"L2 unmodulated carrier"?  As in a CW tone?

Dana

On Wed, Jun 21, 2023 at 12:13 PM Lux, Jim via time-nuts <
time-nuts@lists.febo.com> wrote:

On 6/23/23 2:09 PM, Dana Whitlow via time-nuts wrote:

P/Y code on I, nothing on Q (so "bpsk" as opposed to QPSK).  The table
says "unmodulated"

talking about GNSS, someone here might know....

I have a problem which I can see either one hemisphere or the other, but
not both simultaneously  - from the one antenna.

It takes two antennas. Consequently, my 3D fixes are in the toilet.

Is there a mode where two receivers  separately correlate all the sats
they can see independently and then talk to each other to use the sats
from the 'other' receiver to get a better all sky 3D fix ? or get 
enough raw data out of them and post process elsewhere ?

glen.

On 22/06/2023 10:31 am, Lux, Jim via time-nuts wrote:

On 6/29/23 10:36 PM, glen english LIST via time-nuts wrote:

Not easily - you might be better off just running the two antennas into
a power combiner.

But, there's a variety of software that will take raw observables and
combine them to make a single fix.  GipsyX at JPL does this, but it is
non-trivial to do.

There's also RTGx (Real Time Gipsy) which can probably do it on the fly
(RTGx is part of GIPSYx). Or, at least, I know it can combine
observables from the signals from multiple antennas at the same time. 
Granted it's all one receiver, usually, with multiple input channels,
but I'm pretty sure that RTGx doesn't care - it would help if both
receivers share a common clock.  (isn't that a particularly Time-Nuts
kind of statement in general)?

https://gipsyx.jpl.nasa.gov/

In theory, if you've got a research application, you can get a free
non-commercial license from JPL.

You can get a pdf from here:

https://www.sciencedirect.com/science/article/pii/S0273117720302532

Willy Bertiger, Yoaz Bar-Sever, Angie Dorsey, Bruce Haines, Nate Harvey,
Dan Hemberger, Michael Heflin, Wenwen Lu, Mark Miller, Angelyn W. Moore,
Dave Murphy, Paul Ries, Larry Romans, Aurore Sibois, Ant Sibthorpe, Bela
Szilagyi, Michele Vallisneri, Pascal Willis,

GipsyX/RTGx, a new tool set for space geodetic operations and research,

Advances in Space Research,
Volume 66, Issue 3,
2020,
Pages 469-489,
ISSN 0273-1177,
https://doi.org/10.1016/j.asr.2020.04.015.

Full disclosure, I'm the project manager of NASA's SunRISE mission:
we're using RTGx on board the six space vehicles AND doing post
processing of the observables on the ground to get position to ~1 meter
and time to ~1 ns. The Precision Orbit Determination (POD) also uses
GIPSYx/RTGx, and gives us the clock bias estimate. SunRISE is a
constellation of 6 small space vehicles (called the observatory) that
will record radio signals between 100 kHz and 23 MHz in an orbit about
300km above GEO.  The six space vehicles fly about 1-15 km apart, and we
use interferometery to image the source of RF from Type II and Type III
bursts from the Sun. Type IIs come with CMEs.  We're launching in 2025,
theoretically.  The space vehicles are all built and tested, and are
basically in storage "waiting for our ride". Basically in the last few
weeks.

There will be some papers coming out over the next months (SmallSat in
August, BigSky in March) describing various aspects of how the design
came together and was tested. There's been a series of papers at BigSky,
and a set of 4 posters at AGU last year that describe it fairly well.

Giving plenty of credit to Time-Nuts - the conversations on this list
have been invaluable in feeding into making SunRISE a reality.  After
all, what we're doing with SunRISE is basically a GPS observed
oscillator (we don't adjust the oscillator), and there's been a LOT of
discussion on the list about that.  In fact, those discussions are
partly why we don't discipline - we don't have to worry about backing
out the disciplining.

And if that's not cool enough - We've been working on concepts for
50-100 satellites out at L4 or L5, and there's no handy GNSS out there
for position nav and timing. So now, we need to know relative position
to a meter, time to a nanosecond, and orientation relative to the sky,
completely within the observatory in space.

Solar radio bursts are bright (up to 40 dB over galactic background),
and the Sun doesn't put out much energy in that band, except from the
bursts.  But there's a lot of interest in low frequency radio astronomy
for planetary, or beyond solar system - you can't observe it from the
Earth's surface, because the ionosphere either blocks it, or is
sufficiently non-isotropic that imaging doesn't work.

Hi

This is sort of several questions rolled into one. Various answers:

1. Are there dual antenna receivers? Sure there are/ They assume you can see all the
   sats from both antennas at the same time.

2. Can you get 3D fixes from multiple independent antennas? Sure, but not with a stock
   receiver. The delays and phase shifts into multiple receivers from multiple inputs via multiple
   cables really mess with things.

3. Can receivers chat with each other? Yes, it’s RTK mode. Like 1 above to be useful, it
   wants both receivers looking at the same set of sat’s.

Bottom line, AFIK, you need an antenna that can see at least 4 sats all at the same time. It
needs to do that long enough to get a reasonable location fix. Once you have that fix, you
can go over to fixed location / timing mode. That will give you timing information. This mode
is not available on all GPS receivers or modules.

Bob

Thanks for the replies so far. and very Interesting Jim !

application is improve the GPS fix for field portable installed
TABS/conspicuity in experimental aircraft with metal skins or carbon
fibre skins. In a nutshell they broadcast a 1 sec intervals on ADSB 1090
MHz the posiiton, and info) . they are usually suction cup fitted.

There are conspicuity units that suction cup to the inside rear window
and they are terrible for fix, +/- 300m in H, +/- 200m in V.  even worse
if a high wing and a wing in the way that is full of fuel .

It's passable on the front of the instrument panel at the forward end
where it can see a fair bit of the sky through the windshield, but not
great for vis, and also affects compass swing . The units are very
popular with recreational experimental, and assist to avoid traffic.

Real TSO GPS require GPS antenna on the top of the skin to get an all
sky view.  (like my PA28)

Possible solution is 2 or 3  antennas looking at different parts of the
sky but my feeling is most likely just one antenna on either side window
left and right would eb a substantial improvement. I am yet to
QUANTIITIVELY see exactly why it is so poor- I need to put one of my
UBLOX radios there in the device's place and get some data.

sidenote - As an RF guy, I would have thought combining the signal from
both GPS antenna (downstream of their LNAs)  would lead to a fair bit of
infighting, and be highly sub optimal  . maybe not as bad as I imagine.

-glen

On 1/07/2023 8:35 am, Lux, Jim via time-nuts wrote:

On 6/30/23 4:52 PM, glen english LIST via time-nuts wrote:

Assuming the feedlines are equal length, and you combine them. You'll
get classic grating lobes where one is exactly in or out of phase with
the other.  BUT.. since the pattern is already rolling off (a typical
patch has a 90-100 degree HPBW), both of them are fairly far down at the
"dividing line" equi distant, so it may not be as bad as you think.

This is easy to model in a spreadsheet - Just assume the antenna has a
cos^2(theta) kind of pattern and then calculate the sum of the two
signals (including the phase difference for the angle off vertical,
which depends on the spacing between the antennas).

There's enough other lumps and bumps in the pattern from surroundings,
the grating lobes might be in the same general magnitude.   Or just try it.

One thing to watch out for: if you're using antennas with built in LNAs

• it's easy to build an oscillator - the amplified output of antenna 1
  goes through the splitter and comes out the port to antenna 2 and if the
  reverse isolation of the LNA isn't good enough, it radiates back to
  antenna 1.  Fortunately, the antennas are narrow band, so it might not
  happen to oscillate.  But if you get weird results, try covering one of
  the antennas with aluminum foil (or absorber, if you have some).

And these days, you can get a spectrum analyzer that goes up to 2GHz for
about $100  - it's not awesome, but it sure would see an oscillation at
GPS frequencies, and would prevent all the GPS users around you from
hunting you down like a rogue wolf.

Hi Jim

Seems this question of mine is well covered in  the ether.

Yes, might not be as bad as I think. I'll write a program to compute
various scenarios and also will be interested to see how the fixes get
affected  with loss of signal over some AZEL patch of the sky.

The other thing- these narrowband antennas, over 24 MHz ish, I would
suspect they are NOT phase  coherent, and that a combined pair of
signals over identical sky would be anythign but flat frequency response.

Yes, - the LNA. some of these have 40dB gain, golly. I have some 1500
MHz ferrite isolators, also. probably unnecessary as you say depends on
the S12 for the LNA   and thus the match the antenas see into the
splitter is likely important. Have spec-an to 50 GHz so.....

Time now I think to set up a couple of receivers and get some raw data.,
and compare with single receiver and combined antenna. Patch antennas
generally suck I find for all the usual reasons, I am a fan of
quadrafilar helicies.... but patches are a dime a dozen and 3what is in
use,  so that's what I will use for the tests and report back.

-glen

On 30/06/2023 06:36, glen english LIST via time-nuts wrote:

Geln,

Assuming you mean half-hemisphere....

Why not get a receiver capable of all three main systems - GPS, Galileo, and
Beidou - and configured to use them.  As Bob said, all you need is a reasonable
number of satellites, and receiving all three systems increases the changes of
that happening.

Getting both the 2.3 and 1.5 GHz signals can increase accuracy as well.

Perhaps you need something better than that can provide - I don't know.

Cheers,
David

SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv

On 6/30/23 6:40 PM, glen english LIST via time-nuts wrote:

Oh, they're probably phase coherent "enough" - and it may not make much
difference.

Say your antennas are separated by a meter - that's 5 wavelengths at L
band (20cm). If they were ideal, you'd get a series of fans to form
grating lobes - the fans would be long in the direction crossing the
line between antennas, and narrow in the direction parallel to the
baseline between antennas.

Non ideal antennas make the grating lobes wiggle or be wavy along their
long axis. Of course what YOU care about are the nulls (and to a lesser
extent the phase smoothness as you traverse the pattern).  I've looked a
lot of these kind of models and your saving grace is that the nulls are
deep only when the signals from the antennas are equal strength, which
doesn't really happen much.

If you were trying to do real time kinematic surveying to millimeters,
and are depending on smooth phase response - yeah, probably not going to
work.  Those folks obsess about apparent phase center displacements of
millimeters over a hemisphere.  Good multiband choke ring or artichoke
antennas are where it's at.

Or, if you need precise position calculation, then the "multiple
receivers and post process" is probably a better approach, because that
can explicitly address that the antennas are not co located.

Indeed - sounds like fun.

I'll bet if you get raw observables out of your receiver, you could
probably sort them (based on the look direction to each SV) into which
antenna is more likely, and then run them through any of the online
processors with some excision of "observations likely to be wonky"

(Which is a lot of work, but..)