Very few Android phones have a pressure sensor.
When you remove the filter, it returns 12177 results. So only ~5% of phone models include a barometer.
That seems unlikely in an environment with no cellular / wifi signal. Theoretically possible, but expensive for battery and probably disallowed by the OS.
146/1109
now correct it for number of sold phones each (estimated by looking at the 70 most popular models) and we'll get why they said 1/4.
Note that's "sensitive", not "precise". The atmosphere changes pressure with the weather, so you don't get accurate absolute altitude - at least not without knowing the local "pressure altitude". Barometric pressure can vary in a way that represents plus or minus 300 or 400m of altitude, and can swing between high and low fairly rapidly in extreme weather events.
But over short timeframe the change in barometric altitude can be very useful. Glider pilots (including paragliders and even RC gliders) often use very sensitive barometric pressure sensors to detect when they're in rising to sinking air, down at the few meters per minute range of sensitivity.
Because the SATs only give you a pseudorange distance between you and the sat, so each say is most accurate solving for distance to/from that sat, and much less helpful resolving angle to the sat.
With a clear SkyView, around just under half of the sats are hidden by the earth.
This means that you get a full 360 degrees of data that can be near the horizon helping resolve lat/long.
But only about half of that sky is helpful for altitude. The birds you can't hear directly below the earth would be the most helpful for improving the altitude fix if you could hear them.
Baro is handy because you can take the absolute altitude from GPS as a low frequency baseline and use the baro for high frequency changes. Then when GPS says we teleported +200 feet when a new sat comes into view, we can temper that that with baro information.
A trivial example would be people on different floors of a skyscraper—although I suppose gps works poorly indoors anyway. Still, even outdoors there are peaks and crevices, and on a steep slope a very trivial change in lat/lon could lead to a major change in altitude.
https://www.ncesc.com/geographic-pedia/why-is-my-gps-elevati...
But elevation maps are not detailed enough and position is not accurate enough to get accurate elevation. Think about standing on trail along steep slope. The position not being that accurate is fine since you know you are on trail. But altitude could vary wildly going up or down slope, or even up or down trail. It is probably similar to GPS vertical accuracy, but were going for more accuracy that barometer provides.
You also probably want an accelerometer.
[1] https://www.nstb.tc.faa.gov/reports/2020_Q4_SPS_PAN_v2.0.pdf...
Edit: at least the excellent Physics Toolbox Sensor Suite gives me a barometer signal indicating 102570 Pa right now.
I wonder if it'll Just Work [eventually, given enough repetitions], or if the crowd-sourced network location algorithms will filter it because it is dynamic.
I'd have to have missed the title of the post, not read the post itself, not read GP's comment, not thought about why there'd be a pressure change, to have missed that particular detail. I appreciate you trying to be helpful though. :)
The problem is, it was basically useless. The main use case for heart rate monitoring is continuously throughout the day/night, or during exercise. A watch is very good at this. An optical sensor on the back of your phone is not.
Periodically checking your heart rate by holding your phone in a specific way is not a useful feature for that many people.
Turns out, when you have a known luminance, white balance, and frame-rate... the DSP to grab heart rate from a finger is trivial.
But on the Galaxy S7 that I think is being alluded to here, it was definitely a separate sensor -- a MAX86902, IIRC.
Conclusion, Fitbit and Google heart rate monitors on those wearables are hot garbage. Cue some snooty googler insisting I'm doing it wrong somehow.