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.
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.
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.
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
- Decision calculations based on
the knife edge model are performed to produce the Fresnel-Kirchoff
diffraction parameter ν.
parameter then substituted into Lee's approximation of attenuation over single
- 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)
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.
- Epstein-Peterson model below considers each significant
knife edge individually and sums each loss over the diffracting path
- Giovanelli method below identifies a dominant edge and calculates each loss
with respect to it, but creates separate observation planes for each edge
- Deygout model below identifies a dominant knife edge and calculates all losses with respect to it.
- 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.