Lecture 5: Sensors and Orbits

Professor Menglin Jin

San Jose State University

Sensor types (classification) in the following two diagrams

•Most remote sensing instruments (sensors) are designed to measure photons

•we concentrate the discussion on optical-mechanical-electronic radiometers and scanners, leaving the subjects of camera-film systems and active radar for consideration elsewhere

Non-Photographic Sensor Systems

• 1800 Discovery of the IR spectral region by Sir William Herschel. • 1879 Use of the bolometer by Langley to make temperature measurements of

electrical objects. • 1889 Hertz demonstrated reflection of radio waves from solid objects. • 1916 Aircraft tracked in flight by Hoffman using thermopiles to detect heat

effects. • 1930 Both British and Germans work on systems to locate airplanes from their

thermal patterns at night. • 1940 Development of incoherent radar systems by the British and United States

to detect and track aircraft and ships during W.W.II. • 1950's Extensive studies of IR systems at University of Michigan and elsewhere.

1951 First concepts of a moving coherent radar system. • 1953 Flight of an X-band coherent radar. • 1954 Formulation of synthetic aperture concept (SAR) in radar. • 1950's Research development of SLAR and SAR systems by Motorola, Philco,

Goodyear, Raytheon, and others. • 1956 Kozyrev originated Frauenhofer Line Discrimination concept. • 1960's Development of various detectors which allowed building of imaging and

non-imaging radiometers, scanners, spectrometers and polarimeters. • 1968 Description of UV nitrogen gas laser system to simulate luminescence.

Passive and Active Sensors

• Passive Sensor:

energy leading to radiation received comes from an external source, e.g., the Sun

• Active Sensor

energy generated from within the sensor system is beamed outward, and the fraction returned is measured; radar is an example

Imaging and non-imaging sensor

• Non-imaging:measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance

• Imagingthe electrons released are used to excite or ionize a substance like silver (Ag) in film or to drive an image producing device like a TV or computer monitor or a cathode ray tube or oscilloscope or a battery of electronic detectors

Principal: photoelectric effect • There will be an emission of negative particles (electrons) when a

negatively charged plate of some appropriate light-sensitive material is subjected to a beam of photons. The electrons can then be made to flow as a current from the plate, are collected, and then counted as a signal

• Albert Einstein’s experiment (see lecture 2) • Thus, changes in the electric current can be used to measure

changes in the photons (numbers; intensity) that strike the plate (detector) during a given time interval.

• The kinetic energy of the released photoelectrons varies with frequency (or wavelength) of the impinging radiation

• different materials undergo photoelectric effect release of electrons over different wavelength intervals; each has a threshold wavelength at which the phenomenon begins and a longer wavelength at which it ceases.

two broadest classes of sensors

• Passive sensorenergy leading to radiation received comes

from an external source, e.g., the Sun

• Active Sensor energy generated from within the sensor

system is beamed outward, and the fraction returned is measured

Example: radar

• Radiometer is a general term for any instrument that quantitatively measures the EM radiation in some interval of the EM spectrum

• spectrometer When the radiation is light from the narrow spectral band including the visible, the term photometer can be substituted. If the sensor includes a component, such as a prism or diffraction grating, that can break radiation extending over a part of the spectrum into discrete wavelengths and disperse (or separate) them at different angles to an array of detectors

•spectroradiometer The term spectroradiometer is reserved for sensors that collect the dispersed radiation in bands rather than discrete wavelengths

•Most air/space sensors are spectroradiometers.

Moving further down the classification tree, the optical setup for imaging sensors will be either an image plane or an object plane set up depending on where lens is before the photon rays are converged (focused), as shown in this illustration.

Field of View (FOV)

• Sensors that instantaneously measure radiation coming from the entire scene at once are called framing systems. The eye, a photo camera, and a TV vidicon belong to this group. The size of the scene that is framed is determined by the apertures and optics in the system that define the field of view, or FOV

Scanning System

• If the scene is sensed point by point (equivalent to small areas within the scene) along successive lines over a finite time, this mode of measurement makes up a scanning system. Most non-camera sensors operating from moving platforms image the scene by scanning

Cross-Track Scannerthe Whiskbroom Scanning

A general scheme of a typical Cross-Track Scanner

Essential Components of Cross-track Sensor

• 1) a light gathering telescope that defines the scene dimensions at any moment (not shown)

• 2) appropriate optics (e.g., lens) within the light path train • 3) a mirror (on aircraft scanners this may completely rotate; on spacecraft

scanners this usually oscillates over small angles) • 4) a device (spectroscope; spectral diffraction grating; band filters) to break

the incoming radiation into spectral intervals • 5) a means to direct the light so dispersed onto an array or bank of

detectors • 6) an electronic means to sample the photo-electric effect at each detector

and to then reset the detector to a base state to receive the next incoming light packet, resulting in a signal stream that relates to changes in light values coming from the ground targets as the sensor passes over the scene

• 7) a recording component that either reads the signal as an analog current that changes over time or converts the signal (usually onboard) to a succession of digital numbers, either being sent back to a ground station

Note: most are shared with Along Track systems

pixel The cells are sensed one after another along the line. In the sensor, each cell is associated with a pixel that is tied to a microelectronic detector

Pixel is a short abbreviation for Picture Element

a pixel being a single point in a graphic image

Each pixel is characterized by some single value of radiation (e.g., reflectance) impinging on a detector that is converted by the photoelectric effect into electrons

• NASA, Terra & Aqua– launched 1999, 2002– 705 km polar orbits, descending (10:30

a.m.) & ascending (1:30 p.m.)• Sensor Characteristics

– 36 spectral bands (490 detectors) ranging from 0.41 to 14.39 µm

– Two-sided paddle wheel scan mirror with 2330 km swath width

– Spatial resolutions:• 250 m (bands 1 - 2)• 500 m (bands 3 - 7)• 1000 m (bands 8 - 36)

– 2% reflectance calibration accuracy– onboard solar diffuser & solar diffuser

stability monitor– 12 bit dynamic range (0-4095)

MODerate-resolution Imaging Spectroradiometer (MODIS)

MODIS Onboard Calibrators

Fold Mirror

Space View Port

Blackbody

Spectral Radiometric Calibration Assembly

Nadir (+z)

Solar Diffuser

Scan Mirror

MODIS Optical System

Visible Focal Plane

Trac

k

Scan

SWIR/MWIR Focal Plane

NIRFocal Plane

LWIRFocal Plane

Shortwave IR/Midwave IRVisible

Longwave InfraredNear-infrared

Four MODIS Focal Planes

MODIS Cross-Track Scan on Terra

MODIS_Swath

MISR_Swath

Along-track Scannerpushbroom scanning

the scanner does not have a mirror looking off at varying angles. Instead there is a line of small sensitive detectors stacked side by side, each having some tiny dimension on its plate surface; these may number several thousand

Along-track, or Pushbroom, Multispectral System Operation

Multi-angle Imaging SpectroRadiometer (MISR)

• NASA, EOS Terra– Launched in 1999– polar, descending orbit of 705 km,

10:30 a.m. crossing• Sensor Characteristics

– uses nine CCD-based push-broom cameras viewing nadir and fore & aft to 70.5°

– four spectral bands for each camera (36 channels), at 446, 558, 672, & 866 nm

– resolutions of 275 m, 550 m, or 1.1 km

• Advantages– high spectral stability– 9 viewing angles helps determine

aerosol by µ dependence (fixed )

MISR Pushbroom Scanner• Orbital characteristics

– 400 km swath– 9 day global coverage– 7 min to observe each scene at

all 9 look angles

• Family portrait– 9 MISR cameras– 1 AirMISR camera

MISR Provides New Angle on Haze

• In this MISR view spanning from Lake Ontario to Georgia, the increasingly oblique view angles reveal a pall of haze over the Appalachian Mountains

spectral resolution

• The radiation - normally visible and/or Near and Short Wave IR, and/or thermal emissive in nature - must then be broken into spectral intervals, i.e., into broad to narrow bands. The width in wavelength units of a band or channel is defined by the instrument's spectral resolution

• The spectral resolution achieved by a sensor depends on the number of bands, their bandwidths, and their locations within the EM spectrum

Spectral filters Absorption and Interference. Absorption filters pass only a limited range of radiation wavelengths, absorbing radiation outside this range. Interference filters reflect radiation at wavelengths lower and higher than the interval they transmit. Each type may be either a broad or a narrow bandpass filters. This is a graph distinguishing the two types.

Orbits

The Afternoon ConstellationThe Afternoon Constellation“A-Train“A-Train””

The Afternoon constellation consists of 7 U.S. and international Earth Science satellites that fly within approximately 30 minutes of each other to enable coordinated science

The joint measurements provide an unprecedented sensor system for Earth observations

Video

• Terra_orbit

Satellite Orbits

• At what location is the satellite looking?

• When is the satellite looking at a given location?

• How often is the satellite looking at a given location?

• At what angle is the satellite viewing a given location?

Video: EOS orbits

Low Earth Orbit Concepts

Equator

South Pole

Ground track

Ascending node

Inclination angle

Descending node

Orbit

Perigee

Apogee

Orbit

Sun-Synchronous Polar Orbit

Satellite

Orbit

Earth Revoluti

on

• Satellite orbit precesses (retrograde)– 360° in one year

• Maintains equatorial illumination angle constant throughout the year– ~10:30 AM in this example

Equatorial illuminatio

n angle

Sun-Synchronous Orbit of Terra

Period of orbit

• Valid only for circular orbits (but a good approximation for most satellites)

• Radius is measured from the center of the Earth (satellite altitude+Earth’s radius)

• Accurate periods of elliptical orbits can be determined with Kepler’s Equation

T2= r342

Gme

Period of orbit

Gravitational constant Mass of the Earth

Radius of the orbit

The orbital period of a satellite around a planet is given by

where 0 = orbital period (sec)

Rp = planet radius (6380 km for Earth)

H = orbit altitude above planet’s surface (km)

gs = acceleration due to gravity (0.00981 km s-2 for Earth)

Definition of Orbital Period of a Satellite

T0 2(Rp H )Rp H

gs Rp2

Spacing Between Adjacent Landsat 5 or 7 Orbit Tracks at the Equator

Timing of Adjacent Landsat 5 or 7 Coverage Tracks

Adjacent swaths are imaged 7 days apart

Types of orbits

• Sunsynchronous orbits: An orbit in which the satellite passes every location at the same time each day– Noon satellites: pass over near noon and midnight– Morning satellites: pass over near dawn and dusk– Often referred to as “polar orbiters” because of the

high latitudes they cross– Usually orbit within several hundred to a few

thousand km from Earth

Sun Synchronous (Near Polar)

• Video –TERRA/AQUA

Sunsynchronous image (SMMR)

Sunsynchronous image (AVHRR)

Types of orbits

• Geostationary (geosynchronous) orbits: An orbit which places the satellite above the same location at all times– Must be orbiting approximately 36,000 km

above the Earth– Satellite can only “see” part of hemisphere

Geostationary Image (GOES-8)

Space-time sampling

• Geostationary– Fixed (relatively) field of view– View area of about 42% of Earth’s surface

• Sunsynchronous– Overlapping views– See each point at several viewing angles

• Other orbits (“walking orbits”)– Passes each location at a different time of day– Earth Radiation Budget Satellite– Useful when dirunal information is needed

A precessing low-inclination (35°), low-altitude (350 km) orbit to achieve high spatial resolution and capture the diurnal variation of tropical rainfall

– Raised to 402 km in August

2001

Tropical Rainfall Measuring Mission Orbit (Precessing)

TRMM Coverage

1 day coverage 2 day coverage

Landsat 7 Goals & ObjectivesLand use and land cover change

– Agricultural evaluations, forest management inventories, water resource estimates, coastal zone appraisals

– Growth patterns of urban development, Spring run-off contaminants in lakes, land use in tropical rainforests, health of temperate conical forests

Vegetation patterns– Annual cycle of vegetation dynamics, drought stress, and

floodingGlaciers and snow cover

– Growth and retreatGeological surveys

– Volcanic hazardsLaunched April 15, 1999

Enhanced Thematic Mapper Plus (ETM+)

• NASA & USGS, Landsat 7– launched April 15, 1999– 705 km polar orbit, descending

(10:00 a.m.)• Sensor Characteristics

– 7 spectral bands ranging from 0.48 to 11.5 µm

– 1 panchromatic band (0.5-0.9 µm)

– cross-track scan mirror with 185 km swath width

– Spatial resolutions:• 15 m (panchromatic)• 30 m (spectral)

– Calibration:• 5% reflectance accuracy• 1% thermal IR accuracy• onboard lamps, blackbody,

and shutter• solar diffuser

Landsat Thematic Mapper Bands

• Landsat collects monochrome images in each band by measuring radiance & reflectance in each channel

– When viewed individually, these images appear as shades of gray

TRMM Satellite

1998-2005 Mean Monthly Rainfall (5°x5°)

Polar-Orbiting Satellite in Low Earth Orbit (LEO)

Example from Aqua

SatelliteAltitude

(km)Inclination

(°)Orbital Period

(min)Repeat

Coverage Orbits/dayJason-1 1336 66 112.3 10 12.8Meteor-3M/SAGE III 1020 99.5 105.5 13.7Landsat 1-3 907-915 99.2 103.1 18 14.0SPOT 832 98.7 101.5 26 14.2NOAA 850 98-99 102-104 11 14.0QuikScat 803 98.6 100.9 14.3ACRIMSAT 720 98.1 99.1 14.5Landsat 4-7 705 98.2 98.8 16 14.6Terra, Aqua, Aura 705 98.2 98.8 16 14.6

ICESat 600 94 96.6 – 14.9UARS 585 57 96.3 – 14.9ERBS 610 57 96.8 – 14.9SORCE 640 40 97.5 – 14.8TRMM 402 35 92.6 – 15.6TRMM 350 35 91.5 – 15.7

Orbital Characteristics of Selected MissionsLow Earth Orbit & Precessing Missions

Earth Science Mission ProfileEarth Science Mission Profile1997-20031997-2003

eospso.gsfc.nasa.gov

Earth Science Mission ProfileEarth Science Mission Profile2004-20102004-2010

eospso.gsfc.nasa.gov

The Afternoon ConstellationThe Afternoon Constellation“A-Train“A-Train””

The Afternoon constellation consists of 7 U.S. and international Earth Science satellites that fly within approximately 30 minutes of each other to enable coordinated science

The joint measurements provide an unprecedented sensor system for Earth observations

Satellites in Geosynchronous Orbits are used as Relay Satellites for LEO

SpacecraftImaging

System (e.g., Landsat)

Communication relay system

Communication relay

system (e.g., TDRSS)

GEO

LEOGround station

SectorSatellites in Orbit

(+mode) Operator LocationLaunch date Status

MTSAT-1R (Op) Japan 140°E 2/26/05 Fully functionalMTSAT-2 (B) Japan 145°E 2/18/06 Back-up to MTSAT-1RGOES-9 (B) USA/NOAA 160°E 5/99 Dissemination not

activatedEast-Pacific GOES-11 (Op) USA/NOAA 135°E 5/00 GOES-West

GOES-10 (B) USA/NOAA 60°W 4/97 South America coverageGOES-12 (Op) USA/NOAA 75°W 7/01 GOES-EastGOES-13 (P) USA/NOAA 89.5°W 5/06 In commissioning

Meteosat-6 (B)EUMETSAT 10°E 11/93 Rapid scan anomalyMeteosat-7 (B)EUMETSAT 0°E 2/97 To be relocated to 57.5°E

Meteosat-8 (Op)EUMETSAT 3.4°W 8/28/02 EUMETCASTMeteosat-9 (P)EUMETSAT 6.5°W 12/21/05 In commissioning

Meteosat-5 (Op)EUMETSAT 63°E 3/91 Functional but high inclination mode

GOMS-N1 (B) Russia 76°E 11/94 Standby since 9/98FY-2C (Op) China/CMA 105°E 10/19/04 Functional

Kalpana-1 (Op) India 74°E 9/12/02 DedicatedINSAT-3A (Op) India 93.5°E 4/10/03 Operational

West-Pacific

West-Atlantic

East-Atlantic

Indian Ocean

Geosynchronous Meteorological Satellites

WMO Member States

QuickTime™ and aH.264 decompressor

are needed to see this picture.

Hurricane WilmaOctober 2005

Sample Calibration Curve Used to Correlate Scanner Output with Radiant

Temperature Measured by a Radiometer

• The human eye is not sensitive to ultraviolet or infrared light–To build a composite

image from remote sensing data that makes sense to our eyes, we must use colors from the visible portion of the EM spectrum—red, green, and blue

Color Composites

Chesapeake & Delaware BaysR =0.66 µmG =0.56 µmB =0.48 µm Baltimore

Washington

May 28, 1999

“False Color” Composite Image• To interpret radiance measurements in the infrared portion of the electromagnetic

spectrum, we assign colors to the bands of interest and then combine them into a “false color” composite image

Terra

ASTER

Launched December 18, 1999

MODIS

CERESMISR

MOPITT

• NASA & MITI, Terra

– 705 km polar orbit, descending (10:30 a.m.)

• Sensor Characteristics

– 14 spectral bands ranging from 0.56 to 11.3 µm

– 3 tiltable subsystems for acquiring stereoscopic imagery over a swath width of 60 km

– Spatial resolutions:

• 15 m (bands 1, 2, 3N, 3B)

• 30 m (bands 4 - 9)

• 90 m (bands 10 - 14)

– 4% reflectance calibration accuracy (VNIR & SWIR)

– 2 K brightness temperature accuracy (240-370 K)

Advanced Spaceborne Thermal Emission & Reflection Radiometer

(ASTER)

SWIR

VNIR (1,2,3N)

VNIR (3B) TIR

Wavelength RegionBand No. Spectral Range

(µm)Band No. Spectral Range

(µm)VNIR 1 0.45-0.52

1 0.52-0.60 2 0.52-0.602 0.63-0.69 3 0.63-0.693 0.76-0.86 4 0.76-0.90

SWIR 4 1.60-1.70 5 1.55-1.755 2.145-2.185 7 2.08-2.356 2.185-2.2257 2.235-2.2858 2.295-2.3659 2.360-2.430

TIR 10 8.125-8.475 6 10.4-12.511 8.475-8.82512 8.925-9.27513 10.25-10.9514 10.95-11.65

Terra/ASTER Landsat 7/ETM+

Comparison of Landsat 7 and ASTER

Synergy Between Terra and Landsat 7 DataSynergy Between Terra and Landsat 7 Data(same day 705 km orbits ~ 30 minutes apart)(same day 705 km orbits ~ 30 minutes apart)

spatial resolution (275, 550, 1100 m)

Landsat ETM+ input to Terra data• Vegetation classification for MODIS & MISR biophysical products• Focus on global change hotspots detected by MODIS & MISR• Linking Terra observations with 34+ year Landsat archive• Radiometric rectification of MODIS data

183 km

2330 km swath widthspatial resolution (250, 500, 1000 m) global coverage⇒2 days

360 km global coverage⇒9 days

spatial resolution (15, 30, 60 m)Landsat 7 16 day orbital repeatglobal coverage⇒seasonally

spatial resolution (15, 30, 90 m)ASTER 45-60 day orbital repeatglobal coverage⇒months to years

60 km swath

MODIS

MISR

Terra input to Landsat ETM+ data• Use of MODIS & MISR for improved atmospheric correction of ETM+• Use of MODIS & MISR for temporal interpolation of ETM+ data• Cross-calibration of ASTER, MISR, and MODIS

AquaLaunched May 4, 2002

MODIS

CERESAIRS

AMSR-E

AMSU

HSB

• NASA, Aqua– launched May 4, 2002– 705 km polar orbits, ascending

(1:30 p.m.)• Sensor Characteristics

– 12 channel microwave radiometer with 6 frequencies from 6.9 to 89.0 GHz with both vertical and horizontal polarization

– Conical scan mirror with 55° incident angle at Earth’s surface

– Spatial resolutions:• 6 x 4 km (89.0 GHz)• 75 x 43 km (6.9 GHz)

– External cold load reflector and a warm load for calibration

• 1 K Tb accuracy

Advanced Microwave Scanning Radiometer (AMSR-E)

AMSR-E Conical Scan on Aqua

AMSR-E Composite Sea Surface Temperature

June 2002

-2

28

°C35

Orange colors denote temperature necessary for hurricane formation

Satellite online visualization (class Activity)

• Satellite rainfall observations are very useful to reveal the rain intensity and spatial distribution over the globe. Tropical rainfall measurement mission (TRMM) is one NASA program to monitor rainfall from the space bake to 1998. Use the Monthly TRMM and Other Data Sources Rainfall Estimate (3B43 V6) (http://disc2.nascom.nasa.gov/Giovanni/tovas/TRMM_V6.3B43.shtml), to answer the following questions:

– Plot spatial distribution of rainfall at CA area (25-40°N, 110-125°W) using data from May 1998 to May 2009. Where do you see the highest rainfall in this area? How much there?

– Plot the time series of accumulated rainfall for the same CA area above during the same time. Which month does CA have the highest rainfall and which month CA have the lowest rainfall? How much are the highest and lowest rainfall respectively?

– Plot the rainfall over the globe spatial distribution (180°W-180°E, 50°N-50°S) for July 2008 and December 2008, respectively. Describe at least three major differences of the rainfall pattern of these two months.