All satellites carry both PAN and MS sensors. PAN with high resolution(Spectral less) and MS with Spectral Rich (low spatial resolution). To improve the resolution of MS image, Pan-sharpening techniques has been developed. But My question is instead of carrying PAN sensor, why can't we calibrate the MS Sensor to acquire both spectral and spatial rich information in single image?
As I understand, it is difficult to obtain an image with both high spatial and spectral resolution. Because, if you want to get high spectral resolution you have to divide spectral region into narrow spectral bands, that means the amount of energy reflected from a pixel in each band is small, therefore you have to enlarge the pixel size to increase the amount of reflected radiance, then sensor can detect and record it. Some hyperspectral sensors can provide image with 0.5m spatial resolution and 30-100 spectral bands, such as CASI, however the spatial information is not as good as a RGB aerial photo with the same spatial resolution
But We have Hyper spectral image which gives narrow band information of features, but the spatial resolution of hyperspectral is not so good when compare to PAN sensor image. so it is clear that the sensor can provide high spectral information(eg: Hyperspectral image) So my question is why cant we calibrate the sensor in such way to provide both high spatial and spectral resolution.
To answer your question you should understand why PAN image has high spatial resolution and why hyperspetral sensor provide high spectral resolution image:
1. PAN image is acquired in 1 wide band in VIS spectral region, ex: LANDSAT 8 PAN band has wavelength of 0.50 - 0.68 µm. In such a wide wavelength, the radiance reflected by ground area is great enough to allow the sensor detect reflected radiance from small area, that mean it can provide high spatial resolution image.
2. In case of hyperspectral sensor, in order to get high spectral information it is designed to detect reflected radiance in narrow band, that means the reflected energy is also divided into smaller portions. This leads the detectors cannot detect the reflected radiance from small ground area because it is too small to be sensed (small area, small reflected energy), or if possible the sensor should has very high sensitivity, or has to be mounted on low orbit platform (e.g. smaller swath width).
But building such a sensor is challenging and costly. Therefore, if you want an image with rich spatial information you have to sacrifice the spectral information and vice versa.
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