8/2/2019 Ti Rem

1/4

SpecificationsTIREM calculates the propagation loss along a path over irregular terrain at frequencies between 1 MHz and 40 GHz .TIREM also models near-field considerations, power density , and field strength equations . This commercially availablesoftware product was initially created in 1967 and has been continuously enhanced and maintained by Alion scientists andengineers since then. To see a larger LOS Path image, click 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 theLand / Sea Path illustration, click here .

Based on the geometry of the profile, TIREM selects all appropriate modes of propagation for paths with endpoints areeither within line-of-sight ( LOS ) or beyond-line-of-sight ( BLOS ). The figure at right illustrates both LOS and BLOS zonesin a propagation path. To see a larger image of the Profile Evaluation model, click here .

The figure at right illustrates TIREM's modeling of multiple knife-edge diffraction . It is one of the physical phenomena thatextends the signal's propagation into the BLOS zone. To see a larger image of the Multiple Knife-Edge diffractionillustration, click here .

The figure at left illustrates troposcatter , another of the physical phenomena that extend the propagated signal into theBLOS 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 atmosphericTroposcatter, click here .

http://www.alionscience.com/images/tirem_LOSpath.jpghttp://www.alionscience.com/images/tirem_LOSpath.jpghttp://www.alionscience.com/images/tirem_LOSpath.jpghttp://www.alionscience.com/images/tirem_LandSeaPath.jpghttp://www.alionscience.com/images/tirem_LandSeaPath.jpghttp://www.alionscience.com/images/tirem_LandSeaPath.jpghttp://www.alionscience.com/images/tirem_ProfileEval.jpghttp://www.alionscience.com/images/tirem_ProfileEval.jpghttp://www.alionscience.com/images/tirem_ProfileEval.jpghttp://www.alionscience.com/images/tirem_MultKnifeEdgeDiffraction.jpghttp://www.alionscience.com/images/tirem_MultKnifeEdgeDiffraction.jpghttp://www.alionscience.com/images/tirem_MultKnifeEdgeDiffraction.jpghttp://www.alionscience.com/images/tirem_troposcatter.jpghttp://www.alionscience.com/images/tirem_troposcatter.jpghttp://www.alionscience.com/images/tirem_troposcatter.jpghttp://www.alionscience.com/images/tirem_troposcatter.jpghttp://www.alionscience.com/images/tirem_MultKnifeEdgeDiffraction.jpghttp://www.alionscience.com/images/tirem_ProfileEval.jpghttp://www.alionscience.com/images/tirem_LandSeaPath.jpghttp://www.alionscience.com/images/tirem_LOSpath.jpg

8/2/2019 Ti Rem

2/4

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 thestandard 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 brokendown for each. Selecting a frequency and a zone of sight reveals the model selection methods employed by TIREM. Toview 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 propagationpackage 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 descriptiongiven below is only a very brief summary. More detailed information can be found in the references at the end of thisappendix. The currently version of TIREM is maintained and updated by the Electromagnetic Compatibility AnalysisCenter (ECAC).

Version 7 1996-2008, EDX Wireless, LLC - All rights reserved. EDX Wireless Software Appendices Appendix A: Propagation Models A-5Basically, 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, itcalculates the degree of obstruction of the Fresnel zone and uses this to proportionally adjust the amountof 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 fromthe two methods are proportionally averaged. The attenuation calculated by the methods is adjusted by thedegree 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 clearancesin between 0.5 and 1.5 Fresnel zone radii, the additional path loss is a linear proportion between 0 and 100percent.In the LOS mode, TIREM also calculates what portion of the path is over smooth earth and what portionis over rough earth. If part of the path is over smooth earth, the path loss (on the entire path) is calculatedusing 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, smoothearth is defined as elevations equal to zero (sea level) or some other smooth earth elevation (a lake surface,

http://www.alionscience.com/images/tirem_PathLossGuide.jpghttp://www.alionscience.com/images/tirem_PathLossGuide.jpghttp://www.alionscience.com/images/tirem_PathLossGuide.jpghttp://www.alionscience.com/images/tirem_PathLossGuide.jpg

8/2/2019 Ti Rem

3/4

for example) which the user can define. In the EDX implementation of TIREM, only zero elevations (sealevel) are considered to be smooth earth.

When the path is obstructed, TIREM uses a multiple knife-edge diffraction loss routine like that describedunder 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 mentionedabove) and combine it proportionally with the loss found from the multiple knife-edge calculation. Thismethod comes in to play for obstructed paths over the ocean where the horizon at each point can presenta low angle obstacle. The EDX version of TIREM fully implements this approach for low angle multipleobstacles.In the obstructed path case, TIREM also modifies path loss with a "height-gain" function that is based onthe assumption that the rough terrain profile can be replaced with four circular arcs (earth radii). This is atenuous 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. Thepath loss from the troposcatter path is combined with the diffracted path loss as described above toproduce a net path loss to the receiver (net field strength). The troposphere path essentially is anotherhighly inefficient path from the transmitter to the receiver, resulting in a field strength that is slightly higherthan 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 thediffracted path. For this reason, the troposcatter mode in the NTIA version of TIREM is not implementedin the EDX version of TIREM.More detailed explanations of the TIREM model can be found in references [2] through [5] listed at theend

8/2/2019 Ti Rem

4/4