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Lboro Geography > Remote Sensing and GIS (Year 2) - Part 2 - Remote Sensing > Flashcards

Remote Sensing and GIS (Year 2) - Part 2 - Remote Sensing Flashcards

1
Q

Define Remote Sensing

A

Obtaining information about the Earth, without physical contact, but by measurement of electromagnetic radiation (e.g. visible light, infrared, ultraviolet, micro waves, or thermal energy)

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2
Q

What are the two types of remote sensing?

A

Passive and Active

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3
Q

What is passive remote sensing?

A

Measures radiation coming from a natural source -> e.g. sun or earth

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4
Q

What is active remote sensing?

A

Measures radiation coming from an artificial source -> generated by the sensor itself and measures what come back

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5
Q

What is radiation?

A

Transfer of heat and energy through empty space

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6
Q

All objects with a temperature above absolute 0 do what?

A

Radiate energy

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7
Q

What is electromagnetic radiation?

A

Form of sinusoidal waves which propagate through space at the speed of light, by analogy with water waves

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8
Q

How are individual electromagnetic waves categorised?

A

Frequency v (how many waves pass per second) and wavelength (the distance between successive peaks)

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9
Q

Short wavelength of electromagnetic waves equals what frequency

A

large frequency

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10
Q

What is the electromagnetic spectrum made up of?

A

A ‘family’ of EM waves, each has a characteristics wavelength and frequency

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11
Q

Hotter bodies generate more of what type of radiation?

A

shortwave

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12
Q

Colder bodies generate more of what type of radiation?

A

longwave

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13
Q

What is the atmospheric window/window regions?

A

Windows where energy radiation passes through the atmosphere (most EMR passes through)

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14
Q

What happens to EMR as it passes through the atmosphere?

A

Scattering and absorption

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15
Q

What are the three interactions that occur EMR reaches the earth’s surface?

A
  • reflected: when a ray of light strikes a non-transparent surface and bounces back
  • absorbed: the thermodynamically irreversible transformation of radiant energy into heat (this energy is later re-emitted from the surface)
  • transmitted: when radiation passes through a substance without significant attenuation
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16
Q

What does the proportion reflected, absorbed and transmitted depend on?

A
  • the nature of the surface – city vs jungle
  • the wavelength of the EMR – wavelengths interact with different surfaces
  • the angle of illumination – angle the energy is coming in at
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17
Q

Why is the wavelength (λ) dependency of interactions of EMR with the earth’s surfaces critically important?

A
  • An important concept for Earth observation is that different surfaces have a different “spectral signature”
  • Surfaces have different responses due to different interactions with portions of the EMR spectrum (i.e. specific ‘lights’, or rather, specific wavelengths)
  • City heat maps -> based on wavelength -> how do cities vary?
  • Can tell different spaces apart based on their spectral signature -> response to light (their wavelength)
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18
Q

What are the two types of remote sensing range configurations?

A

Ground based and airborne systems

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19
Q

What is the ground based configuration for remote sensing?

A

E.g., measuring reflection of natural surfaces. In situ measurements are widely used to calibrate and atmospherically-correct airborne or satellite-derived data being acquired at the same time -> Cranes and towers enable measurements to be taken over larger surface areas than handheld instruments

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20
Q

What is the airborne systems configuration for remote sensing?

A

Are used to collect data from a higher vantage point. Fixed-wing aircraft are the platform of choice for most applications, as they provide a stable, flexible platform for small-area, high resolution survey, and testing of sensors before they go into space. More recently = drones

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21
Q

What is the most common remote sensing configuration?

A

Satellite based RS

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22
Q

What is the main purpose of remote sensing?

A

RS is helping us understand Earth as a system at largest scales

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23
Q

Define sensors

A

(not cameras) – detecting instrument

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24
Q

Define platform

A

sensor is attached to a platform (satellites themselves)

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25
Q

What is the relationship between sensors and platforms?

A

Sensors are the detecting instruments that are mounted on platforms (i.e. the satellites, or spacecraft)

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26
Q

Define spatial resolution in relation to remote sensing

A

pixel size on ground (km/m)

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27
Q

Define temporal resolution in relation to remote sensing

A

repeat pass (e.g. daily repeat)

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28
Q

Define spectral resolution in relation to remote sensing

A

number/width of bands of EMR identified

29
Q

Define spatial coverage in relation to remote sensing

A

area of earth imaged (km)

30
Q

What is the difference between each Landsat?

A

They each have a different sensor

31
Q

How do sensors work?

A
  • Sensors have detectors in which incident EMR (behaving as a particle) kicks electrons out of silicon, which the electronics register
  • EMR is dispersed into subcomponent bands using dispersive elements (like prisms)
  • Each colour/band of light then picked up by a different detector
32
Q

What are the two types of sensors?

A

Passive - detecting solar light (direct or reflected) or terrestrial longwave radiatiation (most are passive)
Active - with own radiation source “fired” at target, then the return signal is sensed

33
Q

What are multispectral sensors?

A

If they sense 2 to 9 bands of EMR

34
Q

What are hyperspectral sensors?

A

If they sense more than 10 bands of EMR

35
Q

What do sensors ultimately measure?

A

measure where atmos transmission is high

36
Q

What are panchromatic sensors?

A

Sensors that just senses one band

37
Q

How is EMR data stored via images and pixels?

A

Stored as digital numbers through binary scales (e.g. 8 bit data)

38
Q

What are the two types of satellite orbits?

A

Geostationary and polar

39
Q

What is the geostationary orbit? (satellites)

A

Above fixed point on equator; sees around 1/3 of surface but not poles e.g. like the moon

40
Q

What is geostationary orbit satellite information useful for?

A

This orbit useful when want wide coverage e.g., meteorological satellites, can watch weather systems develop

41
Q

What is a polar orbit? (satellites)

A
  • 200-1000 km above surface

- With Earth rotation under satellite track, whole Earth eventually covered

42
Q

What is faster a geospatial or polar orbit?

A

Geostationary view

43
Q

What are sun synchronous polar orbits?

A

Is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet’s surface at the same local mean solar time. For right choice of altitude and inclination of orbit, angle between sun and satellite orbit plane is maintained for every pass, every day.

44
Q

Why are sun synchronous polar orbits used?

A
  • Results in fixed equatorial crossing time and same illumination conditions every pass
  • Important for earth surface change detection
45
Q

What are the two types of swath view measurement techniques? (polar orbit)

A
  • Pushbroom imaging

- whiskbroom imaging

46
Q

Explain how pushbroom imaging works?

A

Uses a detector array that makes an image based composed of individual viewed pixels

47
Q

Explain how whiskbroom imaging works?

A

Uses a scanning mirror e.g., like pushing a brush forwards while sweeping from side to side

48
Q

What are polar orbit sensors categorised according to?

A

So polar orbit sensors are characterised according to: push/whisk broom imaging number of spectral bands, pixel size e.g., spatial resolution and temporal resolution (defined by swath width).

49
Q

What is the spatial coverage, spatial resolution and temporal resolution for MODIS (terra and aqua satellites)?

A

Spatial coverage (swath): 2330km
Spatial resolution: 1km, 500m & 250m
Temporal resolution: Daily repeat

50
Q

What is the spatial coverage, spatial resolution and temporal resolution for the Thematic Mapper (Landsat 5)?

A

Spatial coverage: 185km
Spatial res: 30m & 60m
Temporal res: 16 day

51
Q

What is a global navigation satellite system (GNSS)?

A

It uses satellites to determine geospatial position of a receiver

52
Q

What is selective availability?

A

Way of degrading effectiveness of mapping up to 100m to prevent public for using it in a negative way. This was later turned off, became accurate to 10m for public.

53
Q

What are the advantages of using GNSS?

A
  • All weather
  • Night or day
  • Instant
  • Provides absolute global position
  • No line-of-sight required to a reference position
54
Q

How many GNSS are there? List them

A

1) NAVSTAR – usually referred to as GPS, American constellation of 24 satellites
2) GLONASS (Global Navigation Satellite System)– Russian -> while relation between Russia and US is positive, the GLONASS and NAVSTAR satellites work together and share data
3) Galileo – EU
4) BeiDou – China -> global view

55
Q

What are the three segments to GNSS?

A
  1. User segment
  2. Space segment
  3. Control segment
56
Q

What is the space segment of GNSS made up of?

A

Satellites:

  • GPSs
  • Solar powered
  • Batteries
  • 4 onboard atomic clocks (accurate to a billionth of a second)
  • Radio transmitters
  • 12-year life
57
Q

How many satellites are needed for a full constellation of the space segment of GNSS?

A

24 satellites

58
Q

What is the control segment of GNSS made up of?

A

Global monitoring stations

59
Q

What is the control segment of GNSS purpose?

A
  • Stations check precise satellite location and atomic clocks
  • Synchronise atomic clocks and upload to satellite location information (ephemeris): altitude, speed, position and health
  • Atomic clocks important as they need to be super precise as tiny amounts of time errors result in much larger spatial errors
60
Q

What is the user segment of GNSS made up of?

A

A Receiver which consists of:

  • Antenna
  • Reciver processor
  • Accurate clock
  • Display screen or logger
61
Q

Satellite broadcast a signal that contains what information?

A
  • Satellite position
  • Time
  • Other parameters: satellite status, possible inaccuracies, information about other satellites
62
Q

If we know where the satellite in space what can you calculate from this and how?

A

How far away from us it is
using:
- Satellites send a signal to earth with a very precise time
- Difference in time signal sent from the satellite, and the clock GPS receiver, tells us how long the signal has been taken to arrive to the receiver (travel time)

63
Q

How is the travel time from a satellite calculated?

A
  • Satellite broadcast a long digital pattern, and receiver plays it
  • Travel time is distance between the time the sateliite broadcasts and delays of when a signal received
  • Requires a very accurate atomic clock on a satellite and receiver uses this accuracy
64
Q

What is Local Area Augmentation System?

A

An all-weather aircraft landing system based on real-time differential correction of the GPS signal.

65
Q

How does Local Area Augmentation System work?

A
  • Errors in GPS positions calculated for ground stations at the airport
  • Known errors are sent by radio signal to aeroplanes
  • User’s equipment corrects the raw GPS signal
66
Q

What is Differential GPS (DGPS)?

A

An enhancement to the Global Positioning System (GPS) which provides improved location accuracy, in the range of operations of each system, from the 15-meter nominal GPS accuracy to about 1-3 cm in case of the best implementations.

67
Q

What is Differential GPS (DGPS) used for?

A

Used for high-precision surveying -> surveying industry

68
Q

How does Differential GPS (DGPS) work?

A
  • A reference GPS station is established at a known point
  • The difference between the actual and computed location is calculated
  • The difference is assumed due to system errors and atmospheric effects
  • A rover GPS unit is used to do the survey
  • Computed rover locations are corrected based on calculated errors at reference station
  • Precision decreases the further the rover is from the reference –> closer the rover to the base station the more accurate the results are
  • Correction may be applied in real-time (like LAAS), or post-processed

Lboro Geography (4 decks)

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