March 30, 2017

Ground segment

The mission ground segment consists of two mission centres (JPL and CNES) and a network of X-band stations provided by the CNES multi-mission ground segment (INUVIK + KIRUNA, with supplementary passes over Aussaguel and HBK). More than 20 passes per day are necessary for transmitting the 7.9 TBits of data produced daily. A specific server is used to separate and transfer the data to the two processing centres.

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SWOT ground system architecture.

CNES is responsible for processing data from the nadir payload which it sends to JPL for archiving and distribution by the two agencies.

The two agencies provide level 1 and 2 KaRIn data (geo-located water elevation levels once known errors have been removed), completed by a river discharge product.

There are two types of processing (and two types of products), corresponding to different resolutions and themes:

  • The HR (high-resolution) mode designed for continental hydrology, characterised by a very high spatial resolution, necessary for precise identification of fine structures (rivers) and small structures (lakes). This mode is active only over land (from coast to coast).
  • The LR (low-resolution) mode dedicated to oceanography, characterised by a very high altimetric precision, but with reduced planimetric resolution compared to the hydrology mode. However, the planimetric resolution is still much better than that of today’s nadir altimeters. This mode is constantly active, even over land.

The difference between the HR products intended for hydrology and the LR products intended for oceanography is due to the differences in onboard and ground processing.

CNES and NASA contributions to operational processing (guaranteeing the performance level, availability and distribution of level 2 products) comply with the following principles:

  • The two mission centres exchange raw, calibration and ancillary data
  • The two mission centres exchange raw, calibration and ancillary data
  • CNES is responsible for processing the high-resolution KaRIn data (Hydro) for EURASIA
  • NASA is responsible for processing the high-resolution KaRIn data (Hydro) for the other continents. The division of hydrology processing meets the need to pool processing efforts
  • CNES will process the nadir altimeter data as for conventional altimetry missions,
  • CNES will provide precise orbit determination (POD)
  • NASA is responsible for processing the low-resolution KaRIn oceanography data

A joint French-US algorithm development team (ADT) was set up in early 2014. Its task is to prepare, propose and carry out algorithm development before validating and providing the operational algorithms and test cases. It is the ADT which guarantees the delivery of operational algorithms to be integrated in the processing systems.