Main sources of errors:

Instrumental errors:

They are characteristics of the precision of the instruments and accuracy of the altimeter pointing. They are also representative of the quality of the retracking processing.

Different corrections are included in the data processing in order to minimize as much as possible these errors. They consist in :

• Mispointing correction: it allows to take into account possible mispointing of the altimeter measurement with respect to the nadir direction
• Doppler effect correction that takes into account the motion of the satellite
• The tracking bias that allows to take into account the imprecision of the different algorithms
• The correction of the Ultra Stable Oscillator (USO) that correct the drift of the instrument
• The internal calibration that takes into account the transit time of the data in the antenna.

Still, the models and corrections used to mitigate the effects are not perfect, and there might remain some residual errors. They are mainly characterized by uncorrelated measurement noises (see specific chapter).

Environmental & sea state errors:

The path of the electromagnetic signal goes through the atmosphere that influences the measurement. In the same way, the sea state bias (presence and shape of the waves and roughness at the surface) also introduce an error on the measurement.

Different corrections are used in the data processing in order to correct the measurement from atmospheric and sea surface effects:

• The dry and wet troposphere corrections that correct the path delay effects linked to the presence of dry gases and water vapor in the atmosphere.
• The ionospheric correction that allows to take into account the effect of the ions present in the atmosphere.
• The sea state bias correction that correct the effects of the sea surface state on the reflection of the altimeter signal on the surface.

In spite of these different corrections, part of the environmental errors can still be observed in the along-track SLA signal. They can be spatially and temporally correlated.

Geophysical errors:

They mainly consist in subtracting from the measurement some physical signal that cannot be accurately sampled with the altimeter (due for instance to the inconsistency between the temporal sampling of the altimeter and the temporal scales characteristic of the signal considered) or that are not of interest for the study of the dynamical signal. In that way, different geophysical corrections are applied of the altimeter measurement (geoid, ocean tide, inverse barometer and high frequency wind and pressure effects, etc.). See Homegenization processing step for the details of these corrections.

The quality of the different corrections that depend on numerical models, can lead to uncertainty in the different geophysical corrections applied. They are considered as errors on the SLA product, for the main part correlated in space and time.

DT vs NRT DUACS products

The NRT along track L3 products are usually less accurate than the DT ones. The main sources of differences come from the different quality of the L2p altimeter products used in input:

• The Orbit estimation is usually more precise in delayed time conditions due to more precise environmental model (pole position, solar activity) and different techniques (DORIS; GPS) used in real time or delayed time conditions. These differences can induce several cm differences on the Sea level.
• The quality of the Dynamic Atmospheric Correction is improved in DT thanks to a better centering of the filtering windows. For some period the input atmospheric model can also be improved (ERA Interim reanalyzed fields)
• The measurement calibrations (radiometer, altimeter, …) processing are usually more accurate in DT conditions. It is the case for example for the radiometer measurement that can be impacted by drift or significant jump induced by inaccurate NRT calibration processing. Additionally, possible altimeter standard changes in the altimeter L2p products used in input of the DUACS processing can induce jump in the SLA field. The management of these jumps, necessary to ensure a seamless transition for the users, is more precise in DT processing.

Additionally, part of the DUACS processing is also less preferment in NRT conditions.

• The multi-mission cross-calibration processing (e.g. Orbit Error Reduction (EOR) and Long Wavelengths Error (LWE) (see “homogenization and cross-calibration processing”) is more accurate when using a centered temporal window. This is not possible in NRT processing since measurements in the future are not available.

For these reasons, we do not recommend to use NRT products for climatic scale signal studies (e.g. MSL trend, see FAQ).