Lecture 8

Question 1

Pixel

Answer 1

The smallest, non-divisible element in an image. The spatial resolution of a sensor is usually represented by a single pixel.

Question 2

Instantaneous Field of View (IFOV)

Answer 2

The ground area covered by a solid angle of a satellite sensor.

Question 3

Bits

Answer 3

Each digital value is recorded as a series of binary digits known as bits. In radiance–often a digital number=brightness value.

Question 4

Brightness or Intensity Level

Answer 4

The number of discrete levels for describing the radiance level of a pixel, scaled from zero brightness to the highest brightness that would be expected. The more discrete levels (or bits of information), the more precisely the radiance of the scene can be measured. We typically deal with 8-bit brightness measurements or 256 levels, also referred to as DYNAMIC RANGE.

Possible Combinations with: 
1 bit: 0 or 1= 2 or 2^1
2 bits: 0,0, 0,1 1,0 1,1= 4 or 2^2 
3 bits: 0,0,0 0,0,1 0,1,0 0,1,1= 8 or 2^3
N bits = 2^N

Question 5

Spectral Band

Answer 5

In a digital image
represents a narrow slice of radiance in a given wavelength range. e.g. “a range of wavelength”

The brightness level in a given spectral band is measured using a sensor that is responsive only in that band or by placing a filter in front of a broad band sensor.

Question 6

Four types of RESOLUTIONS in remote sensing:

Answer 6

spectral, spatial, temporal, radiometric

(1) spectral: the number and dimension of specific wavelength intervals in the EM spectrum to which the instrument is sensitive.
(2) spatial: the smallest angular or linear separation between two objects that can be resolved by the sensor (IFOV).
3) Temporal: the repeat frequency of information gathered at a specific point.
(4) Radiometric: sensitivity of the sensor to different signal strengths in radiant flux.

Question 7

Spatial resolution

Answer 7

A measure of the smallest object that can be resolved by the sensor, or the linear dimension on the ground represented by each pixel or grid cell in the image.

Question 8

Trends in improving spatial resolution

Answer 8

see slide

Question 9

Temporal resolution

Answer 9

how often a sensor can obtain imagery of a particular area of interest.

High temporal resolution: < 24 hours - 3 days Medium temporal resolution: 4 - 16 days
Low temporal resolution: > 16 days

Question 10

Spectral resolution

Answer 10

number and dimension of specific wavelength intervals in the EM spectrum to which the instrument is sensitive

• The ability of a sensor to define fine wavelength intervals
• The finer the spectral resolution, the narrower the wavelength range for a particular channel or band

Question 11

Multi-Spectral resolution

Answer 11

see slide

Band 6–thermal infrared is EMITTED. NIR was absorbed and emitted as heat.

Question 12

Hyperspectral resolution

Answer 12

see slide

100s of bands

Question 13

Radiometric resolution

Answer 13

Refers to the number of possible brightness values in each band of data and is determined by the number of bits into which the recorded energy is divided. In 8-bit data, the brightness values can range from 0 to 255 for each pixel (256 total possible values). In 7-bit data, the values range from 0 to 127, or half as many possible values

Question 14

Imaging (Scanning) Systems–definition and two main modes

Answer 14

Many ELECTRONIC (as opposed to photographic) remote sensors acquire data using scanning systems, which employ a sensor with a narrow field of view that sweeps over the terrain to build up and produce a two- dimensional image of the surface.

There are two main modes or methods of scanning employed to acquire multispectral image data - across- track scanning, and along-track scanning.

Question 15

Whiskbroom Scanners

Answer 15

Across-track scanners scan the Earth in a series of lines. The lines are oriented perpendicular to the direction of motion of the sensor platform (i.e. across the swath). Each line is scanned from one side of the sensor to the other, using a rotating mirror.
• The IFOV (C) of the sensor and the altitude of the platform determine the ground resolution cell viewed (D), and thus the spatial resolution. The angular field of view (E) is the sweep of the mirror, measured in degrees, used to record a scan line, and determines the width of the imaged swath (F).
• Because the distance from the sensor to the target increases towards the edges of the swath, the ground resolution cells also become larger and introduce geometric distortions to the images.

Question 16

Whiskbroom Scanners: Dwell Time

Answer 16

The amount of time a scanner has to collect photons from a ground resolution cell:
(scan time per line)/(#cells per line) depends on:
– satellite speed
– width of scan line – time per scan line

Question 17

Whiskbroom vs. Pushbroom

Answer 17

WHISKBROOM

• Wide swath
• Complex mechanical system
• Simple optical system
• Filters and sensors
• Shorter dwell time
• Pixel distortion

PUSHBROOM

• Narrow swath
• Simple mechanical system
• Complex optical system
• Dispersion grating and CCDs
• Longer dwell time
• Less pixel distortion

Question 18

Pushbroom Scanners

Answer 18

• Pushbroom scanners use a linear array of detectors (A) located at the focal plane of the image (B) formed by lens systems (C), which are “pushed” along in the flight track direction (i.e. along track).
• Each individual detector measures the energy for a single ground resolution cell (D) and thus the size and IFOV of the detectors determines the spatial resolution of the system.