The question of how GPS receivers get the time is a fascinating one and it
turns out to be absolutely integral to how they determine position. That
is: it is impossible to solve for position without solving for time, and
vice-versa.

One key technical requirement is that all the satellites carry extremely
precise clocks are synchronized with each other. This allows each one to
send a signal to a receiver, essentially timestamped with the time the
signal was sent according to the GPS timescale. A receiver can then look at
the set of received signals along with their timestamps. The range from
each satellite is not initially known, because we do not yet know the
difference between our (receiver's) clock and the shared GPS clock.
However, because we know that all the GPS clocks are synchronized with each
other, we know that there's just a single time bias value to solve for, not
one for each received signal.

So, we can essentially ask: what clock bias would make all the measured
range values converge?

I drew a picture of this for a presentation I did some 20 years ago:
https://www.circlemud.org/jelson/writings/localization/sld020.htm . Let's
think about the problem in two dimensions first. The 3 dots in the picture
are satellites. If we somehow knew the exact range to each satellite, we
could draw a circle around it and know we were somewhere on that circle. If
we had two circles, they'd intersect at our location. Since we don't know
the range, but the range plus an unknown bias, we can add a third satellite
and then solve for the bias: the key insight is that only a single bias
values will make all the circles converge. In the picture, some
incorrectly-computed bias is shown as the dotted circles, you can see the
three dotted circles do not meet at a single point. There is just a single
correct bias value, shown as the solid circles, that causes the circles to
converge -- and thus we have solved for both our position (the point where
the circles intersect) and the time (the bias values that caused the
circles to intersect).

In three dimensions, the circles are actually spheres, and you
theoretically need four satellites instead of three to account for the
extra dimension.

Of course, because of errors, the imaginary spheres never actually
intersect in one place. More and more satellites let us get better and
better estimates of the true bias and true location because it lets us
average away non-systematic errors.

With this model in mind, you might also now see why "survey mode" works
well for timing receivers. If we eliminate the position as a variable, but
assume it is known, the system is even more overconstrained; we can use 3
more satellites to average away errors rather than to solve for position.
In fact, if we theoretically knew our position a priori, we could determine
the time with just a single satellite.

Taking this argument to its extreme, if we know our position and the
time, we need "0 satellites", i.e., we can determine the clock error on the
satellite itself! And this is how the clocks in the GPS constellation are
set -- a receiver that has canonical USNO time (e.g., because it's at the
observatory) and a surveyed position listens for transmissions from a
satellite, determines the time error, and sends back a message with a clock
correction.

-Jeremy N3UUO

On Thu, Jul 22, 2021 at 6:01 PM Robert DiRosario ka3zyx@comcast.net wrote:

Hi,
put the GNSS Ant. on the ground. If you use a magnetic antenna then use a smale metal reflector on the bottom.
Than use only Sat’s with 30deg elevation (clear sky) After 24h your location 3D Fix should be good for a proper 1PPS/ToD depends of your GNSS RX. Check DOP Value.

Thanks
Uwe

Gesendet mit der mobilen Mail App

Am 23.07.21 um 03:01 schrieb Robert DiRosario

Hi Robert,

Antenna mounting... it kind of depends. First priority is having a good unobstructed sky view.  (Imagine) lying on your back watching the sky, or stars at night. If there are only minimal obstructions down low in elevation along the horizon you have a perfect sky view. Another criteria is avoiding multi path. Having a close high rise building with a reflective wall will make your receiver get a strong reflected signal sometimes. This could be hard for the receiver to distinguish from the direct line of sight signal and cause position/time errors.

Modern high sensitive receivers, they work fine with less perfect sky view.

There is no need to elevate your GPS antenna if you already have a good sky view from a lower height.

Test your antenna site by watching SNR at your preferred site. There are control software (like Lady Heather) that will plot az/el vs SNR and by running that for a day you will get good information on how your receiver likes that antenna spot.

/Björn

Sent from my iPhone

Jeremy, thank you, especially for covering the 0-satellite case.

I wish stuff like this could be on a web page or FAQ or something, where it
was easily discoverable.

--
Sanjeev Gupta
+65 98551208    http://www.linkedin.com/in/ghane

On Fri, Jul 23, 2021 at 9:18 AM Jeremy Elson jelson@gmail.com wrote:

Hi

The most basic question is: how crazy do you want to go? This is TimeNuts so
crazy has very few limitations here …. :)

As mentioned in other posts, get to the point you have a clear view to the sky.
Ideally you want it clear to within 10 degrees of the horizon. You are concerned
both with blocking signals and with multipath. If things are clear to 10 degrees,
you stop using sats at 20 degrees (in the ultimate crazy approach …).

GPS for timing has turned into GNSS (multiple systems) for timing. These days
I would suggest putting up an antenna that covers multiple bands and multiple
systems. They aren’t terribly expensive from China. They do get a bit costly
from the name brands. L1 / L2 / L5 would cover GPS. An antenna that covers
those bands likely also gets the appropriate Glonass and Galileo bands.

Will your receiver use all the bands / all the systems? That depends on what you
use. The older cell tower gear is L1 GPS. The price of multi band / multi system
gizmos is coming down quickly. It’s a pretty good bet you will get one at some
point.

If a 10’ mast is the only way to get a reasonable view, go for it. Maybe put a
couple guys on it to keep it from wandering a lot.

With an antenna in a good (clear view) location, you will always have enough
sats in view to do a proper solution for time. If you are right at the north or south
pole GPS may struggle a bit.

Antenna gain does matter a bit. So does feed voltage. The survey folks seem
to like high gain antennas. The telecom guys seem to like low gain. You can
either put an amp in front of survey gear or an attenuator in front of a telecom
device. Neither solution is ideal. If you can, go for an antenna that is happy with
3 to 15V feed. An isolated DC feed is a simple way to handle any antenna.

Fun !!!

Bob

On Thursday, July 22, 2021 9:05:47 PM EDT Robert DiRosario wrote:

Well, applying the rule of thumb, 1 foot = 1 nanosecond at the speed of light,
which would make you think that two or three inches of wiggle (250ps) would be
down below most of your other noise sources.

Andrew KC2G