◆ LIT — verified / checkable
Doppler positioning (the Transit system, 1960s): as a satellite crosses on a known track, the received frequency shifts by Δf/f = −v_radial/c. The radial velocity sweeps from + (approaching) through 0 (at closest approach) to − (receding), so the frequency traces an S-curve whose zero-crossing marks the time of closest approach and whose steepness at that crossing is inversely related to your perpendicular distance from the track. Read the crossing-time and the slope off one pass and your position is fixed. The instrument builds the real S-curve live from the geometry.
▲ AMBER — the figure
A single-pass 2-D toy with a fixed transmitted frequency and no clock/ephemeris error; the real Transit fix integrated the Doppler count over a pass and solved with the satellite's precise orbit. The shape of the curve — zero-crossing at closest approach, steepness set by your offset — is the exact geometry.
a point is fixed from THREE — and the point they fix is Top, at the centre.