GPS Uncertainty Circle

See Where Your Rocket Actually Might Be

GPS is good, but it's not perfect. When your flight computer reports a position, the rocket is somewhere within a few meters of that point — not exactly on it. AltosUI now shows you that uncertainty directly, as a circle around the rocket's reported position in both the AR HUD and the Recovery Map. The circle represents the area where your rocket is most likely sitting.

What You See

In the AR HUD — A dashed, semi-transparent circle surrounds the diamond targeting reticle. The circle is small when you're far away (because a few meters of uncertainty doesn't matter much at 500 meters). As you walk closer, the circle grows on screen — because that same uncertainty becomes a bigger part of what you're looking at. By the time you're 20 meters away, the circle honestly shows you that "somewhere in here" is the best GPS can do.

On the Recovery Map — A dashed circle overlay appears around the rocket annotation, drawn at the actual accuracy radius in real-world meters. As you zoom into the map, the circle scales naturally. It gives you a realistic sense of the search area when you arrive at the reported landing coordinates.

Both views use the same color language: a semi-transparent fill with a dashed border, visually distinct from the "this is the rocket" indicator but clearly tied to it.

How It Works

The circle's radius comes from the flight computer's GPS receiver, specifically a value called HDOP — Horizontal Dilution of Precision. HDOP measures how well the visible satellites are arranged in the sky. When satellites are spread across the sky, geometry is strong and HDOP is low (tight circle). When satellites are clustered together, geometry is weak and HDOP is higher (wider circle).

The estimated accuracy in meters is roughly HDOP multiplied by about 2.5 meters. In practice:

• Clear sky, good fix: 3-5 meter radius (HDOP around 1.0-2.0)
• Typical conditions: 5-8 meter radius (HDOP around 2.0-3.0)
• Obstructed sky, poor geometry: 10-25 meter radius (HDOP above 4.0)

More satellites in view generally helps — not just because of count, but because more satellites means better geometric spread. Going from 5 to 8 satellites often improves accuracy noticeably. Going from 12 to 15 matters less if the geometry is already good.

When It Works Best

This feature is most valuable during the last 50-100 meters of recovery. At long range, GPS uncertainty is tiny compared to the distance — you're navigating to a general area and the circle barely matters. Up close, it's the difference between "the rocket is right here" (it might not be) and "the rocket is somewhere in this 10-meter patch" (now you're searching the right area).

It's also useful as a sanity check. If you see a very large circle, the GPS fix is poor — maybe the rocket landed in a gully or dense trees where satellite reception is weak. That's good information. You'll want to search a wider area rather than trusting the pin location.

What You Need

This feature works automatically whenever GPS telemetry is active. No additional settings or configuration. The uncertainty circle appears in both the AR mode and Map mode on the Recover tab.

If the flight computer's GPS packets don't include HDOP data, the circle defaults to a 7-meter radius — a reasonable conservative estimate for consumer GPS.

GPS accuracy visualization is part of the experimental recovery tools. We're refining how it's displayed and would appreciate your observations from real recovery walks.