5. Spectral resolution
Spectral resolution refers to the number and dimension of specific wavelength intervals that a sensor is collecting data within. In satellite imagery, we refer to these wavelength intervals as bands.
VHR satellites collect data primarily in the Visible and Near Infrared (VNIR) portions of the electro-magnetic spectrum. WorldView-3 also collected in Short Wave Infrared (SWIR).
The satellites will also collect a panchromatic (black & white) band, which will have a much wider bandwidth.
There are a few factors to be aware of with regards to spectral definitions and resolution, particularly if we are using the data for spectral analysis.
For earth observation applications, we measure wavelength in nanometers (nm).
Relationship between spatial and spectral resolution
At longer wavelengths, you will notice that satellites collect at a lower spatial resolution. This is because there is less energy in the photons at these longer wavelengths, meaning you need more of them to collect a signal at the sensor. This is achieved by collecting over a wider bandwidth, and by collecting at a lower spatial resolution.
For VHR satellites, the panchromatic band is always the highest resolution (the 30cm band!). Meanwhile, the 'colour' near infrared bands are between x and x for MAXAR satellites.
For the panchromatic data to achieve this high spatial resolution, it collects over a very wide wavelength, at the expense of having any spectral precision. Thus it is also known as the black & white band.
This trade off between spectral and spatial resolution, also explains why the short-wave infrared imagery that is collected by WorldView-3 (a VHR satellite) is only available at a lower resolution that the visible and near infrared data.
Band definition and precision
The bands that make up the "visible" portion of the spectra vary for different sensors, and even the band definitions are slightly different. So what the "green" or "red" band is for one sensor, will not be the same for another sensor. For instance WorldView-2 red band is 630-690 nm while the GeoEye-2 red band is 655 - 690 nm. This makes spectral comparisons between sensors difficult.
Number of bands
Spectral resolution can also be defined by the number of bands a satellite acquires.
Earlier VHR satellites acquired panchromatic imagery plus 3 or 4 VNIR bands, whereas now, WorldView-3 acquired 8 multispectral bands, 8 shortwave infrared bands and 12 CAVIS bands.
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