ITS Irregular Terrain Model (ITM)

This code repository contains the ITS Irregular Terrain Model (ITM). ITM predicts terrestrial radiowave propagation for frequencies between 20 MHz and 20 GHz based on electromagnetic theory and empirical models developed by Anita Longley and Phil Rice. Propagation mechanisms considered include free space loss, diffraction, and troposcatter. Specifically, ITM predicts attenuation as a function of distance (greater than 1 km), terminal heights, meteorological conditions, terrain effects, and the variability of the signal in time and in space.

Note: Version 1.3 of this code base is functionally identical to version 1.2.2 of the FORTRAN source, which has been archived here. ITS plans to apply all future ITM updates to this C++ code base.

Quick Start

Users of ITM have two options to immediately begin using ITM:

1. The ITM software is made available to the user as a precompiled DLL in the Release page. A user can add this DLL to their own software project as a dependency and call ITM through its function definitions. Additionally, a C#/.NET software wrapper is provided. Distribution and updates of this wrapper code are provided through the published NuGet package.

2. A supporting command-line driver is also included in this repository, with its own corresponding readme. This tool allows a user to call ITM without any programming required through the use of ASCII input/output files.

A select set of example inputs and outputs are provided for testing purposes. This is not a comprehensive validation test set. For ITM in Point-to-Point Prediction Mode, p2p.csv defines the set of inputs and outputs with pfls.csv containing the corresponding set of terrain profiles. For Area Prediction Mode, inputs and outputs are defined in area.csv.

Inputs

ITM can be called in either Area Prediction Mode or Point-to-Point Prediction Mode. Additionally, variabilities can be defined by either time/location/situation or confidence/reliability. The below inputs are organized accordingly.

Common Inputs

Variable	Type	Units	Limits	Description
h_tx__meter	double	meter	0.5 <= h_tx__meter <= 3000	Structural height of the TX
h_rx__meter	double	meter	0.5 <= h_rx__meter <= 3000	Structural height of the RX
climate	int		enum	Radio climate of the path <ul><li>1 = Equatorial</li><li>2 = Continental Subtropical</li><li>3 = Maritime Subtropical</li><li>4 = Desert</li><li>5 = Continental Temperate</li><li>6 = Maritime Temperate Over Land</li><li>7 = Maritime Temperate Over Sea</li></ul>
N_0	double	N-Units	250 <= N_0 <= 400	Minimum monthly mean surface refractivity reduced to sea level
f__mhz	double	MHz	20 <= f__mhz <= 20000	Frequency
pol	int		enum	Polarization <ul><li>0 = Horizontal</li><li>1 = Vertical</li></ul>
epsilon	double		1 < epsilon	Relative permittivity
sigma	double	S/m	0 < sigma	Conductivity
mdvar	int		enum	Mode of variability <ul><li>0 = Single Message Mode</li><li>1 = Accidental Mode</il><li>2 = Mobile Mode</li><li>3 = Broadcast Mode</li></ul> Additionally, +10 if location variability is to be eliminated, and +20 if direct situation variability is to be eliminated

Point-to-Point Mode Specific Inputs

Variable	Type	Units	Limits	Description
pfl	double[]			Terrain profile data in PFL format, from TX to RX <ul><li>pfl[0] : Number of elevation points - 1</li><li>pfl[1] : Resolution, in meters</li><li>pfl[i] : Elevation above sea level, in meters</li></ul>

Area Mode Inputs

Variable	Type	Units	Limits	Description
d__km	double	km	0 < d__km	Path distance
delta_h__meter	double	meter	0 <= delta_h__meter	Terrain irregularity parameter
tx_siting_criteria	int		enum	Siting criteria of TX <ul><li>0 = Random</li><li>1 = Careful</li><li>2 = Very Careful</li></ul>
rx_siting_criteria	int		enum	Siting criteria of RX <ul><li>0 = Random</li><li>1 = Careful</li><li>2 = Very Careful</li></ul>

Variability Inputs

A user either supplies values for time/location/situation or confidence/reliability.

Variable	Type	Units	Limits	Description
time	double		0 < time < 100	Time variability
location	double		0 < location < 100	Location variability
situation	double		0 < situation < 100	Situation variability
confidence	double		0 < confidence < 100	Confidence variability
reliability	double		0 < reliability < 100	Reliability variability

Outputs

Variable	Type	Units	Description
A__db	double	dB	Basic transmission loss
warnings	int		Warning flags

Intermediate Values

Internal intermediate values can be extracted from ITM via functions that are suffixed with _Ex. These functions will populate the IntermediateValues data structure with intermediate values from the ITM calculations.

Variable	Type	Units	Description
theta_hzn	double[]	radians	Terminal horizon angles
d_hzn__meter	double[]	meter	Terminal horizon distances
h_e__meter	double[]	meter	Effective terminal heights
N_s	double	N-Units	Surface refractivity
delta_h__meter	double	meter	Terrain irregularity parameter
A_ref__db	double	dB	Reference attenuation
A_fs__db	double	dB	Free space basic transmission loss
d__km	double	km	Path distance
mode	int		Mode of propagation <ul><li>1 = Line of Sight</li><li>2 = Diffraction</li><li>3 = Troposcatter</li></ul>

Error Codes and Warning Flags

ITM supports a defined list of error codes and warning flags. A complete list can be found here.

Notes on Code Style

• In general, variables follow the naming convention in which a single underscore denotes a subscript (pseudo-LaTeX format), and where a double underscore is followed by the units, i.e. h_tx__meter.
• Variables are named to match their corresponding mathematical variables in the underlying references.
• Wherever possible, equation numbers and source documentation are provided.

Configure and Build

C++ Software

The software is designed to be built into a DLL (or corresponding library for non-Windows systems). The source code can be built for any OS that supports the standard C++ libraries. A Visual Studio 2019 project file is provided for Windows users to support the build process and configuration.

C#/.NET Wrapper Software

The .NET support of ITM consists of a simple pass-through wrapper around the native DLL. It is compiled to target .NET Framework 4.7.2.

References

• G.A. Hufford, A.G. Longley, W.A. Kissick, A Guide to the Use of the ITS Irregular Terrain Model in the Area Prediction Mode, NTIA Technical Report TR-82-100, April 1982.
• G.A. Hufford, The ITS Irregular Terrain Model, version 1.2.2 Algorithm.
• G.A. Hufford, 1985 ITM Memo, Dr. George Hufford's 1985 Memo describing the changes to ITM version 1.2.1 (dated April, 1979) in ITM version 1.2.2 (dated September, 1984)
• G.A. Hufford, The Irregular Terrain Model, The "definitive" representation of the ITS Irregular Terrain Model. It contains both the source code and rather extensive documentation.
• A.G. Longley and P.L. Rice, Prediction of Tropospheric Radio Transmission Loss Over Irregular Terrain: A Computer Method - 1968, NTIA Technical Report ERL 79-ITS 67, July 1968.

Contact

For technical questions about ITM, contact Paul McKenna, pmckenna@ntia.gov.