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Atmospheric Modeling Using PcModWin©/MODTRAN®
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Book Description

MODTRAN® is a powerful program capable of describing atmospheric absorption, scattering, and emission paths, but it has a complicated computer-card input structure; a custom user interface, PcModWin©, was introduced to minimize setup and error-mitigation issues. The instruction manuals for these programs provide over 80 test cases, which are instructive for intermediate to advanced users but do not provide detailed explanations. This book is intended to help beginners learn how to perform analysis using MODTRAN/PcModWin by exploiting these test cases. Specifically, many of the cases are used to illustrate how to navigate the PcModWin menus to solve the given problems. The book also provides the underlying mathematics and atmospheric physics incorporated into MODTRAN.

Book Details

Date Published: 30 December 2019
Pages: 362
ISBN: 9781510628052
Volume: PM307

Table of Contents
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Chapter 1 Introductory Material
1.1 Introduction
1.2 Radiometric Parameters
     1.2.1 Radiant flux (radiant power)
     1.2.2 Radiant intensity
     1.2.3 Radiance
     1.2.4 Radiant flux density (irradiance and radiant exitance)
     1.2.5 Bi-directional reflectance distribution function
     1.2.6 Directional hemispheric reflectance
     1.2.7 Specular surfaces
     1.2.8 Lambertian surfaces and albedo
     1.2.9 Blackbody radiation sources
1.3 Blackbody Radiation Sources
1.4 Mie Scattering Theory
1.5 Atmospheric Transmittance and Scattering
1.6 Essential Concepts in Radiative Transfer
     1.6.1 First-order RTE solution
     1.6.2 Radiative transfer equation for a single, homogeneous particulate layer
     1.6.3 Radiative transfer equation for multiple, homogeneous particulate layers
1.7 References

Chapter 2 MODTRAN Modeling Capability
2.1 Introduction
2.2 A Short MODTRAN History
2.3 New MODTRAN User Interface
2.4 General MODTRAN Calculation Details
2.5 MODTRAN Transmittance and Radiance Model Definitions
     2.5.1 VSWIR modeling
     2.5.2 MWIR-LWIR modeling
     2.5.3 Maritime remote sensing modeling
2.6 MODTRAN Spherical Refractive Geometry
     2.6.1 MODTRAN geometrical layout: general problem
     2.6.2 MODTRAN geometrical layout: uniform path
2.7 Selecting Atmospheric Properties
     2.7.1 More on aerosol characterization in MODTRAN
     2.7.2 Available aerosol models
     2.7.3 Clouds
     2.7.4 Fog
     2.7.5 Aerosol angular scattering functions
2.8 The NOVAM Aerosol Model
2.10 MODTRAN LBL Radiative Transfer Option
2.11 References

Chapter 3 Transmittance Calculations
3.1 Introduction
3.2 General Comments on MODTRAN Light Transmission in the Atmosphere
     3.2.1 Molecular absorption
     3.2.2 Molecular scattering
     3.2.3 Aerosol extinction
     3.2.4 Atmospheric transmission
3.3 Effects of Wavenumber Resolution on Atmospheric Transmittance Representation
3.4 Example MODTRAN Calculation Procedure
3.5 MODTRAN Test Cases
3.6 Calculating Spectral Transmittance: 1500mGround Test Case
3.7 Spectral Resolution Selection
3.8 Calculating Spectral Transmittance: New Model Atmosphere
     3.8.1 Calculating spectral transmittance: CD2c3_USS.ltn test case
     3.8.2 MODTRAN A-Plus cards (FLEXIBLE AEROSOL MODEL) and PcModWin
     3.8.3 Aerosol extinction profiles: MODTRAN input entries
     3.8.4 NMA user-defined properties: AuxilSpecies.ltn test case
     3.8.5 Angstrom's law
 Relationship between Angstrom's law and the Koschmeider equation
 Spectral transmittance with Angstrom's law
3.9 Spectral Transmittance: Navy Maritime Aerosol Model
3.10 Spectral Transmittance: NOVAM Aerosol Model
3.11 Long-Path Refraction/Surface Meteorological Range (Visibility)
3.12 Important Sensor-Related Functions in PcModWin/MODTRAN
     3.12.1 Scanning function
     3.12.2 MODTRAN Filter Function
3.13 References

Chapter 4 Atmospheric Spectral Radiance
4.1 Introduction
4.2 General Comments on Solar and Lunar Calculations in MODTRAN
4.3 Plotting Options in MODTRAN
4.4 Calculating Spectra Transmittance and Radiance: Radiance with Scattering Test Case
4.5 Calculating Spectral Radiance: Radiance with No Thermal Scattering Test Case
4.6 Calculating Spectral Radiance: Thermal Radiance Only Test Case
4.7 Calculating Spectral Irradiance: Direct Solar Irradiance Test Case SolarIrrad.ltn
4.8 Calculating Spectral Transmittance: AnLBLTemplate.ltn Test Case
4.9 Calculating Spectral Transmittance: lblThmNoMS1600wn_highres.ltn Test Case
4.10 Calculating Spectral Transmittance: VertStructAlg.ltn Test Case

Chapter 5 Multiple Scattering in MODTRAN
5.1 Introduction
5.2 Correlated k-Distribution
     5.2.1 k-distribution method
     5.2.2 Correlated k-distribution method
     5.2.3 Correlated k-distribution method implementation in MODTRAN
5.3 Calculating Spectral Radiance: CorrelatedK.ltn Test Case
5.4 Calculating Transmittance and Spectral Radiance: SolarIsaacsCK.ltn Test Case
5.5 Calculating Spectral Radiance: SolarScaledCK.ltn Test Case
5.6 Multiple-Lines-of-Sight Option
     5.6.1 CARD 3D for the DISORT algorithm
     5.6.2 Calculating transmittance and spectral radiance: ElevAngMWIRthm.ltn test case
     5.6.3 Calculating spectral radiance: lut01ASC.ltn test case
     5.6.4 Calculating spectral radiance and transmittance: lut01thm.ltn test case
     5.6.5 Calculating spectral radiance and transmittance: lut02thm.ltn test case
5.7. References

Chapter 6 Surface Reflectance in MODTRAN
6.1 Introduction
6.2 Background
6.3 Spectral BRDF and Lambertian Options in MODTRAN
6.4 Spectral Lambertian and BRDF Options in PcModWin
6.5 Calculating Spectral Scattered Radiance: Lambertian.ltn Test Case
6.6 Calculating Spectral Scattered Radiance: MODIS.ltn Test Case
6.7 Calculating Spectral Scattered Radiance: BRDF.ltn Test Case
6.8 Calculating Spectral Scattered Radiance: seaBRDF*.ltn Test Cases
     6.8.1 User-defined line of sight
     6.8.2 Spectral aerosol profile (SAP)
     6.8.3 Compute segment radiance: DISORT
6.9 References

Chapter 7 Clouds and Fog in MODTRAN
7.1 Introduction
7.2 Clouds and Fog Basics
7.3 Engineering Equations for Light Propagation through Clouds
7.4 PcModWin Cloud Settings for Aerosols and Clouds Type Selection
7.5 Calculating Transmittance and Spectral Radiance: UserDefinedCir.ltn Test Case
7.6 Calculating Transmittance and Spectral Radiance: UserDefinedCld.ltn Test Case
7.7 PcModWin Cloud Settings for Aerosols and Clouds Type Selection
7.8 Calculating Transmittance and Spectral Radiance: CirrusProfile.ltn Test Case
7.9 PcModWin FOG Settings for Aerosols and Clouds Type Selection
7.10 Use of the Army Vertical Structure Algorithm with Aerosols and Clouds/Fog
7.11 References

Chapter 8 Special Topics
8.1 Introduction
8.2 Using MODTRAN with Radiosonde Data
8.3 Key Focal Plane Array Metrics and MODTRAN/PcModWin
     8.3.1 Signal-to-noise ratio definitions
     8.3.2 Responsivity
     8.3.3 Optical signal-to-noise ratio
     8.3.4 Noise-equivalent power
     8.3.5 OSNR for an extended source with background optical radiation
     8.3.6 Noise-equivalent temperature difference
8.4 Koschmeider Equation Versus MODTRAN
8.5 References


Atmospheric modeling has been important to systems analysis and remote-sensing applications for decades. It affects a divese set of technical areas, electro-optical sensor development and evaluation; airborne- and satellite-image calibration and correction; hyper-spectral terrain categorization; laser spectroscopy; laser communications; and lidar/ladar system development. One of the major modeling tools available to researchers, scientists, and engineers is MODTRAN®, which is short for MODerate-resolution atmospheric TRANsmission. This tool calculates the transmission and emission from a set of pre-stored or user-defined atmospheres to analyze electro-optical/infrared system performance and natural phenomena. It uses a generalized geometry package to describe atmospheric absorption, scattering, and emission paths, leveraging either included databases describing (1) atmospheric profiles, (2) atmospheric aerosols, and (3) water- and ice-cloud representations, or user-defined atmospheres and characteristics. Its spectral coverage spans the optical wavelength range from the near-ultraviolet (UV) through the far-infrared (IR).

MODTRAN is a complete model for calculating absorption and emission. It has several hundred interconnected inputs. Its 1960s "computer card" and JSON input structure is rather involved, requiring precision to enter the entries in the right place and format. It also offers no easy means to see if your inputs are correct because of its antiquated data input structure. More significantly, it requires expertise to choose the set of inputs and parameters to employ, which exacerbates the problem setup, input file proofing, and error determination. The major error sources in MODTRAN atmospheric modeling include omissions of major effects, incorrect definition of the problem, and inadequate description of the atmosphere.

Ontar's PcModWin© was introduced in 1982 as a graphical user interface (GUI) for MODTRAN that minimizes the problem-setup and error-mitigation issues. It allows the user to quickly review and understand the parameters used by their input file. Unfortunately, neither program's instruction manual provides beginners with the expertise and understanding of what numbers to use in calculations. They do, however, offer over 100 test cases, albeit with limited/negligible explanation of input selections. These cases are derived from the sample input files supplied by the Air Force Research Laboratory as part of the MODTRAN package. Unfortunately, there is no explanation or discussion of them in MODTRAN. The PcModWin addendums give some insight, but those sections are meant for intermediate and expert atmospheric modelers.

Most PcModWin/MODTRAN users are scientists and engineers who design or evaluate electro-optical sensors, or who analyze the data from a sensor. Atmospheric radiative transport typically is not their field of expertise, but they need accurate estimations of the atmospheric effects to do their "real job." They may only use PcModWin/MODTRAN a few times a year when they need to perform a calculation quickly. This book is designed primarily for these users: the theories of molecular absorption, aerosol scattering, multiple scattering, BDRF, etc., are discussed but not to the extent of more theoretical texts. This book will tell you how to quickly set up your scenario calculation and interpret the results with a high degree of confidence. Specifically, this book uses many of these test cases to illustrate and explain the standard way to fill out the PcModWin screens for the problem associated with each test case. This book also will help beginners learn how to perform analysis using PcModWin/MODTRAN with discussions of the various possible inputs, featuring these test cases as examples. In fact, given the number and breadth of the included test cases, at least one of them may approximate the problem a beginner needs to solve.

As in many fields, the material described in this book is based on the tremendous work of others. We would like to thank Alexander Berk, David Robertson, and Lawrence Bernstein of Spectral Sciences, Inc. Additional SSI contributors are noted in their comments that follow.

Likewise, many people have been responsible for the success of PcModWin and its predecessors over the past 35 years. This is the 17th major release of the software that includes PcTran©, Oncore©, PcModWin©, and PcModWin2© through PcModWin6©. Its success belongs entirely to the following people: Bob Haimes, Gretchen Slemmons, Joe Kristl, Luke Biberman, Sabrina Harvey, Martine Voltaire, Paul Noah, Meg Noah, Johnson Chen. John Selby, Andy McCann. Cheryl Tibaudo, Leon Varvack, Kuilian Tang. Scott Theleman. Michelle Fischer, Alyssa Douglass, Dylan Payne, and Austin Sharpe.

Most importantly are the following US government personnel who have made significant contributions to MODTRAN and related models, in alphabetical order: Len Abreu, Gail Anderson, Jeannette van den Bosch, Jim Chetwynd, Jr., Tony Clough, Earl Good, Bob Fenn, Stu Gathman, John Garing, Frank Kneizys, Bob McClatchey, Dick Picard, Bob Roberts, Larry Rothman, John Selby, and Eric Shettle. We have known them as colleagues and friends for many years.

In addition, we would like to thank Michael Soel for his helpful comments and suggestions. Schroeder would especially like to remember Lucien M. Biberman of IDA. His keen intellect and blunt common-sense advice served as an inspiration that is sorely missed today.

Any deficiencies in this book are attributed to the authors alone. We want to make the book a useful tool and appreciate any feedback, both good and bad.

Larry B. Stotts
John Schroeder
June 2019

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