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    24-Jul-2014
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2.5 Level 0 products

2.5.1 RA-2 Level 0 products

2.5.1.1 Definition

All the information relative to the Measurement, BITE and IF Calibration modes of the instrument are packed in binary format and sent to ground in the form of Source Packets. They are generated by the instrument at a rate of one every 1.114 seconds. Approximately 6000 Source Packets are sent each orbit.

Each Source Packet contains 75712 bits in normal operation (no individual echoes are transmitted) and 101312 bits whenever individual echoes are transmitted.

The Level 0 products, used as input for the L1b processing, will be a stream of RA-2 source packets in the same format as the instrument delivers them to the multiplexer of the ENVISAT satellite. The data will have overlaps and similar artifacts removed, although they may still contain gaps.

2.5.1.2 Structure

The generalised structure of ENVISAT products consists of a Main Product Header, a Specific Product Header and a certain number of Data Sets, each composed of Data Set Records, as shown in the following figure:

 

 

 

The number of Data Sets attached in the L0 RA2 products is always 1, although there are DSs referred to in the SPH through DSDs for the auxiliary data used to build them.

The RA2 Level 0 Data Set records are composed of some FEP and PF-HS annotations, and quality indicators, common to all ENVISAT instruments, and a dedicated part called Instrument Source Packets.

The Source Packets consist of two major fields:

  • the Packet Header, of fixed length, which provides the standardized control information like Packet Identification, Packet Sequence Control and Packet Length;
  • the Packet Data Field which contains a Header and the scientific information distributed among 20 Science Data Blocks,
  • a Calibration Field (always present but is effectively filled up only during the tracking phase of the measurement mode)
  • an Individual Echoes Field ( present only when the transmission of individual echoes to ground is commanded by macrocommand)

 

The 20 Science Data Blocks will contain data relative to Acquisition or Tracking phase of the Measurement mode as well as data derived from operation in BITE and IF calibration. The internal structure of the Science Data Blocks and of the Data Field Header is function of the specific mode and phase within the mode; during tracking, for example, the samples of the averaged Ku and S waveforms are stored in the Science Data Blocks with auxiliary information gathered over 55.7 msec.

2.5.1.3 Packet Header

The Packet Header contains information of the ISP, and not DB dependant.

Parameters such as source sequence count (to be used for the OBDH/USO datation checks and for the PTR processing) and packet length (to be used for the packet length check) are stored in this part of the ISP.

2.5.1.4 Packet Data Field

The Packet Data field is composed of Data Field Header and Source Data Field.

The Data Field Header, whose format depends on the instrument operative mode, contains in Measurement mode (when at least one DB is in Tracking), parameters like the OBDH/USO datation, mode id, redundancy vector, averaged noise power, AGC attenuation for NPM, Delta Offset, etc.

The Source Data field contains the Data Blocks, with the Ku/S and extra DFT samples and all the information that is DB dependant, like coarse and fine Rx distance values, coarse and fine AGC values, chirp bandwidth, fault identification, etc.


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