In this post we take a look at the AMSR-E Level 2A data. Specifically, we now want to be able to write pseudocode that reads the file, checks for errors, and grabs the raw temperature data. In order to get to this level of understanding of the data, we're going to use the Level 2A Data Fields web page. It describes the fields in the Level 2A data file for the AMSR-E instrument.
Additional Posts In This Series
Aqua Satellite Data ProcessingA Note On The UAH And RSS Raw Data
How UAH And RSS Temperatures Are Measured
Overview Of The Aqua Satellite
Looking At The Aqua Satellite Data
UAH Satellite Data
Dangit! More Climate Stuff. UAH and RSS Raw Data
Each AMSR-E Level 2A data file contains a singe scan at low resolution and a single scan at high resolution.
For low resolution, the AMSR-E scans 1994 times in the same direction as the satellite is moving. Each of these 1994 scans contains 243 readings perpendicular to the movement of the satellite (called Footprints), and each of these 243 readings contains 5 channels, each at a different frequency.
This gives a total of 2,422,710 readings in a single Level 2A file. This data has been "smoothed" as part of its processing from Level 1B to Level 2A.
For high resolution scans the AMSR-E scans 1994 times in the same direction as the satellite is moving. Each of these 1994 scans contains 486 readings perpendicular to the movement of the satellite, and each of these 486 readings contains 1 channel at a frequency of 89.0 GHz.
This gives a total of 969,084 readings in a single Level 2A file. This data has not been smoothed. It's in its original Level 1B form.
NOTE: The high resolution scans were designed to be collected by two "horns" on the AMSR-E instrument, called Horn A and Horn B. However, recall from this post that Horn A failed on November 11th, 2004. A software change was made to gather all high resolution data from Horn B. Horn A still has data fields in the level 2A file, but they are all set to -32,768.
NOTE: Both the low resolution and high resolution temperatures are stored in a scaled offset format. To obtain the actual temperature in degrees Kelvin multiply the stored values by 0.01 and add 327.68 to the result.
Data Quality
There are several fields in the AMSR-E Level 2A data file that contain the results of Quality Assurance (QA) checks. These fields tell if various pieces of data in the file can be used with confidence. Here are the important QA fields to check when processing temperature data:
For Low Resolution Scans
- Scan Quality Flag A series of 1994 flags indicating the quality of each scan line. If a flag is not zero, the corresponding scan line should be rejected.
- Channel Quality Flag 6 To 52 A series of 1994 arrays containing 12 flags each. There are two flags for channels 6.9, 10.7, 18.7, 23.8, and 36.5. The last two fields are not used as there is no channel 52. These flags indicate the horizontal and vertical scan validity for each scan footprint. If either flag for a footprint is not zero, the corresponding footprint should be rejected.
For High Resolution Scans
There are two high resolutions scans in the Level 2A file, one for Horn A and one for Horn B. Horn A is broken and its data should be ignored.
- Scan Quality Flag 89B A series of 1994 flags indicating the quality of each scan line for Horn B. If a flag is not zero, the corresponding scan line should be rejected.
- Channel Quality Flag 89B A series of 1994 arrays containing 2 flags each. These flags indicate the horizontal and vertical scan validity for each scan footprint. If either flag for a footprint is not zero, the corresponding footprint should be rejected.
Pseudocode For Reading Raw AMSR-E Data
The following pseudocode shows how to process a AMSR-E Level 2A file, checking for QA errors, and grabbing the temperature data. Recall that temperatures are provided in scaled offset degrees Kelvin and that Horn A is always bad.
Open File
Read Low Resolution Record Into Memory
Foreach Of The 1994 Scanlines
If Scan Quality Flag Is Not 0 Then Reject Scanline
Foreach Of The 5 Footprints
If Either Of The Channel Quality Flags Are Not 0 Then Reject Footprint
Read Each Of The 243 Footprints For the Channel
End Foreach
End Foreach
Read High Resolution Horn B Record Into Memory
Foreach Of The 1994 Scanlines
If Scan Quality Flag Is Not 0 Then Reject Scanline
If Either Of The Channel Quality Flags Are Not 0 Then Reject Footprint
Read Each Of The 486 Footprints For the Channel
End ForeachReferences:
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