StarFix Sextant works out where you are by photographing the night sky. No GPS. No network. No compass. Everything is computed on your iPhone, and nothing ever leaves it.
HOW IT WORKS
Aim the camera at a clear patch of sky and keep the phone still. StarFix Sextant stacks a burst of frames, finds the point sources, and identifies the star field against a catalogue stored on the device — starting from no idea where on Earth you are.
The stars alone do not tell you your position. They only tell the phone how it is oriented in space. What carries the position is gravity: expressed in the frame of the stars, your local vertical points at one spot on the celestial sphere. Its declination is your latitude, its right ascension your local sidereal time. Subtract Greenwich sidereal time and you have your longitude.
WHAT ACCURACY TO EXPECT
5 to 15 km in good conditions. 25 to 60 km in ordinary use. That is the accuracy of a bubble sextant, not of a GPS — and the app says so on its home screen rather than in the small print.
Every result carries its error ellipse, a 95% confidence radius, and a plain-language list of what degraded it: too few stars, a field aimed too low, a phone that moved, an uncertain clock.
LATITUDE AND LONGITUDE ARE NOT EQUALLY RELIABLE
Latitude does not depend on the time at all. Longitude depends on it entirely: one second of clock error moves it by 464 metres. StarFix Sextant shows each with its own error bar, and tells you when longitude is the weaker of the two.
WHAT IS INSIDE
• Fully offline — no network request of any kind
• Automatic star identification, no starting position needed
• Manual exposure, ISO and focus, with multi-frame stacking
• Multi-sighting protocol that measures and removes camera misalignment
• Error ellipse, confidence score and explicit caveats on every fix
• Diagnostics: every matched star and its residual, arcsecond by arcsecond
• Camera calibration mode
• Night-vision theme that preserves your dark adaptation
• History kept on the device alone
• Built-in self-test over the whole mathematical chain
• Simulation mode, to see the method work indoors or by day
WHAT IT CANNOT DO
It cannot work by day, through cloud, or without visible stars. It cannot reach GPS accuracy — three orders of magnitude separate 5 metres from 10 kilometres. It cannot tell you the time from the stars: the clock is an input, never an output. And it does not correct the deflection of the vertical, which adds a bias of 0.15 to 1.8 km depending on local terrain.
StarFix Sextant is a backup instrument, and an instrument for the curious. It is not a substitute for GPS, and it never pretends to be one.
