Irregular Lunar Model (ILM)

Note: This software repository has been made available as an active workspace in support of on-going discussions during the 2024 ITU-R Study Group 3 meetings. The work presented here represents an example implementation of the ILM. Software releases in this repository are best-effort attempts to augment conversation and support collaboration and data sharing in the context of NTIA's scientific culture that values Open Science and holds that openness in science is fundamental to scientific and research integrity. The contents of this repository, including any software releases, have not undergone ITS standard quality review procedures such as code-reviews and unit testing.

This code repository contains a US implementation of the Irregular Lunar Model (ILM). ILM 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 and diffraction. Specifically, ILM predicts attenuation as a function of distance (greater than 1 km), terminal heights, terrain effects, and the variability of the signal in location.

Inputs

ILM can be called in either Area Prediction Mode or Point-to-Point Prediction Mode. 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
f__mhz	double	MHz	20 <= f__mhz <= 20000	Frequency
pol	int		enum	Polarization 0 = Horizontal 1 = Vertical
epsilon	double		1 < epsilon	Relative permittivity
sigma	double	S/m	0 < sigma	Conductivity
p	double		0 < p < 100	Location variability

Point-to-Point Mode Specific Inputs

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

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 0 = Mobile 1 = Fixed
rx_siting_criteria	int		enum	Siting criteria of RX 0 = Mobile 1 = Fixed

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 ILM via functions that are suffixed with _Ex. These functions will populate the IntermediateValues data structure with intermediate values from the ILM 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
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 10 = Line of Sight 20 = Diffraction, Single Horizon 21 = Diffraction, Double Horizon

Error Codes and Warning Flags

ILM 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.

References

• United States WP 3J input contribution 3J/26

Contact

For technical questions about ILM, contact Billy Kozma, wkozma@ntia.gov.