Remote Sensing



Remote Sensing

: The quantitative and qualitative observation and interpretation of the interaction of some form of wave energy (electromagnetic or sound) with the atmosphere.  Sensors are categorized as: 

Active

: Sensor emits wave energy and detects a return of the emitted energy after interaction with atmospheric constituents. 

Passive

: Sensor only detects energy emitted, scattered, or reflected by atmospheric constituents.

 Remote Sensors are needed to provide space and time coverage of the atmosphere to the degree not capable by traditional (in situ) sensors.  Interactions of the signal with the atmosphere:  Spreading losses  Scattering losses  Reflection losses  Absorption losses

 Degree of loss depends to a great extent on the wavelength of the signal.



Acoustic waves

: • Interact strongly with gaseous portion of the atmosphere. • Weaker interaction with aerosols (unless in high concentration to significantly affect density). • Aerosols are suspensions of minute particles in the atmosphere.

• Density differences affect acoustic waves greatly.

• Have short ranges.



Electromagnetic waves

:  Weaker interaction (than acoustic) with gaseous portion in visible and radio wavelengths.

 Absorption of certain bands in visible and IR spectrum is important.

 Interact more strongly with aerosols (closer the size of the wavelength to the size of the aerosol - the stronger the interaction).



Weather Radar

Emits electromagnetic waves of a particular wavelength.

• Wavelength used is determined by the desired target.

 The shorter the wavelength, the greater the attenuation that occurs and the shorter the effective range.

 Receives returning reflected energy • Must be amplified  Orientation of antenna determines azimuth angle to target.

 Time of travel for emitted energy to return determines range from radar location.

 Intensity of returned energy determines strength (size or concentration) of target. Typical usable range is about 200 km for detecting precipitation in the troposphere.

• Radar signal travels in nearly a straight line.

• Earth curves away from the signal.

• Signal path affected by refractive properties of atmosphere.

– May produce anomalous propagation.

 Consider a 3 GHz, 10-cm radar wave with a pulse duration of 2 m s (0.000002 seconds).

 This microwave oscillates at 3,000,000,000 cycles per second (3,000 MHz).

 The radar listens for returning echoes for 0.000999 seconds before transmitting another pulse.

 The 2 m m pulse will contain 6000 cycles and will be 300 meters long (along its path of radiation).

 Consider that the beam is focused to 1 degree diameter.

 At a range of 50 km, the beam will spread to a diameter of 873 meters since:

S



R

f where: S = arc length, or width of spread,  So: R = radius, or distance along beam, f = angle of spread in radians

S

 50,000

m

 1

o

 2  360   = 872.67 meters  The pulse will reflect off a volume of raindrops (or other targets) that is about 873 meters in diameter with a depth of 150 meters (half of the 300 meter pulse length which is folded by reflection).

 In the case of precipitation, the radar illuminates a large number of individual targets. The average returned power from this volume of raindrops is where:

r



P t G

  2 3  2

r

4 

i



bi

P t  Peak transmittted power , G  Antenna gain ,   wavelength of transmitted energy r = range to targets 

b

= single target cross section  The summation is over the volume from which power is returned simultaneously.

 For spherical targets uniformly distributed over the volume illuminated with an incident wavelength,  , large compared to the radius, a, of the target, the radar back-scattering cross section,  b , of the volume is given byRayleigh’s law to be: 

b

 64   4 5

a

6  Returned power is strongly dependent on the radar back-scattering cross section which is dependent on the 6th power of the size of the raindrops.

 The returned power, intensity, is expressed in decibels of reflectivity.

 The returned power, intensity, is expressed in decibels of reflectivity.  20 - 30 dB: Weak, steady rain  30 - 40 dB: Showers  50 dB or more: Severe storms.  The intensity for each range interval moving outward from the radar (perhaps 1 kilometer in length) is stored in a bin. After a number of successive pulse transmissions (e.g., 32), the bin values are averaged, resulting in this for our example.

 Thus, each bin receives a single intensity value for all the precipitation falling within that volume, regardless whether some reflects high intensity and some reflects low intensity.

Doppler Radar

 Named after Christian Doppler, Austrian physicist who discovered the principle.



General Types

: 

Continuous

: Frequency shift between emitted signal and returning signal is a measure of the target radial velocity. 

Pulsed

: Measures phase shift between emitted pulse and returning signal • Phase shift is related to the radial velocity of the target. • Measures return time of pulse energy which gives range.

• Sorts velocities in range bins, averages, and displays velocities according to color/shades.

 If the emitted frequency is “ n ” and the wavelength is “  ” and “c” is the speed of light, then: n 

c



 When a target is in motion with radial velocity (towards or away from the radar) of n , the reflected signal has a frequency of n ’ given by: n ' 

c

  2 n where + means target is moving towards the radar, and - is target moving away.

 The difference in frequency, the Doppler shift, is:  n

D

 n '  n   2  n

 If n is expressed in knots, Hz, then:  n

D

  103  n  in cm, and n D in  The 144 o shift relates to about 10 m/s.

 Typically, clutter (from buildings, trees, etc. which would indicate zero velocity) is picked up in the range bins and must be removed before averaging occurs.

 Nexrad (WSR-88D)(~164)  Pulsed Doppler (~1300 pulses emitted each second)  During course of 1 hour, pulses are transmitted for a total of about 7 seconds and radar listens for returning signal about 59 minutes and 53 seconds.

  10 cm (S-Band) Can operate in “Clear Air” or “Precipitation” mode •

Precipitation Mode

: When precipitation is occurring. Rotation rate is faster. Moves through increasing elevation angles from 0.5

o up to 19.5

o to obtain a volume scan (volume coverage pattern) for precipitation.

•

Clear Air Mode

: Rotation rate slower. Samples volume of air longer. Can detect airborne dust and particulate matter. Better at detecting snow. Volume coverage pattern angles only go up to 4.5

o .

Terminal Doppler Weather Radar

 FAA  5 cm (C-Band)  41 Major airports (may rise to 47)  Primarily to detect microbursts and wind shear

Airport Surveillance Radars

 ASR-9 Weather system Processor  ASR-11  Digital Survellance Radar with monopulse second surveillance radar  Has weather system processor



Installed at airports which do not have TDWR

 Approximately 34 operational and 3 support systems  Weather processor provides warnings of wind shear and microbursts near runways  Used to predict arrival of gust fronts and storm track motion.

 Operated in KU band (14 - 16 GHz)

Wind Profiler

 Vertical pointing Doppler radar  Types:

UHF

(300-3000MHz) 915 MHz (33 cm), 404.37 MHz (74 cm),

VHF

(30 - 300 MHz), 50 MHz (600 cm)  Fluctuations in atmospheric density cause reflection of signal  Three beams are used to determine components of wind along the beams.

 U, V, W components are determined from the beam components.

Vre



U

cos 75

o



W

sin 75

o Vrn



V

cos 75

o



W

sin 75

o Vrz



W

 Height of wind is determined from elapsed time for returning signal.

 Angle of beam is achieved by delaying pulse to individual antennas across the profiler.  Typically operates in 2 modes: low mode - 1 2/3 microsecond pulse for low levels, 6 2/3 microsecond for higher altitudes.

Palestine, TX, profiler data

Laser Radar (Lidar/Ladar)

 Light Detection And Ranging: Operate in the ultraviolet, visible or infrared portion of the spectrum.

 Operation may be Continuous (always on) or Pulsed (like a strobe light).

 Returned echo detected either:  Incoherently: Without reference to the emitted signal • Detects backscattered radiation • Range can be determined by – Focusing the energy at a particular range – Measuring the angle the reflected light makes with a baseline. E.g., for determining cloud height.

• Concentration level of target is determined from the intensity of the returned signal.

 Coherently: With reference to the emitted signal.

• Detects backscattered radiation • Range is determined by measuring the time of return of reflected signal.

• Speed of target is determined by frequency shift between transmitted and returned signal.

• Concentration level of target is determined by the intensity of the returning signal.

 Medium for different types of lasers include: gases (Helium Neon, Zenon Fluoride); solid state diodes, dyes and crystals (ND:YAG = Neodymium: Yttrium Aluminum Garnet)  Photomultiplier tubes detect the backscattered radiation and convert the quanta of light into electric currents and then into photocounts, digital values that can be stored on a computer. Currents generated are on the order of picoamps (1 pA = 10 -12 A).

 A 60W light bulb draws 0.5 Amps.

 Photocounts received at fixed time intervals after a lidar pulse. Fixed time intervals represent heights above the lidar unit when aimed vertically.

 Used in detection of Clear Air Turbulence, aerosols, concentrations of gases (water vapor, carbon dioxide, methane, chloroflurocarbons, ozone), Clouds, temperature profiles, winds, etc.

Differential Absorption Lidar (DIAL)

 Based on the fact that absorption of wavelengths of light by constituents in the atmosphere is different for different wavelengths.

 Measures intensity of returned lidar signal at the “on line” frequency as compared to the “off line” frequency.

 “On line” is a frequency that a particular gas in the atmosphere will absorb the the lidar energy. “Off line” is where it does not absorb the energy.  Gives a measure of the presence/concentration of a particular gas which absorbs at the on line frequency.

 LITE Experiment: Lidar Technology Experiment.

 Flew on shuttle mission STS-64 in Sep. 1994.

 Demonstrated use of perating a lidar from space to detect clouds and aerosols.

 GALE: Giant Aperture Lidar Experiment   A layer of alkali metals exist from 80-120 km in atmosphere from meteorites. Sodium atoms when radiated with 589nm light fluoresce. Lidar’s receivers measures this fluorescence. By shifting the wavelength of transmitted light a tiny amount, will create a Doppler shift due to the motion of the sodium atoms which will then give a measure of wind.

 GALE results:  Notice how the wind changes from 40m/s westward (-40) near 90 km to 80 m/s eastward near 97 km at 8:30 UTC.

 Pressure profiles can be determined based on the amount of “pressure broadening” of the actual absorption vs. wavelength curve as compared to a standard absorption vs. wavelength curve for the particular gas under study.

 Temperature profiles can be determined in two ways, one by the sodium resonance-fluorescence scattering as in the GALE system, secondly by Rayleigh-scattered light. Rayleigh scattered light is responsible for the blue sky observed on clear days.

 Lidar photocounts in range bins can be related to atmospheric density by:     

z

2   where, b is a constant which depends on the individual lidar system, the type of scattering (Rayleigh) and the transmission of the atmosphere, z is the height, N(z) is the number of photocounts at each height.

 Once the density profile is known, temperature is determined assuming hydrostatic equilibrium.

 Clear Air turbulence is detected by sudden changes in the velocity of aerosols as measured by the phase shift between the transmitted and returned laser energy.

 Pollution:  Lidar can track pollutants by detecting the backscattered energy from the pollutant aerosol  Utilizes a specific wavelength for different types and sizes of pollutants.

Sodar

 Detects sound energy which has been scattered by density differences in the atmosphere.

 Ranges: ~300 m  Studies near-surface boundary layer moisture, temperature, wind variations.

 Cut-away of a Sodar unit.

 Some systems have a series of sound units pointed upward.

 .



Radio Acoustic Sounding System (RASS)

 Combines Wind Profiler and Sodar  Sodar transmits soundwave vertically.

 Wave front compressions and rarefactions of air provides a target the wind profiler can detect.

 The rate of movement of a sound wave is dependent on the virtual temperature of the air through which the wave moves.

T v

 

M



yR

 

Vac

2   T v =Virtual Temperature M=Molecular weight of air molecules  Y=ratio of specific heat  R=Universal gas constant  V ac =acoustic speed of sound wave front  Temperature profile determined from data taken during first 10 minutes of each hour  Wind profiles are determined from the next 50 minutes of data

 End