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Monday, Dec 11 2017 02:08 UT

VHF and UHF Area Prediction Tool Details

Digital Elevation Models and Raster Imagery

  • Main input data source being DEM (Digital Elevation Models) derived from Satellite and LIDAR sources.
  • DEMs measure the highest point below a nominal observer hovering the earth (data can include buildings and trees).
  • Imported into software in square tile or irregular format.
  • Variable resolution from 5m to 1km.
images: satellite, LIDAR airoplane, satellite map of Tasmania

Mapping and Coordinate Systems

  • GIS requirements for use on Australian terrain data.
  • Incorporating Australian AGD66, AGD84 and GDA94 datums (GRS80 ellipsoid) and equivalent UTM projections for grid coordinates AMG66, AMG84 and MGA94.
  • Operations to perform conversions between Grid coordinates (Eastings/Northings) and Geographical (Latitude/Longitude) using Redfearn's formulae.
  • Distance and Height Scale factors for accurate distance calculations on the ellipsoid.
diagrams and equations

Empirical Propagation Models

  • ITU recommended Empirical Pathloss models such as Okumura-Hata and Longely-Rice
  • Okumura-Hata model variations for Large Cities, Medium Cities, Suburban Area and Open/Rural Areas. Valid for:
    • 150MHz < f < 1500MHz
    • 30m < Htx < 200m
    • 1m < Hrx < 10m
    • 1km < d < 20km
  • COST 231 Hata model for 1500MHz-2000MHz.
equations

Knife Edge Diffraction

  • Semi Deterministic pathloss models employ knife edge diffraction for evaluating hilly terrain and finding losses in shadowed regions.
  • A terrain cross section profile is produced between the Tx and Rx which is then passed through a convex hull function to find diffracting radio path.
  • Decision calculations based on the knife edge model are performed to produce the Fresnel-Kirchoff diffraction parameter ν.
  • Fresnel-Kirchoff parameter then substituted into Lee's approximation of attenuation over single diffracting edge.
  • Used in conjunction with the Friis transmission equation for pathloss (dependent upon Fresnel zone clearance).

Path loss (dB) = 32.44 + 20 log d (km) + 20 log f (MHz)

diagrams and equations

Each model differs in its approach to determining the inputs to the Fresnel-Kirchoff diffraction parameter ν equation, and for what edge contributes most to the loss.

  • Bullington model below takes simplest, least accurate approach and reduces the profile to a single knife edge.
    Bullington model diagrams
  • Epstein-Peterson model below considers each significant knife edge individually and sums each loss over the diffracting path
    Epstein-Peterson model diagram
  • Giovanelli method below identifies a dominant edge and calculates each loss with respect to it, but creates separate observation planes for each edge
    Giovanelli method diagram
  • Deygout model below identifies a dominant knife edge and calculates all losses with respect to it.
    Deygout model diagram

Antenna modeling

  • Vertical and Horizontal gain patterns loaded in from a manufacturers antenna data file.
  • Pattern multiplication performed for an approximate 3D representation.

  • Full gain pattern can be incorporated into propagation model via a simple ray-trace function and added to the pathloss equation.
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