What is the difference between the SIR, AVE, and GRD algorithms?
The Scatterometer Image Reconstruction (SIR) algorithm is an iterative recontruction algorithm at creates high resolution images from irregullarly spaced measurements. The AVE algorithm is the first iteration of SIR and is computed by defining a high resolution pixel grid. A value for each pixel is created by weighted averaging all of the measurements that have a non-zero spatial response function for the pixel. The weighting function is the normalized value of each measurement's spatial response function for the particular pixel, see D.G. Long, P. Hardin, and P. Whiting, "Resolution Enhancement of Spaceborne Scatterometer Data," IEEE Transactions Geoscience Remote Sensing, vol. 31, pp. 700-715, 1993 and D.S. Early and D.G. Long, "Image Reconstruction and Enhanced Resolution Imaging from Irregular Samples," IEEE Transactions on Geoscience and Remote Sensing, Vol. 39, No. 2, pp. 291-302, 2001.
GRD stands for "gridded" image. This is a non-spatially enhanced image created by gridding the measurements into a fixed, low-resolution grid using "drop-in-the-bucket" (DIB) techniques. Each pixel value consists of the average of only the measurements whose center falls within the spatial extent of the particular pixel. GRD images have the coarsest resolution, but the lowest noise, while SIR images have the highest spatial resolution. AVE images fall in between. Typically the dimensions of AVE and SIR images are 1/5 the size of GRD images. The same projection is used for all three image types so each GRD pixel corresponds to a 5x5 area if AVE or SIR pixels.
How can I display a BYU .SIR image file in ENVI?
There are several ways to display BYU. SIR files in ENVI. One is to convert the image to a GEOTIFF first. Second, the IDL file readers can be used. Alternately, for just displaying the image, follow theses steps after starting ENVI: First, you have to determine the file size (X by Y) in pixels. This can be determined using the IDL reader code. Then,
From the File menu select, "Open image file", and select the file to read after gunzipping it.
A pop-up widow appears. Enter the X file size in "samples".
Enter Y file size in "lines".
Enter 1 in the "bands" and 512 in the "offset"
Select "Unknown" for "File type" and "Network (IEEE)" for the "Byte Order".
Select "Integer" for the "Data Type" (leave "BSQ" for "Interleave").
Select the "Gray Scale" radio button and press "Load Band".
Note: the file will be displayed flipped vertically since ENVI defines the image origin (pixel 1,1) at the upper-left rather than lower-left..