writer: Edward Carolipio, Ph.D.
Position solutions accurate to a meter represent the holy grail of car navigation. For top-of-the-line systems, meter-level accuracy means knowing which lane the car is traveling in and which direction the car's going. For entry-level systems, meter-level accuracy immediately translates into performance rivaling today's top-shelf systems for a fraction of the cost. The improved accuracy also could have other unforeseen benefits.
With the suspension of Selective Availability (see the February 2001 issue of CA&E), the nominal GPS user now can get position solutions accurate to within 10 meters, compared to about 50 meters previously. Selective Availability was a purposeful degradation of GPS accuracy, so its removal simply was a matter of executive order. The next largest hurdle to meter-level accuracy is the ionosphere, and this physical barrier is more difficult to overcome.
The ionosphere slows down the GPS signal, introducing an error in the measured user-to-satellite estimate as large as several meters; also, the ionosphere attenuates the GPS signal and tweaks the carrier wave making it more difficult to lock on to and track the signal. A better receiver design likely can overcome these latter two effects, but single-frequency GPS users have difficulty accounting for the high variability of the ionospheric delay. A couple of options exist to correct for ionospheric delay, with differential GPS being the most viable path to achieving meter-level accuracy.
This article discusses the impact of the ionosphere on the GPS user. The first part is a brief description of the ionosphere, followed by a section on how the ionosphere delays, attenuates, and modifies the GPS signal. The third section discusses how the attenuation and modification of the signal can be alleviated through better receiver design and describes methods of compensating for the ionospheric delay. A summary is also included.
[The Ionosphere]The ionosphere is a dissipative layer that surrounds the Earth between 50 and 1,000 km above the surface (Figure 1). (Compare that height to the 20,000 km altitude of GPS satellites.) The sun's ultraviolet rays ionize gases in the upper atmosphere, freeing up bound electrons and creating a weakly ionized gas. This mixture of positively charged ions and negatively charged electrons is called a plasma. The composition is highly variable and depends strongly on the presence of sunlight. The total number of electrons, or total electron content, of the ionosphere is a measure of the degree of ionization and the parameter that best characterizes ionospheric conditions. During the day, the electron content and the mean height of the ionosphere increase as the number of ionized particles increases: the opposite is true when night falls.
The most visible evidence of the ionosphere are the auroras: the aurora borealis in the northern hemisphere; and the aurora australis (Figure 2) in the southern hemisphere; also referred to as the northern and southern lights, respectively. Charged particles tend to follow the Earth's magnetic field, which points into the ground in the polar regions. Auroras are the result of charged particles radiating light when they impact the ionospheric layer while flowing along the magnetic fields into the atmosphere near the poles.
Shortwave radio enthusiasts and AM radio listeners should be familiar with the ionosphere. This layer acts as a mirror for electromagnetic waves in these frequency bands and makes it possible to pick up broadcasts originating from beyond the horizon.