3.4 Airborne Multispectral Systems
The mechanism of airborne multispectral sensors is similar to the Landsat MSS and TM. The airborne sensor systems usually have more spectral bands ranging from ultraviolet to visible through near infrared to thermal areas. For example, the Daedalus MSS system is a widely used system that has 11 channels, with the first 10 channels ranging from 0.38 to 1.06 Ám and the 11th is a thermal channel (9.75 - 12.25 mm).
Another airborne multispectral scanner being used for experimental purposes is the TIMS - Thermal Infrared Multispectral Scanner. It has 6 channels: 8.2 - 8.6; 8.6 - 9.0; 9.4 - 10.2; 10.2 - 11.2; 11.2 - 12.2 Ám.
Canada Centre for Remote Sensing developed the Multispectral Electro optical Imaging Scanner (MEIS-II). It uses 1728 - element linear CCD arrays that acquire data in eight spectral bands ranging from 0.39 to 1.1 mm. The spatial resolution of MEIS-II can reach up to0.3 m.
Advantages of multispectral systems over photographic systems are
- Spectral range: photographic systems operate between 0.3 - 1.2 mm while Multispectral systems operate between 0.3 - 14 mm.
- Multiband photography (photographic system) uses different optical systems to acquire photos. This leads to problems in data incomparability among different cameras. MSS, on the other hand, uses the same optical system, eliminating the data incomparability problems.
- Electronical process used in MSS is easier to calibrate than photo chemical process used in photographic systems.
- Data transmission is easier for MSS than for photographic systems which require onboard supply of films.
- Visual interpretation for photographic systems v.s. digital analysis for MSS systems--Visual analysis is difficult to analyze in 3-dimension.
False Color Composite
Each time, only three colours (red, green and blue) can be used to display data on a colour monitor. The colours used to display an image may not be the actual colour of the spectral band that is used to acquire the image. Image displayed with such colour combinations are called false colour composite. We can make many 3-band combinations out of a multispectral image.
where Nc is the total number of 3-band combinations and nb is the number of spectral bands in a multispectral image. For each of these 3-band combinations, we can use red, green, and blue to represent each band and to obtain a false-colour image.
Digital Photography with CCD Arrays
Videographic imaging includes the use of video cameras and digital CCD cameras. Video images can be frame grabbed,or quantized and stored as digital images; however, the image resolution is relatively low (up to 550 lines/image). Digital CCD cameras use two-dimensional silicon-based charge coupled devices that produce a digital image in standard raster format. CCD detectors arranged in imaging chips of approximately 1024 X 1024 or more photosites produce an 8-bit image (King, 1992).
Digital CCD photography compare favorably to other technologies such as traditional photography, videography, and line scanning. Comparing to photography, digital CCD cameras have linear response, greater radiometric sensitivity, wider spectral response, greater geometric stability, and no-need for film supply (Lenz and Fritsch, 1990, King, 1992). Matching with the fast development of softcopy photogrammetry, they have the potential to replace the role of aerial photography and photogrammetry for surveying and mapping.
Imaging spectrometry refers to the acquisition of images in many, very narrow, continuous spectral bands.
The spectral region can range from visible, near-IR to mid-IR.
- The first imaging spectrometer was developed in 1983 by JPL. The system called Airborne Imaging Spectrometer (AIS) collects data in 128 channels from 1.2 Ám to 2.4 mm. Each image acquired has only 32 pixels in a line.
- The Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) represents an immediate follow-up of the AIS (1987). It collects 224 bands from 0.40 - 2.45 mm with 512 pixels in each line.
- In Canada, the first system was the FLI - Flourescence Linear Imager manufactured by Moniteq, a company that used to be located in Toronto, Ontario.
In Calgary, the ITRES Research is producing another imaging spectrometer called the Compact Airborne Spectroscopy Imager (CASI) (Figure 3.21).
Figure 3.21. The two dimensional linear array of the CASI.
For each line of ground targets, there will be nb x ns data collected at 2 bytes (16 bits) radiometric resolution where nb is the number of spectral bands and ns is the number of pixels in a line.
Due to constraint of data transmission rate, these nb x ns data cannot be transferred completely. This leads to a division into two operation modes of CASI, spectral mode and spatial mode.
In spectral mode, all 288 spectral bands are used, but only up to 39 spatial pixels (look directions) can be transferred.
In the spatial mode, all 512 spatial pixels are used, but only up to 16 spectral bands can be selected.
Where to obtain remote sensing data?
See the Appendix in Lillesand and Kiefer (1994).