Hi, this is Jeromy.

This is where I keep notes about I’m working on, working with or thinking about when I’m, ah, not working. Please see my bio for info about me.

collection of overlaid photos of Jeromy Evans

Pool of Errors

A pool of errors is a navigation aid that represents the accumulation of errors between where you knew your were and where you think you are now.

Sometimes I feel I’m drifting within a pool of errors.

One way a navigator estimates their position is by obtaining a fix. A visual fix identifies the relative direction (bearings) of a few reference points, draws lines to them and finds their position from the intersection.

illustration of a position fix between to islands

The navigator estimates position from the bearings to good reference points

In modern navigation, a constellation of GPS satellites provides the reference points. The GPS receiver calculates the fix, the time of day, altitude and velocity. However, in some environments the GPS system isn’t available or not good enough; for example, when it breaks down, is submerged, at high speeds and in space. A pool of errors is a technique used between fixes to accumulate the potential navigation errors over time. It tracks the worst-case position.

A submarine can’t use a GPS when submerged unless it raises a mast above water, which may be detected. A submarine can calculate fix from seabed features, but these active transmissions (sound) are also detectable. When submerged, a submarine’s primary navigation sensor are gyroscopes and accelerometers that use the boat’s own inertia to estimate position relative to movement from where it was. This calculation contains a degree of error that gets worse the longer it’s used.

illustration of an estimated pool of errors

The navigator needs to get a new position fix before the pool of errors intersects an obstacle

Anyway, the point of all this, is that sometimes my life feels like a pool of errors and I’m waiting on my next fix.