This deals with the different types of imaging sensors, its constituents, thermal control systems, satellite communication, sensor detector assemblies, sensor design.
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A STUDY OF INFRA-RED IMAGING SENSORS AND INSTRUMENTS ON GEO-STATIONARY SATELLITES
1. INFRA-RED IMAGING
SENSORS AND INSTRUMENTS
ON GEO STATIONARY
SATELLITES
AASHEESH TANDON AND PRIYANKA GOSWAMI
INSTITUTE OF TECHNOLOGY
NIRMA UNIVERSITY
2. INTRODUCTION
• Infrared (IR) Imaging from Satellites is an advanced field of space
science and technology
• The first satellite with thermal imaging capability was by
NOAA/NASA in 1970’s
• Images are generated by emitted IR radiation from the Earth surface
• Satellite Thermal imaging systems exploit MWIR (3-5um) and LWIR
(8-14um)
• Applications includeWeather Prediction, Global climate studies,
Cyclone monitoring and warning, Earth resource studies, detection of
forest fires, and many more applications
3. GEOSTATIONARY SATELLITES (GEO-S) FOR IR
IMAGING
• GEO-S placed in geostationary Earth orbit
or geosynchronous equatorial orbit (GEO) circular orbit 35,786
kilometers above the Earth's equator and following the direction of
the Earth's rotation
• GEO-S Imaging sensors can see 1/3 of the earth sphere only, centered
with the satellite location
• USA, Europe, India, Japan, China have deployed operational GEO-S
for earth imaging, mainly for meteorological applications
4. IR IMAGING SENSOR TYPES
A. Imager/Radiometer
• Imagers provide images for the entire Earth disc or a part of it.
• They have much wider bands, and fewer imaging channels
• Resolutions are 500mts-2.5km
• Imaging intervals can be 1min to 20 min.
• Generally 48 or more images can be generated and transmitted per
day
5. IR IMAGING SENSOR TYPES
B. Sounder
• Special type of multi band IR sensor.
• Related with emission or absorption of EM/IR radiation of
atmospheric water vapor, CO2, Ozone.
• Sounder instruments have resolutions of 4-10km
• Provide up to 6-8 images per day.
6. CONSTITUENTS OF A SATELLITE IR IMAGING
SENSOR
A.Telescope and Scanning systems
• Telescope configuration: Cassegrain or RC type
• Focal length that covers the Earth disc and its surrounding space
• The primary mirror can be “scooped” to make it light-weighted
• “Scan Mirror” is required to be placed before the telescope
7. CONSTITUENTS OF A SATELLITE IR IMAGING
SENSOR
B. Pre – detector optical system
• Several IR imaging detectors are required to cover the different IR
bands, to provide the images of features on Earth
• “Beam splitters” are placed, to only transmit that part of the IR
radiation required for a specific detector
• e.g. Gold di-chorics are used with different thickness to split 8-14um
bands and glass filters are also used for 2.5-14 um radiation bands
8. CONSTITUENTS OF A SATELLITE IR IMAGING
SENSOR
C. Electronic IR Detectors and Cryogenic coolers
• There are two basic types of photon (radiative energy)
detectors:
1. Majority (electron): detector is photoconductive
2. Minority (hole): detector is both photoconductive and
photovoltaic
• Most GEO satellites with IR imaging use cryogenic cooled
HgCdTe (photoconductive) detector
• IR detectors being photoconductive, the performance is
governed by Planck’s radiation law, Stephan-Boltzzman
black body law, and ElectronicWork Function law.
• The detector elements have to be operated at the lowest
possible cryogenic temperatures to obtain meaningful
electronic signal
9. THERMAL CONTROL SYSTEM
• This system is cardinal to achieve optimum performance and
it ensures efficient equipment operation
• The generated heat is transferred by TCS from the
spacecraft or space station to the space.
• The TCS system has two subsystems – a Passive Thermal
Control System (PTCS) and an Active Thermal Control
System (ATCS).
• The cooling mechanism in GEO satellites can be of two
types:
1. Active, using cryogenic electro-mechanical coolers (sterling cycle)
2. Passive, using radiant cooler
10. SATELLITE COMMUNICATION
• Line of Sight
Communication
• Uplink Station on ground ->
Signal Processing -> Data
Transmission -> Modified
Signal Received at the
Satellite -> Signal
Processing ->
Retransmission of Signal
back to Earth -> Ground
Equipment Receives the
Signal
11. DATA TRANSMISSION
• Analog Signal from Sensor -> Pre-processing ->ADC -> Serial
Stream of Data -> Modulation ->Transmission to Earth
• Data Processing Software Solutions:TeraScan, Geomatica,
Idrisi, GRASS GIS
• Different product services include data acquisition, sensor
conversion, visualization, algorithm generation, data export,
environmental monitoring, data cataloguing, etc.
12. SENSOR DETECTOR ASSEMBLY (CASE STUDY)
A.Visible Infrared Imaging Radiometer Suite (VIIRS) [Raytheon]
• Inspecting radiometer – accumulatesVisible and IR imagery, and
radiometric measurements of land, atmosphere, oceans and,
cryosphere.
• 22 band optomechanical radiometer - uses a cross track rotating
telescope fore optics design.
• Spectral range from 0.41 to 12.5 microns.
• The observation scene is imaged onto 3 focal planes, splitting the
VNIR, SWIR, MWIR, andTIR energy.
13. SENSOR DETECTOR ASSEMBLY (CASE STUDY)
B.Thermal Infrared Sensor (TIRS)
• Mounted on in the Landsat 8 [formerly LDCM] mission by NASA
• Collects image data over a 185 km swath for two narrow spectral
thermal bands with 100 meters resolution.
• A push broom type sensor – Uses focal plane with long arrays of
photo-sensitive detectors.
• A refractive telescope having four elements and focuses a f/1.6
beam of thermal radiation onto a cooled focal plane.
• Two sate cryocooler is employed to let detectors function at the
required temperature of 43K.