Remote Sensing Systems Overview

Transcription

1 Remote Sensing Systems Overview Remote Sensing = Measuring without touching Class objectives: Learn principles for system-level understanding and analysis of electro-magnetic remote sensing instruments (primarily optical) for use in atmospheric, earth, planetary, solar, or space science, or free-space communications, etc. page.jsp?page=122&id=33 Topics: Radiometry Detectors & SNR Calibration Resolution Polarimetry Atmospherics Passive sensors Active sensors how to calculate how much light reaches a detector converting light into electrical signal of known quality relating electrical signal to radiometric quantities discretizing spatial, spectral, and radiometric measuring polarization state of the light how the atmosphere affects EM/optical propagation examples of sensors that measure natural radiation examples of sensors that transmit their own radiation J. A. Shaw 1

2 Motivation Imagine you are hired for one or more of the following tasks: 1) calibrate a thermal infrared camera to measure the distribution of temperature in a scene; 2) calculate the electricity generated by a solar cell. 3) determine if a laser beam can detect a high-altitude airborne object. We will learn how to do all of this and more! 2

3 Sensor system performance Signal-to-noise ratio: SNR ( λ) = Detected signal power (flux) = Noise Equivalent power AΩ Δλ L s λ * ( λ) D ( λ) T ( λ) T ( λ) A Δf d a o Signal-to-background ratio: SBR ( λ) = signal power background power = s s ( AΩ) L ( λ) T ( λ) T ( λ) λ Δλ bg bg ( AΩ) L ( λ) T ( λ) T ( λ) λ Δλ a a o o dλ dλ This course explores the role of radiometry, atmospheric effects, optical systems, and detectors in the design and operation of passive and active optical & infrared sensor systems. A = entrance-pupil area (light-gathering area) Ω = FOV projected solid angle L s λ = spectral radiance of source L bg λ = spectral radiance of background Δλ = optical bandwidth of sensor system D* = specific detectivity of detector T a = atmospheric transmittance T o = optical transmittance of sensor system A d = detector active area Δf = electrical bandwidth of sensor system λ = optical wavelength A obj Ω s A Ω bg A det 3

4 Electromagnetic sensors & atmospheric effects detector optics signal electronics algorithms data products Not the focus of this course Radiance at sensor L s = reflected solar + surface + atmospheric emission emission reflected + + scattered addition of scattered light into the beam - extinction from absorption & scattering As a beam of light propagates through the atmosphere, it can gain light through emission and scattering lose light through absorption and scattering. 4

5 Grading Homework (problems, hands-on experiments, literature discussions) 20% Exam 1 20% Exam 2 20% Sensor project (building, calibrating, and using solar radiometer) 20% Sensor analysis report 20% What matters most is honest effort and conceptual understanding. Late assignments: 2 late periods during semester; otherwise no late assignments. (talk to me if traveling for research, etc.). Student interaction is encouraged. However, cheating or plagiarism will result in a failing grade for at least that assignment (copying from the web is plagiarism). 5

6 Electromagnetic waves and photons Photon energy is proportional to frequency and inversely proportional to wavelength. X-rays & gamma rays ultraviolet Infrared mm waves & THz λ [μm] energy 0.38 vis 0.7 NIR SWIR MWIR LWIR Note: band designations (e.g. LWIR, etc.) can vary widely across disciplines 1. waves wavelength = speed of light / frequency c λ = ν 2. photons photon energy Q p = hν = hc λ h = Planck s constant = x J s 6

7 Passive and active sensors Passive Sensors measure naturally occurring radiation. photographic camera measures reflected sunlight (vis) EUV imager measures emitted & scattered solar radiation (EUV) night-vision camera measures thermal emission (LWIR) Active Sensors transmit their own radiation. Radar measures scattered radio waves Lidar measures scattered laser light 7

8 Solar radiometer A solar radiometer (sometimes called a sun photometer ) measures extinction of direct solar spectral irradiance [W/(m 2 nm)]. By measuring how much sunlight is absorbed by gases and scattered by molecules (Rayleigh) or aerosols and clouds (Mie), we can determine the amount of absorbing gas or scattering particles. Wavelength bands are defined by optical interference filters. τ (λ) θ z Scattering and absorption τ (λ) secθ d f 8

9 Optical depth A unitless and dimensionless quantity that describes the amount of the total extinction (absorption + scattering) in an integrated medium (e.g., an entire atmospheric path). Optical depth = τ = σ ( z) N( z) dz 0 [ ] [m 2 ] [m -3 ] [m] σ = absorption, scattering, or extinction cross section [m2 ] N = gas molecule or particle number density [m -3 ] z = range (distance) along optical path [m] Optical depth is also used to describe the net effect of scattering by aerosols or absorption by gases either a specific gas or of all gases combined. Hence, you will hear mentioned the aerosol optical depth, the ozone optical depth, the Rayleigh (scattering) optical depth, the cloud optical depth, etc. All optical depths vary with wavelength (and with gas and/or particle distribution) 9

10 Irradiance varies exponentially with optical depth If the Beer-Bouguer-Lambert Law (often called Beer s Law ) is valid, then the Irradiance of a light beam is reduced exponentially with distance and the optical depth is the quantity in the exponent. Spectral irradiance = E 0 σ ( z, λ ) N ( z, λ ) dz τ ( λ ) ( λ) = E ( λ) e = E ( λ) e 0 0 [W/(m 2 nm)] E 0 (λ) is the initial spectral irradiance at wavelength λ. In solar radiometry this usually represents the exo-atmospheric solar spectral irradiance (incident at the top of the atmosphere). Integrated over wavelength, this becomes the solar constant (~1370 W/m 2 ). Example: If we have 1000 W/m 2 incident outside the atmosphere and the optical depth of the atmospheric path is τ = 0.3, then what reaches our sensor on the ground is τ 0.3 E = E0 e = 1000e = 741 [W/m 2 ] 10

11 Air mass Air mass is a term used to describe the optical path length through the atmosphere relative to the zenith path length (looking straight up). Air mass = sec(θ z ) z θ z z/cos(θ z ) = z sec(θ z ) Air mass = 1 at the zenith Air mass = at the horizon J. Shaw 11

12 Langley plot calibration of solar radiometers If we plot the natural logarithm of the solar irradiance as a function of air mass, the exponentially varying signal can be fit to a straight line. The zero-intercept of this line (at air mass = 0) is the exo-atmospheric irradiance and the slope is the optical depth. ln(i 0 ) ln(i) τ E d i d = i 0 e τ sec ln( i ) = ln 0 ( θ ) ( i ) τ sec( θ ) z z E 0 zenith OD air mass air mass 8 We use the calibration coefficient i 0 with a measurement i d to find τ at another location and time. G. E. Shaw, Sun photometry, Bulletin American Meteorological Society 64(1), 4-10 (1983) 12

13 Measuring optical depth with a calibrated solar radiometer If the radiometer is sufficiently stable, you can use the intercept from a previous calibration to measure the optical depth at a later time and place. ln(i 0 ) τ = ( i ) ( i m ) sec( θ ) ln 0 ln z The optical depth is found as the slope of a line connecting the calibration value ln(i 0 ) and the newly measured value ln(i m ) (illustrated in this case at air mass = 4). ln(i m ) measurement Data can be recorded vs. time and air mass calculated from the Sun s position at that time and place: air mass 8 At large air mass ( 6) a correction to the sec(θ z ) air mass model is required to account for refraction. A. T. Young, Air mass and refraction, Applied Optics 33, (2004). 13

14 Solar radiometer project Objective: build and calibrate a simple solar radiometer and use it to measure atmospheric extinction (absorption + scattering) caused by gases and aerosols (dust, soot, and other particles in the air). This project will determine 20% of your grade. Timing: Now ASAP Feb 5 Feb-March March 31 Start designing your sensor and buying parts Build a prototype and verify that it works Demonstrate to me that your sensor works Collect calibration and measurement data Turn in sensor (in working condition) with report showing and discussing your calibration and measurement data. 14

15 LED-based solar radiometer Advantage: use low-cost LED in place of expensive detector and filter. In the 1990s, Forrest Mims III demonstrated the use of a light-emitting diode (LED) as a very low-cost detector and optical filter (the reverse of the typical use of an LED as a spectrally tuned light source). Mims created a very simple solar radiometer that will provide a starting point for the design of your sensor system. Visit the Haze-span webpage for details: F. Mims III, Sun photometer with light-emitting diodes as spectrally selective detectors, Appl. Opt. 31(33), (1992). 15

16 LED absorption & emission spectra can differ An LED solar radiometer relies on the use of an LED as a detector. However, LEDs have much broader spectra than the usual solar radiometer bands, which makes data analysis more difficult. Furthermore, the LED detection spectrum can be different from its emission spectrum. Y. B. Acharya, Spectral and emission characteristics of LED and its application to LED-based sun photometry, Optics & Laser Technology 37, (2005) 16

17 Langley plots for LED solar radiometer LED solar radiometers can be calibrated and used to take quite accurate data. Y. B. Acharya, Spectral and emission characteristics of LED and its application to LED-based sun photometry, Optics & Laser Technology 37, (2005) 17

18 Wavelength variation of optical depth Optical depth generally decreases at longer wavelengths. Usually the wavelength variation of optical depth can be estimated with Angstrom s turbidity formula: τ λ λ = τ λ 0 0 α with τ λ = optical depth at wavelength λ τ 0 = optical depth at wavelength λ 0 α = the Angstrom exponent You can use optical depth measurements at two wavelengths to estimate the Angstrom exponent: α = τ 1 ln τ 2 λ 1 ln λ2 18

19 Aerosol optical depth vs. wavelength Obtaining the aerosol optical depth requires: 1) Measure the total optical depth using a calibrated radiometer; 2) subtract Rayleigh & ozone optical depths, etc. 3) Correct for actual Earth-Sun distance (irradiance ~ 1/R 2 )... Angstrom exponent is inversely proportional to mean particle size. G. E. Shaw, Sun photometry, Bulletin American Meteorological Society 64(1), 4-10 (1983) 19

20 References G. E. Shaw, Genesis of sun photometry, J. Appl. Rem. Sens. 1, (2007). Y. B. Acharya, Spectral and emission characteristics of LED and its application to LED-based sun photometry, Optics & Laser Technology 37, (2005) B. N. Holben, T. F. Eck, I. Slutsker, D. Tanre, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. J. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, A. Smirnov, AERONET a federated instrument network and data archive for aerosol characterization, Remote Sens. Environ. 66, 1-16 (1998). A. R. Ehsani, J. A. Reagan, W. H. Erxleben, Design and performance analysis of an automated 10-channel solar radiometer instrument, J. Atmos. Ocean. Technol. 15, (1998). G. E. Shaw, The Arctic Haze phenomenon,: Bull. Am. Meteorol. Soc. 76(12), (1995). K.J. Thome, M. W. Smith, J. M. Palmer, J. A. Reagan, Three-channel solar radiometer for the determination of total columnar water vapor, Appl. Opt. 33(24), (1994). F. Mims III, Sun photometer with light-emitting diodes as spectrally selective detectors, Appl. Opt. 31(33), (1992). G. E. Shaw, Sun photometry, Bulletin American Meteorological Society 64(1), 4-10 (1983) G. E. Shaw, Solar spectral irradiance & atmospheric transmission at Mauna Loa Observatory, Appl. Opt. 21(11), (1982). G. E. Shaw, Transport of Asian desert aerosol to the Hawaiian Islands, J. Applied Meteorology 19, (1980). M. D. King, D. M. Byrne, B, M. Herman, and J. A. Reagan, Aerosol size distributions obtained by inversions of spectral optical depth measurements, J. Atmos. Sci. 35(11), (1978). G. E. Shaw, Error analysis of multi-wavelength sun photometry, Pure and Appl Geophys. 114(1), 1-14 (1976). F. E. Volz, Economic multi-spectral sun photometer for measurements of aerosol extinction from 0.44 μm to 1.6 μm and precipitable-water, Appl. Opt. 13, (1974). G. E. Shaw, J. A. Reagan, and B. M. Herman, Investigations of atmospheric extinction using direct solar radiation measurements made with a multiple wavelength radiometer, J. Applied Meteorology 12(2), (1973). Important websites for solar radiometry: * Aerosol Robotic Network (AERONET): * Haze span: * NOAA Solar position calculator: 20