ATS 351 Lecture 8 Satellites

Electromagnetic Waves

Electromagnetic Waves

• Consist of an electric field and a magnetic field

• Polarization: describes the orientation of the electric field.

Remote Sensing

• Passive vs Active sensors– Active sensors emit radiation and measure what

is sent back to them• Radars are active sensors.

– Passive sensors detect naturally occurring radiation

• Most satellites are passive sensors.

Satellite Basics• Satellites are in many

different types of orbits.– Geosynchronous (36,000 km)

– LEO (300-1000 km)

– MEO (10,000 km)

– Polar (LEO orbit)

– Sunsynchronous

– Molinya

– Constellations

• Ascending vs Descending Nodes

Satellite Basics

How many times a day a particular point is seen by a sunsynchronous satellite

Satellite Basics

Satellite Formations fly multiple satellites in the same orbit.

Satellite constellations fly multiple satellites in varying orbits to provide maximum earth coverage.

Satellite Basics• Most satellites carry

multiple sensors– Saves money

• Instrument design• Instrument launch• Multiple funders

– Allows many earth/atmosphere parameters to be observed at one time

Satellite Basics• Current weather satellites include radiometers, radars, and

sounders– Radiometers sense emitted radiation– Radars sense reflectivity in the vertical rather than horizontal– Sounders sense profiles of radiance from many layers of the

atmosphere.

• Radiometers sense emitted radiation in the infrared and microwave

• Brightness temperature (Tb) – The temperature of an equivalent blackbody emitting radiation.– Infrared and microwave sensors measure the emitted radiation and

report it as a brightness temperature– Insert LOTS of complicated math here.– Tb = Ts

Emissivity

• Emissivity – amount of absorbed radiation that is emitted by a body– Has values between 0 and 1

– Ocean has an emissivity of about 0.5

– Land and cloud cover have an emissivity > 0.8

– Over land, emissivity is dependent on soil moisture and vegetation cover, and is very difficult to determine.

– Over ocean, emissivity is dependent on wind speed and sea surface temperature

Retrievals• A retrieval is a method by which we infer information about

the atmosphere based on measured brightness temperatures• Retrievals can be used to infer

– Cloud liquid water– Water vapor content– Concentrations of atmospheric gases (CO, CH4, CO2)– Land surface properties (Soil Moisture, vegetation)

• If y is the observed brightness temperature, x is the state of the Earth/Atmosphere system, and f is a forward model that predicts y based on x:

y = f(x) x = f-1(y)

What is x?

atmosphere

surface

(So/4) (1-α) ơ TA4

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Visible Satellite Imagery• Visible satellites basically take pictures of the

earth.• Most visible satellites are in geosynchronous orbit. • In the US, we use the GOES satellites.

– GOES: Geostationary Operational Environmental Satellites

– GOES satellites carry 5 sensors• Visible (.6 microns)• Shortwave IR (3.9 microns)• Water Vapor (6.7 microns)• Infrared (11 microns)• Infrared (12 microns)

GOES West GOES East

• http://rammb.cira.colostate.edu/ramsdis/online/goes-west_goes-east.asp

Visible Satellite Imagery

• Advantages– Always located in the same place (homogeneous

sampling)– View at an angle to give some information about

vertical development of clouds– Can be used to monitor snow cover– High spatial resolution (GOES Vis: 1km)

• Disadvantages– Cannot be used at night– Views of the poles are limited due to the viewing angle– Not very useful for retrievals

Infrared Satellite Imagery• Displays the brightness temperature calculated

based on the amount of sensed infrared radiation.

• Useful for determining whether clouds are convective or stratiform.

• Land surfaces appear warmer than cloud.• Land surface emissivity is very close to 1 in

the infrared.

• http://rammb.cira.colostate.edu/ramsdis/online/goes-west_goes-east.asp

Infrared Satellite Imagery

• Advantages– Can infer cloud heights by temperature– Can be used at night– Can be combined with visible imagery to infer

cloud types– Can be color enhanced – Can be used in retrievals

• http://rammb.cira.colostate.edu/ramsdis/online/goes-west_goes-east.asp

Some satellites using IR frequencies

• MODIS (MODerate resolution Infrared Spectroradiometer)

• AIRS (Atmospheric Infrared Sounder)• GOES (Geostationary Operational

Environmental Satellite)• AVHRR (Advanced Very High Resolution

Radiometer)• MOPITT (Measurements Of Pollution in

The Troposphere)

Southwest Asian Dust Storm image from MODIS at 500m resolution

AIRS Retrieved Temperature Isotherms over Southern Europe viewed from the west, September 8, 2002

Water Vapor Imagery

• Retrieved water vapor based on sensed emissivity

• Shows where significant weather systems are located

• Uses infrared wavelengths– Remember: water vapor is a strong absorber of

IR radiation.

Microwave Satellite Imagery

• Senses emitted radiation in the microwave frequencies

• Also frequently used in retrievals.• Land surface emissivity in the microwave

averages 0.95.• Most microwave sensors carry radiometers in

multiple frequencies– Allows for varying resolutions– Useful in retrievals

• Atmospheric water vapor absorbs at ~22 GHz

Microwave Sensors• AMSR-E (Advanced

Microwave Scanning Radiometer for EOS)

• TRMM (Tropical Rainfall Measuring Mission)– TMI (TRMM Microwave Imager)– TRMM-PR (Precipitation Radar)– VIRS (Visible and Infrared

Satellite)

• SSM/I – Special Sensor Microwave Imager

• AMSU – Advanced Microwave Sounding Unit

Cloudsat

• Cloud Profiling Radar

• 94 GHZ

• Measures the power backscattered by clouds as a function of distance from the radar

Where to find images

• http://virga.sfsu.edu/crws/archive/satimgs_month_arch.html (archived GOES imagery)

• http://trmm.gsfc.nasa.gov/ (extreme events link at bottom of page)

• http://modis.gsfc.nasa.gov/gallery/ (MODIS image gallery)

• http://daac.gsfc.nasa.gov/AIRS/gallery.shtml (AIRS image gallery)

• http://www.eorc.jaxa.jp/en/imgdata/latest/index.html (images from multiple sensors/events)