ISS Tracker: Visible Passes

ISS Tracker works out when the International Space Station passes over your place so that you can see it: when it comes up, how high it climbs, which way it goes, and the moment it slips into the earth's shadow and goes out, often high in the sky.

A pass can only be seen when three things hold at once: the station is above your horizon, it is in sunlight, and you are in the dark. The app shows the three separately on every pass, and names the one that fails when a pass cannot be seen, rather than leaving a gap in the list.

Every pass is computed on your phone. The only thing fetched is the station's published orbit, once a day at most, from CelesTrak, with nothing about you or your place in the request. Without a connection the app keeps working from the last orbit it has, and says how old it is.

PASSES

• The next days of passes for your place, on your place's clock

• For each: the visible window, greatest altitude, duration, and the direction it rises and sets

• Where to look first, and when and where it enters the earth's shadow

• Where the Moon is and how much of it is lit

• Passes that cannot be seen are shown dimmed, with the reason: daylight, a sky still too bright, or the station in shadow

SKY

• One pass drawn on the sky from horizon to zenith, north up and east on the left, as you see it lying on your back

• The sunlit part of the track solid white, the part in shadow grey, and the moment it goes out marked with its time

NOW

• The point on the ground under the station this second, its height and speed, and whether it is in sunlight

• How far it is from you, and in which direction

WHEN YOU ARE NOT LOOKING

• A home-screen widget with the next visible pass and the age of the orbit it came from

• A note about ten minutes before each visible pass, with where to look and when it goes out

• A log of the passes you saw

READING

• Seven short articles: why the station disappears mid-sky, why passes crowd around dusk and dawn, how an orbit goes stale, why no pass lasts more than ten and a half minutes, the midsummer problem in the north, why the earth is not a ball, and how to tell the station from an aircraft

YOUR LIMITS

• The lowest altitude that counts (0 to 30 degrees) and how dark the sky has to be (the Sun 6, 9 or 12 degrees down)

• Red night mode: one dim red on black, and no ads, to keep your eyes adapted outside

• Several saved places; the place comes from a city list, typed coordinates, or the phone's rough position, and never leaves the phone

HOW IT IS COMPUTED

The orbit is propagated with SGP4, the model every published element set is fitted with, written from the published algorithm and checked row by row against the official verification set of "Revisiting Spacetrack Report #3": nine near-earth test satellites, 158 positions, largest difference 7 micrometres. The earth's shadow is computed as the cone it is, umbra and penumbra, and you stand on the WGS-84 ellipsoid rather than on a sphere.

The limit on accuracy is the orbit itself, not the arithmetic. An element set drifts as it ages and is wrong after the station raises its orbit, so the app always shows its age and asks you to refresh when it is old.

No magnitude is shown: a satellite's brightness has to be measured, and none is invented. Distance and the angle of the sunlight are shown instead.

Free to install, with ads. Everything in the app is free: both stations, a week of passes, the note before a pass and the log of passes seen. A single one-time purchase removes the ads and nothing else.