C.1 Introduction

Calculating propagation loss is a fundamental tool in systems engineering for GNSS, since this loss relates the power at a source (e.g., a satellite transmitter or an interferer) to the power at a destination (e.g., a GNSS receiver). The propagation loss typically depends on the distance between source and destination, as well other factors.

The simplest common expression for propagation loss is called free-space propagation loss, since it applies in free space (source and receiver are located in a vacuum or equivalent, with no other objects in the vicinity). Although this expression is often employed, there are widespread misunderstandings of its applicability (under what conditions does it apply?) and its technical characteristics (e.g., in what sense is free-space propagation loss frequency dependent?).

Entire texts (e.g., [1]) are devoted to radio wave propagation predicting, measuring, and compensating for its effects. This appendix only touches on one simple and common model for radio wave propagation free-space propagation loss. It also addresses a related topic how to convert back and forth between power flux densities (PFDs) and power spectral densities (PSDs).

C.2 Free-Space Propagation Loss

Propagation loss is defined as the ratio of the power transmitted in the direction of the receive antenna to the power at the terminals of a receive antenna, for a unity-gain receive antenna. If the receive antenna has gain other than unity, the received power is divided by the receive antenna gain in taking this ratio. The transmit antenna actually radiates P