# Benefits and Lessons Learned from the Sentinel-3 Tandem Phase
## 2 Motivation and Design of the S3B Tandem Phase  
### Tandem Inter-Comparisons from a Metrology Point of View  
Metrology, the science of measurement, provides a framework for creating a robust, long-term, interoperable framework for climate records (see, e.g., [9]). Metrology has three basic principles:  
* Traceability of measurements to a stated metrological reference.
* Comparisons of measurements to confirm traceability and validate uncertainty analysis.  
In the domain of earth observation, the availability of significant comparisons is a critical issue. Although it is common practice to compare measurements obtained by different satellites, the comparisons are limited by:  
* Temporal variations of the geophysical signal;
* Variations of the observation geometry;
* And variations of atmospheric conditions.  
Inter-satellite comparisons often rely on simultaneous nadir observations (for optical data) or cross-over measurements (altimetry). This is possible only in a few specific locations, which leads to a poor and possibly biased statistical sampling. Atmospheric and bi-directional geometry effects can be estimated through models or ancillary measurements, and temporal variations of the geophysical signal can be limited by looking for pseudo-invariant sites. This allows the relaxation of simultaneity requirements, increasing the number of measurements. However, it also introduces additional uncertainty in the comparison process and, therefore, reduces the quality of the inter-comparison. This is particularly true for altimetry as the sea surface height (used to retrieve geostrophic currents) is estimated, thanks to the knowledge of several oceanic and atmospheric geophysical parameters, e.g., the water vapor content in the atmosphere, the air pressure on the oceanic surface, the surface elevation induced by ocean tides. The high spatial and temporal variability of these parameters leads to increased uncertainty in inter-comparison methods.  
When two satellites are placed in a tandem configuration on the same orbit at a short time interval, all measurements are (at first order) directly comparable. This allows robust statistical analysis of the measurements with good control of the uncertainty.  
For an observation system based on a constellation of satellites, a tandem phase during the commissioning phase is, therefore, the best way to (a) ensure the traceability of measurements obtained by the different satellites of the constellation, (b) validate the uncertainty on the measurements. Both aspects are critical to building reliable long-time series as needed by, e.g., climate change studies. This has been recognized early on by the satellite altimetry community. As a result, tandem inter-calibration phases were implemented for the JASON satellite series to precisely estimate the absolute biases between the two different altimeters and construct a reliable and long time series of the mean sea level (MSL) [10; 11]. These considerations naturally led to consider a tandem phase with Sentinel-3A during the commissioning phase of Sentinel-3B. It became evident that this would also bring significant benefits to the optical missions (OLCI and SLSTR).  
While the tandem phase provides opportunities to acquire measurements with the same viewing conditions, the drift phase between the tandem and nominal positions allowed earth scenes to be observed with increasing differences in viewing angles. Of particular interest is a moment in time where the dual view of the SLSTR-B overlaps with the nadir view of SLSTR-A. In order to take advantage of this specific geometric configuration, the drift rate was kept low, while the swath of the A and B optical instruments was overlapping and increased subsequently.