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Chapter 10
Optical Methods for Assessing Fade Margins
10.1 Background
Until recently, several measurement methods were used by investigators for assessing
fade margins for the following types of land-mobile earth-satellite scenarios: (1)
propagation paths with clear line-of-sight in a multipath environment, (2) paths through
shadowing media such as canopies of trees, and (3) paths which are blocked by buildings
or structures. The methods involved radiation from transmitter platforms on towers,
aircraft, and satellites that are stationary and non-stationary. Although significant
information has been derived from these measurements as described in the other chapters
of this text, the methods are generally labor intensive and expensive as they require a
transmitter platform, vehicle tracking systems, and complex receiver/data acquisition
system which are frequency and bandwidth dependent. More recently, Akturan and
Vogel [1997] and Vogel [1997] describe a unique measurement technique for estimating
fade margins. The method involves photographing the roadside image of ambient
structures, image reduction and analysis, and application of existing models assuming a
known constellation of satellites. The methods involved in imaging, photographing, and
analyzing the ambient scene are also referred to in this chapter as “photogrammetry”.
The models are associated with scenarios pertaining to propagation for (1) clear line-ofsight
with multipath reflections, (2) the shadowed state [Loo, 1985], and (3) the blocked
state [Karasawa et al. 1995]. In this chapter we describe the various experiments,
methodologies for reduction and analysis, and subsequent results that have hitherto been
derived employing this optical method. The “shadowed state” is here defined as a
condition where propagation takes place over the first Fresnel zone through a medium
which attenuates the signal (such a tree canopy). The “blocked state” is defined as the
condition where the propagation path within the first Fresnel zone is completely
obstructed such as by a building or a mountain, and the mechanism of signal reception is
accomplished through diffraction and multipath.