On Jan 22, 10:37*am, Greg Arnold wrote:
309 wrote:
[...]

The 277 nm radius at FL250 roughly covers San Francisco to the west, San
Diego to the south, and Salt Lake City to the east. *I am not sure that
this information is terribly helpful for pilots (power as well as
soaring) who would like to know if they can believe what their GPS is
telling them.


I am skeptical.


But pilots should always be skeptical about black box navigation.

The GNSS (Global Navigation Satellite System, a name that encompasses
all satellite navigation systems) community is well aware that jamming
has an effect on civil air traffic, powered and glider. That's why
the AIM outlines a procedure for medical flights to request that the
tests
be stopped. I have done a lot of medical flying, and a lot of those
small mountain airports have GPS approaches. If I had needed
the approach I could have asked ATC to ask the controlling agency to
stop the test. I'm sure this would have been an expensive proposition
for them, but there is also a strong life preservation ethic among
GNSS
engineers and operators. They go to work with the motto that
"Someone,
somewhere is depending on my work to save his life."

The latest word is that enhanced LORAN will be preserved as a
backup to GPS. I have never heard of a glider with a LORAN
receiver, nor have I heard of a handheld LORAN receiver, but
it might be something to consider. Many aircraft have inertial
reference units to supplement GPS; these are light but drift, so
usually there is some kind of Kalman filter blending the
solutions. One can also use DME/DME updating if there
are enough DME stations in sight, but DME uses a ton of
power and is unsuitable for gliders.