The GPS uncertainty circle — what the radius means

Walk up to where your rocket landed and you notice something: the reported GPS point is a single pin on the map, but the actual airframe might be a few meters east of it, or fifteen meters north. The GPS Uncertainty Circle is the dashed ring AltosUI draws around that pin to show you how much ground “last reported position” really covers.

Where the radius comes from

The circle is sized by the flight computer’s HDOP — horizontal dilution of precision — which is the number the GPS receiver itself reports about how confident it is in the fix. Good satellite geometry and a clear view of the sky give a small HDOP, which draws a small circle. Poor geometry, obstructed sky, or a weak signal give a larger HDOP, which draws a larger circle.

In practice the radius tends to fall into three bands: 3–5 meters in clear sky, 5–8 meters in typical field conditions, and 10–25 meters when the sky is obstructed or the satellite geometry is poor. If a flight computer’s GPS packets don’t include HDOP at all, AltosUI defaults the circle to 7 meters — a reasonable conservative estimate rather than a fake “zero uncertainty” display.

How it looks on the map vs. in AR

On the Recovery Map the circle is drawn at its real accuracy radius in meters, and it scales with zoom. Zoom out for context and the ring shrinks on screen; zoom in for the walk-up and it grows, covering real ground at the same rate you’re covering it with your feet. You’re seeing the true uncertainty area, just at different display sizes.

In the AR Recovery HUD the circle is drawn in the camera view at the rocket’s actual GPS position. It’s perspective-compressed by viewing angle — thin and far away when you’re hundreds of meters out, expanding into a meaningful patch of ground as you approach. That’s the point: when you’re standing close, the ring honestly shows you how wide “somewhere in here” is.

Reading a large circle

A big circle isn’t a software problem — it’s information. It tells you the rocket was reporting a weak or geometrically poor fix when the last GPS packet landed, which usually means one of two things. The rocket may have come down in a spot with obstructed sky: under tree canopy, in a gully, beside a tall structure, or behind terrain that blocked satellites from the receiver.

Or the fix quality degraded during descent — tumbling under drogue can rotate the antenna through poor attitudes, and low-altitude multipath near the ground can drop satellite count. Either way, the move is the same: search wider. Don’t stop at the pin; expect to sweep the area inside the ring, because the rocket’s true position is somewhere in that patch.

How to use the circle in the field

Treat the circle as your search radius when you get close. Walk to the reported point, then expand outward to the edge of the ring and quarter it visually. If the ring is small — say three or four meters — the rocket is almost certainly within a step or two of the pin. If the ring is twenty meters and the recovery terrain is tall grass or scrub, plan on sweeping for longer before you spot it.

The circle also helps you calibrate expectations before you start walking. Glance at the Recovery Map before you leave the flight line: a tight circle means a quick walk-up; a wide one means budget more time and probably bring a spotter.

Why this isn’t a standalone feature

On the site the GPS Uncertainty Circle is labeled a Technical Reference, not an Experimental feature. That’s deliberate: the circle doesn’t introduce its own interaction for you to learn — it just makes the AR HUD and the Recovery Map more honest about what the GPS fix actually means. The features waiting on field validation are the AR Recovery HUD, the Recovery Map, and the Flight Report. If you have feedback on how the circle behaves in practice during one of those, the experimental-features feedback guide is the place — frame it as feedback on the host feature, since that’s where the ring shows up.