I was curious to see that the StateVector type in Numeric.Estimator.Model.SensorFusion doesn't include estimation of the local relationship between barometric pressure and altitude. (Or, maybe it does, but it's implicit and I overlooked it?)

That state, QNH, is disseminated by voice in piloted flight, but it can quickly be inferred from GPS altitudes and barometer readings. It seems like it would be useful for small UAV flight too, since it would allow you to ride through lengthy GPS signal interruptions without losing absolute altitude information? (Perhaps you could pick a relatively safe horizontal navigation strategy like circling, hovering, or climbing above some mapped equivalent of the minimum vectoring altitude, and you'd have a decent chance of avoiding CFIT even with large horizontal position variances.

Downloading estimator-1.0.0...
Configuring estimator-1.0.0...
Building estimator-1.0.0...
Preprocessing library estimator-1.0.0...
[ 1 of 10] Compiling Numeric.Estimator.Model.Symbolic ( src/Numeric/Estimator/Model/Symbolic.hs, dist/build/Numeric/Estimator/Model/Symbolic.o )
[ 2 of 10] Compiling Numeric.Estimator.Quaternion ( src/Numeric/Estimator/Quaternion.hs, dist/build/Numeric/Estimator/Quaternion.o )
[ 3 of 10] Compiling Numeric.Estimator.Model.Pressure ( src/Numeric/Estimator/Model/Pressure.hs, dist/build/Numeric/Estimator/Model/Pressure.o )
[ 4 of 10] Compiling Numeric.Estimator.Model.Coordinate ( src/Numeric/Estimator/Model/Coordinate.hs, dist/build/Numeric/Estimator/Model/Coordinate.o )

src/Numeric/Estimator/Model/Coordinate.hs:37:90:
Can't make a derived instance of ‘Traversable NED’
(even with cunning newtype deriving):
You need DeriveTraversable to derive an instance for this class
In the newtype declaration for ‘NED’

src/Numeric/Estimator/Model/Coordinate.hs:56:90:
Can't make a derived instance of ‘Traversable XYZ’
(even with cunning newtype deriving):
You need DeriveTraversable to derive an instance for this class
In the newtype declaration for ‘XYZ’
Failed to install estimator-1.0.0

$ cabal --version
cabal-install version 1.19.1
using version 1.19.1 of the Cabal library
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3