General information and background

The first Earth Resources Technology Satellite (ERTS-1, later renamed Landsat-1) was launched in 1972 and carried the Return Beam Vidicon (RBV) sensor and the Multispectral Scanner Subsystem (MSS). The RBV sensor consisted of three spectral bands and were called "band 1", "band 2", and "band 3". The MSS sensor had four spectral bands and were named "band 4", "band 5", "band 6", and "band 7". Band 4 approximately corresponded to human visible green, band 5 approximately corresponded to human visible red. Band 5 and band 6 are near infrared bands. Since the launch of ERTS-1, most images displayed are the "standard" false color infrared with which we are all so familiar. The main charasteristic of these images are that chlorophyll (vegetation) are displayed in reds. Water is shown as dark blues or black and urban areas as shades of blue/grey.

A new sensor appreared on the Landsat 4 and Landsat 5 satellites. The RBV sensor was dropped and the Thematic Mapper (TM) sensor was added. The TM sensor was a vast improvement in the MSS scanner in that its sensor had a more narrow bandwith and also included more spectral channels. The most important of these, as far as making pseudo natural color images, was the addtion of band 1, which approximately corresponds to the human visible blue wavelength. So, to produce pseudo natural color images with the TM scanner, one would use bands 1, 2, and 3 displayed as blue, green, and red. (Personally, I like to make a new green band by combining 95% of band 2 along with 5% of band 4, the chlorophyll band, but that is another story.)

Except for the TM scanner, most of the multispectral scanners in orbit and active today do not include a blue channel. A good example is the SPOT satellite. Even though it has a higher spatial resolution than MSS or TM, it does not have a blue channel, so it is not possible to produce a pseudo natural color rendition of the imagery.

This past year, I discovered someone who was able to produce believable pseudo natural color images from color infrared imagery. My supervisor attended the IGARRS meeting in Singapore where he optained a CD-ROM produced by SPOT Asia Pte. Ltd. and the Center for Remote Imaging, Sensing, and Procesing (CRISP) at the National University of Singapore. More information can about CRISP and SPOT Asia can be found on the acknowledgment web page which is a part of this "report".

The CD-ROM includes some sample SPOT images, some of which display in pseudo natural color. I sent an e-mail to the developers of the CD-ROM asking how they produced the images as well as requested a copy of the CD-ROM. They were giving out the CD-ROMs free of charge, so I tried to procure a copy. (It has a pretty good remote sensing tutorial which I wanted to take a closer look.) When I heard back, they didn't say anything about procedures of making pseudo natural color images, but they did ask for some money to send a copy of the CD-ROM.

I was pretty discouraged and placed the idea on the "back burner". Eventually, my supervisor gave me his copy of the CD-ROM and I once again started taking a look at the fine images and tutorials on the disk. Buried within the appendixed of the tutorial, they briefly described how they make the pseudo natural color images. Eureka! I tried the procedure out with some of the false color infrared images on the disk. It worked! Then I tried it out on a SPOT mosaic that Tom Thaller and I produced of the Island of Hawai'i. That worked also. Hummm, I wondered. Maybe this procedure will work on other data sets. I obtained a one-kilometer resolution AVHRR images from Dr. Tom Logan and tried the procedure. That worked fairly well also. Finally, I tried the procedure on a MSS image. That worked as well. You can see the results of these "tests" on the associated web pages in this "report".

The only "addition" that I made to the procedure described on the CD-ROM was that the resulting images have a yellow cast. I imported the images into Adobe Photoshop and did some "quick and dirty" contrast enhancement. The results are quite acceptable. You can see another image data set at http://www-dial.jpl.nasa.gov/~steven/hawaii/hawvp1.html. This page shows the pseudo natural color Hawai'i SPOT mosaic used as a color texture map within Vistapro. (Vistapro is a low-cost 3D prespective rendering program available from RomTech, Inc..)

Spectral bands of various satellites