Aqua Satellite Raw UAH Data, Part 2

In this post we take a more detailed look at the AMSU Level 1B data that serves as the raw data for UAH. 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 AIRS Version 5.0 Released Files Description PDF. It contains a section for each AIRS instrument on the Aqua satellite and each section describes every field in the Level 1B data file for that instrument.

Additional Posts In This Series

Aqua Satellite Raw UAH Data, Part 1

Satellite Summary

Aqua Satellite Data Processing
A 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

Scan Information

Each AMSU Level 1B data file contains a a collection of scans that take place over 6 minutes.

In these 6 minutes, the AMSU scans 45 times in the same direction as the satellite is moving. Each of these 45 scans contains 30 readings perpendicular to the movement of the satellite (called Footprints), and each of these 30 readings contains 15 channels, each at a different height above the Earth's surface.

NOTE: These values are collected by three different pieces of hardware. Receiver A11 handles channels 6, 7, and 9 through 15. Receiver A12 handles channels 3, 4, 5, and 8. Receiver A2 handles channels 1 and 2. Also, recall from Part 1 that channel 7 is always bad. 

This gives a total of 20,250 readings in a single Level 1B file. These 20,250 readings are stored as a contiguous block of data in the file that, in the parlance of C programmers, can be read into computer memory as a three dimensional array.

There are actually two sets of this 20,250 piece block of information in the AMSU Level 1B files. The first set is called antenna temperatures and is the raw data in degrees Kelvin as read by the satellite modified by calibration information (specifically, antenna temperatures = calibration coefficient/instrument readings²).

The second set is called brightness temperatures consists of slightly modified antenna data.The brightness temperatures have a companion set of data, 20,250 error estimates called brightness temperature errors that estimate the accuracy of the brightness temperatures.

Either the antenna temperatures or the brightness temperatures can be considered to be the raw data used to create the UAH data.

Data Quality

There are several fields in the AMSU Level 1B data file that contain the results of Quality Assurance (QA) checks. In fact, much of the file is QA data. 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:

• State 1 Indicates the state of the AMSU-A1 unit for each scan line. If this value is anything other than 0, the data in the scan line should be rejected.
• QA Receiver A11 and QA Receiver A12 These flags indicate the quality of the AMSU temperature receivers for each scan line. If either are non-zero, the data in the scan line should be rejected.
• SATGEO QA, GLINTGEO QA, and MOONGEO QA These fields contain QA information for each scan line. If a non-zero value is present, the data in the corresponding field should be rejected. 
• FTPTGEO QA, ZENGEO QA, and DEMGEO QA These are satellite position QA flags and occur for each of the 30 field of view footprints for each of the 45 scan lines. If a non-zero value is present, the data in the corresponding field should be rejected. 
• QA Channel This is a series of 15 flags indicating the reliability of each channel of the AMSU. Channels with non-zero values should not be used. Recall from Part 1 that channel 7 is always bad. 

Pseudocode For Reading Raw UAH Data

The following pseudocode shows how to process a AMSU Level 1B file, checking for QA errors, and grabbing the temperature data. Recall that temperatures are provided in degrees Kelvin and that channel 7 is always bad.

Open File
Read AMSU Record Into Memory

Foreach Of The 45 Scanlines
   If State_1 Not Equal To 0 Then Reject Scanline
   If QA_Receiver_A11 Not Equal To 0 Then Reject Scanline
   If QA_Receiver_A12 Not Equal To 0 Then Reject Scanline
   If SATGEO_QA Not Equal To 0 Then Reject Scanline
   If GLINTGEO_QA Not Equal To 0 Then Reject Scanline
   If MOONGEO_QA Not Equal To 0 Then Reject Scanline

   Foreach Of The 30 Footprints
      If FTPTGEO_QA Not Equal To 0 Then Reject Footprint
      If ZENGEO_QA Not Equal To 0 Then Reject Footprint
      If DEMGEO_QA Not Equal To 0 Then Reject Footprint

      Foreach Of The 15 Channels
         If QA_CHANNEL Not Equal To 0 Then Reject Channel

         Read Antenna Temperature for this Scanline/Footprint/Channel
         Read Brightness Temperature for this Scanline/Footprint/Channel
         Read Brightness Temperature Error for this Scanline/Footprint/Channel
      End Foreach
   End Foreach
End Foreach

References:
Aqua Satellite Raw UAH Data, Part 1

Satellite Summary

Aqua Satellite Data Processing
A 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
AIRS Version 5.0 Released Files Description