem, Walker and colleagues are developing a system of SI-traceable radiometric measurements that can be used to determine the brightness temperature (TB) of a microwave source, which can then be used as the basis for determining the temperature of any "grey body," including the Earth's atmosphere, land, and sea surfaces. (TB is the temperature at which an ideal blackbody would have to be to emit the same radiance that arrives at the sensor.) Satellites do not measure temperature directly; instead they measure the amount of radiation incident on their sensors at various key wavelengths and sometimes polarizations associated with different meteorological phenomena and different sections of the atmosphere or surface. Those data are then processed by different groups using various methods to yield temperature equivalents.
A TB standard can take the form of either a source (radiance standard) or a receiver (irradiance standard, also called a "radiometer"). Creating a fully traceable radiance TB standard requires a microwave source of well-known and independently verifiable brightness. PML is working on this type of standard in collaboration with researchers at the University of Colorado and NASA Goddard Space Flight Center. While significant progress has been made towards that goal, the final realization is still to come.
The new method, tested at PML's anechoic chamber in Boulder, CO, used a blackbody target 33 cm in diameter and a rectangular horn antenna. (See Figure 3, below) The target temperature, monitored with calibrated thermometers embedded in the backside, was increased in approximately 10 K increments from 296 K to 352 K. Distance from target to receiver was varied from 50 cm to 500 cm. Three frequencies (18 GHz, 22.5 GHz, and 26 GHz) were measured in each configuration. The data – "we took a lot of data," Walker says – produced plots with relatively limited uncertainties, and the new technique circumvents problems in conventional methods related to precise knowledge of the antenna pattern for all possible directions. In all, "this procedure is well-suited for pre-launch calibration of blackbody targets at microwave frequencies," the team concludes in a forthcoming paper.
Any progress is welcome. "Although the majority of satellite data are taken at optical frequencies," Walker explains, "microwave readings provide information that is not available through optical measurements. For example, microwave monitoring provides total, continuous coverage of the Earth because microwave sensors can 'see' through clouds. Moreover, microwaves provided quantitative information about the moisture content in the atmosphere, which serves as an essential check on other measurements.
"For example, radar altimetry from satellites in space is used to determine sea-level changes with an accuracy of a few millimeters . Those measurements depend critically on understanding and quantifying the moisture-dependent atmospheric path-delay variations, which can be as large as a few centimeters. The path-delay measurement is made using passive microwave radiometry, which NIST's research addresses."