Horizontal error
Satellite in view (drag me)
Below mask (excluded)
Elevation-mask ring
Every GNSS receiver — the same navigation chip inside a phone, a car, or a ship — solves the same small linear-algebra problem dozens of times a second: given ranges to several satellites scattered across the sky, back out its own 3D position and clock error. This 2D companion draws the receiver's classic sky plot — zenith at the center, horizon at the rim, azimuth clockwise from north — and lets you drag satellites around it by hand. Drop them into a tight cluster and the geometry matrix nearly stops being invertible, so GDOP/HDOP/VDOP blow up and the horizontal error ellipse balloons; spread them across the whole sky and the same ranging noise collapses into a small, tight ellipse. It is the same geometry that makes a GPS fix drift in a dense urban canyon and lock down tight under open sky.