The erroneous GPS signal
When the codes no longer in line
The measurements based on the GPS signal are affected by errors from various sources, which can be classified according to whether they are linked to satellites, receivers or the propagation of the signal in the Earth's atmosphere. It was this last source of error that the two researchers decided to examine.
The ionosphere: a battleground between waves and electrons
To understand how a GPS signal might interact with the ionosphere, we must remember that the composition of the latter results from two complex processes. First of all, there is an ionisation process which is initiated by radiation from space, mainly the sun’s ultraviolet rays and x-rays. The photons (luminous particles) in these rays contain enough energy to strip the electrons (negative charge) from the neutral atoms and atmospheric gases. Some free electrons are then captured by positive ions according to a second process known as recombination. The result is a continuous competition between the ionisation and recombination processes, thus determining the overall electronic density of the ionosphere. The concentration of electrons can therefore vary at any time and depends on two main factors: on the one hand, the density of neutral atoms and molecules (the recombination process is less pronounced at high altitudes because there is very little pressure there) and, on the other hand, the amount of sunlight received from space. While the pressure gradient (which is governed by a physical law dependent on altitude) remains stable and regular, this is far from the case for sunlight. Indeed, daytime (day/night) and seasonal (summer/winter) variations as well as solar activity (solar eruptions, 11-year cycles, etc.) will considerably modify the concentration of electrons, and consequently the propagation of the electromagnetic waves, including the GPS signals.
(1) Remember that in a given environment, a particle or a signal can travel faster than light in this environment. Therefore, it isn’t a question of exceeding the speed of light in a vacuum!