Satellite Geometry Matters
Two receivers can see the same number of satellites, with the same individual signal quality, and still produce solutions of very different quality, because of one factor that has nothing to do with signal strength: where those satellites happen to sit relative to each other in the sky.
Satellite geometry describes how spread out, or how clustered, the visible satellites are across the sky. Well-spread geometry sharpens a position solution. Clustered geometry amplifies the same underlying measurement errors into a noticeably worse position.
Why Spread Matters More Than Count
Imagine trying to pin down a point using several measuring tapes stretched from different directions. Tapes pulled from widely separated directions intersect sharply, pinning the point down precisely. Tapes pulled from nearly the same direction intersect at a shallow, uncertain angle, and small measurement errors translate into a much larger uncertainty in where they actually cross. Satellite ranging works the same way. Satellites spread widely across the sky, at a range of elevations and directions, intersect sharply. Satellites clustered together, even several of them, intersect shallowly, and the same small ranging error produces a much larger position error.
Adding more satellites to a shallow, clustered geometry helps less than moving even one satellite to a completely different part of the sky. Geometry is about spread, not simply about count.
PDOP, and Why It Is Worth Knowing About
This effect is commonly summarized by a single number called PDOP, position dilution of precision, with a related figure called GDOP that also folds in the receiver clock. A low PDOP, generally under two or three in modern multi-constellation environments, indicates strong geometry, where ranging errors are not amplified much. A high PDOP indicates weak geometry, where the same ranging errors translate into a noticeably larger position error. Most field software displays this number, though it is easy to overlook once FIX is showing.
A high PDOP does not necessarily prevent a receiver from reaching FIX. It can still resolve integer ambiguities under weak geometry. What suffers is the underlying position quality and the reliability of that resolution, not necessarily the FIX status itself.
Why Vertical Suffers More Than Horizontal
GNSS satellites only ever appear somewhere in the upper half of the sky, never below the horizon. That structural limitation means the geometry available for solving the vertical component of a position is always inherently weaker than the geometry available for the horizontal component, regardless of how many satellites are in view. This is the same underlying reason that RTK vertical solutions are typically noisier than horizontal ones — a limitation baked into the geometry itself, not a correctable flaw in any particular receiver.
A receiver reports FIX with eleven satellites in view, but PDOP is unusually high. Is FIX still meaningful? It can be, but the position it represents carries more uncertainty than the same FIX status would under stronger geometry. The satellite count alone does not tell the whole story.
Satellite geometry, not satellite count alone, determines how much ranging error gets amplified into position error. PDOP summarizes that geometry in a single number, and vertical positioning is structurally more vulnerable to weak geometry than horizontal positioning, because satellites only ever occupy the upper half of the sky.
Geometry is one factor RTK cannot fully control. Distance from the correction source is another, and it shapes performance in its own distinct way.