McIDAS Learning Guide
Version 2015

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Satellite Imagery - Basic Concepts

SSEC receives real-time satellite images from geostationary and polar orbiting satellites. Geostationary satellites remain above a fixed location on the earth's surface, appoximately 35,800 km above the equator. Because the satellites rotate with the earth, they always observe the same portion of the globe. Typically, there are operational geostationary satellites from the United States, the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), and the Japan Meteorological Agency (JMA).

At most times, the United States has a satellite called GOES-East, which monitors North and South America and the western Atlantic Ocean, and one called GOES-West, which monitors North and South America and the eastern Pacific Ocean. EUMETSAT has two satellites (METEOSAT) which monitor Europe, Africa, Asia, the Indian Ocean, and the Atlantic Ocean. JMA has a satellite (MTSAT) which monitors eastern Asia, Australia, the western Pacific Ocean and the eastern Indian Ocean.

Roll your mouse over the image below to see the coverage of each geostationary satellite:

 

Polar orbiting satellites orbit at much lower altitudes (800-900 km). Their path is 2,400 km wide centered at the orbit path. With each orbit, the satellites observe a new path. SSEC typically receives real-time imagery from the operational POES satellites plus others like Metop, Aqua and Terra.

See the SSEC Data Center's website for a list of all current and archived satellite data available from SSEC.

Satellite Data Storage

In McIDAS, satellite data is stored in locations on disk called areas. You can copy, change, display, and delete areas. Areas contain both data and area directories. Area data can be displayed in McIDAS image frames. The area directory contains descriptive information, such as the sensor source, image date, picture start time, and image coordinates. To see this information, use the IMGLIST command, as shown below: 
 IMGLIST GV4.1 FORM=ALL                                                          
 Image file directory listing for:GV4                                           
  Pos Satellite/         Date       Time      Center      Res (km)   Image_Size   
      sensor                                 Lat  Lon    Lat   Lon                
  --- -------------  ------------  --------  ---- ----  ----- ----- ------------  
    1  GOES-7        13 MAR 93072  17:01:00    26   87                            
     Band: 1   0.65 um Daytime cloud detection           4.39  3.64  1335 x 1608  
      proj:    0 created: 1993208 191703  memo: AAA-MSI TO VIS      VIA ARCHIVE   
      type:VISR     cal type:BRIT                                                 
      offsets:  data=     768 navigation=  256 calibration=    0 auxiliary=    0  
      doc length:   0   cal length:   0   lev length:   0 PREFIX=   4             
      valcod: 1104210435 zcor:  1 avg-smp: A                                      
      start yyddd: 1993072  start time:170105  start scan:   56                   
      lcor: 1121  ecor:  7585  bytes per pixel: 1  ss: 32                         
      Resolution Factors (base=1):   Line=    4.0   Element=    4.0               
 IMGLIST: done

Area Naming Conventions

Areas use the naming convention AREAnnnn where nnnn is the four digit area number. For example, AREA0003 is the name of the file that contains area 3. Most of the older McIDAS commands use only the area number. However, you must use the AREA prefix with the DMAP command or when using Unix commands to copy, move, or delete areas.

In ADDE, sequences of AREA files are grouped together in a dataset. For example, in a previous lesson, you created the dataset MYDATA/IMAGES, which contains the AREA file numbers from 1 to 9999. Individual AREA files can be accessed with their position number within the dataset. In the case of MYDATA/IMAGES, position 3 (MYDATA/IMAGES.3) would relate to the file AREA0003. However, in the dataset MYDATA/TEST-IMAGES that was created in the previous lesson, the dataset defined the AREA files ranging from 4000 to 4004, so position 3 (MYDATA/TEST-IMAGES.3) would be AREA4002. (This would be equivalent to MYDATA/IMAGES.4002)

Coordinate Systems

McIDAS references image data in four different, but interconnected coordinate systems:

Image Coordinates

The image coordinate system forms the basis for the other McIDAS coordinate systems. A full image is a sequence of lines and elements arranged from top to bottom. The top line and leftmost element have image coordinates (1,1). Therefore, each pixel has a unique pair of line and element values that are its image coordinates. The figure below represents a full image, image sector and displayed area. The upper-left image coordinates of the full image are (1,1) and the upper-left coordinates of the image sector are (3500,5000).

Area (File) Coordinates

Area coordinates (file coordinates in ADDE) are based on the size of the area only. Like image coordinates, area coordinates are referenced as lines and elements. The first pixel has area coordinates (0,0) as shown in the image sector below. The bottom-right pixel has area coordinates (LSIZ-1, ESIZ-1) where LSIZ and ESIZ are the number of lines and elements in the area.

Earth Coordinates

If the displayed image is navigated, the image coordinates can be converted to earth coordinates (latitude and longitude). Earth coordinates are specified in degrees, minutes, and seconds in the form DDD:MM:SS. Southern latitudes and longitude east of Greenwich are negative. Latitudes run from -90:00:00 to +90:00:00 and longitudes run from -180:00:00 to +180:00:00.

TV Coordinates

The pixels on the McIDAS image frames are arranged by raster lines and pictel elements. The raster lines run horizontally across the frame and the pictel elements run vertically across the frame. The pixel in the upper-left corner of the frame is numbered (1,1) which means (raster line 1, pictel element 1). The total number of raster lines and pictel elements on the frame is determined by the frame size. The lower-right corner of the default-sized frame is (480,640) in TV coordinates.

Image Navigation

Navigation, as applied to satellite imagery, means the conversion between satellite image coordinates (line and element) and earth coordinates (latitude and longitude). This is usually done when the data is ingested.

If a displayed image sector is navigated, McIDAS can convert the image coordinates of a specified pixel to earth coordinates. The E command lists the earth, TV and image coordinates at the cursor center.

Image Resolution

Image resolution is measured in kilometers and is highest at the subsatellite point. The highest available resolution depends on the satellite and image type:

Satellite Visible Infrared
GOES 1 km 4 km
METEOSAT 2.5 km 5 km
MODIS 0.25 km 1 km
MSG 1 km

3 km
MTSAT (HiRID format) 1.25 km 5 km
POES 1 km 1 km

This means that a single GOES infrared pixel at the subsatellite point represents a 4km x 4km square on the earth's surface, and a single METEOSAT visible pixel at the subsatellite point represents a 2.5km x 2.5km square on the earth's surface, etc. The further a pixel is located from the subsatellite point, the lower its resolution.

You can display imagery at a resolution other than that stored in the area. Specifying a positive magnification factor in McIDAS commands (e.g., IMGDISP) enlarges or blows up the data by replicating pixel values; a negative magnification factor produces a blow down by sampling pixel values. For example, if you choose a magnification factor of 16, the value of each pixel in the area is duplicated 256 times (in a 16 x 16 box of pixels) when displayed on the frame. If you choose a magnification factor of -4, the value of every fourth element along every fourth line is displayed as one pixel on the image frame.

Other Digital Imagery

Although geostationary satellites, polar orbiting satellites and radar images are usually displayed, you can digitize any data and display it on McIDAS. For example, cell proteins have been digitized for biochemistry studies and human photographs have been digitized to aid in forensic work.

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