sensor technology

Question 1

What is an active sensor

Answer 1

Provides its own energy source for illumination

Question 2

What are the advantages of active sensors

Answer 2

• Can obtain measurements anytime, regardless of the time of day or season,
• Examine wavelengths that are not sufficiently provided by the sun (e.g. microwaves)
• Depending on wavelength, can see through clouds (e.g. radar)

Question 3

Give an example of an active sensor

Answer 3

LiDAR (Light detection and ranging) sensors
▪ Transmit a pulse of energy (red, green or NIR) to the surface this:
▪ Maps surface height (digital elevation models)
▪ Derives information on vegetation canopies (height, biomass, forest structure)
iSAT - maps ice surface thickness

Question 4

What is a passive sensor

Answer 4

measure the energy naturally available

Question 5

what are the limitations of passive sensors

Answer 5

• Require sunlight/emission from ground objects
• As they require sunlight they only collect data in the day
• can’t see through clouds

Question 6

How is the sensor/ instrument classified

Answer 6

▪ Ground based
▪ Airborne
▪ Spaceborne

Question 7

What does The distance between the target being imaged and the platform depend on? think how we’d choose the sensor.

Answer 7

• The spatial resolution required (how much detail is needed?)
• The area coverage required (how big is the area of interest?
  So a 1/2km could fly drone or plane
  much bigger area = satellite but in less detail.

Question 8

Space borne

Answer 8

• Cheaper - Expensive to build and launch but cheaper in the long-term
• Long operation - Most satellite missions >5years so measurements are consistent over this time
• Repetitive orbits - Consistent long-term observations of same place
• Global coverage - Climate change models need global data

Question 9

Geostationary satellites

Answer 9

– don’t provide global coverage
* High altitude (~36,000 km)
* Moving above the equator in synchronization with the earth - Remains in same position above the Earth (not global coverage)
* Used by meteorological (weather) and communications satellites
* Provides the same view of the Earth every 15-30 minutes
* monitor weather over entire hemispheres of the Earth

Question 10

Polar orbiting satellites

Answer 10

• Low altitude (orbit lower in the atmosphere) (approximately 800km)
• Revisit time varies from twice daily to ~16 days to observe the same region
• Mostly used for environmental remote sensing
• Global coverage

Question 11

Name the 4 types of resolution

Answer 11

• Spatial resolution * ability to discern objects on ground
• Spectral resolution * width and number of spectral bands
• Radiometric resolution * signal precision
• Temporal resolution * sensor revisit frequency

Question 12

Spatial resolution

Answer 12

▪ The smallest size of an object that can be picked out from its surrounding objects or features
▪ Expressed in units of km, meters, cm etc
▪ Higher the spatial resolution the more detailed image (able to resolve smaller features).
250m = low spatial resolution, to measure urban areas you would need high spatial resolution to pick out roads and urban areas. Deforestation Landsat or sentinel is ok

Question 13

Spectral resolution

Answer 13

• the number and size of spectral regions (or
  frequencies) the sensor records data in, e.g.
  blue, green, red, near-infrared, thermal infrared.

Question 14

What is panchromatic

Answer 14

Panchromatic Consist of one wide band that encompasses a large spectral range (often the entire visible spectrum). High spatial resolution

Question 15

Multispectral

Answer 15

Consists of relatively narrower several bands of which two or three bands in the visible range and also have a few near-infrared or middle infrared bands.

Question 16

Hyperspectral

Answer 16

Hyperspectral Consists of large number of narrower bands spread across the EMR. On a graph would have smoother curve. high spectral resolution - more wavebands.

Question 17

Why is landsat good

Answer 17

useful to compare older and newer data.

Question 18

Radiometric resolution

Answer 18

sensitivity of detectors to small
differences in electromagnetic energy.
Defines the number of values that an image can store. expressed as bits. the higher the radiometric resolution, the better the data quality ▪ Bit is expressed as power of 2, and numbers start from 0
▪ 1 bit = 21 = 2 values (0 and 1) ▪ 2 bit = 22 = 4 values (0,1,2 and 3) ▪ 8 bit = 28 = 256 values (0 to 255)

Question 19

Temporal resolution

Answer 19

how often the sensor acquires data, e.g.,
every 30 days.

▪ However, because of some degree of overlap in the imaging swaths of adjacent orbits for most satellites and the increase in this overlap with increasing latitude, some areas of the Earth tend to be re-imaged more frequently.
Longer to build up images over equator than the poles – cover the poles regularly.
High temporal resolution is needed for weather minutes (meteorology) low temporal resolution needed for urban areas (every year).

Question 20

Name 2 types of satellite orbit

Answer 20

• Geostationary – same place, very quick
• Polar orbit – global coverage, takes longer

Question 21

What is spatial resolution?

Answer 21

The smallest size of an object that can be picked out from its surrounding objects or features