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When flying on commercial flights for the past several years I've used GNSS on my smartphone to track my location for fun. The signal strength in the cabin isn't ideal, but it's generally good enough to maintain a lock for most of the flight. However, on most flights I will lose my lock shortly after takeoff until we finish our ascent, and again as we begin our descent until shortly after landing. This doesn't happen every time, but often enough that I expect it at this point. And I've noticed it happen with several different smartphones I've owned over the years.

Is there any reason why GNSS signal should be weaker in the ascent and descent stages? Is it harder for it to track significant vertical changes? Is the signal weaker in some of the atmospheric layers we pass through? Does the aircraft itself put out signals that might disrupt my phone's ability to pick up GNSS signals? Or is this just confirmation bias?

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    $\begingroup$ In theory yes, the wing, no longer horizontal, may hide satellites low on the horizon. This, added to the fact one half of the sky is already hidden by the fuselage, may prevent to lock on the four satellites required to compute a fix. Most GNSS receivers have a graphic plot showing the azimuth of the satellites in sight, this would be useful to confirm this possibility. $\endgroup$
    – mins
    Commented Feb 29 at 14:58

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Confirmation bias.

However, the aluminum fuselage does provide shielding from the signal. Your phone should be next to the window. But even then it's only receiving satellite signals from satellites "in view", and just a turn away once the phone has a lock on a particular satellite constellation, may force it to start searching for satellites again and you lose the position signal.

You will generally see GPS systems installed with a dedicated antenna on the roof somewhere so it's above the sheet metal.

My own airplane has a bubble canopy and doesn't need a dedicated antenna, and the built in antenna in my phone works fine for VFR navigating with no sheet metal to shield it.

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    $\begingroup$ And, on the ground, the phone can use cell phone towers to get positioning information. That assistance is lost shortly after takeoff. $\endgroup$
    – Ralph J ♦
    Commented Feb 27 at 4:09

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