ITU-R P Aeronautical Propagation Model Guide

Transcription

1 ATDI Ltd Kingsland Court Three Bridges Road Crawley, West Sussex RH10 1HL UK Tel: + (44) Fax: + (44) ITU-R P Aeronautical Propagation Model Guide Author: Title: Cyprien de Cosson Product Manager Dated: 29 February 2008 Issue: 2.00 Advanced Topographic Development and Images Ltd Registered in England No Registered Office: 19 New Road, Brighton, BN1 1EY

2 0. Contents 1. Introduction ATDI Aeronautical Model Core Transmission Model Obstacle Diffraction Model Getting Started Software Installation ICS Telecom & HTZ Warfare Technical Support Further Information References... 7 Appendix (i) Core Model Validation

3 1. Introduction The ATDI Aeronautical Propagation Model is an extension model to ATDI planning tools for use in the planning and coordination of aeronautical mobile systems. The model is compliant with ITU-R P and includes an optional model in order to account for terrain obstructions that give rise to excess losses with respect to smooth earth conditions. 2. ATDI Aeronautical Model The model is an hybrid based upon a core transmission model and an optional obstacle diffraction algorithm. The core model reproduces the published set of reference curves in ITU-R recommendation entitled Propagation Curves for Aeronautical Mobile and Radionavigation Services using the VHF, UHF and SHF bands. The obstacle model is based upon the recommendation ITU-R P Propagation by Diffraction, and if selected will be added to the loss of the core transmission loss. The model manifests itself as a dll that may be used in conjunction with ATDI planning tools. 2.1 Core Transmission Model The core transmission loss model is based upon an implementation of the IF-77 propagation algorithm, from which the original curves of ITU-R P were obtained. This means that heights and frequencies that do not exist in the published curves are computed rather than interpolated. The model is constrained to give basic transmission loss for 5%, 50% and 95% of the time for antenna heights in the range 0.5 to at least 50,000m. The frequency range of the implementation is limited from 100MHz to 20GHz and is applicable to paths of up to 1800km. The core loss is calculated using a smooth (terrain parameter h 0) earth with an effective Earth radius factor k of 4/3 (surface refractivity N s 301) along with compensation for the excessive ray bending associated with the k 4/3 model at high altitudes. Constants for average ground, horizontal polarization, isotropic antennas, and long-term power fading statistics for a continental temperate climate are specified, which may be considered either reasonable or worst-case for many applications. With the exception of a region near the radio horizon, values of median basic transmission loss for within-the-horizon paths are obtained by adding the attenuation due to atmospheric absorption to the transmission loss corresponding to free-space conditions. Within the region near the radio horizon, values of the transmission loss are calculated using geometric optics, to account for interference between the direct ray and a ray reflected from the surface of the Earth. The two-ray interference model is not used exclusively for within-the-horizon calculations, because the lobing structure obtained from it for short paths is highly dependent on surface characteristics (roughness as well as electrical constants), atmospheric conditions (the effective Earth radius is variable in time), and antenna characteristics (polarization, orientation and gain pattern). Such curves would often be more misleading than useful, i.e., the detailed structure of the lobing is highly dependent on parameters that are difficult to determine with sufficient precision. However, the lobing structure is given statistical consideration in the calculation of variability. For time availabilities other than 50%, the basic transmission loss, does not always increase monotonically with distance. This occurs because variability changes with distance can sometimes overcome the median level changes. Variability includes contributions from both hourly-median or long-term power fading and within-the-hour or short-term phase interference fading. Both surface reflection and tropospheric multipath are included in the short-term fading

4 2.1 Obstacle Diffraction Model The core model accounts for smooth earth diffraction and no account is taken of irregular terrain obstacles. Whilst IF-77 has an obstacle model, this is not applied, because it is a simple model that suffers from a limitation that the highest antenna height must be greater in altitude than the highest obstacle. This constraint makes the model of limited use where low altitude aeronautical platforms are to be modelled. In order to overcome this limitation, the ATDI Aeronautical Propagation Model contains an optional algorithm based upon the cascaded knife edge method as specified in of ITU-R P to account for the excess diffraction loss attributable to irregular terrain effects. This model upon the geometry of the terrain profile between the ends of the path and is based on the Deygout method limited to a maximum of 3 edges. For line-of-sight paths it differs from the Deygout construction in that two secondary edges are still used in cases where the principal edge results in a non-zero diffraction loss. In order to avoid the double counting of smooth earth diffraction losses, the diffraction method is applied twice, once to a smooth earth profile, and the second time to an irregular terrain profile. The excess irregular terrain diffraction loss is taken as the difference between the diffraction losses computed in the two different cases

5 3.1. Getting Started 3.1. Software Installation This software is currently targeted for use with ATDI planning tools running under Windows 2000, 2003, XP or Vista. Other hardware and software requirements are as per these tools. This version is currently compatible with ICS Telecom and HTZ Warfare. To install the model run the setup on the supplied disk. This will install the ATDI Aeronautical Propagation Model to the default locations for ICS Telecom and HTZ Warfare. The installer creates shortcuts to ITU-R P Curve Viewer application and to this guide under Programs > ATDI Software > Aeronautical Model. Please ensure that you have read and agreed to the software licence agreement shown on the splash screen of any installed software prior to use. 3.2 ICS Telecom & HTZ Warfare In order to use the model in ICS Telecom or HTZ Warfare, the model must be loaded using the Propagation Models dialog by clicking on the small button near the end of the models list and selecting prop528.dl from the files listed.. This should display the following splash screen which contains the licence agreement

6 After accepting the agreement, this screen disappears and the model specific settings will be displayed as shown following. This dialog contains few options which must be selected as appropriate to the task in hand. The Time availability should be selected as recommended by the ITU. Typically an availability of 95% should be specified for wanted services and 5% for unwanted services. The height reference should normally be set to sea level. Where ground level is specified the Fast mode option performs an interpolation between curves rather than an exact calculation and should be used for coverage calculations where a speed gain of several orders of magnitude may be obtained. This mode normally yields an error of a fraction of a decibel, but may be greater in the transition region from the line of sight to diffraction region. The height reference should be synchronised with the options on the left hand side of the main screen of ICS Telecom. The LND setting is for all heights referenced to ground level and the R/S option for the receiver referenced to sea level. Once the model specific settings have specified, then click OK to save them. They may be recalled by the user at any time by clicking on the prop528 label from the Propagation models screen. The settings are saved in the file prop528.cfg which is stored in the same directory as the dll and not in the main parameter file of the planning tool. In order to copy the setting from one machine to another, then this file must be copied. Once the model specific settings have been selected, the other options in the main propagation dialog should be set. The most important option to check is the Flat earth profile sent to DLL. This must be ticked for correct operation as shown below. All other diffraction, sub path and climate models should be disabled. Clutter options may be set from Tools > Clutter options... by setting clutter excess height for each category, which will be used to modify the path profile altitudes used by the ATDI Aeronautical Model. Clutter attenuations should not be specified, since these are generally only applicable where the receiver is located in or just above ground clutter such as in land mobile systems

7 4. Technical Support To obtain technical support please make a note of the operating system in use, hardware components together with a brief description of the issue and how to reproduce it if there is a fault. If possible please include sample files and screen shots and send them via to: technical.support@atdi.co.uk Support may also be obtained by telephone on +44 (0) and fax +44 (0) Further Information For further information on this product or other propagation models, including the supply of this model as a component for use in other systems, then please contact the author by telephone on +44 (0) or by using the address cyprien.de.cosson@atdi.co.uk 6. References (i) (ii) Recommendation ITU-R P Propagation Curves for Aeronautical Mobile and Radionavigation Services using the VHF, UHF and SHF bands, International Telecommunications Union, Geneva, Switzerland Recommendation ITU-R P Propagation by Diffraction, International Telecommunications Union, Geneva, Switzerland - 7 -

8 Appendix (i) Core Model Validation This section contains a series of comparisons between values derived from the ATDI implementation of the aeronautical curves and those found in Annex II of ITU-R P The curves in the ITU recommendation were derived from the IF-77 propagation model upon which the ATDI implementation is based. These curves were originally produced by the Institute of Telecommunication Science and appear in a document entitled An Atlas of Basic Transmission Loss for to 15.5GHz published in This document describes a number of limitations of published curves which are related to the fact that the IF-77 is an hybrid model that accounts for a number of factors that affect propagation including long-term power fading, surface reflection multi-path and tropospheric multi-path. The model is therefore blended at the transitions between the line-of-sight, diffraction and scatter regions. These complications and the use of simple linear interpolation to obtain the plotted curves result in some bumps and discontinuities. For the most part these are minor, but some are severe enough to have been manually smoothed in the published document, particularly at higher frequencies. Furthermore, the curves do not exist in tabular format and capture of them in tabular form is difficult due to the skewed graphics. Therefore the curves have been validated with the ATDI implementation, by overlaying the calculated values from IF-77 onto the published graphics. These following pages show the curves plotted from IF-77 on the lefthand side, and then overlaid onto of each of the graphs from Annex II of ITU-R P on the right-hand side. Bearing in mind all of these limitations, there is generally excellent agreement. Height code h1 (m) h2 (m) A B C D E F G H I

9 - 9 -

10 - 10 -

11 - 11 -

12 - 12 -

13 - 13 -

14 - 14 -