[time-nuts] Do ordinary clouds adversely affect GPS reception?

[Fiorenzo Cattaneo]

Any kind of atmospheric disturbance has a measurable effect on GPS
space and time precision, the question is more about how much the
impact is. Presuming that the GPS receiver is stationary, the antenna
has a clear visibility of the sky, then "ordinary weather" like
clouds, moderate rain or snow, should not have a large effect. The GPS
signal will be attenuated by rain and snow, but precision should not
be impacted provided that the GPS receiver is operating in stationary
mode (I'm referring to stationary mode as I presume the goal here is
to get precise GPS or UTC time, not position).

The signal quality will definitely be very poor if keeping the antenna
window mounted, especially if there are nearby metal surfaces. I see a
significant difference in quality between an antenna by the window in
my basement (up to and including complete periodic loss of GPS lock)
versus keeping the antenna in the attic (composition roof in my case).
In the basement there is only 1/4 sky direct view as opposed to the
attic which has essentially unobstructed view. The antenna in the
attic never gave me trouble.

I have not compared the difference in signal quality between an attic
mounted antenna versus an externally mounted antenna so I don't know
about that.

I haven't seen loss of GPS lock under light to moderate rain, let
alone simple clouds. Heavy rain doesn't seem to be a big problem
either. Your friend's experience of losing GPS lock on cloudy days
seems very, very strange to me. Barring malfunctioning receiver or a
defective antenna, I would first make sure to have the antenna
externally mounted, or at least in the attic if the roof is
composition or wood shingles.

Your friend could collect NMEA data on PDOP, HDOP, satellites view
with signal strengths and how many satellites are tracked and part of
the GPS solution. I definitely see better signal quality with the
BG7TBL GPSDO as opposed to a Garmin 18x with serial output and PPS.
The Oscilloquartz Star IV GPSDO seems to give even better results than
the BG7TBL, although having just recently bought them, I don't have
enough statistical data.

Another thing to try would be setting the minimum elevation threshold
for a satellite to be included in the position and time solution to
make sure to avoid surrounding hills or mountains, if any. Both the
Oscilloquartz Star IV GPSDO as well as the plain old UBLOX-7 GPS
receiver with PPS allow you to set the minimum elevation threshold.
The BG7TBL GPSDO does not allow you to do that. You should look at the
specifications of the two GPS receivers.

At the very high end of measurements, there are observable effects
during weather fronts -- which are of course much more severe than
simply ordinary clouds or light rain. This article is rather dated but
very interesting. With dual frequency receivers (L1 and L2 bands) they
can observe precision errors down to the order of about 3 - 10
centimeters  :

http://geodesy.unr.edu/publications/gpsworld.may98.pdf



-- Fio Cattaneo

Universal AC, can Entropy be reversed? -- "THERE IS AS YET
INSUFFICIENT DATA FOR A MEANINGFUL ANSWER."

On Mon, Oct 21, 2019 at 11:01 PM Dana Whitlow <k8yumdoober at gmail.com> wrote:
>
> A friend of mine living in SE lower Michigan recently bought
> a Geppetto GPS clock, and swears that it tends to lose
> satellite lock on cloudy days but does OK on sunny days.
>
> He is admittedly using a very poorly-sited antenna,
> placed in a window because his house has aluminum
> siding.  He reports that his Garmin handheld GPS
> has much less trouble acquiring and maintaining lock
> on cloudy days than does the Geppetto, but still tends
> to show higher levels of probable position error on
> cloudy days.  I don't yet know if he takes the Garmin
> outside for these comparisons.
>
> Is this a real phenomenon, or is my friend just imagining
> things?
>
> Meanwhile I think I have finally persuaded him to install
> the antenna outside on the roof.
>
> Dana    (K8YUM)
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