Sensors and Imagery 3

What is radiant flux (Φ)?

Total radiant energy per unit time (watts).

What is irradiance?

Amount of radiant flux incident upon a surface from all directions per unit area (W/m²).

What is exitance?

Amount of radiant flux leaving a surface into all directions per unit area (W/m²).

What is radiance (L)?

Radiant flux per unit solid angle leaving a source in a given direction per unit projected area (W/m²·sr). It is what satellites measure.

What is reflectance?

Dimensionless ratio of radiant flux reflected from a surface to radiant flux incident on it. It is what we want to use.

Types of resolution in remote sensing

Spatial, Spectral, Radiometric, Temporal.

What is spatial resolution?

Smallest feature that can be detected; corresponds to pixel size in a raster. Depends on the sensor's IFOV.

What is IFOV?

Instantaneous Field of View; determines the ground area seen by the sensor at a given altitude.

Effect of cell size on spatial resolution

Large cell size = low resolution; small cell size = high resolution.

What is spectral resolution?

Ability of a sensor to define fine wavelength intervals. Finer resolution = narrower bands.

Example of low spectral resolution

Panchromatic: entire visible spectrum lumped together (cannot distinguish red, green, blue).

Example of high spectral resolution

Bands isolate narrow portions of the spectrum, e.g., green only.

What is multispectral imagery?

Multiple discrete bands across the spectrum (e.g., Landsat: red, green, NIR, etc.).

What is hyperspectral imagery?

Very high spectral resolution with ~200+ continuous narrow bands.

Limitations of hyperspectral imagery

Adjacent bands can be highly correlated (redundant information); computationally expensive.

What is radiometric resolution?

Number of signal levels a sensor can distinguish; sensitivity to differences in energy.

Examples of radiometric resolution

2-bit = 4 values; 8-bit = 256 values; 16-bit = 65,536 values.

What is temporal resolution?

How often a sensor acquires data of the same area (revisit time).

Factors affecting temporal resolution

Orbit type, swath width, number of satellites in a constellation.

How do constellations improve temporal resolution?

Multiple satellites (e.g., PlanetScope) reduce revisit time by covering the same area more frequently.