Remote Sensing I Summer Term 2013 Lecturers: Astrid Bracher ,

Remote Sensing I

Summer Term 2013

Lecturers:

Astrid Bracher,Mathias Palm and Christian Melsheimer

Contact:Prof. Dr. Astrid Bracher Dr. Mathias Palm Dr. Christian Melsheimer Office: U-3215 (NW 1) Office: U-3235 NW 1) Office: N-3371 (NW 1)Phone: 0421-218-62112 Phone: 0421-218-62179 Phone: 0421-218-62181Email: [email protected] [email protected] melsheimer@uni-

bremen.de

Photograph taken from ISS by Donald Pettit, Space Station Science Officer

OutlineLecture 1 Introduction & EM Radiation 04.04.2013 Bracher

Lecture 2 EMR II & Radiative Transfer 11.04.2013 Bracher

Lecture 3 Retrieval Techniques, Inverse Methods 18.04.2013 Palm

Lecture 4 Satellite Remote Sensing (RS) 25.04.2013 Bracher

Lecture 5 Spectroscopy 02.05.2013 Bracher

Lecture 6 Infra-red Techniques 16.05.2013 Palm

Lecture 7 UV-visible Atmospheric RS I 23.05.2013 Bracher

Lecture 8 UV-visible Atmospheric RS II 30.05.2013 Bracher

Lecture 9 Ocean Optics 06.06.2013 Bracher

Lecture 10 Ocean Color Remote Sensing 13.06.2013 Bracher

Lecture 11 Microwave RS 20.06.2013 Palm

Lecture 12 Sea Ice Remote Sensing 27.06.2013 Melsheimer

Lecture 13 Summary & Lab Tour 04.07.2013 Bracher/MW Group

Exam: 11 July 2013 10-12

Remote Sensing I, Bracher Palm Melsheimer 2

Lecture 4: Satellite Remote Sensing


Principle of Satellite Remote Sensing


ENVISAT: Launched 1 March 2002

Geostationary orbit• Circular orbit in the equatorial plane, altitude ~36,000km• Orbital period ~1 day , orbit matches Earth’s rotation

Advantages• See whole Earth disk at once due to large distance• See same spot on the surface all the time i.e. high temporal

coverage• Big advantage for weather monitoring satellites (knowing the

atmospheric dynamics is critical to short-term forecasting and numerical weather prediction - NWP)

DisadvantagesLow spatial resolution


Meteorological satellites:A combination of OES-E, GOES-W, METEOSAT (Eumetsat), GMS (NASDA), IODC (old Meteosat 5)

GOES 1st gen. (GOES-1 - ’75 GOES-7 ‘95); 2nd gen. (GOES-8++ ‘94)

Geostationary orbit


• METEOSAT - whole earth disk every 15 mins

Geostationary orbit


Orbital Disadvantages ofGEO

• typically low spatial resolution due to high altitude: e.g. METEOSAT 2nd Generation (MSG) 1 kmx1 km visible, 3 kmx3 km IR (used to be 3 x 3 & 6 x 6, respectively)

• spatial resolution at 60-70° several times lower• not much good beyond 70°- cannot see the poles very

well (orbit over equator)

Other geosynchronous orbits which are not GEO: same period as Earth, but not equatorial


Lower Earth Orbit (LEO):Polar & near polar orbits

Advantages• full polar orbit inclined 90° to equator• typically few degrees off, so poles not covered• orbital period, T, typically 90 – 110 min

– near circular orbit between 300 km and 1000 km (low Earth orbit) – typically higher spatial resolution than geostationary– rotation of Earth under satellite gives (potential) total coverage

• ground track repeat typically 14-16 days


Lower Earth Orbit (LEO)Ground track of SCIAMACHY (on ENVISAT with 98° inclination and 780 km orbit height) nadir at 1 day


Sun elevation at local noon

Sun elevation angle at local noon at the four seasons

21 Dec 21 Mar/22 Sep 21 Jun

Bremen, 53°N 14° 37.5° 61°

Delhi, 28°N 39° 63° 85°

Singapore, 1°N 65.5° 90° 67.5°

(over S horizon) (zenith) (over N horizon)


Lower Earth Orbit (LEO):Inclination (tropical) orbits

• orbit inclined >0° to <90° to equator• Determined by the region of Earth that is of most interest (e.g. low

inclination angle for tropics) • Orbital altitude typically a few hundreds km• Orbital period around a few hours• These satellites are not sun-synchronous view a place on Earth

at varying times


Orbital Disadvantages forLEO

• need to launch to precise altitude and orbital inclination• orbital decay at LEOs (Low Earth Orbits) < 1000 km

– drag from atmosphere causes orbit to become more eccentric– drag increases with increasing solar activity (sun spots)– ~ solar maximum (~11yr cycle) drag height increased by 100km!

Lower Earth Orbit (LEO)


Swath describes ground area imaged by instrument during overpass

Instrument’s Swath


Lower Earth Orbit (LEO)Ground track of SCIAMACHY (on ENVISAT with 98° inclination and 780 km orbit height) nadir at 1 day


ENVISAT: 1 March 2002 - 12 Apr 2012)SCIAMACHY UV/Vis/NIR grating spectrometers: 8 channels, 240 - 2380 nm Moderate spectral resolution: 0.2 –

1.5 nm Measurement Geometries :

nadir viewing+ limb + solar / lunar occultation

Polar, sun-synchronous orbit, 10:00 Global coverage in 6 days During eclipse calibration and limb

measurements Spectroscopy is used to derive trace

gas distributions in the troposphere, stratosphere and mesosphere


Overview of satellite observations geometries

Measured signal:Reflected and scattered sunlight(Thermal emission from Earth)

Measured signal:Directly transmitted solar radiation(Thermal emission from Earth)

Measured signal:Scattered solar radiation(Thermal emission from Earth)

Overview of satellite observations geometries


Swath describes ground area imaged by instrument during overpass

Instrument’s Swath


Broad Swath• MODIS, POLDER, AVHRR etc.

– swaths typically several 1000s of km– lower spatial resolution– Wide area coverage– Large overlap obtains many more view and

illumination angles (much better BRDF sampling)– Rapid repeat time

MODIS:• Note across-track “whiskbroom” type

scanning mechanism• swath width of 2330 km (250-1000m

resolution)• Hence, 1-2 day repeat cycle


Narrow Swath• Landsat TM/MSS/ETM+, IKONOS, QuickBird etc.

– swaths typically few 10s to 100s km– higher spatial resolution– local to regional coverage NOT global– far less overlap (particularly at lower latitudes)– May have to wait weeks/months for revisit

Landsat:• 185km swath width, hence 16-day repeat

cycle (and spatial res. 25m)• Contiguous swaths overlap (sidelap) by

7.3% at the equator• Much greater overlap at higher latitudes

(80% at 84°)


Narrow Swath: IKONOS & QuickBird - very local view!


• Single or multiple observations• How far apart are observations in time?

– One-off, several or many?• Depends (as usual) on application

– Is it dynamic?– If so, over what timescale?

• Examples– Vegetation stress monitoring, weather, rainfall

• hours to days– Terrestrial carbon, ocean surface temperature

• days to months to years– Glacier dynamics, ice sheet mass balance

• Months to decades

What temporal resolution chosen for measurements?


• Sensor orbit– geostationary orbit – good temporal sampling over

same spot: BUT due to large orbit height nearly the entire hemisphere can be viewed (e.g. METEOSAT)

– Near-polar orbit – less temporal sampling, but can use Earth rotation to view entire surface

• Sensor swath– Wide swath allows more rapid revisit

• typical are moderate resolution instruments for regional/global applications

– Narrow swath == longer revisit times• typical of higher resolution for regional to local applications

What determines the temporal sampling


• Coverage (hence spatial and/or temporal sampling) due to combination of orbit and swath– Mostly swath - many orbits nearly same

MODIS and Landsat have identical orbital characteristics:Inclination 98.2°, h=705 km, T = 99minsBUT swaths of 2400 km and 185 km, repeat of 1-2 days and 16

days, respectively– Most EO satellites typically near-polar orbits with

repeat tracks every 16 or so days– BUT wide swath instrument can view same spot

much more frequently than narrow• Tradeoffs again, as a function of objectives

Summary: spatial and temporal resolution


End of Lecture 4


Documents

Remote Sensing I Summer Term 2013 Lecturers: Astrid Bracher ,