Satellites nearest a place

nearest and within in @johnmorrisdotca/chikyu-earth/satellites answer from the positions SGP4 already gives (exact). One call places each satellite once. It does not run again by itself, and nothing is drawn until a page asks.

import { nearest, within } from "@johnmorrisdotca/chikyu-earth/satellites";

const date = Date.now();
const near = nearest(stations, { place: { lon: -0.13, lat: 51.51 } }, { date, by: "space", count: 5 });
console.log(near[0].name, near[0].distance.kilometres.toFixed(0));
const overhead = nearest(stations, { place: { lon: -0.13, lat: 51.51 } }, { date, by: "sky", count: 3 });
console.log(overhead[0].angle.toFixed(1));

• A place. { place: { lon, lat } } and by: "space" is the slant range. A satellite straight overhead is its height away. by: "sky" is the angle from straight up.
• A direction. { azimuth, elevation, observer } and by: "sky" is the angle from that direction. Azimuth is degrees clockwise from north.
• Another satellite. { satellite: index } and by: "space" is the distance between them. by: "sky" is the angle at observer, or at the ground under the target when no observer is given.
• How sure. distance.uncertaintyKm is 1 km at the elements' epoch and 1 km more for each day they have aged. It is a rough figure of this package, not a published error.
• How close they come. closeApproach(group, a, b, from, to) samples every 60 seconds (or the step you give, at least 1 second). Two placements a sample. The uncertainty is how much further the samples on either side were.
• What it costs. One placement per satellite per call. Measured for one satellite (the ISS, elements of 2026-11-09): 0.15 ms. A catalogue of a few thousand is on the order of a second. Do not call it on a timer.
• On the globe. ringSatellites on <chikyu-space> rings and names the ones you pass, with the same rings as a labelled group. The list is kept. One that is outside the picture is ringed on the first later draw that shows it, and nothing is drawn at rest just to wait for it. placeSatellite says which edge it lies beyond (edge) and whether it is behind the camera (behind), so a page can mark the edge itself. The demo lists the five nearest when you pick a satellite and rings them.