Accurate determination and monitoring of sea-level changes are of fundamental value and crucial in understanding the ocean’s influence on our weather patterns and long-term climate change of our planet. For more than 30 years, satellite altimetry has been monitoring changes in sea level and climate, unequivocally, over regional to global scales with [mm/yr] accuracy and reliably, with respect to the center of mass of the Earth.

• Fiducial Reference Measurements for Altimetry (FRM4ALT) are essential so that monitoring of the ocean and its changes is maintained:
• Objective, continuous, homogeneous and reliable;
• Free of errors and biases for any mission;
• Uninterrupted and tied from one mission to the next.

This action will add value and deliver confidence in users when monitoring the Earth and its changes and, at the same time, maximize the Return-On-Investment for these missions.This FRM4ALT project will set the fundamental ground for monitoring and controlling altimetry responses as regards their quality, biases, errors, drifts, as well as establish relations among various missions. Future altimetry is to be based on a common, reliable, absolute and unquestionable reference system, maintained over a long period of time but also traceable to international standards.

The Permanent Facility for Altimetry Calibration (PFAC) established and operating over 15 years in west Crete, Greece will act as proving ground of this FRM4ALT concept. It will not simply be providing in-situ data for ground truth verification of the parameters measured by altimetry, but:

It will not simply be providing in-situ data for ground truth verification of the parameters measured by altimetry, but:

• It will be the permanent facility to serve as the “yardstick” for evaluating performance of all altimetry missions with documented procedures, protocols, and with results tied to international standards in an open and transparent way.
• For each calibrated parameter (i.e., sea surface height), efficient location and uncertainty measures will act as indicators of the satellite altimeter performance.

A three-step approach is followed to estimate the true calibration parameter:

1. Review of Components in Cal/Val to achieve FRM quality;
2. Procedures, Protocols and Best practices for FRM;
3. Define, Establish and Implement Procedures to Maintain FRM.

From present activity, the following questions need to be evaluated:

1. Altimetry measures “ranges” from satellite to the sea surface. Parameters are determined based on these satellite observations and used to monitor changes in the sea surface and in the Earth’s climate. Are these ranges “true” to an absolute sense? Are these earth monitoring values correct, invariant to influences and errors, but also unbiased to lead us in the long term, reliably and securely, for monitoring climate change?
2. How do we measure and put into action the parameter “range” in the altimetry system to monitor the oceans? Is the present measuring principle going to change in the future? If a new technology and altimetry system is invented what influences are expected for the altimetry results? How can we make the altimetry system invariant to measuring principles and errors?
3. What reference system for time, i.e., satellite, terrestrial, geocentric, inertial, etc., generates values for altimetry ranges but also sets time-tagging of the altimetry observations in the satellite orbit?
4. What clocks are engaged in altimetry to realize time and how they influence range observations? How do we reliably trace effects in satellite clocks by variations in satellite velocity, gravity, and changes of earth shape?
5. How do we determine calibration parameters (e.g., sea level anomaly) to evaluate altimetry performance invariably and objectively? How accurately, for example, the sea surface and or the transponder are established to act as reference targets for calibration to determine altimetry biases, drifts, etc. What effects and errors may vitiate our calibration results? How these errors are identified, quantified and then isolated?

6. How can we provide parameters for calibrating altimetry reliably, but also traceable and measurable to international reference standards (e.g., speed of light)? Is monitoring of satellite performance accompanied by error budgets, reliable statistical measures, quality indicators, etc., in an objective manner?

 Stay tuned to FRM4ALT to check the upcoming Web Story for Step 2: Procedures, Protocols and best practices to attain FRM quality of satellite altimetry measurements.