Giovanni-3 Operation Technical Summary: A-Train Across Track Subset

A-Train Across Track Subset

The Giovanni A-Train Across (CloudSat) Track operation performs an analysis of science parameters along the CloudSat orbit track. Only CloudSat science parameters can be supplied directly to Giovanni along the CloudSat orbit track. Giovanni-3 requires data from all platforms other than CloudSat to be pre-subsetted along the CloudSat orbit track or undergo subsetting during Giovanni-3 processing. The Across-Track Swath Subsetter performs this service on those datasets which have not already been subsetted along the CloudSat orbit track.

Input parameters

• Data in Satellite Swath Format, stitched together along  the orbit track
• CloudSat orbital elements

Output Description
Data in Satellite Swath Format, subsetted along the CloudSat orbit track.

Output Parameters
Same as input.

File Type
Input: HDF4
Output: HDF4

Sub-Definitions

• Frame: individual data element along the scan line. 
• Swath: 1- or 2-dimensional data structure, typically representing
  an array of measurements distributed along the center pixel of a
  satellite track. 
• Scan Line: horizontal (across-track) dimension of swath. 
• TLE (Two-Line Elements): orbital elements of the platform along
  which the across-track subset is to be performed.

Description of This Operation

1. Use the CloudSat TLE to compute the geolocations of the CloudSat orbit
   track that spans the swath being processed.
   
2. Estimate when CloudSat passes closest to the central scan line of the
   swath to be subsetted.
   
3. Perform an exhaustive search along this scan line for the frame whose
   geolocation lies closest to the CloudSat orbit track.
   
4. For each remaining scan line, identify the frame that lies closest
   to the CloudSat orbit track. Narrow the search using the previous
   collocation solution.
   
5. Complete the process by subsetting each science parameter along the
   CloudSat orbit track. Extract a pre-determined number of frames
   surrounding the collocation frame within each scan line found in the
   swath being subsetted. If the data consists of profile data, the
   profile about each colloaction frame is extracted. The swath width
   of the subset is roughly 200km.
   
6. Write the subset in Satellite Swath Format.

Algorithm Details
 IDL is used because array-based operations may be performed
 easily in that language.

Caveats/Warnings
-  The vertical and horizontal dimensions for swaths described here are the transpose of the arrangement used in HDF4 files.

- Only Aqua AIRS science parameters undergo the across-track subsetting process, because the data from all other platforms currently processed by the A-Train Giovanni instance have already been subsetted along the CloudSat orbit track.

Origin/Software
Implemented in IDL.

Other Information
 This document currently describes only the mathematical approach used
 in the computation.

This Technical Summary page is a draft version -- for information on this operation while
the document is being prepared, please send a message to GES DISC Help.