GNSS Precise Positioning

One of the most common applications of satellites is positioning. Your car now shows where you are and guides you to your destination. For such a performance, your positioning system needs to get linked with at least four satellites of Global Navigation Satellite Systems (GNSS in jargon, of which the most famous is the GPS). GNSS signals are precious for space weather and space climate monitoring purposes as they allow to estimate the total electron content along the wave path, from which the state of the Earth’s ionosphere can be deduced. But if a solar or geomagnetic disturbance occurs, the electron content of the ionosphere changes and can became erratic. This changes the opacity of the air for the waves. Instead of coming to you in a straight line, the waves can be refracted and diffracted causing issues in keeping the tracking with the affected radio-links and degrading your estimated position with significant error. You have all experienced it: suddenly, the screen in your car positions you next to the highway you are driving on. You laugh: "this GPS is crazy". No, it's not: an electronic cloud due to a burst of solar activity has just passed between you and one (or more) of the positioning satellites. So, you know it's a horizontal error of a few meters. This is not a big deal, you think, because you know perfectly well that you are on the highway. But if your car was driven by a robot guided by this positioning system, how would you feel? Or if you were one of those truck drivers in the foggy Siberian tundra? The most sophisticated systems use countermeasures to avoid these errors. They have two frequencies on the satellites. If they give the same positioning, everything is fine. If they deviate from each other, they still manage to make corrections that give them an accuracy of about fifteen centimetres horizontally, but still several meters vertically, up to eight in case of a big magnetic storm. Again, this might seem ridiculous. It is not. A few meters vertically are the difference for a missile between a target and the school next door... And even with multiple frequencies, some applications are still subject to the vagaries of space weather and space climate. Many cities throughout the world are located on a plain at the foot of a mountain range. Numerous dams dominate them. If one of them breaks, sirens will sound in the city and the inhabitants will have to evacuate as soon as possible. These dams are therefore constantly monitored. Thanks to GNSS positioning sensors. It goes without saying that it is better to know the weather conditions in space than to evacuate four hundred thousand people because of a solar flare that would mislead the measurements...

Satellite Tracking

For decades, it has been accepted that a satellite at the end of its life would deorbit and burn up in the atmosphere. On average, one satellite or launcher stage per week enters the atmosphere. This represents only about one third of the initial mass. Since the beginning of the space era in 1957, there have been about 5600 launches generating more than 220 satellites fragmented in orbit, or about 8800 tons of space objects. This generated more than thirty-four thousand fragments of more than ten centimetres, circulating at about eight kilometres per second, ten times bigger and ten times faster than a bullet from a large calibre rifle, enough to damage a solar panel, to tear off an antenna, to perforate a cabin: no armour resists to particles of more than two centimetres of cross section launched at this speed. We have been able to catalogue and track 22,300 (as of February 2020) of these objects, "only", one would be tempted to say, but what about the 900,000 fragments of one to ten centimetres, and the one hundred and twenty-eight million smaller objects? They circulate haphazardly like so much dangerous dust above our heads. Space is gigantic and the probability of encounter remains low. But it grows exponentially with time. Thus, it is estimated that there have already been five hundred failures, collisions or abnormal events resulting in fragmentations and debris. And recently - in January 2007 - the number of space debris increased by 25% when China destroyed one of its own meteorological satellites, FengYun-1C, using also its own satellite as projectile. Only the USA had carried out such testing space destruction in 1985, attacking one of its solar wind observation satellites, well outside of densely occupied orbits. How does this relate to space weather and space climate? When a solar flare occurs, the strong emission of X-rays and extreme ultraviolet radiation that accompanies it heats the upper atmosphere on the dayside. The atmosphere expands and carries the debris with it. The two global orbital debris tracking centres - NASA's Langley and ESA's Darmstadt - lose sight of them. Their trajectory is now distorted by turbulent currents of the thermosphere, with the risk of damaging other satellites or, worse, the International Space Station. The movie "Gravity" has widely popularized this theme. It is serious and real. So, what to do? It took years for the UN to take up this problem. The UN alone had the legal authority to do so. It created the Committee for the Peaceful Use of Outer Space (COPUOS) which drew four lines of force: to know the situation, to protect from satellites, not to create debris and finally to clean up space. The first two points are undoubtedly the prerogative of space weather and space climate. Protecting from satellites requires, for example, to be able to calculate the orbits in case of a random re-entry, during a solar event. It is better to be warned that a flare is going to occur, because the radiation, once emitted, takes only eight minutes to begin its deleterious effects. Even if this is no longer a matter for space weather and space climate, space “cleaning” raises an important legal point. Suppose a given country retrieves a decommissioned satellite from another country, packed with electronics and, who knows, classified data. Who owns it? According to the laws of the sea, to first one. But does that apply up there? In 2008, France passed its own space law that includes binding requirements on space debris. After a transition phase, this law is in force as of December 10, 2020. A satellite can only be launched if, during its complete life and its decommissioning (not launch), the probability of causing at least one casualty is less than or equal to 0.00002. Is this low? That is one death for every 50,000 inhabitants, a little more than a thousand in a populated area like France. In case of an uncontrolled re-entry, this number is multiplied by five. It helps that space weather forecasts rapidly improve…