Looking at a cheap new barometric pressure sensor, the LP25H from ST.
It's got a built-in 24-bit ADC! And "High-resolution mode: 1 Pa RMS"! Awsumz!
So... is this good enough to fill in the ever-problematic Z axis for a blind and/or robotic landing, given a sufficiently accurate reading on local barometric pressure at the runway? Like, maybe a ground station with a similar sensor?
Scroll down... while the resolution may be (1000 hPa) / 224 or thereabouts, the absolute accuracy over pressure, at shirtsleeve temperature, is ±0.2 hPa. Plug some numbers into a handy online calculator: if the true pressure is 1000 hPa, the altitude is 363.6 feet; an error of ±0.2 hPa gives you a range of 358.1 to 369.2, so off by -5.5 to +5.6 feet.
Um. It looked worse at first glance. This is getting close to useful, actually.
Still, if the ground station's error runs the other way, that's still rather a large uncertainty as to when your landing robot should flare. So you need plenty of runway, or some better way of getting altitude clues to the robot. Like maybe a vision system and some colored lights.
Some of the new GPS modules claim ±1.4 meters (4.6 feet) horizontal accuracy; I haven't seen a vertical spec yet.
Oh, yes: the pressure sensor in question costs $4, quantity one. And the extra-accurate GPS module, a bit over $18. Cool, no? As usual, I'm thinking of applications both naughty and nice. The nice ones, I'm keeping to myself for the moment, in case I manage to do something with them. The naughty ones, I don't publish, 'cause I'd rather make evildoers do the work of coming up with their own diabolical schemes. (And not in any way because I'm thinking of selling ⌧⌧⌧⌧⌧ ⌧⌧⌧⌧⌧⌧⌧⌧ to ⌧⌧⌧⌧ ⌧⌧⌧⌧⌧⌧⌧.)