S3NET: Distributed and Fractionated Radar Based Earth Observation Analysis

Abstract

The prototype radar mission selected for detailed assessment within S3NET consists of a bistatic formation to enhance observation capabilities of a potential future Sentinel-1 follow-on mission. A chief satellite equipped with a transmit-receive C-band SAR - orbit and radar parameters being taken from Sentinel-1 - is augmented by pairs of receive-only deputies. The bistatic baseline between chief and each deputy is assumed to be sufficiently large (from several km up to 200km) to allow safe formation flying, whereas the baselines between deputies is only in the order of few 100m. This can be adjusted to allow the generation of digital elevation models and/or the measurement of ocean surface currents by means of single-pass interferometry (SP-InSAR), but also the retrieval of glacier flow velocities by means of repeat-pass interferometry (RP-InSAR) or Speckle tracking. It is shown that DEM performance is comparable to the one achieved by the TanDEM-X mission. Retrieval of 2D-displacements is possible in case of large bistatic baselines and deployment of two sets of deputies, one ahead and one behind the chief illuminator. The selected formations and the bistatic SAR operating mode require the development of specific algorithms, which has been adopted in S3NET: - bistatic SAR image formation: a numerical least-squares kernel has been adopted and validated for C-band. Its reversed order of operations is used for simulating SAR raw data of extended scenes. - Synchronisation of the oscillators of the Tx-sensor and the different Rx-only deputies: onedirectional synchronisation links are assumed for the direct transfer of chirped signals, which are evaluated to compute correction parameters for the received bistatic SAR raw data. The developed algorithms have been integrated and validated in the frame of DLR's bistatic end-toend SAR simulator, meant to support development activities and performance analysis for different bistatic SAR mission concepts from X- to L-band. In addition, a monostatic SAR image formation algorithm is specified for the assessment by S3NET partners of onboard processing in support of near-realtime (NRT) applications, together with a multilook algorithm for data reduction purposes.