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3.2 Spacecraft Sensors – Introduction to Sensors
Earth, Moon, Mars, and Beyond
Dr. Jinjun Shan, Professor of Space Engineering
Department of Earth and Space Science and Engineering
Room 255, Petrie Science and Engineering Building
Tel: 416-736 2100 ext. 33854
Email: [email protected]
Homepage: http://www.yorku.ca/jjshan
Introduction
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Classification of sensor
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Optical sensors
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Microwave sensors
Prof. Jinjun Shan
Spacecraft Sensor – Introduction to Sensors 2
Classification of Sensor
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λmax = 2898 / T
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Spacecraft Sensor – Introduction to Sensors 3
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Passive sensors detect the reflected or emitted electro-magnetic
radiation from natural sources
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Active sensors detect reflected responses from objects which are
irradiated from artificially generated energy sources, such as radar.
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A sensor classified as a combination of passive, non-scanning and
non-imaging method is a type of profile recorder, for example a
microwave radiometer. A sensor classified as passive, nonscanning and imaging method, is a camera.
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Sensors classified as a combination of passive, scanning and
imaging are classified into image plane scanning sensors, such
as TV cameras and solid state scanners, and object plane
scanning sensors, such as multispectral scanners (opticalmechanical scanner) and scanning microwave radiometers.
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An example of an active, non-scanning and non-imaging sensor is
a profile recorder such as a laser spectrometer and laser altimeter.
An active, scanning and imaging sensor is a radar, for example
synthetic aperture radar (SAR), which can produce high resolution,
imagery, day or night, even under cloud cover.
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In remote sensing, the most popular sensors are the camera, solid
state scanner, such as the CCD (charge coupled device) images,
the multi-spectral scanner and in the future the passive synthetic
aperture radar.
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Laser sensors have recently begun to be used more frequently for
monitoring air pollution by laser spectrometers and for
measurement of distance by laser altimeters.
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Most common sensors and their spectral bands
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Optical Sensors
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Those sensors which use lenses in the visible and reflective infrared region, are
called optical sensors.
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Optical sensors are characterized specified by spectral, radiometric and
geometric performance.
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Definitions of Optical Sensor’s Characteristics
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Spectrometer
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A spectrometer is an optical instrument used to measure
properties of light over a specific portion of the
electromagnetic spectrum. The variable measured is most
often the light's intensity but could also, for instance, be the
polarization state.
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Spacecraft Sensor – Introduction to Sensors 9
Prism spectrometer
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This Figure shows a simple prism spectrometer for examining a light
source with a focusing lens to produce parallel light (plane waves) which is
dispersed by the prism. The spectrum of plane waves can be focused onto
a photographic plate. The plate must be tilted to keep all the colors in
focus because of the change in focal length of the lens with wavelength.
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Spacecraft Sensor – Introduction to Sensors 10
Diffraction spectrometer
Different diffraction based
spectrometers:
Reflection optics
Refraction optics
Fiber optics
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Michelson interferometer
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The Michelson interferometer is the most common configuration for
optical interferometry. The Michelson interferometer produces interference
fringes by splitting a beam of monochromatic light so that one beam
strikes a fixed mirror and the other a movable mirror. When the reflected
beams are brought back together, an interference pattern results.
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Fabry-Pérot interferometer or etalon
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Fabry-Pérot interferometer or etalon is typically made of a transparent
plate with two reflecting surfaces, or two parallel highly-reflecting mirrors.
(Technically the former is an etalon and the latter is an interferometer)
http://www.physics.uq.edu.au/people/mcintyre/applets/fabry/fabry.html
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Application of Spectrometer
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In astronomy, spectrographs are widely used. These are installed at the
focus of a telescope which may be either in a ground-based observatory
or in a spacecraft.
The Mars Exploration Rovers each contained a Mini-TES - a Miniature
Thermal Emission Spectrometer (i.e. infrared spectrometer).
The forthcoming James Webb Space Telescope will contain both a nearinfrared spectrograph (NIRSpec) and a mid-infrared spectrometer (MIRI).
This image shows the martian terrain through
the eyes of the Mars Exploration Rover Spirit's
mini-thermal emission spectrometer, an
instrument that detects the infrared light, or
heat, emitted by objects. The different colored
circles show a spectrum of soil and rock
temperatures, with red representing warmer
regions and blue, cooler. The mini-thermal
emission spectrometer data are superimposed
on an image taken by the rover's panoramic
camera.
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Detectors
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An element which converts the electro-magnetic energy to an electric
signal is called a detector. There are various types of detectors with
respect to the detecting wavelength.
Photo tube and photo multiplier tubes are the examples of the photo
emission type which has sensitivity in the region from ultra violet to
visible light.
Optical excitation types have sensitivity in the infrared region. Photo
diode detectors utilize electric voltage from the excitation of electrons,
while photo transistor and photo conductive detector utilize electric
current.
Thermocouple bolometers and pyroelectric bolometers are examples
of the thermal effect type, which has sensitivity from near infrared to
far infrared regions.
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Cameras
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Cameras may work with the visual spectrum or other portions of the
electromagnetic spectrum.
Examples of space cameras, are the Metric Camera on board the Space Shuttle
by ESA, the Large Format Camera also on board the Space Shuttle by NASA,
and the KFA 1000 on board COSMOS by Russia.
Multispectral cameras with several separate film scenes in the visible and
reflective IR, are mainly used for photo-interpretation of land surface covers.
Panoramic cameras are used for reconnaissance surveys, surveillance of electric
transmission lines, supplementary photography with thermal imagery, etc.,
because the field of view is so wide.
LE-900 reusable space camera is used as part of a Video Guidance Sensor System for
docking of the Spartan Satellite. The camera has supported several flights aboard the space
shuttle including a variety of satellite applications.
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Cameras - Russia
1.
2.
3.
4.
5.
6.
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Luna-3
Mars-1
Zond-3, Luna-12, Venera-2
Mars-5 Orbiter
Experimental
Experimental
7.
8.
9.
10.
11.
12.
Luna-9, Mars-3 Landers
Luna-19, Luna-22
Mars-5, Venera-9 Orbiter
Earth orbiter (Meteor)
Earth orbiter
Venera-9 Lander
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Long Wave Infrared Camera - Clementine
Mass:
2100 grams
Size:
15 x 15 x 40 cm
Avg. Power
26 W
Wavelength:
8.0 - 9.5 microns
Field of View:
1.0 x 1.0 degrees
Pixel Format:
128 x 128
The long wave infrared camera for
Clementine, a lightweight camera
operating in the thermal infrared
region of the spectrum.
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Optical Mechanical Scanner
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An optical mechanical scanner is a multispectral radiometer by which two
dimensional imagery can be recorded using a combination of the motion of the
platform and a rotating or oscillating mirror scanning perpendicular to the flight
direction.
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Optical mechanical scanners can be carried on polar orbit satellites or aircraft.
Multispectral scanner (MSS) and thematic mapper (TM) of LANDSAT, and
Advanced Very High Resolution Radiometer (AVHRR) of NOAA are the examples
of optical mechanical scanners.
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Optical mechanical scanners are composed of an optical system, spectrographic
system, scanning system, detector system and reference system.
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Function of the elements
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Optical system: Reflective telescope system such as Newton,
Cassegrain or Ritchey-Chretien is used to avoid color aberration.
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Spectrographic system: Dichroic mirror, grating, prism or filter are
utilized.
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Scanning system: rotating mirror or oscillating mirror is used for scanning
perpendicular to the flight direction.
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Detector system: Electromagnetic energy is converted to an electric
signal by the optical electronic detectors. Photomultiplier detectors utilized
in the near ultra violet and visible region, silicon diode in the visible and
near infrared.
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Reference system: The converted electric signal is influenced by a
change of sensitivity of the detector. Therefore light sources or thermal
sources with constant intensity or temperature should be installed as a
reference for calibration of the electric signal.
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Multispectral scanner -Skylab
Multispectral Scanner (S192). (a)
Cutaway diagram. (b) Scanner Optics.
(c) Lens system.
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Atmospheric Sensors
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Atmospheric sensors are designed to provide measures of air
temperature, vapor, atmospheric constituents, aerosols etc. as well
as wind and earth radiation budget.
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Laser Radar or Lidar is an active sensor which is used to measure
air pollution, physical characteristics of atmospheric constituents in
the stratosphere and its spatial distribution using laser. The theory
of laser radar is also utilized to measure distance, so that for this
application it is called laser distancemeter or laser altimeter.
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Laser altimeters have been used by Apollo 15, 16, 17 to provide
ranging data for use in determining the altitude of the CSM above
the lunar surface.
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The Clementine Laser Image Detection And Ranging (LIDAR)
experiment was designed to measure the distance from the
spacecraft to a point on the surface of the Moon.
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Microwave Sensors
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Many of the earth observation satellites launched after 1992 are
equipped with onboard microwave sensors.
Passive and active sensors
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Frequency of passive microwave sensor for monitoring
major targets - I
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Spacecraft Sensor – Introduction to Sensors 24
Frequency of passive microwave sensor for monitoring
major targets - II
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Active Sensors
Active sensors
can be classified
into more types
in terms of the
target with
respect to
horizontal or
vertical
polarization.
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Frequency of active microwave sensor for monitoring
major targets
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Imaging radar
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Imaging radar is classified further into Real Aperture Radar (RAR) and
Synthetic Aperture Radar (SAR).
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RAR transmits a narrow angle beam of pulse radio wave in the range
direction at right angles to the flight direction (called the azimuth direction)
and receives the backscattering from the targets which will be transformed
to a radar image from the received signals.
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The resolution in the range direction depends on the pulse width. The
resolution in the azimuth direction is identical to the multiplication of beam
width and the distance to a target. As the resolution of azimuth direction
increases with shorter wave length and bigger antenna size, a shorter
wavelength and a bigger antenna is used for higher azimuth resolution.
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However as it is difficult to attach such a large antenna, requiring for
example a 1 km diameter antenna in order to obtain 25 meters resolution
with L band (=25 cm) and 100 km distance from a target, a real aperture
radar therefore has a technical limitation for improving the azimuth
resolution.
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Side-looking airborne radar operation
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SAR
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Compared to RAR, Synthetic Aperture Radar (SAR) synthetically
increases the antenna's size or aperture to increase the azimuth
resolution though the same pulse compression technique as
adopted for range direction. Synthetic aperture processing is a
complicated data processing of received signals and phases from
moving targets with a small antenna, the effect of which is to
should be theoretically convert to the effect of a large antenna,
that is a synthetic aperture length.
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In the case of SAR, unsuitability of satellite velocity and attitude
will reduce the Doppler effect. Therefore the satellite with SAR is
required to be high, because the correction for synthetic aperture
processing due to instability at lower altitudes is very difficult.
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Relationship between RAR and SAR
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Characteristics are of importance
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Two factors of microwave characteristics are of importance:
frequency (or wavelength) and polarization.
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In microwave remote sensing, various wavelengths (or
frequency) such as L-band, C-band, X-band, P-band etc. will be
used ranging from millimeter wavelengths (1 mm - 1cm) up to
about 30 cm.
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Polarization is defined as the oscillating direction involved in an
electric field. Usually transmitted microwave and received
microwave will have a choice between horizontal polarization
and vertical polarization. Therefore four combinations: HH, HV,
VH and VV can be used for SAR.
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Radarsat 2
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Microwave Radiometer
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Microwave radiometers or passive type microwave sensors
are used to measure the thermal radiation of the ground
surface and/or atmospheric condition.
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Brightness temperature measured by a microwave radiometer
is expressed by Reyleigh-Jean's law, which is the resultant
energy of thermal radiation from the ground surface and the
atmospheric media. Multi-channel radiometers with multipolarization are used to avoid the influences of unnecessary
factors to measure the specific physical parameter.
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Jason Microwave Radiometer
The image shows a mapping of preliminary channel 1 antenna temperatures
from the first five days of operation.
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Microwave Scatterometer
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Microwave scatterometers can measure the received power of surface
backscattering reflected from the surface of objects. According to a
narrow definition, a microwave scatterometer may be a space borne
sensor to measure the two dimensional velocity vectors of the sea wind,
while according to the wider definition, it also involves air-borne sensors,
as well as ground based sensors to measure the surface backscattering
as well as volume scattering, such as rain radar.
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Microwave scatterometers are classified as two types, pulse type and
continuous wave type (CW). The pulse type uses wide band which has
restrictions in obtaining a license to operate and in avoid obstructions.
CW type has the advantage that the band width can be reduced to 1/100
times that of the pulse type and the price becomes cheaper.
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SEASAT-SASS (Seasat-A Satellite Scatterometer) is one of the typical
scatterometers. SASS has four fixed antennas to transmit the pulse in a
fan beam of 14.5 GHz to four different angles, and to receive the
backscattering in subdivided cells through a Doppler filter.
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3-band microwave scatterometer
X-Band：9.6 GHz
C-Band：5.4 GHz
L-Band：1.2 GHz
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Microwave Altimeter
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Microwave altimeters or radar altimeters are used to measure
the distance between the platform (usually satellite or aircraft)
and the ground surface. Some applications of microwave
altimetry are in ocean dynamics of the sea current, geoid
surveys, and sea ice surveys. Therefore, precise measurement
of the satellite orbit and the geoid should be carried out.
The principle of satellite altimetry
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