Both one-satellite and all-in-view methods are used for time
transfer. But all other things being equal you get the typical
sqrt(N) improvement with multiple concurrent SV in view so
it's more than just a redundancy trick.

Your concerns with multipath are valid; I suspect the more
serious the user the more attention is paid to the antenna
design and placement (e.g., choke ring antennas, remote
sites, dual frequency, etc.).

The traditional method of GPS common view avoids this by
carefully picking which SV to track at which specific times. Thus
there is no switching in the middle of any 13 minute track.

I'm not sure what the all-in-view time transfer solutions do for
this case. Someone can check FCS, ION, or PTTI abstracts
for a paper on this. Somewhere there must be a nice plot
showing measured accuracy by SV as a function of angle.
This would be a worse issue, except that even since the first
GPS receivers, provision is made for elevation mask angle.

I'm guessing low elevation is more than just a multipath
problem. The ionosphere correction may also be less accurate
at low angles. Perhaps Tom Clark can tell us about this.

Brooke, one cool thing you could do with your Oncore and a
cesium is to change your elevation mask angle, say once a
day, or day and a half, going all the way from 0 to 90 over a
week or two.

That single plot will tell a good story. You might guess that you
will get poorer results near 0 (obstructed view or excessive
multipath) and also near 90 (not enough satellites in view). But
where the sweet spot is, and how many tens of degrees wide
it is will be revealing.

/tvb

Hi Brooke,

Thank you for your suggestions, that page is probably ripe for a rewrite,
but I may just add a link to your page for now :-)

I would have thought the GPS receiver averages the readings when it locks on
multiple satellites. Are you saying it only uses one at a time? If so,
what's the point of a self survey?

I talk about WWV because most ham operators have a receiver that receives
WWV, so it is an inexpensive and convenient way to get an absolute
reference, even though it is not as good as GPS.

I am sure there is a reason why timing receivers track all the way down,
instead of switching to a better bird when one is available, but by setting
the mask angle fairly high (appropriate for your location and obstructions),
you can take care of this problem.

Didier KO4BB

Didier
Didier Juges wrote:

A weighted average is actually used as the measurement
quality/precision/accuracy varies from SV to SV due to geometry and
other factors.
Not one at a time (unless only one is visible) but the receivers with
less than 12 channels may not replace one of the 8 being tracked with a
"better" new candidate SV until one of the tracked SVs vanishes below
the horizon.
Only early GPS receivers tracked one SV at a time, swichting between
them periodically. Some carrier phase tracking receivers still do this
tracking each visible SV for a short while before switching to the next
in sequence.

Bruce

Hi Didier:

If you have a high elevation mask, as I do to avoid most of the tree tops, then
it's a common occurrence that there's only one SV in view.  But in position
hold mode the receiver can compute the time and the 1 PPS output is good.

I'm not sure how the receiver combines all the SVs that it's tracking, but all
that are being tracked are part of the solution.

The more accurate the position used to lock the receiver the better the
results.  It's about 1 ns per foot in air.  I hired the local surveyor who does
GPS to locate my antenna and a few spots around the house that are on even
second Lat and lon numbers, like 39:11:25.00000.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

Didier Juges wrote:

"Didier Juges" didier@cox.net wrote:

Average is perhaps too simple of a word.

The good GPS receivers (here I'm assuming we aren't talking about Trimble
SV6!) look at all the satellites in view and in mask, throw out obvious
outliers, and average among the remaining.

??? If you're wrong by 1000 feet about where you are, then that could
be hundreds of nanoseconds in time error. And that hundreds of nanoseconds
would jump around like the dickens as you chose different satllites
or the chosen satellite pans across the sky.

WWV is great. But again, you gotta know where you are to have the
right time. Fluctuations in propogation will change path length
and if you don't know the path length, it is NOT an absolute reference
in the nanosecond or even microsecond range. For typical conditions
the propogation delay for WWV cannot be estimated much better than
the tens of microseconds; combining WWV with a better "absolute"
clock in fact lets you study propogation by studying the shifting
of the various ionospheric layers. (There have been ham operators
who do this, including me!)

Tim. (N3QE)

Tim,

That's what I meant I suppose. The interesting part is that the timing GPS
receivers don't continualy try to select the best satellites from all those
it can see, so other than eliminating outliers, there may still be some that
are not far out enough to be eliminated, but will negatively affect the
average. What is the criterion to determine an SV is an outlier? How far off
should it be?

An algorithm could select the best 3 at all times for instance, even if 5
are visible and unmasked. I am not sure that would be better, but it's a
possibility. On the other hand, I suppose that's what the mask if for. Maybe
we need a dynamic mask so that we could force the receiver to never look at
more than the best 3 or 4?

I agree, but to do a self-survey, you need to look at more than one
satelite, which was my point.

OK, once again I confused "frequency standard" and "time standard". I
promise that's the last time :-)

Yes, it's interesting to look at the WWV signal wander on the HP 3586,
compared to it's OCXO, and while listening to it on the speaker or on a
receiver. I need to plot it over a 24 hour period or so.

Didier KO4BB

"Didier Juges" didier@cox.net wrote:

Didier -
I think this is what many are doing when they set their timing
receiver to only use satellites above a given elevation.

Oh, all the timing receivers I know of refuse to self-survey without
at least 6 satellites in view. By definition it's impossible to do
without at least 4. Presumably those who advocate very picky elevation
angles dial the angle up only after getting a decent survey. I think
the Motorola timing receivers specifically disable any custom elevation
angle mask before doing a self-survey; at least that's the way the one
in my Z3801A works (although to be honest it might be the firmware
in the Z3801A and not on the GPS board).

It depends on where you are and how long you average over, but
by eye I see WWV on 10 and 15 MHz going up and down by tenths of a Hz
in the several minute timespan from here on the East Coast.

Dave Mills has done a lot of DSP-type work on extracting the
leading edge of the second and minute pulse tones from WWV in
autocorrelator fashion.

I think that overall, you and I need to better quantify needs before
deciding on a solution as suitable :-). It's easy to overkill in
time and frequency, and I think the point of this list is to
do even more overkill!

Personally I am so delighted with the abilities of any but the
cruddiest GPS timing receivers that I see no need to tweak elevation
angles. (I think I already said that I think that several consumer
non-timing GPS receivers common out there are not worth crap
for timing applications. Remember the SV6, it's PPS will go in and
out by six milliseconds as satellites go in and out of view!! Most
new stuff is better...)

Tim.

----- Original Message -----
From: "Tim Shoppa" shoppa@trailing-edge.com
To: time-nuts@febo.com
Sent: Monday, December 17, 2007 8:37 AM
Subject: Re: [time-nuts] Time Transfer

snip------

Its also instructive to use a scope triggered off a local PPS and
watch the timing of the 1 second pips on WWV bounce around.

DaveB, NZ

Hi Dave:

The HP 114 is a 1 PPS trigger generator with offset and was made for use with
WWV, see: http://www.prc68.com/I/TF_rack.html
The audio from an HF receiver is fed to the V input on the scope and you can
adjust the sweep speed as fast as propagation will allow.  The 114 time offset
moves the trace left or right.  Then watching how long it takes to drift a
division you can see what your house standard is doing.

An interesting way to use HF for time transfer would be to use a chirp based
system.  Most chirp transmitters now use GPS control of their timing.  Chirp is
used to determine what the propagation is between a given transmitter and
receiver.  Since the chirp receiver knows exactly what the propagation is it
should make for a more acccurate HF timing receiver.
http://www.prc68.com/I/RCS-5A.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

Dave Brown wrote: