1.10   Opening Envisat Scenes

Until now you have been dealing with monthly data-sets that have been produced by combining SST data from many satellite overpasses. Such images are the result of lengthy processing to calculate temperatures, masking pixels with invalid data and combining images from a number of overpasses to calculate the monthly average temperatures. This is known as Level 3 data, or composite, gridded data.

In this section you will study an AATSR image at a lower stage of processing (Level 1). The image comes from a single overpass by the European Space Agency's Envisat satellite and shows a scene from the southwest coast of Africa. The file name is ATS_TOA_1COLRA20040201_082356_000001252023_00478_10051_1629.N1

Envisat file names

All Envisat file names contain information about the origin and processing of the data. In this instance the file name , ATS_TOA_1COLRA20040201082356_000001252023_00478_10051_1629.N1 can be broken down as follows:

ATS_TOA_1C is the Product ID. In this case
- ATS shows that the data comes from the AATSR sensor
- TOA_1 tells you that this is Top Of Atmosphere level 1 data
- C shows a customer request (standard products have a P).

OLRA contains information about the data processing:
- O is a processing stage flag in the range N-V (N is Near Real-time, V is validated).
- LRA identifies the processing centre (others are UPA, UPE, PDK ... )

20040201 is the date (in the format YYYYMMDD).

082356 isthe start time in UTC

This is followed by a series of numbers giving - duration (00000125 seconds ), phase number (2) and cycle number (023),
- the relative (00478) and absolute (10051) orbit numbers, and
- a quick-reference file ID (1629).

Finally the file extension .N1 identifies this as an Envisat image, stored in the Envisat data format. This extension is used by Bilko to recognise an Envisat scene.

The N1 file structure

Open the AATSR image ATS_TOA_1COLRA20040201_082356_000001252023_00478_10051_1629.N1

As you can see from the file structure window this is another hierarchical data format, similar to HDF in many ways. It contains not just the satellite data, but also extra information needed for further processing and data interpretation, such as orbit information, viewing configuration, flags, ancillary data-sets and information about geophysical units and conversion factors.

In the left frame you can see the overall structure of the file content. In this case there are four main sections. Metadata and Bands are common to all the N1 file types you will encounter in the Envisat Module; most will also contain the Tie Point Grids needed for the geo-referencing of the image data, and some will contain processing flags that provide information about data quality.

When you open the file the Metadata folder is open and its content is displayed in the right frame, with information about the data format (text, table), its size and the number of attributes (properties) associated with the data. The two top folders MPH (Main Product Header) and SPH (Special Product Header) are files you will find in all Envisat N1 files, regardless of sensor and processing level. The number and type of the other metadata sets varies from product to product.

Activity / Question 1
Open the text files in the Metadata folder by double clicking on the folder icons (or right-clicking and selecting Open Items). Use the information you find there to answer the following questions:
a)	If this file had been renamed, where would you find the original file name?
b)	Where would you find information about the date and time when the scene was recorded, and how long did this scene take to record?
c)	What was the orbit number of the overpass?

Band properties

The Bands folder contains the remote sensing data. As you can see by selecting it in the left frame there are several bands:
- 7 bands recorded in nadir view (look-angle perpendicular to the Earth's surface)
- 7 bands recorded in the forward view
- 4 sets of flags: quality flags and cloud flags for each look-angle

You will be looking at the nadir brightness temperature in the 11µm channel, but first it may be worth looking at the properties of these data-sets:

1. In the left frame select Bands.
2. In the right frame highlight btemp_nadir_1100 by clicking on it once.
3. Right-click on the highlighted folder, and select Properties from the pop-up menu.

All the bands have similar property text-files associated with them. These are the 5 "attributes" referred in the last column of the table in the right frame of the file structure window.

Activity/Question 2
Use the property files to answer the questions below. You may also want to refer to the sidebar information about the electromagnetic spectrum
a)	At what wavelength interval is btemp_nadir_1100 recorded?
b)	What temperature scale is used?
c)	How would you get from the 16-bit values in the file to actual temperatures?
d)	What are the wavelengths of the other 6 AATSR channels?
e)	Why are some of these channels referred to as reflectance, others as brightness temperature?

Examining the data

1. Open the btemp_nadir band by double-clicking on it in the right frame of the file structure window.
2. When the Extract dialog appears, accept the default settings (whole image at full resolution)
3. In the Redisplay dialog set the Null value to -5, and apply the settings to open the image display.
4. When the image opens, right-click to zoom and select Preserve shape so that you can see the whole image.

Brightness temperatures give a good idea of whether the radiation that reaches the sensor comes from land, sea or cloud. You will probably have noticed already that the land in this image is much brighter (warmer) than the ocean. This is as expected. January is the height of the southern hemisphere summer, and by 08:23 local time (which is 09:23 local time at 15°E), the sun has already warmed the land considerably. Sea temperatures remain much more constant, cooler than land during the day, and often warmer than land at night.

You may also have noticed some dark areas near the top of image, with the typical shape of clouds. Infrared radiation is absorbed by clouds and re-radiated at a temperature typical of the cloud top altitude. As air temperature usually decreases with height above the Earth's surface, cloud temperatures will be colder than temperatures at the Earth's surface, and therefore appear dark.