Specifications TIREM™ calculates the propagation
loss along a path over irregular terrain at frequencies between 1 MHz and 40 GHz. TIREM also models near-field considerations, near-field considerations, power power density, density , and field and field strength equations. equations . This commercially available software product was initially created in 1967 and has been continuously enhanced and maintained by Alion scientists and engineers since then. To see a larger LOS Path image, click here. here.
The figure at right shows a representative path over a curved earth involving both rough earth terrain and sea water expanses. TIREM examines the terrain type and elevation profile and automatically selects the optimum model(s) for calculating propagation loss along the path. If sea water does not lie in the path, TIREM will not use that model. If the propagation path is entirely sea water, TIREM will not use the earth model in the calculation. To see a larger image of the Land / Sea Path illustration, click here. here.
Based on the geometry of the profile, TIREM selects all appropriate modes of propagation for paths with endpoints are either within line-of-sight (LOS) or beyond-line-of-sight (BLOS). The figure at right illustrates both LOS and BLOS zones in a propagation path. To see a larger image of the Profile Evaluation model, click here. here.
The figure at right illustrates TIREM's modeling of multiple of multiple knife-edge diffraction. diffraction . It is one of the physical phenomena that extends the signal's propagation into the BLOS zone. To see a larger image of the Multiple Knife-Edge diffraction illustration, click here. here.
The figure at left illustrates troposcatter , another of the physical phenomena that extend the propagated signal into the BLOS zone. In this case, the electromagnetic signal is reflected off of the earth's troposphere. TIREM models troposcatter as a function of frequency and other environmental and equipment factors. To see a larger image illustrating atmospheric Troposcatter, click here. here.
The effects of atmospheric absorption are also included in calculations for frequencies above 10 GHz. Long-term power fading can be considered for climatic conditions representing most parts of the world. Modeling variability as a result of the standard deviation of the model from measured data can also be considered.
The figure at right presents a matrix of TIREM's modeling methods. The left column is frequency in ascending order. Across the top, the two zones of LOS and BLOS are shown with the terrain types of sea, sea/land mixture, and land broken down for each. Selecting a frequency and a zone of sight reveals the model selection methods employed by TIREM. To view the matrix of TIREM's modeling methods, click here. The current release of TIREM includes an add-on to enable the consideration of the attenuation due to rain. The current release of TIREM does not consider rain-scatter coupling , ducting phenomena, or ionospheric (skywave) propagation. Attenuation due to rain, foliage, or man-made obstacles are also not considered.
A.1.6. TIREM-EDX Model
TIREM stands for Terrain Integrated Rough Earth Model, one of several propagation models in a propagation package developed by the National Telecommunications and Information Administration (NTIA) in conjunction with various branches of the US Department of Defense. This model is widely used in US government and military organizations as well as by private companies. The model is one of the more complex currently used; the description given below is only a very brief summary. More detailed information can be found in the references at the end of this appendix. The currently version of TIREM is maintained and updated by the Electromagnetic Compatibility Analysis Center (ECAC).
Version 7 ©1996-2008, EDX Wireless, LLC - All rights reserved. EDX Wireless Software Appendices – Appendix A: Propagation Models A-5 Basically, TIREM considers each path and makes an initial decision on whether it is "line-of-sight" (LOS) or obstructed. Based on this decision it uses one of two approaches to find path loss. In the LOS mode, it calculates the degree of obstruction of the Fresnel zone and uses this to proportionally adjust the amount of additional path loss (over free space path loss) which results from one of two propagation loss models. For frequencies above 200 MHz, the Longley-Reasoner model is used. For frequencies below 150 MHz, the Longley-Rice model is used. At frequencies in between 150 and 200 MHz, the losses calculated from the two methods are proportionally averaged. The attenuation calculated by the methods is adjusted by the degree of path clearance over the terrain. If the path clearance is less than 0.5 Fresnel zone radius, 100% of the attenuation is used. If the clearance is greater than the 1.5 Fresnel zone radius at all points, no additional attenuation is applied from these methods, only basic free space path loss is used. At clearances in between 0.5 and 1.5 Fresnel zone radii, the additional path loss is a linear proportion between 0 and 100 percent. In the LOS mode, TIREM also calculates what portion of the path is over smooth earth and what portion is over rough earth. If part of the path is over smooth earth, the path loss (on the entire path) is calculated using Bullington's smooth earth path loss method. The loss for the smooth earth and rough earth (Longley-Rice or Longley-Reasoner) are proportionally combined based on the percentage of the path which is smooth and the percentage which is rough. In the NTIA implementation of TIREM, smooth earth is defined as elevations equal to zero (sea level) or some other smooth earth elevation (a lake surface,
for example) which the user can define. In the EDX implementation of TIREM, only zero elevations (sea level) are considered to be smooth earth. When the path is obstructed, TIREM uses a multiple knife-edge diffraction loss routine like that described under RMD in Section A2.1. However, for paths where the average loss per obstacle is less than 10 dB, TIREM will calculate a smooth earth path loss (using the Bullington smooth earth algorithm mentioned above) and combine it proportionally with the loss found from the multiple knife-edge calculation. This method comes in to play for obstructed paths over the ocean where the horizon at each point can present a low angle obstacle. The EDX version of TIREM fully implements this approach for low angle multiple obstacles. In the obstructed path case, TIREM also modifies path loss with a "height-gain" function that is based on the assumption that the rough terrain profile can be replaced with four circular arcs (earth radii). This is a tenuous assumption for most cases. For calculation efficiency, this element of the NTIA version of TIREM is not implemented in the EDX version. The NTIA version of TIREM has a third propagation mode that calculates troposcatter path loss. The path loss from the troposcatter path is combined with the diffracted path loss as described above to produce a net path loss to the receiver (net field strength). The troposphere path essentially is another highly inefficient path from the transmitter to the receiver, resulting in a field strength that is slightly higher than the field strength resulting from the diffracted path over the terrain obstacles. For path lengths of interest to EDX software users (i.e., less than about 200 km), the troposcatter route is a relatively unimportant contributor to the field strength at the receiver when compared to the field strength from the diffracted path. For this reason, the troposcatter mode in the NTIA version of TIREM is not implemented in the EDX version of TIREM. More detailed explanations of the TIREM model can be found in references [2] through [5] listed at the end
