Passive Microwave Sensors LIDAR Remote Sensing Laser Altimetry 28 April 2003
Outline Passive Microwave Radiometry Rayleigh-Jeans approximation Brightness temperature Emissivity and dielectric constant Sensors and applications LIDAR Laser Altimetry
Passive Microwave Radiometry Microwave region: 1-200 GHz (0.15-30cm) Uses the same principles as thermal remote sensing Multi-frequency sensing Weak energy source so need large IFOV and wide bands
At long wavelengths, such as in the microwave region, the relationship between spectral emittance and wavelength can be approximated.
Rayleigh-Jeans Approximation L l = e 2kcT l 4 spectral radiance is a linear function of kinetic temperature k is Planck s constant, c is the speed of light, e is emissivity, T is kinetic temperature This approximation only holds for l >> peak (e.g. l > 2.57mm @300 K)
Microwave Brightness Temperature Microwave radiometers can measure the spectral radiance received (L l ) at an antenna This is called the brightness temperature and is linearly related to the kinetic temperature of the surface The Rayleigh-Jeans approximation provides a simple linear relationship between measured spectral radiance temperature and emissivity
Brightness temperature can be related to kinetic temperature through emissivity T b = et kin Thus, passive microwave brightness temperatures can be used to monitor temperature as well as properties related to emissivity
Monitoring Temperatures with Passive Microwave Sea surface temperature Land surface temperature
Passive Microwave Sensing of Land Surface Emissivity Differences Microwave emissivity is also a function of dielectric constant Dielectric constant is related to electrical conductivity of the material (intrinsic property) Most earth materials have a dielectric constant in the range of 1 to 4 (air=1, veg=3, ice=3.2) Dielectric constant of liquid water is 80 Thus, moisture content affects brightness temperature
Snow Emissivity Example T b dry snow (2) Soil Dry Snow snow water equivalent Wet Snow (1) Soil (3) Soil Wet snow is a strong absorber/emitter
SSM/I Northern Hemisphere snow water equivalent (mm of water)
Atmospheric Effects Atmospheric attenuation increases with frequency from 1 to 1000 GHz At frequencies less than 50 GHz, there s little effect of clouds and fog on brightness temperature (it sees through clouds) Thus, PM can be used to monitor the land surface under cloudy conditions In atmospheric absorption bands, PM is used to map water vapor, rain rates, clouds
Atmospheric Mapping Mapping global water vapor (85 GHz)
Passive Microwave Radiometry Passive microwave sensors use an antenna to detect photons which are then converted to voltages in a circuit Scanning microwave radiometers mechanical rotation of antenna or platform phased array of smaller antennas (signals are advanced by electronic steering,so that radiation arriving from a particular direction is combined in phase)
Scanning Microwave Radiometers Scanning Multichannel Microwave Radiometer (SMMR) 1981-1987 Special Sensor Microwave/Imager (SSM/I) 1987- present Tropical Rainfall Measuring Mission (TRMM) 1997-present Advanced Microwave Scanning Radiometer (AMSR) 2002-present
Special Sensor Microwave/Imager (SSM/I) Near-polar, sun synchronous orbit, 98.8 o inclination orbital altitude of 860 km
SSM/I scanning geometry
Passive Microwave Applications Soil moisture Snow water equivalent Sea/lake ice extent, concentration and type Sea surface temperature Atmospheric water vapor Surface wind speed Cloud liquid water Rainfall rate only over the oceans
LIDAR and Laser Altimetry LIDAR = Light detection and ranging Based on transmission and measurement of laser light (typically green or NIR) Return power is indicative of reflectance of the surface Shape of return pulse provides estimate of vegetation canopy structure and density Timing of return pulse gives information on surface elevation
LIDAR Laser + Receiving System = LIDAR Laser radar depends on knowing the speed of light Distance = (Speed of light x Time) / 2
Waveform Information
Airborne LIDAR sensors Laser Vegetation Imaging Sensor (LVIS) Scanning LIDAR imager of Canopies by Echo Recovery (SLICER) Airborne Topographic Mapper (ATM) rotating mirror is used for coverage
ICESat NASA s Ice, Cloud, Elevation Satellite (ICESat) Launched 13 Jan 2003 Primary mission is to map ice sheet elevation and changes over the next 3-5 years Instrument is called GLAS
Geosciences Laser Altimeter System (GLAS) Frequency-doubled laser system: 1064 nm (NIR) laser wavelength is used for altimetry 532 nm (green) laser wavelength is used for atmospheric characterization (aerosols) 40 pulses per second are transmitted 70m diameter ground footprint, 175m spacing Returned photons are collected through a 1- meter diameter telescope Return time, return power, and waveform provide important information about the surface
ICESat Orbit near polar orbit, 94 o inclination 600 km altitude On-board star cameras and gyros provide spacecraft and laser pointing direction. GPS provide spacecraft location
Tentative: Vegetation Canopy Lidar (VCL) 1-meter accuracy