## 1 Introduction  
Topex/Poseidon, Jason and Envisat are currently orbiting, delivering data for ocean monitoring and forecasting. Addition of spatial radar altimetry observations to in-situ measurements and models has given evidence of success for the last decades (see [3]). However, a limit has been reached that nadir altimetry cannot overcome without launching several radars at the same time. The two main drawbacks of nadir altimetry are lack of spatial and temporal coverage and a limited observed swath (typically 2 km for T/P and Jason). Ocean mesoscale phenomena with spatial scales that can go down to a few tens of kilometres and amplitudes in the range of 50 centimetres play a major role in the ocean dynamics: they remain largely undersampled.  
A new and revolutionary altimetric measurement has been assessed for the past few years (see [1, 2]). Indeed, mixing altimetry and interferometry, it increases by almost a hundred times the observation swath and aims at keeping a typical measurement error budget of about a few centimetres to catch most of the ocean circulation (see [3] and [4]). The Wide Swath Ocean Altimeter (WSOA) was originally expected to be implemented onboard the Jason-2 satellite in 2004, but the WSOA is not now going ahead (WSOA main parameters are recalled in the table of Figure 24). A global study of the performances of this instrument and its interactions with any external factors was driven by CNES (Centre National d'Etudes Spatiales) in cooperation with NASA/JPL (Jet Propulsion Laboratory) since the end of 2003. Every error domain has been checked and the outcomes have led to the definition of a dedicated mission to such an instrument.  
In the first section, we give an overview of the geometric layout of such concepts mixing altimetry and interferometry. We assess the derivation of the observed target features (position and height). Subsequently, based on the physics of the interferometric measurement for a given configuration (frequency, chirp bandwidth, antenna length, interferometric base, etc.), we set-up the instrumental error budgets leading to the performance estimation. Instrumental height error profiles are finally simulated for several processing configurations. The above-mentioned approach is primarily applied to the nominal profile of the WSOA onboard Jason-2.  
In the second section, the impact of external error sources is evaluated: platform (attitude, atmospheric drag), and propagation media (ionospheric and tropospheric effects). Subsequently, we focus on the required quality of a specific ground processing procedure dedicated to the roll angle determination, which is shown to be a critical aspect of the concept.  
A data simulation case is presented in the third section. It is based on real data acquired during the tsunami episode of December 26\({}^{\text{th}}\), 2004. Some of the possibilities offered by such concepts are illustrated.  
Finally, the last section develops a new set of instrument characteristics and dedicated mission by taking into consideration the outcomes of the previous error budget studies. An overall satellite configuration (platform + payload) is proposed, which optimizes the existing technologies in order to much better answer the scientific objectives.