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Remote Sensing > 3 Sensors > Flashcards

3 Sensors Flashcards

1
Q

Recap

The first requirement for remote sensing is a source of energy

A

– This energy is in the form of electromagnetic radiation.

– Any object with temperature above 0 k emits EM energy.

– EM radiation travels through vacuum and mediums.

– It interacts with materials, i.e. atmosphere and earth surface.

– The energy reflected or emitted from earth’s surface reaches the sensor.

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2
Q

Remote Sensor

A

A device that captures EM energy, quantify it as digital values and records it.

– The source of energy can be external or with the sensor.
– It might record different parts of EM spectrum.
– The sensor is mounted on a platform.

Passive sensors:

  • gammaray spectrometer
  • multispectral scanner (visible domain)
  • imaging spectrometer (visible domain)
  • satellite line camera (visible domain)
  • aerial camera (visible domain)
  • video camera (visible domain)
  • thermal scanner (optical domain)
  • passive microwave radiometer (microwave domain)

active sensors:

  • laser scanner (visible domain)
  • radar altimeter (microwave domain)
  • imaging radar (microwave domain)
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3
Q

Platforms used for remote sensing (based on their altitude)

A

• Ground-based platforms
– laboratory, Hand-held, tripod, vehicles, towers, Drones
– Up to ~50 m

• Airborne
– Balloons , Airplanes, Helicopters
– Up to ~50 km

• Space-borne
– Rockets, satellites, shuttles
– ~100-37000 km

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4
Q

Space-borne platforms advantages

A

✓ Stable platform
✓ Rapid data acquisition
✓ Globally uniform quality
✓ Regular revisit

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5
Q

Space-borne platforms disadvantages

A

✗ High cost of satellite systems
• Landsat 8 launched in 2013 cost 855 million USD.

✗ The process of develop -> build -> test -> launch can take several years.
• 10 years for Landsat 8.

✗ Chance of failure
• Landsat 6 failed to reach orbit.

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6
Q

Space-born platforms thingy (1/2)

A

• Satellite orbits
– An orbit is a curve path that an object in space takes around another object due to gravity.
– A small object orbit around Earth when its speed is balanced by the pull of Earth’s gravity.
– A satellite in a stable orbit is in an equilibrium between the Earth’s gravity and centrifugal force, due to its velocity.
– Satellites orbit Earth at different heights, different speeds and along different paths.
– The speed of satellite is determined by satellite’s orbit and closely tied to the satellite’s altitude.
– Satellites close to the Earth move faster relative to earth comparing to those at higher altitudes.

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7
Q

Space-born platforms thingy (2/2)

A

• A satellite’s orbit always lies in a plane that passes through the center of the Earth.
– The angle between the orbital plane and the
– equator is called the orbit’s inclination.
– Inclination of 0° is directly above the equator, and 90° crosses the poles.

• Orbital altitude
– The distance of a satellite to the Earth’s surface
– Affects the spatial coverage and level of details that can be observed by the sensor.

• Types of orbits
– Geostationary orbit (GEO)
– Low earth orbit (LEO)

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8
Q

Types of orbit: Geostationary orbit (GEO)

A

– Many satellites need to stay above a given point on earth.
– They follow the Earth’s rotation and circle the Earth from west to east.
– GEO satellites travel at the same rate at which Earth rotates.
– Only an altitude of 36,000 km and an inclination of 0° allows a satellite to stay above a certain point.
– From the earth, the satellites seem motionless.
– Examples: weather satellites and communication satellites

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9
Q

Types of orbit: Low Earth Orbit (LEO)

A

– A Low Earth Orbit is an orbit that is relatively close to Earth.
– In contrast to GEO, LEO orbits can have different inclinations.
– Each satellite orbit in LEO takes approximately 90 minutes.
– The rotation of earth provides the satellite the possibility to reach different points above earth.
– Ascending pass: when satellite travels from south to north.
– Descending pass: when satellite travels from north to south.
– LEO satellites are often on sun-synchronous orbits.
– Sun-synchronous orbits remain fixed with respect to the sun.
– The satellite passes over the target at almost the same local time.
– It keeps the angle of sunlight on the surface of the Earth as consistent as possible

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10
Q

Geostationary orbit (GEO) advantages and disadvantages

A

✓ Large spatial coverage
✓ High temporal resolution
✓ Satellite always visible from a certain point on Earth
✓ Always provide an image from the same perspective

✗ Lower spatial resolution
✗ Poor coverage of high latitudes

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11
Q

Low Earth Orbit (LEO) advantages and disadvantages

A

✓ Medium to high spatial resolution
✓ Coverage of high latitudes
✗ Lower temporal resolution

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12
Q

Imaging Sensors - Panchromatic films

A

– Analog photography
– Radiant energy is detected by silver halide crystals
– Silver halide grains turn to metallic silver when exposed to light.
– Each light ray triggers a chemical reaction of some grains.

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13
Q

Imaging Sensors - Charge-Coupled Device (CCD)

A

– A major technology used in digital imaging.
– A CCD captures light and converts it to digital data that is recorded by the camera.
– It is an array of detectors that absorb photons of light and release electrons.
– Electrons associated with each pixel flow an electric current.
– The ELECTRIC SIGNAL is sampled and quantized in a process called Analog to Digital (A/D) conversion.

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14
Q

Imaging Sensors - A/D conversion -> Digital Number

A

• A/D conversion
– Sampling: Reduction of a continuous-time signal to a discrete-time signal.
– Quantization: Mapping signal values with an approximation from a finite set of discrete values.

• The output of A/D conversion is a Digital Number (DN) which is stored by the sensor.
– DN is an integer number within a finite range.
– This fixed range defines the number of grey levels in a digitized image which is called radiometric resolution.

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15
Q

Imaging Sensors - Radiometric Resolution

A

• A higher radiometric resolution provides a higher information content.
– Older sensors stored 8-bit recordings that translates into 256 discrete values (e.g. Landsat 7).
– Modern sensors have a higher radiometric resolution (e.g. Landsat 8 stores 12-bit data with 4096 values).

  • Each recorded DN is assigned to an element called pixel.
  • An array of recorded DNs form a digital image.
  • A single digital image is referred to as a band.

• Multispectral sensors record several bands in different parts of EM spectrum.
– e.g. MOSIS has 36 and ASTER has 14 spectral bands.

• Hyperspectral sensors record hundreds of bands.

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16
Q

Imagining Systems - Framing cameras

A

Take snapshots of an area, projected on an analog film or an array of detectors.

✓ The entire image is acquired at once.
✗ A high number of CCD arrays used to be expensive.
✗ The sensor response over the wide field of view must be consistent.
✗ Difficult to connect successive images.

17
Q

Imagining Systems - Whisk Broom Scanners

A

– Also called across track scanners
– Use a steering mirror to reflect light onto a single detector.
– The mirror swipes back and forth to scan the target pixel by pixel.
– The forward motion of the platform is used to collect sequential scan lines.
– Multiple arrays used for multispectral imagery.
– Instead of one detector per band, several are usually used to scan more lines simultaneously.

✓ Fewer sensor detectors to keep calibrated.
✓ Precise radiometric measurements.
✗ Low exposure time for each pixel.
✗ Expensive sensor design.
✗ Needs fast moving mirror which is more likely to break or wear out
✗ Lower geometric accuracy.

18
Q

Imagining Systems - Push Broom Scanners

A

– Also called along track scanners
– Newer and more reliable technology
– Use a line of detectors perpendicular to the flight direction.
– Each line of image is captures simultaneously
– Multiple linear arrays are used for multispectral imagery.

✓ Better geometric accuracy
✓ More exposure time, allows smaller IFOV and/or narrower bands.
✗ Cross calibration of detectors is more difficult.

19
Q

Examples of passive optical sensors - Landsat

A

• Landsat series of Earth Observation satellites
– Landsat 1: 1972 - 1978
– Landsat 6: 1993 (failed to reach orbit)
– Landsat 9: 2021 (expected)

  • Landsat 1-7 are whisk broom scanners
  • Landsat 8 is a push broom scanner.
  • Scanners carried by different Landsat series improved through time.
20
Q

Landsat mission

A
  • Landsat 1-5 carried sensors called Multispectral Scanner (MSS)
  • Radiometric resolution of 6 bits
  • Landsat 4-5 also carried sensors called Thematic Mapper (TM)
  • Radiometric resolution of 8 bits
  • Landsat 7 carried a sensor called Enhanced Thematic Mapper Plus (ETM+)
  • Radiometric resolution of 8 bits
  • Landsat 7 SLC-off data, Scan Line Corrector (SLC) was responsible to keep the whisk broom module aligned.
  • Landsat 8 carried two sensors called Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS)
  • Radiometric resolution of 12 bits
21
Q

Examples of passive optical sensors - Sentinel-2

A

• Sentinel-2 constellation
– Twin satellites launched in 2015 and 2017
– Sentinel-2 sensors are push broom scanners.
– Radiometric resolution of 12 bits
– Free and open data

• Sentinel-2 is a constellation with twin satellites which carry sensors called multi-spectral instrument (MSI)

22
Q

Examples of passive optical sensors - SPOT

A

SPOT sensors are whisk broom scanners

– SPOT 1: 1986 - 1990

23
Q

Digital number to radiance

A

• Raw remote sensing images are stored as Digital Numbers (DN).
– DNs are not calibrated.
– DNs do not have any physical meaningful unit.
– Data visualization can be done using DNs.
– For quantitative analysis we need to convert DNs to physically meaningful radiance or reflectance.

• DN to radiance conversion
– Sensor dependent

for SPOT
Li = DN/10000

for LANDSAT
Li = Ai x DN + Bi

where 
Li = radiance
DN = Digital Nunber
Ai = Calibration gain value for band i 
Bi = Calibration offset value for band i
24
Q

At sensor radiance

A
  • The energy measured by the sensor is a mix of different radiations arrived at the sensor.
  • Radiometric correction is important to improve degraded raw data.

Remote Sensing (12 decks)

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