Calibration and validation

Calibration geometry adopted within the IMOS calibration/ validation sub-facility.
Calibration geometry adopted within the IMOS calibration/ validation sub-facility.

This IMOS sub-facility directly contributes to the calibration and validation of the Jason-series satellite altimeters within the framework of the NASA/CNES/NOAA/Eumetsat Ocean Surface Topography Science Team (OSTST). This is achieved via direct comparison of the altimeter data with in situ data from the Bass Strait and Storm Bay comparison points, thus providing a continuous calibration and validation record to the OSTST to assess altimeter accuracy.

The sub-facility determines the “absolute bias” of the altimeter for each over flight. The absolute bias is the difference between sea surface height (SSH) measured by the altimeter, and SSH measured in situ (using an independent technique). Understanding the contributions to absolute bias (e.g. spurious estimates of the delay caused by the troposphere, unresolved biases in instrument constants etc) assists in ensuring an accurate and reliable altimeter data record. By monitoring changes or trends in the absolute bias, often called “bias drift”, the sub-facility is able to track any spurious drifts that 

Absolute Bias

To determine the altimeter absolute bias, a range of in situ instruments are used, deployed on an ongoing basis. Our technique involves a novel combination of moored oceanographic instrumentation (pressure gauges, temperature and salinity sensors), surface based GPS equipped buoys, and coastal tide gauges and continuously operating Global Positioning System (GPS) sites that form part of the International GNSS Service (IGS) global array.

The moored oceanographic instrumentation (deployed at specific offshore comparison points under the orbiting altimeter) provides a highly precise SSH record by converting the water pressure measured by the pressure gauge to height (i.e. taking into account the density of the water column), yet lack an “absolute” datum that makes them suitable for direct comparison with the altimeter.

To derive this absolute datum, GPS equipped buoys are deployed approximately every three months over the mooring locations. The combination of the GPS buoy data and mooring derived SSH, enables the generation of a sea level time series that can be compared to the altimeter, for each over flight (once every 9.9 days).

Outside of the period of operation of the mooring, we use a coastal tide gauge that has been tidally corrected and transformed to the datum that exists at the offshore comparison point. This has enabled the generation of absolute bias estimates in Bass Strait back to the time of the launch of TOPEX/Poseidon in 1992.

The schematic process used during processing in order to derive an in situ sea surface height that is relative to an International Terrestrial Reference Frame (ITRF) (the same frame used as the satellite altimeter data) is shown in the Figure below.

Schematic view of how coastal tide gauge, mooring and GPS buoy data is used to derive an in situ sea surface height that is comparable to the satellite altimeter data.
Schematic view of how coastal tide gauge, mooring and GPS buoy data is used to derive an in situ sea surface height that is comparable to the satellite altimeter data.

The fundamental output from this sub-facility is the contribution of a continuous absolute bias data stream, or time series, to the Ocean Surface Topography Science Team (OSTST). These data are vital to enable the OSTST to deliver final ‘calibrated and validated’ Geophysical Data Records (GDRs) to the global oceanographic community through its various data centres.

The contribution of the absolute bias data stream and its technical detail is undertaken between members of the OSTST and at the annual OSTST meetings. The sub-facilities work is also published in various special editions of the journal Marine Geodesy.

The current evolution of the absolute bias time series that spans the TOPEX/Poseidon, Jason-1 and OSTM/Jason-2 missions is shown below. An absolute altimeter bias that is not equal to zero highlights that the altimeter systems, while having very good repeatability and precision can have significant offsets , and gives estimates of sea level either too high (indicated by a positive (+)ve bias value) or too low (indicated by a negative (-)ve bias value).

We place conservative error estimates of ±15 mm on these mean absolute bias values (as determined from a rigorous error budget, see Watson et al 2011 for details).

Absolute bias for the TOPEX side A and B, Jason-1 and OSTM/Jason-2 missions from the Bass Strait validation facility.
Absolute bias for the TOPEX side A and B, Jason-1 and OSTM/Jason-2 missions from the Bass Strait validation facility.

Bias Drift

The computation of bias drift requires an alternate approach that takes advantage of many more comparison points, thereby reducing the noise and revealing subtle trends in the altimeter bias. To achieve this, the altimeter record is compared with specific long running tide gauges across the global network available from the University of Hawaii Sea Level Centre and the Australian Bureau of Meteorology.

An important part of this analysis is the treatment of the vertical land motion of the tide gauges resulting from a range of geophysical phenomena such as tectonics, subsidence from fluid extraction and global isostatic adjustment (GIA) from previous ice mass loads and their subsequent unloading. In many cases, the land motion can be directly observed using analyses of Global Positioning System data that is either recorded co-location with the tide gauge, or in a nearby location. Examples of GPS derived vertical land motion are provided on the SONEL page hosted by the University of La Rochelle in France.

Our most recent result for bias drift confirm that sea levels have risen faster over the altimeter era compared with proceeding decades over the twentieth century as observed by tide gauges.

Second, after taking into account land motion, our results suggest that the first six years of the altimeter record slightly overestimates the sea-level trend. Our revised estimate over the full altimeter period (1993-mid 2014) is +2.6-2.9 mm/yr (with the exact value depending on the method used to estimate vertical land motion), slightly less than the previous estimate of +3.2 mm/yr.

Third, without corrections for bias-drift, the previous record showed a slower rate of sea-level rise over the most recent decade, relative to the proceeding decade. Our revised record is significantly different and suggests that the rate has actually increased in line with accelerating contributions from the Greenland and West Antarctic ice sheets. Our acceleration is also consistent with the acceleration determined from the IPCC sea level projections.

For a detailed description of the methods and discussion of the findings, please refer to our recent paper in Nature Climate Change. An explanatory piece can also be viewed in The Conversation.

The IMOS satellite altimeter calibration and validation team is one of a number of international groups that actively participate in the process of validating and further understanding this important climate data record.

Unadjusted (lower blue/grey combination) and adjusted (upper black/red combination) global mean sea level time series reproduced from the publication in Nature Climate Change in May 2015.
Unadjusted (lower blue/grey combination) and adjusted (upper black/red combination) global mean sea level time series reproduced from the publication in Nature Climate Change in May 2015.