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2.2 Organisation of Products

The suite of RA-2/MWR data products is based on one main Geophysical Data Record (GDR) product. The Envisat general product format is exploited to add sub-structure inside the product to hold additional data such as the averaged waveforms (at 18 Hz), the individual waveforms (at 1800 Hz) and the Microwave Radiometer data set. Thus, the waveform data product (SGDR) is a superset containing the same geophysical data records as the GDR, but with waveform data sets appended. Moreover, following a user requirement survey, these products were made global, independent of the sub-satellite terrain and of the Radar Altimeter measurement resolution mode, thus avoiding artificial boundaries between geographical features like land/sea, land/ice or land/lake transitions. This design ensures that ocean, land, ice, lake or wetland data always ends up in the same (unique, global) data product.

 

The Fast Delivery GDR (FDGDR) product is transmitted in less than three hours, for weather forecasting, sea-state and real-time ocean-circulation applications. An ocean-related parameter subset of the FDGDR called FDMAR (for Marine Abridged Record) is extracted to reduce the volume of on-line data transfers. FDMAR is converted into the BUFR format commonly used by Meteorological Offices. Less than three days later, the so-called Interim GDR (IGDR) for ocean-circulation monitoring and forecasting applications is delivered, replacing the original meteorological predictions with more precise analyses, and the preliminary orbit with an improved orbit solution. The final GDR and SGDR products containing the most precise instrument calibrations and orbit solutions are delivered after 30 days (not more than 50 days). The schematic in Figure 2.2.1 summarises the organisation, the inter-relationships and latency of the product generation. The terminology used to name products is based on the nomenclature traditionally used in Altimetry, with the product names stored in the first field of the specific product header.

 

 

 

 

The Envisat products are categorised into three distinct levels:

- Level 0 (raw): unprocessed data as it comes from the instrument.

- Level 1b (engineering): data converted to engineering units, with instrumental calibration applied (IF filtering, to correct power distortions of echo waveforms, internal range calibrations, corrections for possible drift of reference timing source, no re-tracking); the half-orbit segmented (pole-to pole) product mainly contains: datation (conversion of satellite time to UTC), geo-location, time delay, orbital altitude, sigma-zero, averaged wave-form samples at 18 Hz data rate, individual waveform at full pulse repetition frequency and MWR brightness temperatures.

- Level 2 (geophysical): data converted to geophysical units (with re-tracking); the product mainly contains datation, geo-location, output from re-trackers (range, wind speed, significant wave height, etc.), at 1 Hz plus some 18 Hz parameters (range, orbital altitude). All geophysical products, including the near-real time products, are re-tracked (waveform data are fully processed in the ground processor to extract the geophysical parameters). In order to retrieve the geophysical parameters over all types of surface (ocean, ice, sea-ice, etc.), four specialised re-trackers are continuously run in parallel (over all surfaces):

· Ocean re-tracker: optimised for ocean surfaces, and based on a modification of the Hayne model [Hayne, 1980]

· Ice-1 re-tracker: optimised for general continental ice sheets, it is a model-free re-tracker called the `Offset Centre of Gravity Echo Model'; it is used for ERS and will ensure measurement continuity [Bamber, 1994]

·Ice-2 re-tracker: optimised for ocean-like echoes from continental ice-sheet interior, it is a Brown-based model re-tracking algorithm [Legrésy, 1997]

· Sea-Ice re-tracker: optimised for specular returns from sea-ice, it is a threshold re-tracking scheme for peaky waveforms [Laxon, 1994]

 

The usual necessary geophysical corrections are available in the Level-2 products. The ionospheric correction will come from the dual-frequency altimeter, backed-up by the measurements from DORIS and the Bent model. The wet tropospheric correction will come from the on-board microwave radiometer, backed-up by a value computed from ECMWF fields. Users requiring the Altimeter waveforms will find them conveniently stored in the Level-2 SGDR product, along with the co-located geophysical corrections and the outputs of the four re-trackers. In other words, the SGDR holds the GDR data augmented by averaged and individual waveforms. Therefore, users of Envisat Altimetry need not access the Level-1b products.

 

The Fast Delivery data (FDGDR) will be processed at the ESA receiving stations (PFHS Kiruna and ESRIN) and delivered in less than 3 hours. The Interim Geophysical Data Record (IGDR) and the final precision Geophysical Data Record (GDR) products will be processed offline at the French Processing and Archiving Centre in Toulouse, the so-called F-PAC, with the same algorithms as the Fast Delivery processor. The SGDR is also built at F-PAC.


Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry