Just happen to be evaluating my own GPSDO design and part of it is
evaluating a cheap GPS module
The measurement done is the phase of the PPS versus a Rb standard.
One measurement was done with a roof top GPS antenna, the other with the
antenna at an east facing window.
The difference is striking. Where the phase of rooftop antenna
measurement has very little phase wandering at all (the Rb is a bit out
of tune)
the phase with the east facing antenna wandered 6e-7 s in 30 minutes.
(see attached phase plot)
My location is several hundred kilometers south of Hadrian's wall and
one timezone to the east.
Erik.
On 1-3-2022 23:43, Bob kb8tq wrote:
Hi
A GPSDO with a poor sky view will not do well. They are designed with the
“assumption” that you will have a good lock unless the antenna gets hit by
lightning ( or some other catastrophe ).
Assuming you are south of Hadrian’s Wall, you will have GPS sats overhead
at least occasionally. The bulk of what you will be able to “see” will be to the
south. If you really have a good antenna location and are a bit north, you may
be able to “see” sats on the other side of the north pole. More or less, there
is a big coverage blank area over the poles with GPS.
The closer you get to either pole, you will need to have a fairly good view
closer
to the horizon than you would at the equator. Still something like a 20 degree
view should be “good enough”.
One way to get an idea of what’s what is to buy a $10 USB plug in GPS. Fire
up any of the many tracking / display programs and see what sort of plots you
get.
It might be a bit cold to sit on the roof with a setup like this, but at least
you
will get an idea.
Bob
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