Miratlas · Sky Monitor network

FSOC Demo

Laser links between satellites and ground stations. Clouds cut them, turbulence slows them down: Sky Monitors measure both, so the network always routes through a clear sky.

Simulated UTC

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Inside a Sky Monitor

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Clear sky, sector by sector

The sky seen by the Sky Monitor's all-sky camera — The sky, and the satellites passing

The same frame, with the decor mask in red — Clouds and decor

Red: trees, relief and buildings. A satellite behind them can't carry the link.

When can a laser link reach the ground?

Satellites can send data to Earth by laser: far more bandwidth than radio, no spectrum licence, very hard to intercept. But light has to cross the atmosphere. Clouds stop it. Turbulence scatters it. Every minute, for every ground station, the demo asks three questions. The network is up when at least one station answers yes to all of them.

01 · Geometry

Is a satellite in view?

A ground station (OGS) can only point at a satellite high enough above its horizon: low in the sky, the beam crosses too much air. The demo tracks the highest satellite and keeps it until it sets, as a real station would, since switching satellites costs acquisition time.

\sin ε = \frac{(r_{sat} - r_{ogs}) \cdot {\hat{r}}_{ogs}}{‖ r_{sat} - r_{ogs} ‖}

ε \geq °

ε is the elevation of the satellite; r are positions in an Earth-fixed frame. The constellation is a simulated shell of satellites at km, inclined °.

In the demo: the bright dots in orbit. A linked satellite glows white.

02 · Clouds

Is the sky clear?

A cloud is opaque to a laser: when one sits in the line of sight, the link is simply cut. No amount of power gets through. This is the main cause of outages.

The Sky Monitor sees clouds day and night with a thermal camera looking up. A clear sky is very cold in the infrared (you see the cold of space); a cloud is warm, because it radiates heat back. Part of the air temperature is subtracted to compensate for the sensor warming in sunlight.

T_{sky} = T_{LWIR} - 0.3 T_{air}

clear \Leftrightarrow T_{sky} \leq °C

T_LWIR is the zenith temperature measured at 8–14 µm. Weight and threshold were fitted against the Meteosat cloud mask (the same rule as in SkyOps).

In the demo: the white layer drifting over the globe, drawn from the same weather the links react to.

A blocked link turns red and faint: the beam stops at the cloud tops and the station marker turns red. Working links are drawn green and bold.

03 · Turbulence

How much does the air blur the beam?

With a clear sky, the link closes, but not at full speed. Pockets of warmer and cooler air act like moving lenses: the beam spreads and twinkles (scintillation), so less light lands on the receiver and less data gets through. It is worst on sunny afternoons, when the ground heats the air.

Physicists measure it with the Fried parameter r₀: the size of a patch of air that behaves like a clean lens. Large r₀ means calm air. The Sky Monitor measures it at night on stars (DIMM) and by day on the Sun (scintillation sensor). Looking lower in the sky crosses more air, so r₀ shrinks with elevation:

r_{0} (ε) = r_{0} {(\sin ε)}^{3 / 5}

usable \Leftrightarrow r_{0} \geq cm

The light lost to the spreading, and the data rate that remains:

L = 10 \log_{10} (1 + {(\frac{D}{r_{0} (ε)})}^{2}) dB

R = R_{0} 10^{- L / 10}

D = cm effective receiver aperture, R₀ = Gbps nominal rate. Every 3 dB halves the throughput. This is a simplified long-exposure model, illustrative only: a real link budget also accounts for adaptive optics, pointing and fibre coupling.

In the demo: the shimmering column above each station, green to red as turbulence grows.

A link above the turbulence threshold turns yellow. Every beam thins and flickers more as its loss grows.

04 · Network

Can the network still deliver?

One station under a cloud does not matter if another one, elsewhere, has a clear sky. This is site diversity: the network hands the link over to a station that can close it. The network is available at a given minute when at least one station is up:

A (t) = \max_{i} a_{i} (t)

P_{outage} \approx \prod_{i} p_{i}

a_i(t) is 1 when station i is up at minute t. If sites have independent weather, the chance that all are clouded at once is the product of their individual chances: three sites cloudy 30% of the time each are all cloudy together only 0.3 × 0.3 × 0.3 ≈ 2.7% of the time. Sites far apart are what make weather independent, and Sky Monitors are how you know a site's p_i before you build.

In the demo: the availability bar at the bottom, one slot per minute. On the scripted day the whole network is one region: green when a station is up, yellow when only turbulent links remain, red when no station can. A recorded day has several regions, see below.

05 · Regions

Regions and rerouting

Users are served by the stations near them. Stations within a few hundred kilometres form a region; on the recorded day: Iberia, Provence, North Sea, Alps, US West and US East. While one station of a region has a clear sky, the region carries its own traffic. When clouds close over all of them, its traffic is rerouted over terrestrial fibre to the nearest region that still has a link: users stay connected, over a longer path.

The network is judged by how many regions still use their own sky. The more traffic leans on rerouting, the closer the network is to its limits:

S (t) = \frac{1}{R} \sum_{r} u_{r} (t)

u_r(t) is 1 when region r has a station up (clear sky, turbulence within the criterion) at minute t, and R is the number of regions. The minute is green when S(t) is at least ; yellow when fewer regions are clear but at least still have a link of their own, clear or turbulent; red below that. With six regions: four clear regions keep it green, three or fewer with a link turn it red. Thresholds are a demo choice, set in the configuration.

In the demo: under the network row, one thin row per region (green: its own clear sky, yellow: its own sky through turbulence, red: rerouted). On the globe, a dashed blue trace along the ground, like a circuit-board track, runs from a station of the rerouted region (red, no link) to the nearest station of the region serving it that still has one (green). Click a region's name to fly there.

About this demo

The day shown is real: , minute by minute, as measured by Sky Monitors. Each is named after a nearby large town, and its position is rounded to about 10 km. Over Europe, Africa and the Atlantic, the clouds are that day's Meteosat cloud mask (EUMETSAT). Elsewhere they are illustrative, borrowed from real clouds at the same latitude. Right above each station, its own measurement decides the sky. Minutes a monitor did not measure get a best guess (a neighbour's sky, or the station's own trend), shown dotted in the station panel. Town names: GeoNames (CC BY 4.0).

The day shown is simulated: weather is generated, then scripted so the key moments happen on cue, and the station positions are placeholders. The rules above are the ones applied to real Sky Monitor data in Miratlas' tools, so the same demo can replay a day of real measurements.

What a Sky Monitor sees, looking up

A Sky Monitor is a camera that films the whole sky, horizon to horizon, around the clock. From its pictures it works out, every few minutes and for every part of the sky, whether a laser could get through. This view replays a day and a night it recorded at a dark-sky site in Haute-Provence.

01 · The camera

The whole sky in one picture

The camera points straight up through a fisheye lens. The picture is the sky as you see it lying on your back: the zenith in the middle, the horizon all around, North at the top and East on the left. The distance from the middle tells the height above the horizon.

In this view: the middle window, a new frame every five minutes.

02 · By night

Counting the stars

At night the Sky Monitor knows which stars should be visible, from a star catalogue and the time. It checks how many it actually sees. A sector where all the expected stars shine has a clear, transparent sky; where they vanish, cloud or haze is in the way. That share is the sector's availability T: stars seen over stars expected.

03 · By day

Mapping the clouds

In daylight there are no stars to count, so the Sky Monitor maps the clouds in the picture instead and gives each sector its share of clear sky. By night it measures how transparent the sky is; by day, how much of it is covered. They answer the same question with different evidence, and the timeline keeps them apart.

04 · Sectors

The sky in 32 pieces

The sky is cut into 8 directions and 4 heights (15–25°, 25–40°, 40–60° and above 60°), so a station knows not just whether the sky is clear but where. Nothing is measured below 15°: there, the light crosses too much air for a link anyway.

In this view: the left window. A sector is clear (blue) when at least % of it is clear.

05 · The decor

Trees, relief, buildings

Every site has its own horizon: a hill, a tree line, a roof. The Sky Monitor learns it and paints it red on its overlay. A satellite behind it is out of reach, however clear the sky above.

In this view: the right window.

06 · The link

Picking a satellite

The satellites passing overhead are computed for that moment, the same constellation as on the globe. The ground station links to the highest satellite that is at least ° above the horizon, clear of the decor, and in a clear sector. When clouds close that sector, it looks for another one.

In this view: the green beam in the middle window. Its sector is outlined in the left one.

About this view

The frames and sector measurements are a Sky Monitor's own, recorded at a dark-sky site in Haute-Provence and shown at one of the demo's Manosque stations. The satellites are simulated, the same constellation as on the globe. Night availability is a star count, day availability a cloud map; neither is a loss in dB.