Also, and this applies particularly to flight near the ground, the update rate for GPS is (or was last time I checked) about 1 second.
Update rate depends upon the receiver. My portable Garmin 196 updates several times per second (perhaps 4 Hz, but I haven't measured).
A jet making an approach at 140 kts is going about 225 ft/s. With a 700 fpm descent rate that would correspond to this approach speed on a 3 deg glide slope (VS in fpm is TAS in kts * 10 and divided by 2 as my instructor used to teach) even if the accuracy were perfect you would have already landed 11 ft high or low by the time you realized the error.
Normally, you start ignoring the altimeter and flying visually below 200 ft AGL (on an ILS) or about 500 ft AGL (on a non-precision approach). Some big planes do have autoland, but that probably uses a RADAR altimeter: after all, the regular barometric altimeter is allowed to be off by up to 50 ft either way.
For anything I'd be likely to fly in real life (most of my professional simulator time has been in airliners, space shuttles and now spacecraft)> 300 fpm descent rate (~5 ft/s)
the numbers (Cessna 150 eg) would be....
60 kts approach speed (~95 ft/s)
Yikes, that's a slow approach! Did you get that number by putting the 150 on a 3 deg glide path? I don't think anyone would approach that low in a light, single-engine plane unless on an ILS -- 5 or 6 deg is probably more typical. You want to be high enough that you can glide to the runway in case of an engine failure, and a 3 deg glide path is way too low for that.
Still enough to throw you off. Accuracy has improved dramatically since the elimination of SA, but the time lag is still a bugger.
Again, time lag depends on the receiver, but it's far better than one second with the newer technology. The big change for vertical accuracy is not from GPS itself but from WAAS -- that's why the FAA has approved precision GPS/WAAS approaches down to 250 ft DH (expect to see ILS's start disappearing from some smaller airports over the next decade).
All the best,
David
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