Modern navigational systems that use radio-wave signals reflecting from or propagating through the ionosphere as a means of determining range, or distance, are vulnerable to a variety of effects that can degrade performance. In particular, systems such as the Global Positioning System (GPS), that use constellations of earth-orbiting satellites, are affected by space weather phenomena. In principle, the GPS uses known positions of satellites and their distances from a receiver to determine the location of the receiver.
When charged particles ejected from the Sun arrive at the Earth, they can cause perturbations in the geomagnetic field. Another effect is that in the ionosphere the electron density (number of electrons in a given volume) can vary considerably, both in time and space.
A GPS receiver uses radio signals from several orbiting satellites to determine the range, or distance, from each satellite, and determines from these ranges the actual position of the receiver. The radio signals must pass through the ionosphere and in so doing they are subjected to variations in the electron density structure of the ionosphere. Changes in the electron density due to space weather activity can change the speed at which the radio waves travel, introducing a “propagation delay“ in the GPS signal. The propagation delay can vary from minute to minute, and such intervals of rapid change can last for several hours, especially in the polar and auroral regions. Changing propagation delays cause errors in the determination of the range, or “range errors“.
The performance of single-frequency GPS receivers using Code Phase Tracking techniques can be significantly degraded by the ionospheric propagation delays. Use of dual-frequency GPS receivers can, under some conditions, compensate for most of the ionospheric propagation delays by measuring the different delays at the two frequencies. Ionospheric delay corrections for a region can be determined from a network of precisely-positioned dual-frequency receivers and then be transmitted in near-real-time to users of single frequency GPS receivers in the region. Such a system is operated by the Canadian Active Control System of Natural Resources Canada.
Another GPS technique uses Carrier Phase Tracking. In this technique, the phases of individual cycles of the carrier waves are compared. However, if the electron density along a signal path from a satellite to a receiver changes very rapidly, as a result of space weather disturbances, the resulting rapid change in the phase of the radio wave may cause difficulties for the GPS receiver, in the form of “loss of lock“. Temporary loss of lock results in “cycle slip“, a discontinuity in the phase of the signal. Very rapid variations (less than about 15 seconds) in the signal's strength and phase are known as “ionospheric scintillations“. Scintillations can be particularly troublesome for receivers that are making carrier-phase measurements and may result in inaccurate or no position information. Code-only receivers are less susceptible to these effects.
From another viewpoint, the GPS system provides continuous routine measurements of the Total Electron Content (the aggregate of electrons along each radio wave propagation path from satellite to receiver) along the multitude of varying signal paths to each receiving station in a regional or global network. These measurements permit the mapping of variations in the ionospheric electron density over a region. Such information can be of use for studying space weather phenomena themselves.
For further information on GPS, please visit the Publications of Canadian Spatial Reference System page.