Chapter 2
MERIS Products and Algorithms

2.1 Introduction

2.1.1 MERIS products overview

Figure 2.1 - MERIS Products.

2.1.1.1 MERIS product processing levels

The ENVISAT nomenclature of the products delivered to the public describes three types of processing levels:

• level 1B – are images resampled on a path-oriented grid, with pixel values having been calibrated to match the Top Of Atmosphere (TOA) radiance.
• level 2 – are images deriving form the level1B products, with pixel values having been processed to get geophysical mesurements.
• level 3 – are synthesis of more than one MERIS products (and possibly external data) to display geophysical measurements for a time period.

Figure below (see Figure 2.2 - ) shows the difference between level 1B and level 2 products. See http://earth.esa.int/showcase/env/North_America/HurricaneIsabel_CTP_MER_RR_Orbit07965_20030908.htm for more information about these images.

Examples of level 3 products are provided in section 1.1.2.2.

Figure 2.2 - Level 1B RGB composition (left) and Cloud Top Pressure (right)
Example of hurricane Isabel acquired on 8^th Septembre 2003.

2.1.1.2 Full and reduced resolutions

MERIS data are provided at 3 different levels of processing - Level 0, Level 1, Level 2 - and at 3 different spatial resolutions - Full, Reduced and Low. They may also be sorted and visualised very easily using a browse product (RGB colour coded images of full-resolution data). Child products, including a limited number of measurement data sets, may also be available in reduced and full resolution.

For the same image, a Full-Resolution (FR) image has 4 × 4 more points (pixels) than the same image in Reduced Resolution (RR), and an RR image has 4 × 4 more points (pixels) than the same image in Low Resolution (LR). Accordingly, a pixel in an FR image represents an area of 260 m × 290 m, in an RR image an area of 1,040 m × 1,160 m, and in LR an area of 4,160 m × 4,640 m.

The instrument always takes measurements with full resolution; i.e., 260 m × 290 m ground resolution. Onboard averaging generates the RR images, while LR images are generated in the ground processor by further averaging the RR data.

Full-resolution images can be provided either as scenes or as imagettes. A scene is a half-swath square image. An imagette is a quarter-swath square image.

The different MERIS products primarily hold:

· instrument source packets for a Level 0 Product

· Top Of Atmosphere (TOA) radiances measured in the 15 MERIS bands for a Level 1 product

· geophysical quantities varying according to the underlying surface identified (land, ocean, or clouds) for a Level 2 product.

In addition to the core geophysical data, MERIS Level 1 and Level 2 products contain:

· geometric information to allow the user to locate the image on the Earth's surface

· data describing the Sun and viewing geometry

· additional annotation data such as coastline information

· terrain height

· meteorological data

· scaling factors to allow the user to decode the data from numerical counts to geophysical meaningful values

· flags that address the quality and the validity of the image

The Level 0 products are generated and archived routinely in both RR and FR, whereas the FR MERIS Level 1b and Level 2 products (scenes or imagettes) are generated on request only.

A reduced resolution product contains data on 1,121 columns (1,165 km) with a number of lines that may vary from product to product. An entire orbit is covered with 15,057 reduced resolution lines. A full-resolution product contains data on 2,241 × 2,241 pixels (i.e., 582 km × 650 km) for a scene and 1,153 × 1,153 pixels (i.e., 300 km × 334 km) for an imagette.

2.1.1.3 MERIS product types

Table 2.1 gives an overview of the MERIS products:

Table 2.1 - MERIS Products.

The MERIS product tree is shown here below.

Figure 2.3 - MERIS Product Tree.