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2.6.2 MWR Level 1b algorithms

The key processing algorithms required for the generation of the L1b product starting from the MWR Level 0 product are:

  • Level 0 data extraction and decoding
  • gain processing
  • temperature offset evaluation
  • antenna temperature retrieval
  • brightness temperature evaluation
  • antenna axis registration
  • Level 1b data formatting

 

The MWR Level1b data processing outputs calibrated antenna temperature values using as input scientific raw data, and registers the two channels measurements , as they are not pointing at the same place. Moreover using an apropriate radiative model the brightness temperature is calculated using the calibrated and geolocated antenna temperature. The input of Level 1b processing is the Level 0 scientific data (Telemetry Data provided by High Speed Multiplexer in Low Bit Rate) for MWR.

To calculate the brightness temperature it is necessary, first, to evaluate the temperature as seen by the main antenna, and then to separate the contribution coming from the main lobe from those coming from the secondary lobes affected by spurious contributions.

Moreover to retrieve the main antenna temperature Ta from the output receiver voltage, it is necessary to characterise the radiometer response.

 

The result of these MWR L1b processing steps represents the engineering foundation product enabling the derivation, durig nthe Level 2 processing, of integrated liquid water, water vapour content and altimeter path delay values.

2.6.2.1 Level 0 data extraction and decoding

The Level 0 data needed for the Level 1b processing is extracted from the telemetries, coming from the on board ICU, from the source packet headers, and from the product headers.

Each field of the Source Packet representing the acquired raw data stream is extracted and converted into engineering units according to its binary format. Some consistency checks are performed to validate the extracted parameters at this level; in most cases the validity is checked by simply comparing the expected values with the current ones, within the Level 1b processing itself.

The following steps are performed:

  • extraction of the source packet identifier
  • extraction of Packet version number and validity check (setting of warning header flag if unexpected value)
  • extraction of the Packet type to distinguish telecommand from telemetry
  • extraction of the data field header presence flag (setting of header warning flag if unexpected value)
  • extraction of mode identification check (setting of warning header flag if unexpected value)
  • extraction of the source packet segmentation flag (setting of warning header flag if unexpected value)
  • extraction of the packet length value (setting of warning header flag if unexpected value)
  • extraction of the Data Field header length (setting of warning header flag if unexpected value)
  • extraction of instrument mode check (setting of warning header flag if unexpected value)
  • extraction of number of blocks for each source packet (setting of warning header flag if unexpected value)
  • on board datation consistency check, using the source sequence count fro mcurrent and previous SP (setting of Header and OBDH flag if difference between consecutive OBDH datation values is not in the allowed tolerance)
  • telemetry datation and location (lat/long) for each TM and for each channel. Note that the UTC reference time to be used for this calculation will be leap second corrected (if relevant) before being used for the processing. Location of each data block can be evaluated using orbital parameters and the datation corresponding to each data block. There are two different modes of operation. The first mode corresponds to the case when no Orbit file will be present in input. In this case, the orbit propagator provided by CFI Library will be used and its initialisation will be performed using the reference state vector stored into the Level 0 MPH. The second mode corresponds to the case when at least one Orbit file will be present in input. In this case, the orbit interpolator provided by CFI Library will be used and its initialisation will be performed using FOS or DORIS auxiliary file.
  • extraction of Line_Count from first Telemetry word (consistency check with previous TM value and setting of a warning flag if the difference between consecutive Telemetries is not 1)
  • extraction of calibration status field CS in the telemetries for the individuation of the data source (Main Antenna, Hot Load, Sky Horn or Offset) and of the Calibration status field CF for discrimination of first or not first measurement of the current data type.
  • extraction of Channel 1 and Channel 2 counts for any of the data source types
  • extraction of the telemetry coming from on board thermistors, according to the data these thermistor are relevant to (indicated by Line_Count)
  • extraction of the DHK bits and check on their values (telemetry warning flag raised if different values decoded)
  • extraction of the power bus protection indication, overload and overvoltage protection, normal/redundant ICU channel indication and blanking pulse status
  • setting of the Temperature table full flag if at least 32 telemetriues have been acquired
  • calculation of the on board temperature measurement by using a 10th grade polynomial fitting algorithm
  • calculation of the two channels physical receiver temperatures (and standard deviations) and consistency checks on them (telemetry and temp flag raised if values out of allowed ranges)
  • computation of the temperatures relevant to the Dicke load, Hot Load, IF sections and mixers, for both channels (telemetry and temp flag raised if values out of allowed ranges)
  • CRC check and setting of CRC flag if the L0 value is different from the one calcualted during the processing

2.6.2.2 Gain Processing and Offset Temperature Evaluation

The in-flight calibration is obtained from combination of measurements from sky horn and internal load.

This calibration performs the evaluation of the first order approximation of the current gain, that is the gain relevant to the current intercalibration period.

This is relevant to the calibration of the radiometric response using a non linear slope between the two extreme temperatures, sky horn and hot load, (and the relevant output counts) fixed by the two calibration points. The evaluation takes into account.the dependence of receiver gain to the instrument physical temperature.

Moreover, due to the system noise, there is a temperature offset related to the output counts where no input is present, introducing a temperature bias in every measure. All the parameters involved in the processing and not extracted by telemetry data are expected to be estimated and calibrated on-ground before launch, and stored in the Characterisation/Calibration auxiliary file.

 

The gain and offset are calculated through the following processing steps:

  • selection of the Offset measures (discarding the very first one), only if 32 telemetries have already been acquired
  • calculation of the constant gains for both channels, using the hot load and sky horn counts, receiver physical temperatures and coefficients extracted from the Characterisation auxiliary file
  • calculation of the offset temperatures, using the gain values, hotload and offset counts, Dicke switch temperatures, sky horn radiometric temperatures, hot load temperatures and sky horn calibration switch temperatures, and some coefficients from the Characterisation auxiliary file
  • calculation of the smoothed (averaged) gain and offset temperatures if the calibration number reached the moving window size

NB: if the end of file occurs before the number of calibration measures (Offset) reaches the moving window size, the file shall be rewinded. That is; the new moving window size will be set to the available calibration number, and the only set of smoothed gain and offset temperature values will be used to process the whole L0 data. While rewinding, no antenna data is lost, since the smoothed offset and gain values are used to obtain antenna temperatures from all the data, even from the measures prior to the actual calibration sequence.

 

In case no gain values are calculated during the processing (not enough Offset measures in the L0 product), the output brightness temepratures are directly set to default (0). This shall avoid having negative values of brightness temperautres, that when casted to unsigned values in the L1b product, give temperatures values with no physical meaning.

The user shall know in this case that the gain was never evaluated, since the moving window size in the Configuration auxiliary file will be different from the moving window size in the L1b product.

2.6.2.3 Antenna Temperature Retrieval

Once the radiometric response has been characterised it is possible to perform the main antenna noise temperature calculation, by retrieving its measure by the output counts of each channel, the actual gain and the actual temperature offset, and all the radiometric parameters involved.

This is an estimate and will be affected by an uncertainty due to several causes: residual uncertainty on parameters values after pre-flight calibration or uncertainty on resistances measures and temperature conversion, not completely assessed knowledge of non linear behaviour of the receiver.

For these reasons the retrieval processing is accomplished using an expected value of algorithm accuracy theoretically evaluated, that is compliant with the instrument requirements.

2.6.2.4 Brightness Temperature Evaluation

The geophysical parameters are computed from the temperature seen by the antenna main lobe. To calculate it, it is necessary, first, to evaluate the temperature seen by the antenna, and then to separate the contribution coming from the main lobe from those coming from the secondary lobes.

There are three different algorithms that can be used to remove the effects of the secondary lobes. In the first, ERS-1 like, algorithm, the secondary lobes effects are tabled in a Side Lobe Table auxiliary file, as function of the latitude of the satellite.

In the second, ERS-2 like, algorithm, instead, the side lobes contributions are obtained using the reflector physical temperatures, the transmission coefficients of the reflectors and the global side lobes contributions for each channel.

The third algorithm is proposed by CLS that used one full year of ERS2 brightness temperatures to evaluate the contribution of the land emissivity (directly and after reflection by the platform) inside the measurement cell.

2.6.2.5 Antenna axis registration

In order to obtain geophysical data, it is necessary to combine the observations of Channel 1 (23.8 GHz) and Channel 2 (36.5 GHz) and of the altimeter, when available over the same spot. As the two radiometric channel are not looking at the same spot because of their different off-nadir angles (1.72 and 1.83 respectively) , a registration is necessary.

This is done in the following way:

The brightness temperatures, already evaluated, are co-located using a certain number of samples to be skipped between the two files (Channel 1 and Channel 2) containing all the information (e.g. brightness temperatures, location, datation, record counter, samples since last calibration period, quality flags, OBDH flag, valid data flag) of the two channels.

Then, since the36 GHz channel looks forward in EnviSat, for every sounding of Channel 2, starting from the first valid data, the corresponding sounding in Channel 1 is searched in the record that is shifted a number of samples.

This number of samples to be skipped between the two files has been obtained during the Level 0 data extraction and decoding when the datation and location of the Telemetries were calculated.

At this point, all the co-located averaged data (brightness temperatures and standard deviations) are calculated for both channels and output into the same MWR L1b record.

Information like datation, location, source packet identifier and packet telemetry counter are exported into the output product corresponding to the 4th measure of the 8 elementary measurements that were used for the averaging process. In the case of datation and location, only the information pertaining to the forward channel is copied into the L1b output record.

2.6.2.6 Level 1b Data Formatting

The processed data are stored in a specific layout including some necessary information for the following processing. The output layout shall include file headers, with data relevant information like the functional mode during which the acquisition has been performed, the longitude/latitude co-ordinates and the UTC time, etc., followed by a series of records regarding the output brightness temperature specified by the actual functional mode. As the output layout is the same for Level 1b and Level 2 default values (set to 0) will be written in Level 2 data fields.


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