Achievements and Technology Domain
Over a two year period the CryoSat Plus For Oceans (CP4O) project carried out a study of SAR altimetry, based on CryoSat-2 data, in four thematic areas: Open Ocean, Coastal Ocean, Polar Ocean and Sea Floor Bathymetry. Starting with a through review of SAR altimetry, CP4O has developed and implemented innovative new processing schemes, in all four thematic areas, and developed and evaluated a series of demonstration products to take advantage of this capability. An independent evaluation confirmed that these products based on CryoSat-2 SAR data provide users with significantly improved measurements over the ocean. Also a further technical and scientific understanding of SAR altimetry over the ocean was achieved and recommendations for further work and exploitation have been provided in a Scientific Road Map.
The final CP4O Technical Report provides a summary of all the results and recommendations.

For the activities under the CCN01, additional work under four themes was carried out. Firstly data from the CryoSat-2 SAR interferometric mode data set were used to develop and validate a new processing approach to provide oceanographic measurements in complex coastal topographies. The final technique can be applied to SARin, SAR and conventional “Low Rate” mode altimeter data. Second, a regional Tidal Atlas for the Arctic Ocean was produced and validated, using re-tracked CryoSat-2 data together with historic altimeter data and tide gauge data. Third, improvements to the SAMOSA re-tracker were implemented (better estimation of thermal noise) and validated, confirming an improved performance on the current baseline implemented in the Sentinel-3 SRAL DPM. Finally, an extended evaluation of performance of CryoSat-2 SAR mode in the coastal zone was carried out, confirming that with appropriate processing and filtering accurate sea surface height data could be retrieved to within 2km of the coastline.
The final CP4O CCN01 Technical Report provides a summary of all the results and recommendations.

Results from Main CP4O Contract
The Scientific Road Map provides a comprehensive description of proposed and planned activities ranging from further research needed to support improvement of SAR altimeter processing to the further development and exploitation of higher level products derived from SAR altimeter data.
In this document the following aspects are identified:
SAR Processing Issues

• SAR Waveform Processing
  • Under-sampling peaky waveforms
  • Optimising Doppler processing /selection / weighting
  • Purpose /optimisation of Windowing,
  • SAMOSA Echo Model implementation
  • SAR waveform blurring at high alt rates
• RDSAR Processing
  • Effect of SAR transmission pattern on waveform statistics
  • How to improve waveform statistics / minimize use of correlated echoes
• Investigations into SAR Altimetry Characteristics
  • SWH dependencies / errors
  • Impact of swell
  • Development and evaluation of SAR mode SSB models
  • Dependence of retrieved parameters on roll angle, pitch angle, radial velocity
  • Characteristics of Full Bit Rate echoes, stack data
• Validation activities
  • Comprehensive evaluation of CryoSat SAR data for coastal
  • Larger data sets for SAR and RDSAR validation
  • Sentinel-3 validation planning (mode selection)

Improved Geophysical Corrections

• WTC data set for whole CryoSat mission, global along-track and gridded data sets
• Change ionosphere model used to estimate electron content above CryoSat orbit

Application Specific Processor and Product Development

• Open Ocean
  • Sentinel-3 DPM updated to include best performing implementation of SAMOSA3
  • Further improvements to CPP SAR mode processing scheme
  • SARin processing for oceanography (e.g. across track slope)
• Coastal Ocean
  • SARin investigations, Develop schemes to improve processing of SAR data at coast
• Polar Oceans
  • CryoSat reprocessing (all polar data with consistent Baseline)
  • Develop and publish improved Polar tide model
• Sea Floor Bathymetry
  • Processing of a longer period of SAR mode altimeter data for the Pacific SAR region, using an improved prior bathymetry
  • Investigations in shallow / coastal regions to investigate potential capabilities of data in this environment.

Integration into Operations Use.
The following products have been identified as fully mature and ready for integration into operational use: RADS CryoSat RDSAR (TUDelft), CPP CryoSat RDSAR and SAR (CNES), SPECTRE Ionosphere Correction (Noveltis), COMAPI Regional Tide model (Noveltis).
The following products have been identified as almost mature, needing limited further development but expected to be ready for integration into operation use within one year: SAMOSA Echo Model (ESA/Starlab), SAR Polar Ocean Products – Mean Sea Level, Mean Dynamic Topography, Polar tide Model (DTU Space), Wet Troposphere Correction (U Porto)

A CCN has been approved which will address some of the above issues where further work is needed to complete the development of some products and algorithms initiated in the main CP4O contract, in the area of coastal and polar applications and in the refinement of the SAR altimeter echo model that forms the basis of the re-tracker solution applied in the Sentinel-3 ground segment. These are:

1. SARin for Coastal Altimetry, Improved SARin processing for Test Data Set Generation.
2. Implementation of a Regional Tidal Atlas in the Arctic Ocean.
3. Improvements to the SAMOSA re-tracker implementation and Evaluation- Optimised Thermal Noise Estimation.
4. Extended evaluation of CryoSat-2 SAR data for Coastal Applications

Activities (1) and (4) will enable better application of CryoSat-2 SAR data in coastal areas, in terms of developing and evaluating improved processing approaches. Activity (2) will provide a state of the art regional tidal atlas for the Polar Ocean, necessary as results from the main CP4O project have demonstrated current tide models have significant errors. Activity (3) will support a significant improvement to the implementation of the SAR Altimeter Echo Model (SAMOSA) that is applied within the Sentinel-3 altimeter processing chain. Results in the main CP4O project have shown that the estimate of Significant Wave Height in particular will be improved.

Results from CP4O CCN 01
Some recommendations for further work related to the activities in the CCN01 are highlighted:
For the New Approach for Coastal Processing of Altimeter data, developed using CryoSat-2 SARin mode data.

• The SAR mode enables the possibility of a dedicated stack processing that could lead to a less contaminated multi-look waveform. The algorithms developed within this CP4O CCN activity could be combined with a stack processing in order to give the re-tracker not only a seed and a cut waveform to avoid contamination, but also work with an enhanced multi-look waveform pre-processed for better further SSH retrievals. For echoes very close to the coastline, steering could also be applied to further improve results. These would be major modifications to the current processing as they would require modification of the L1b SAR/SARin processing.
• An interesting option for further studies is to combine the two above approaches in a synergic solution that takes the best from both, and derive an enhanced SSH coastal mapping.
• An interesting application of this new processing approach could be to develop a mean sea surface database of a selected area by processing a data series over a period of several years.
• A validation exercise of such a dataset would be very interesting. From it, we would be in position to better discriminate the real measurement noise from the SSH variations due to the ocean dynamics (e.g. MSS variations in the Area of Interest).

For the Regional Arctic Tidal Model

• The model is recommended “as is” for ocean modeling and forecasting in the Arctic Ocean. It can also provide an improved tidal correction in the CryoSat-2 ocean products, and for altimetry missions with high-inclination orbits, such as Envisat, SARAL/AltiKa and Sentinel-3. The new atlas can also benefit the Copernicus Marine Environment Monitoring Service (CMEMS), which includes the Arctic Ocean as one of five priority European regional seas, and to other European Arctic observing systems.
• A key limitation on the quality of the model is the availability of a reliable and accurate bathymetry. There are differences between the two available sources (IBCAO and R-Topo), indicating that both contain errors. It is believed that more accurate bathymetry models have been generated, but are not available for scientific use. It is strongly recommended that efforts are made to support the availability of improved bathymetric models for scientific use.
• Further improvements could be made to the Arctic Model, by modifications to the hydrodynamic model, to provide a better representation of the main diurnal components (K1 and O1), and by adding a stage to the processing of the altimeter data to remove the annual variation in sea level prior to the estimation of ocean tide parameters.

For implementation and further development of the SAMOSA re-tracker:

• It is recommended to incorporate the improved estimate of thermal noise in the DPM for the Sentinel-3 SRAL processing chain
• A common way of computing the misfit should be applied and incorporated into standard SAR altimeter products. 

• It is a priority to develop a robust re-tracker that will operate reliably on uncontaminated open-ocean SAR echo data. 

• There is still a problem in accurately modelling SAR echoes at low wave heights. 
A further investigation into the performance of the SAMOSA re-tracker at low wave heights is 
needed.
• The high SSH noise in SAR datasets away from the coast underlines the need of including geophysical corrections and mean sea surface to derive SSH statistics, and of determining the 
reason (geophysical or instrumental) of such a high variability within 7 km of along-track data. 


The extended evaluation of the capability of CryoSat-2 SAR mode data in the coastal zone identified the following recommendations for future application:

• Across-track and along-track distances from the coast are more suited than the ‘angle to coast’ as independent variables for this assessment. The angle to coast is ambiguous where the coastline is complex and its definition has a degree of subjectivity.


• The adoption of a specific processing configuration (Hamming filter, zero padding and extended range window) improves the noise characteristics especially in the “last few km” from the coast. 


Deliverable documents are available at: http://www.satoc.eu/projects/CP4O

CCN2 Arctic bathymetry and tides
The CryoSat Plus for Oceans (CP4O) project, under the ESA STSE program, aims to develop and evaluate new ocean products from CryoSat-2 data and so maximize the scientific return of CryoSat-2 over oceans. The main focus of CP4O has been on the additional measurement capabilities that are offered by the SAR mode of the SIRAL altimeter, with further work in developing improved geophysical corrections, such as a regional tidal model in the Arctic Ocean. The Arctic Ocean is a challenging region, because of its complex and not well-documented bathymetry, together combined with the intermittent presence of sea ice and the fact that the in situ tidal observations are scarce at such high latitudes. In 2016-2017, the CP4O initiative successfully implemented the Arctide2017 regional tidal model in the Arctic Ocean. Some possibilities of improvements were identified, that are addressed in the current initiative. First, the improvement of the Arctic bathymetry ingested by the hydrodynamic model, by using the near 7 years of Cryosat-2 high quality and high resolution ”geodetic” SAR altimetry all the way up to 88°N. Second, the use of improved Cryosat-2 derived harmonic tidal constituents for assimilation into the regional tide model. The project runs during 2017 and in this presentation we focus on the evaluation of existing bathymetry datasets in the Arctic (R-TOPO2, IBCAO etc) and the development of a new dataset including CryoSat-2 data. We also present the methodology to develop the improved regional tidal model in the Arctic Ocean.