Copernicus satellite missions are designed to serve all Copernicus services by providing systematic measurements of Earth’s oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics. Satellite measurements provide a unique and quasi synoptic view of the Earth’s climate system from a near- global perspective. For satellite data to contribute fully and effectively to the determination of long-term records, missions must be implemented and operated in an appropriate manner to ensure adequate stability, accuracy, and homogeneity. The key characteristics required of a multi-satellite data set are:
1. Continuity, homogeneity and overlap of satellite observations;
2. Enhanced orbit control;
3. Calibration and instrument characterisation, and validation of products;
4. Sampling strategy; and
5. Sustained generation of products, data analysis, and archiving.
The Copernicus Space component and Copernicus Services addresses all of these requirements. However, the overlap –which is implicitly implemented by flying 2 satellites concurrently- could be significantly improved by flying satellites for a limited duration in Tandem flight (i.e. one mission follows the other with a small temporal ~30 seconds separation). This is advantageous because oceanographic and atmospheric variability is minimised between successive satellite measurements to the extent that it can be ignored. Consequently the uncertainty in any comparison between different satellite measurements viewing the same targets is minimised. Furthermore, a limited duration Tandem operation significantly reduces the time required to acquire sufficient data to build confidence in our knowledge of relative absolute bias between Sentinel-3A and Sentinel-3B instruments (traditionally this is achieved after many months of comparison to a limited suite of in situ fiducial reference measurements or crossovers at specific locations). Finally, a limited duration Tandem flight directly addresses GCOS (Global Climate Observing System) requirements stated in the GCOS CMP (Climate Monitoring Principles).

This presentation provides a summary of the first Sentinel-3 Tandem Phase activity completed using Sentinel-3A and Sentinel-3B in 2018.

The Copernicus Sentinel-3A mission was successfully launched in February 2016. It is a multi-instrument mission to measure surface topography, sea- and land-surface temperature, ocean colour and land colour with high-end accuracy and reliability. To reach the data coverage required by the Copernicus service, its twin Sentinel-3B was launched on 25th of April 2018. During the commissioning phase, from June to October 2018 the two satellites flighted in tandem configuration over the same ground track with Sentinel-3B 30 seconds ahead from Sentinel-3A. As it was already demonstrated with Jason missions, such a configuration is primordial to characterize with high precision the payload differences. In addition, in the case of the Sentinel-3 tandem phase, such a configuration allows for the first time to study different combination of altimeter modes (SARM, P-LRM, LRM) and the effects of different acquisition modes (Open Loop and Close Loop tracking modes) on the SARM measurements.

In this paper we propose to focus on ocean surface results. On the one hand, the main outcomes obtained in the frame of the Sentinel-3B commissioning activities will be described. They demonstrate how similar are the two altimeters and how such a configuration allows accessing to small magnitude signal in the altimetry error budget. On the other hand, results obtained in the frame of the Sentinel-3 Tandem for Climate (S3TC) project will be presented. Short wavelength differences are analysed to check the assumption of decorrelated white noise between both altimeters. This topic is of high interest, especially given the SARM sensitivity to swell.

While first objective of the tandem phase is to insure an accurate validation of Sentinel-3B mission and a seamless transition between both satellites, it also offers opportunity to further understand different topics of interest: the sensibility of the SARM processing to the SRAL acquisition mode, the comparison of the performances between LRM and SARM observations and between LRM and P-LRM observations that are for the first time available on a collocated data set.

The presentation will combine two abstracts:

1. Authors: Mittaz J., Woolliams E., Hunt S., Merchant C.

One of the primary products from the Sea and Land Temperature Radiometer (SLSTR) is the sea surface temperature (SST) which is based on algorithms developed from the ESA Climate Change Initiative (Advanced) Along Track Scanning Radiometer ((A)ATSR) SST retrievals. The product also comes with an estimate of the SST uncertainty based on a propagation of different uncertainty components both from the SLSTR instrument itself as well as components related to uncertainties associated with the retrieval. It also contains a set of SST quality level flags based on the Group for High Resolution SST (GHRSST) specifications to allow the user to select SSTs over a range of different qualities.

While certain aspects of the SST quality levels and uncertainties are routinely studied as part of the general SST validation process, it is quite difficult to obtain data which can isolate different components of uncertainty because of different spatial and temporal sampling of the observations and how things like atmospheric variability change the retrieval and its uncertainties. Further, in terms of understanding how the Sentinel-3 A/B data can be used in a consistent manner there is also the issue of how changes in instrument behaviour between different sensors impact the SST retrieval. Here we report on a study of the SST and associated uncertainty/quality information using data from the Sentinel-3 Tandem phase where Sentinel-3A and Sentinel-3B fly in close proximity. This enables direct comparisons of the SST and uncertainty/quality information from two independent sources both where differences in the viewing geometry and observation time is small as well as allowing studies of the impact of varying the time difference/viewing angles as the satellites drift apart. We will discuss the differences in the SSTs and analyse the SST uncertainty budget, and look a differences in the products over a range of different parameters such as the SST quality level and variations in the associated cloud mask data with an aim of understanding different possible sources of uncertainty and error in the SLSTR SST retrieval process.

2. Authors: Smith D., Polehampton E., Etxaluze M., Nieke J., Hunt S., Wooliams E.

The Sea and Land Surface Temperature Radiometers (SLSTR) on the Copernicus Sentinel-3 A and B missions are multi-channel scanning radiometers primarily designed to measure global sea-surface temperatures (SST) to an uncertainty < 0.3 K and land-surface temperatures (LST) to an uncertainty <1 K for climate monitoring. The first instrument was launched in February 2016 and was joined by its sister in April 2018. In principle, and to ensure seamless SST and LST data products from the two instruments, the instruments are identical and both are self-calibrating with an uncertainty in brightness temperature of 0.1 K over the main operating range. Nevertheless, small differences between the instrument builds and uncertainties in the calibration will inevitably lead to offsets in the radiometry of the two.

The tandem phase of the Sentinel-3A and B satellites provides a unique opportunity to directly compare the radiometric calibrations of the SLSTR A and B instruments. The tandem comparison data will be analysed for many long-term applications. During the commissioning phase of the SLSTR-B instrument, comparisons of the L1 tandem data have been used to verify the radiometric performance and to adjust the calibration factors of the VIS and SWIR channels to align to SLSTR-A.

Direct comparisons of the L1 products produced by the SLSTR-A and SLSTR-B instruments showed that there is a strong sensitivity to geometric differences, particularly where there are scenes with strong spatial and temporal variability, in particular over clouds. Therefore filtering of individual scenes is needed to reduce the scatter on the pixel-to-pixel comparisons.

An approach used for this paper is to remap the L1 radiometric data to a L3 grid with a resolution of 0.05degrees. This was found to be more efficient that comparing individual L1 images and allows comparisons to be performed over a wider range of scene temperatures/radiances. We present the results of the preliminary comparisons of the analysis.

Few weeks after its launch in April 2018, Sentinel-3B of the European Space Agency has been put in a tandem phase with its twin Sentinel-3A already in orbit. Both platforms were on the same track with the same geometrical conditions to gather acquisitions over the same targets only thirty seconds apart. This tandem phase lasted from early June to mid October 2018 to provide a unique opportunity for each S-3 sensors to increase knowledge of payload differences, reduce uncertainties when comparing data and to homogenise differences by defining appropriate adjustments.
In this presentation we address the outcome of this tandem phase (as well as of the succeeding relatively shorter drift phase) for OLCI, the Ocean and Land Colour Instrument. OLCI is a multi-spectral imaging spectrometer acquiring in the Visible to Near-Infrared domain dedicated to ocean colour and land science at medium resolution (300 m full resolution, 1.2 km reduced resolution). In the continuity of the MERIS mission, it is a good candidate to become a reference instrument for the radiometry in the Visible and NIR, thanks to its advanced on-board calibration device, wide swath and medium spectral resolution. The radiometric validation of the sensor and the inter-unit consistency is therefore critical for the mission.
The tandem phase first allows to perform direct comparisons of the level 1 radiometry and to address geometric and radiometric calibration issues and to inter-calibrate the sensors. Comparisons are presented over any target and then especially over dedicated targets such as open oceans and deep convective clouds which provide ideal conditions respectively for low and high radiances.
By design, the spectral calibration (i.e. the characterization of the acquisition wavelengths of the instrument detectors) of each sensor is not strictly identical and homogenization (which is related to smile correction) needs to be performed in order to accurately inter-compare the absolute calibration of the two sensors. The huge statistics provided by the tandem acquisitions allows to retrieve inter-calibration coefficients which can be used to align OLCI-B to OLCI-A with much smaller uncertainty than by comparing each OLCI to any other mission.
Variability of the inter-calibration coefficients is assessed over the period of the tandem phase so as to derive a harmonization process of OLCI-A and OLCI-B to be effective during the complete tandem phase as well as during the drift phase. The connection with the nominal monitoring of each OLCI sensor provides an assessment of the usefulness of the tandem inter-calibration for the whole mission duration.
While level 1 inter-comparisons are required to understand issues related to the inter-calibration of OLCI-A and OLCI-B, inter-comparisons of level 2 products (water-leaving reflectances for water products, OGVI for land products) provide insight on the sensitivity of the atmospheric correction models to differences on the input L1 radiances. During the drift phase, acquisitions of the same targets are made from different geometry of acquisition so that the handling of the geometrical dependency by the same correction model can also be assessed.
Benefits of the tandem and drift phases on the nominal level 2 processing and products are presented and concludes on the outstanding usefulness of such mission operation.

Over the past decade, there has been a growing interest for altimetry measurements over inland waters. Studying lakes, reservoirs and rivers water level is of prime importance for the hydrology community to assess the Earth’s global resources of fresh water. In the context of the European Union’s Copernicus Programme, Sentinel-3 altimeters are key to provide such global and continuous datasets of water surface height. Indeed, much progress has been made in altimeters capability to acquire quality measurements over inland waters. In particular, the Open-Loop Tracking Command (OLTC) represents a major evolution of the tracking function. This tracking mode’s efficiency has been proven on past missions and it is now stated as operational mode for current Sentinel-3 and Jason-3 missions.

On Sentinel-3, areas of interest define where SRAL is operated in OLTC mode. Outside these areas, SRAL altimeters operate in Close-Loop mode (a.k.a. autonomous tracking mode), with no a priori elevation information used to set the echo reception window.
During Sentinel-3B commissioning phase and its tandem flight with Sentinel-3A, both altimeters were operated either in Close-Loop or Open-Loop, or a combination of those, in different areas of the world. This clever and unique operating mode masks combination was very useful for evaluating both modes capabilities over different types of inland water bodies. These analyses were performed over lakes, reservoirs and rivers worldwide to evaluate the current and future capability of Sentinel-3 to provide continuous and reliable measurements of inland waters surface height.

Furthermore, this tandem phase has provided a unique opportunity to evaluate innovative SAR ground processing techniques over inland waters, such as the Fully-Focused-SAR processing. It has been shown very powerful, especially over small inland water bodies. A dedicated validation campaign was performed over a few rivers and lakes in southwestern France using an airborne lidar flying over the water surface, at the same time as Sentinel-3 overflight.

We present detailed results of this fruitful tandem phase and future perspectives on Sentinel-3A and Sentinel-3B OLTC tables contents for further improving inland waters observation and monitoring.

One of the main objectives of the Sentinel-3 (S3) mission is to measure sea- and land-surface temperature with high-end accuracy and reliability in support of environmental and climate monitoring in an operational context. Calibration and validation are thus key criteria for operationalization within the framework of the Sentinel-3 Mission Performance Centre (S3MPC).
Land surface temperature (LST) has a long heritage of satellite observations which have facilitated our understanding of land surface and climate change processes, such as desertification, urbanization, deforestation and land/atmosphere coupling. The Level-2 SL_2_LST product from Sentinel-3A (S3A) became freely available on an operational basis from 5th July 2017. A rigorous validation is critical in order to assess the quality of the data and the associated uncertainties. While it has been shown to meet the mission requirements for LST of < 1K accuracy, an opportunity arose to evaluate the product with respect to first LST data from Sentinel-3B (S3B). Both utilise the same retrieval algorithm, but the retrieval coefficients are independently determined. Such an evaluation can help reveal specific traits exhibited by each of these Sentinel LST data products.
During the Tandem Phase, the released Level-1 product on 4th September 2018 enabled a comparison between the SL_2_LST product from S3A and a pre-release Level-2 LST from S3B. Here we present the first results from the Tandem Phase of LST comparisons, which highlight across-track differences between the two products. These differences were put into context through both in situ validation of the LST from S3A and S3B, and intercomparison with a third-party LST product (in this case the operational LSA SAF SEVIRI LST). This Tandem Phase has facilitated a more detailed understanding of these differences enabling an improved parameterisation to be implemented in the LST data for better output products for the user community.

In June 2018 Sentinel-3A (S3A) and Sentinel-3B (S3B) entered the tandem phase. The aim of this phase was to collect statistically significant data over a period covering ocean and land to compare the performance of S3A and S3B instruments (SLSTR, SAR, MWR and OLCI) and understand the uncertainties between the instruments and their impact on products. We have investigated the similarity and differences of the OLCI L2 land products during the tandem phase (i.e. the OLCI Global Vegetation Index (OGVI), measuring the fraction of absorbed photosynthetically active radiation (FAPAR), and the OLCI Terrestrial Chlorophyll Index (OTCI), quantifying canopy chlorophyll content). These variables are essential to monitor vegetation condition and health, and are often used as input to models to understand the climate. OLCI continues the legacy of MERIS. This contribution examines OTCI and OGVI differences from S3A and S3B at global scale, over various land cover types and through time. Preliminary results show an overall strong level of agreement between S3A and S3B for both land products (R2 > 0.9). However, some discrepancies possibly associated to pixel collocation, viewing geometry and cloud cover were observed and may require further examination.

Long-term, consistent satellite records of ice sheet change are essential for understanding sea level rise, and for mitigating the social and economic effects of climate change. The longest continuous observational record to date is derived from satellite radar altimeters, which through the combined operations of ERS-1, ERS-2, Envisat and CryoSat-2, now provide a unique 25-year record. Looking to the future, the series of Sentinel-3 radar altimeters, which form part of the EU Copernicus programme, offer the opportunity to continue this record for decades to come. The launch of Sentinel-3A (2016) and Sentinel-3B (2018), provide SAR Ku-band altimeter coverage up to a latitude of ~81.5 degrees, and with a repeat period of 27 days. After the launch of Sentinel-3B in April 2018, the pair of 3A and 3B satellites were operated in a Tandem Phase, whereby for ~ 4 months the satellite flew in close formation, separated only by 30 seconds, or ~ 220 km.

This Tandem Phase offers a unique and invaluable opportunity to systematically inter-compare Sentinel-3A and Sentinel-3B SRAL SAR measurements over ice sheet surfaces, and to investigate any differences in the performance of the two missions ahead of 3B reaching its nominal orbit. Importantly, the tandem phase provides pairs of 3A-3B measurements that are almost exactly co-located in space and time, and acquired with the same viewing geometry. This minimises the influence of spatial and temporal variations in the ice sheet snowpack, and also the effect of anisotropic backscattering of the radar reflection, which can complicate the interpretation of height differences computed at orbital cross-overs. Here, we present analysis of Sentinel-3A and -3B Tandem Phase data over ice sheets to assess the similarities and differences between the two instruments. We will (1) intercompare waveform-shape and backscattered power, (2) assess the shot-to-shot precision of elevations retrieved from both instruments over the flat surface above subglacial Lake Vostok in East Antarctica, and (3) evaluate absolute accuracy through a comparison to Operation IceBridge airborne measurements. These activities will allow us to characterise any differences in the elevation measurements made by the S3A and S3B SRAL instruments over land ice, and thus contribute to the development of consistent, long-term climate records.