Watch the ISS fly over

The Satellites tab predicts when a satellite crosses your sky. It uses SGP4, the standard model for the two-line element sets (TLEs) that the U.S. Space Force publishes for every tracked object. Skyfold's implementation reproduces the official verification cases of Vallado and colleagues (2006) to well under a metre.

Open File → Templates… and choose ISS passes over Utrecht to start with real elements of the International Space Station.

Get orbital elements

Elements go into Two-line element sets (paste one or more). A set has an optional name line followed by two lines that start with 1 and 2 . You can paste several satellites at once.

To get fresh elements, type NORAD catalogue numbers into NORAD catalogue numbers (25544 is the ISS; up to five numbers separated by commas) and press Fetch from CelesTrak. The request goes through the usefulweb.site server, which keeps each answer for two hours — CelesTrak updates its data about that often and asks users not to download more frequently.

You can also open a .tle or .txt file with File → Open file…; Skyfold puts the elements into this tab.

Set the search

• Days to search starts at the date and time in the side panel and runs for up to seven days.
• Minimum elevation (°) skips low passes behind trees and houses. 10° is a good start.
• Only naked-eye visible passes hides passes you cannot see.

Read the pass table

Each row gives the satellite, the date, and three moments with direction: Rise (it climbs above your minimum elevation), Highest (the culmination with its elevation) and Set. Directions are degrees from north with a compass word, for example 216° south-west.

Condition explains whether you can see it:

• visible — the satellite is in sunlight while your sky is dark (Sun at least 6° below the horizon). Visible then shows the time window.
• in daylight — the sky is too bright.
• in Earth's shadow — the satellite is dark. In the middle of a winter night, the ISS often passes unseen.

Below the table a small chart draws the track of the first visible pass (or the first pass) across the sky as you see it looking up.

How long are elements good for?

Element sets lists each satellite's epoch and Age (days). Predictions drift as elements age — a minute or more after a few days, and the ISS reboosts now and then. Skyfold warns when the elements are older than three days; fetch fresh ones before a session.

Keep the passes

Add to plan puts a pass into your observation plan; Export → Satellite passes as CSV writes all passes with UTC times, azimuths, elevations and visibility for other tools.

Next: sun, shadows and the day-and-night map.