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SI Traceability

What is SI traceability?

SI traceability is a technique used for satellite observations that links a satellite’s measurements to internationally-recognized measurement standards. Using this technique, measurements from different satellites may be pooled into one long-term observational record that is free from small drifts in measurements due to slight differences between satellites. Additionally, SI traceability allows scientists to test the satellite observations for small errors that mirror the variability of the climate itself. Testing for these errors provides for a clear interpretation of measurement trends, greater enhancing our confidence in scientific results from satellites.

The SI in SI traceability refers to the International System of Units (Système International d'unités in French). This system of units was developed to provide an error-free means of comparing measurements made anywhere in the world, at any time, to one another. To accomplish this task, SI Units are defined by basic properties of matter. For example, the second is defined by the quantum mechanical properties of the Cesium atom. These quantum mechanical properties will be exactly the same, regardless of at what laboratory, or when, they are measured.

An understanding of SI traceability may be gained by looking at the measurement principles of the CLARREO mission. One of the CLARREO sensors (the GPS/GNSS-RO sensor) measures the changes in the properties of microwave radiation as it makes its way through the Earth’s atmosphere. These changes are detected using a timing measurement. This timing measurement can be directly compared against the timing standard provided by an atomic clock—a clock that uses the quantum mechanical properties of the Cesium atom as its timing mechanism. In this way, the CLARREO mission creates an index of climate that is directly related to the atomic clock. The principles of the atomic clock are well understood by decades of research, and are used in many different kinds of laboratories all over the world. This link provides evidence that can be understood and applied by all disciplines of natural science, to show that CLARREO measurements are clearly capturing long-term climate trends.

Why is SI traceability important for climate science?

The trends in the climate system associated with long-term change are covered up in the short term by climate “noise.” This noise arises from short term changes associated with day-to-day and seasonal weather as well as other events such as El Niño. Uncovering the long-term trends from the noise is essential to decadal climate science since knowing the correct strength of the trends is key to testing climate models and to understanding the changes in detail. Identifying the long-term trends requires satellite observations that are free from small measurement errors (that could even slowly change in time themselves) and that are equal from one satellite to the next.

SI traceability provides strict methods that guide the creation of satellite data that can meet these specific requirements. These methods are built on decades of practice in measurement science and have been continually tested and strengthened by scientists all over the world. It is this foundation that lends the greatest degree of credibility possible to measurements made with these methods.

Past climate change data from satellites have provided invaluable information, but have sometimes been challenged due to controversy related to the accuracy of the measurements. Technological advances of the past decade along with the increasing maturity of the areas of science that support space-based climate measurements, allow scientists to robustly implement SI traceable satellite observations today. By carefully learning from the successes and shortcomings of previous missions, the CLARREO mission looks to directly address the societal objectives of characterizing long-term climate change and testing climate projections to support public and private decision-making with the most reliable information possible.