The Disturbed Ionosphere and Effects on GPS and GNSS Systems

Ionospheric disturbances are sometimes produced by a chain of events starting with the expanding solar atmosphere and sometimes produced by factors internal to the ionosphere and thermosphere. The important results in both cases are electron density irregularities and total electron content (TEC) gradients that affect and disturb trans-ionospheric electromagnetic waves. These disturbances may be regional and predictable on a seasonal basis such as those produced by equatorial spread-F or equatorial plasma bubbles. On the other hand, the disturbances may be associated with geomagnetic storms and have global properties extending from the equator to the poles and several earth radii into space. In yet other cases the disturbances may be very localized and associated with individual auroral arcs or traveling ionospheric disturbances whose physics is not understood. The goal of this paper is to present examples of space weather effects on GPS signals and differentially-augmented Global Navigation Satellite Systems (GNSS) to demonstrate the variety and significance of ionospheric disturbances.

The effect of the ionosphere on electromagnetic signals is produced by two physical processes. The first process is refractive in which TEC produces group delay and phase advance of signals between the transmitter and receiver. Typical, undisturbed values of TEC vary from 10-40 TECU (1 TECU= 10^16 electrons/m2) depending on the solar elevation angle and geomagnetic latitude. One TECU produces 16 cm of range delay and 0.84 cycle of phase advance for the GPS L1 signal at 1.6 GHz. The second effect is diffraction produced by electron density irregularities with spatial scales at the Fresnel length in the ionosphere. Diffraction is the phase-wise addition of signals from multiple, forward scattered paths in the ionosphere. The Fresnel length is where is the wavelength (19 cm for the GPS L1 signal) and d is the distance between the receiver and the scattering volume. Typically the Fresnel length is 350-450 meters depending of the altitude and slant range to the irregularities. Diffraction produces both amplitude and phase fluctuations in the signal and amplitude fluctuations can exceed 30 dB leading to loss of tracking for GPS signals. The effects of both the refractive and diffractive processes are frequently referred to as scintillations.

Spatial and temporal gradients in TEC lead to errors in differentially augmented GPS systems such as the FAA Wide Area Augmentation System (WAAS) or, in extreme cases, lead to loss of tracking. At mid-latitudes these gradients can reach 10 TECU/min during geomagnetic storms which WAAS can not accommodate either in accurately measuring the gradients, the spatial density of reference receivers is too small, or in updating the ionospheric correction model, the data rate is inadequate. When detected, these gradients cause loss of operational availability of WAAS and GPS navigation for commercial air travel up to tens of hours per event. At tropical latitudes similar TEC gradients produced by different physical processes complicate the delivery of WAAS technology and services. At polar latitudes the TEC density gradients associated with auroral arcs are so rapid that the GPS signal phase can not be tracked by dual frequency GPS receivers leading to cycle slips.

Generally, spatial gradients in TEC are also associated with density irregularities whose scale lengths include the GPS L1 Fresnel length. These irregularities are commonly detected by fluctuations in the signal carrier to noise ratio which is characterized by the amplitude and time scale of the fluctuations. Loss of tracking depends on the amplitude of the carrier to noise ratio decrease, called fading, the temporal length of the decrease or fade, and the parameters of the receiver tracking algorithm. The fade amplitude depends strictly on properties of the ionospheric irregularities but the fading time scale is reference frame dependent. To first order fading can be modeled as a spatial pattern translating in the horizontal plane at speeds of 50 m/s to 1 km/s depending on the ionospheric drift velocities. Hence a moving receiver, such as an aviation receiver, can match the velocity of the translating pattern leading to fade lengths of several seconds or longer. If the carrier to noise ratio is small within the fade, loss of tracking can occur and the likelihood of loss is increased during period of intense signal or phase dynamics.

Examples of both refractive and diffractive effects on GPS signals and GPS receiver operation will be presented. In some cases the effects are well characterized and their frequency is predictable over seasonal and solar cycle epochs. In other cases our understanding of the ionosphere, thermosphere, and magnetosphere is inadequate to characterize or predict these effects.