# Benefits and Lessons Learned from the Sentinel-3 Tandem Phase
## 1 Introduction  
The Copernicus Sentinel-3 mission [1] is devoted to the measurement of water surface topography, temperature, and optical radiometry over land and ocean. Its accuracy and revisit are designed tosupport ocean forecasting systems as well as environmental and climate monitoring. The Sentinel-3 satellite accommodates four main instruments:  
* SRAL, the SAR radar altimeter working in bands Ku/C
* MWR, the microwave radiometer, which complements SRAL to perform the sea surface topography mission
* SLSTR, the sea and land surface temperature radiometer, with channels covering VIS, SWIR, and thermal infra-red
* And OLCI, the ocean and land color imager in the VISNIR domain  
Sentinel-3A was launched on 16 February 2016, and Sentinel-3B on 25 April 2018. From June to October 2018, Sentinel-3B operated in close formation (tandem configuration with 30 s overpass time delay) with Sentinel-3A, before rallying its nominal orbital position forming a 140\({}^{\circ}\) orbital angle with respect to Sentinel-3A.  
The Sentinel-3 tandem phase was designed as a short-term period, allowing better understanding and characterization differences between the satellites. This is in contrast with some other missions where formation-flying is a key part of the mission operation concept: e.g., tandem X (two satellites operating at 200 m of distance), or FLEX ([2], to be operated in formation with Sentinel-3). A short-term tandem phase was previously implemented for altimetry missions and provided significant benefits (see e.g., [3; 4; 5; 6]). Sentinel-3 is, however, the first mission with a significant tandem period for optical instruments (OLCI and SLSTR). A short tandem phase (4 days) was implemented with Landsat 5-TM and Landsat 7-ETM+ [7]. The previous ERS-1/2 tandem configuration (with 2 satellites at one-day interval) did not allow a direct comparison of optical instrument measurements.  
The main objective of the Sentinel-3 tandem phase is ensuring the consistency of the data provided by the two satellites. This is important because they are intended to operate as a constellation and to establish reliable long-term climate data records. With this objective in mind, a specific study (\"Sentinel-3 Tandem for Climate\" [8]) was started in May 2019. This study complemented previous analyses performed by the Sentinel-3B commissioning team and the Sentinel-3 mission performance center (MPC), which led to the successful transfer to operations of Sentinel-3B on 23 rd November 2018. An important aspect of the tandem study was the collection and curation of a consistent archive of Sentinel-3A and Sentinel-3B data for all instruments. The data set consisted of operational (Sentinel-3A), pre-operational (Sentinel-3B), or reprocessed data (for optical instruments only) sampled at the rate of one day per week and covering the pre-tandem, tandem, and post-tandem phases.  
The present paper provided an overview of the activities performed in the frame of the Sentinel-3 tandem for climate study. Reference is made throughout the paper to several companion papers presenting specific analyses in more detail. The next chapter describes the motivations and characteristics of the Sentinel-3 A/B tandem phase. The following chapters detail, in turn, the main benefits of the tandem phase analyses to address different aspects of inter-calibration and validation. Each aspect is illustrated by an example for one of the Sentinel-3 instruments, highlighting the unique benefits of the collocated tandem data.  
Finally, we gather lessons learned and recommendations for future tandem operations.