Space Weather Effects on Communication and Navigation

Numerous communications and navigation services for both commercial and military users are currently provided by space-based platforms. Common examples include automobile navigation systems that use GPS signals for positioning data and credit card transactions requiring the exchange of information via satellite communications; the number of technologies relying on satellites continues to expand rapidly. Such services may be vulnerable to space weather effects as the radio waves used to transmit information pass through the earth’s ionosphere. The ionosphere is a partially ionized region of the upper atmosphere. The ionized gas significantly modifies the atmosphere’s refractive index, inducing a variety of propagation effects that may distort the phase and/or amplitude of radio signals and thereby degrade the performance of a given radio frequency (RF) technology. Radio waves transiting the ionosphere experience phase advance and group delay which manifests itself as a ranging error in single frequency GPS and radar applications. Because the delay is dispersive with an inverse square frequency dependence, however, systems that employ two or more appropriately separated frequencies can successfully correct the ionospheric delay; this technique was demonstrated very successfully in dual frequency GPS receivers and has been adopted by every other major global navigation satellite system (GNSS) including GLONASS, Galileo and Beidou. When the ionization exhibits significant structure or irregularities on scale sizes from tens to hundreds of meters, however, variable phase perturbations across the wave front can result in diffraction, essentially constructive and destructive self-interference of the wave. This phenomenon, known as scintillation, is characterized by fluctuations in phase and amplitude that affect both single and dual frequency systems. Here we examine the conditions under which scintillations occur in the natural ionosphere and describe the extent to which they can disrupt the performance of space-based communications and navigation systems, as well as both ground and space-based radar applications. Furthermore, we consider potential strategies to mitigate ionospheric impacts and briefly explore the use of these technologies as sensors to monitor the state of the ionosphere and detect conditions where system performance may be compromised by space weather.