Algorithm Theoretical Basis Document

GCOM-C/SGLI Level-2 Mosaic (G4B)

31 March 2017
Masahiro Hori
Japan Aerospace Exploration Agency

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Contents
1. Introduction
1.1 Objectives
1.2 Development strategy
1.3 Processing targets
2. Theoretical Description
2.1 Processing flow
2.2 Statistics
2.3 QC process
2.4. Sample images
3. Current status and remaining issues

21. Introduction
1.1 Objectives
The objective of the G4B algorithm is to make cloud-free top-of-atmosphere (TOA)
radiance data from Level-2 Tile LTOA (TOA radiance) product. The temporal interval ofthe process is 8-day or 1-month. The spatial resolutions of the tile products are kept thesame.
1.2 Development strategy
The G4B algorithm does not generate statistics of multiple observation days butstores only the data from a single observation day of the most probable clear-sly during
the temporal interval period in output file. Thus, a single output file of the G4A
processing contains TOA radiances of all the channels stored in the input file.
1.3 Processing targets and outputs
Processing targets of the L2 Tile mosaic (G4B) are the LTOA product with the spatialresolution of 250m. Input and output variables are summarized in Table 1.
Table. 1 List of the target products, GVs, and output types of the G4B processing
Product Long Name

L2 Variables*1

Output Variables

Lt_VN01-11, Lt_SW01-04,

The same as the

Lt_TI01-02, Lt_VN08P,

L2 variables

Lt_VN11P, Lt_PI01-02,

shown left.

ID
LTOA

250m TOA radiance

Lt_PQ01-02, Lt_PU01-02,
Sensor_azimuth, Sensor_zenith,
Solar_azimuth, Solar_zenhth,
Sensor_azimuth_IRS,
Sensor_zenith_IRS,
Solar_azimuth_IRS,
Solar_zenhth_IRS,
Sensor_azimuth_PL,
Sensor_zenith_PL,
Solar_azimuth_PL,
Solar_zenhth_PL*1 Lt: TOA radiance, VN: Visible and Near infrared (VNIR), SW: Shortwave infrared, TI: Thermal
infrared, VN08P and VN11P: Polarization channels co-registered to the VNIR channels VN08 andVN11, PI: I component of the polarization, PQ: Q component of the polarization, PU: U component ofthe polarization, IRS: Infrared scanner, PL: Polarization.

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Basically, the same variables are stored in the input and output files. The variables inthe former are daily values whereas those in the latter are the selected values of themost probable clear-sky among the temporal interval period (8-day or 1-month).

2. Theoretical Description
2.1 Processing flow
Figure 1 indicate the flow of the G4B L2 Tile LTOA mosaic processing. Input is the L2
tile daily LTOA products with spatial resolution of 250m. Output is the 8-day or1-month mosaic product of LTOA. The output of the G4B process contains the cloud-freemosaic data of the same L2 variables (i.e., TOA radiances and geometric angles) as
those stored in the input file shown in Table 1.
1/480deg (250m) Tile [EQA](LTOA: TOA radiance)

Input: Daily LTOA × 8-day or 1-month

G4B: L2 Mosaic
MAX of Abs(NDVI-α)1/480deg (250m) Tile [EQA] (L2 Mosaic)(Cloudless mosaic of TOA radiance: 8D,1M)

Output: 8-day or 1-monthcloud-free mosaic

Fig. 1 Flow of the G4B L2 Tile LTOA mosaic processing
2.2 Criteria for the mosaic data selection
We employed the maximum absolute NDVI value composite method for themosaicking process. In this method output data store the TOA radiance of a single
observation day on which the value of |NDVI-α| becomes the maximum during thetemporal interval (8-day or 1-month). The α is the NDVI value at which the histogram ofNDVI has a peak for cloudy pixels (Currently, α of 0.028 is set in the code. We will fix the α after
the launch of GCOM-C/SGLI). See also the appendix-1 for the comparison of the performances withdifference mosaicking criteria.
2.3 QA process“QA_flag” currently stores the same QA_flag value of the daily input LTOA file4selected in the mosaic process.
2.4. Sample images
Figure 2 indicate sample images for the input and output of G4B processing (L2 Tile
LTOA with the spatial resolution of 250m and the temporal interval is 8-day).

Input: Daily Tile LTOA × 8-days

Output: 8-day Tile mosaic of LTOA

Fig. 2 Sample images of the G4B processing. (LTOA, Tile No.: 0528, 8-day)

3. Current status and remaining issues of the G4B code implementation
The G4BA process works well without system errors. Processing speed and memory
size are also within the expectations. Remaining tasks are the implementation of theQA_flag for the output of quality assurance information.

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Appendix 1.
Comparison of the performances with different criteria for mosaicking
MIN of SW03

MIN of VN03

MAX of TI01

MAX of NDVICloud shadow remains

Cloud remains over ocean

Lots of clouds over ocean

MAX of Abs(NDVI-α)

RGB:CH8-5-3

RGB:CH8-11-5
A little bit noisy and hazy

Good performance overland and ocean. Less
cloud shadow over land.
A little bit haze remains
but ocean blooming alsoremains.

BT116