|
MERIS Level 0 Analysis Tool |
Overview
MERLOT is the MERIS Level 0 analysis tool developped under IDL © with the following capabilities :
Check the Packet Header:
- the APID must be consistent w/ the L0 product filename ( e.g. a calibration APID can only appear in a MER_CA__0P file);
- identify any packet that brakes the packet counter sequencing (increment of 1 or rewind to 0 after 16383);
- identify all packets w/ the wrong :
- packet version number (4)
- packet type (0)
- data field header flag (1)
- segment flag (3)
Check the Data Field Header:
- Data field header length field must hold the proper default value;
- Check internal consistency of Instrument mode/format values;
- Format definition is consistent w/ Instrument mode/format field (8 LSB of Instrument mode/format equals Format definition field) and identify any questionable packets.
Any record that fails for these criteria will be considered as suspicious and will be ignored, and all the records that belong to the same frame also, for the APID sequencing analysis that follows.
APID sequencing analysis:
The different segments within the same L0 product are identified on all frames that are not suspicious (if at least one record is suspicious in a frame - see above - than the entire frame is considered as suspicious), each segment is identified w/ the following attributes:
- First packet number (from 0)
- Last packet number
- Offset (in bytes from start of file) of first packet
- OBT count of first packet
- OBT count of last packet
- Duration of segment (in sec)
then for a valid segment:
- APID hexadecimal code
- APID meaning
- number of frames
or for an invalid segment:
- explanation for invalidity (see below)
The full sequencing analysis is presented in a report.
The pakets/frames will be identified as being invalid with the following error messages:
|
Error message
|
Test performed for :
|
Diagnosis
|
|
Invalid Packet Version Number
|
each packet
|
packet version number is not equal to the expected default value (4) |
|
Invalid Packet Type Number
|
each packet
|
packet type number is not equal to the expected default value (0) |
|
Invalid Data field header flag
|
each packet
|
invalid data field header flag is not equal to the expected default value (1) |
|
APID incoherent w/ File Name
|
each packet
|
packet APID is not coherent w/ the filename (that should start w/ MER_RR__0P, MER_FR__0P or MER_CA__0P) |
|
Invalid Segment Flag
|
each packet
|
invalid segment flag is not equal to the expected default value (3) |
|
Packet Count Sequence lost
|
all product
|
packet counter is not increased by 1 wrt the previous packet, or has not been reset to 0 after packet count 16383 |
|
Invalid Packet Length
|
each packet
|
packet length stored in the packet header is not coherent w/ the file type (7685 for FR or calibration, 2135 for RR) |
|
Invalid Data Field Header length
|
each packet
|
invalid data field header length is not equal to the expected default value (286) |
|
Invalid On-board time
|
packets of all complete frames
|
all packets of the current frame (that is complete) donnot have the same on board time - all packets of same frame should be stored at the same time |
|
Invalid Redudancy vector
|
each packet
|
redundancy vector is not equal to the value of the current orbit |
|
Invalid Band Number
|
each packet
|
the packet band number is not within [0,15] |
|
Anomaly in band # sequence
|
each frame
|
band number does not follow the sequence 0-15 |
|
Incomplete Frame
|
within each frame
|
the packet does not belong to a frame because at least one packet is missing in the adjacent packets to build a set of 16 packets of consecutive bands (from band 0 to band 15) |
The MERLOT Tool may either be used with an interface or through the IDL command line; a full description of all the MERLOT functions may be found here.