Test a site before you build on it
It is far cheaper to reveal adverse atmospheric conditions on paper than after installing equipment and running operational tests. Pathfinder lets engineers evaluate every candidate site on the globe at once, and keep only the most promising for on-site characterisation.
Open the toolFour numbers that describe what the atmosphere does to a laser beam
From the distribution of turbulence along the altitude, Pathfinder derives the integrated parameters that size an optical link and the mitigation it needs.
The size of the region over which the wavefront stays roughly undistorted: the strength of turbulence integrated over the whole air mass. Larger means calmer. From a few to several dozen centimetres, typically 4 cm at mild low-altitude sites; it grows with wavelength.
The maximum angle over which the wavefront stays valid across the link: the spatial validity domain of a turbulence state. Usually within 10 arcseconds, inversely proportional to wavelength.
How long the received signal keeps a coherent phase with the transmitted one: the temporal validity domain. A few milliseconds, which sets the maximum response time of the mitigation, adaptive optics, coding.
The distribution of turbulence strength along the altitude, born of temperature, pressure and humidity fluctuations. A typical profile peaks at the tropopause, with strong turbulence in the ground layer.
Local characterisation, global maps, site combinations
Statistics on the turbulence parameters over the years reveal the character of a site: strong or weak turbulence, better by day or by night, in winter or in summer. Averages per hour and per season expose the patterns: here r₀ dips around midday, more so in summer than in winter.
Any parameter can be mapped over the whole world to reveal global patterns: here the daytime cloud cover during the northern-hemisphere summer.
Combining turbulence and cloud cover dismisses the worst locations. Then, from the decorrelation of their weather, Pathfinder proposes the best and smallest subset of sites that reaches a target network availability, 99.9 %, for instance.
The model shortlists, the Sky Monitor confirms
Modelling is limited in temporal and spatial resolution; daytime turbulence is a very local phenomenon where a few metres can change everything; and pollution, aerosols and dust cannot be modelled at all. Pathfinder should therefore be followed by in-situ analysis with a Sky Monitor installed as close as possible to the future station, through the seasons. Only in-situ measurement confirms the availability and capacity of a location.
