wireless: Topographical planning

Introduction

System planning is critical to the successful installation, operation and proper performance of any communication system especially in VHF and microwave systems, for the simple reason that they require a line of sight propagation pattern. Line of sight as used in radio system design describes a condition in which radio system device antennas are within a "visible" range within the radio horizon. Knowing that electromagnetic signals disperses as it moves from source to destination the line of sight clearance must take care of this dispersion and due attention should be paid to objects i.e buildings, small hills, trees and foliage along the signal path. There are many ways through which this can be accomplished i.e. using strobe lights, mirrors and extacts of topographical data from maps.

Feasibility and viability of a microwave link (point-to-point) is dependent on gains (Tx power, Tx & Rx antenna gains, in some analogue systems weighting and emphasis), receiver sensitivity and losses (Tx & Rx feeder/ filter losses and largely path loss). It is apparent therefore that good microwave paths will be relatively high points to give maximum line-of-sight availability, in what is referred to as 'Fresnel Zone' clearance.

Establishing line-of-sight

There are two ways of establishing line-of-sight; namely by creating a path profile, or by surveying the actual path. Conventional radio system planning may require using both methods to make a comprehensive plan. For the simple reason that a path survey undertaken by visiting sites and observing the path helps establish possible obstructions that may not be captured by maps and also helps make note of potential interuptions current or future. A path profile established from topographical maps, by reference to the contours of the map, can be translated into elevation profiles of the land between the two sites. Earth curvature is then added, as can be known obstacles. The Fresnel Zone clearance calculation can then be added and an indication of any clearance problems gained.

Topographical planning tools

Topographical maps (containing contours, co-ordinates i.e. WGS or UTM, appropriate resolutions i.e. 1:10,000 or 1:25,000 etc.

GPS

Binoculars

There are various software tools available, but most are reliant upon the availability of topographical data to the appropriate degree of accurancy being available in digitized format. There are also tools available in excel templetes that may be useful, one of which we are going to demonstrate.