Hi Gregory:

I wonder if anyone has tried using a small parabolic dish, like used for Free To Air satellite TV and aimed it at a GPS
satellite track or at a WAAS geostationary satellite using a feed antenna with reverse polarization from a normal GPS
antenna?
http://www.prc68.com/I/FTA.shtml

--
Have Fun,

Brooke Clarke
https://www.PRC68.com
https://www.end2partygovernment.com/2012Issues.html
axioms:

1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------

Hi

The original “we cracked GPS” paper back in the 1980’s (that unlimitedly lead to the end of SA)
used a medium sized dish ( think of the good old C-band antennas) to pick out a single sat.

Bob

Hi

“Backbone timing” gets done by boxes buried deep in systems. Those systems take years
to design. The boxes that go in them similarly take years to get onto the market. Once designed
deployment is far from instantaneous. Operators are always pressed by cost constraints. Adding
anything beyond the minimums … not going to happen.

The result is that there are no systems out there that use WWVB or WWV other than wrist watches
and wall clock like devices. Utilities (cell phones, internet, finance ) run with something else. Converting
them to a secondary “something” is a many years sort of thing, even if it is technically feasible.

You can pull a bunch of spare GPS sat’s out of storage and get them in orbit way quicker than you can
rebuild every cell tower in the country. In fact, newer designs run their timing in a way that a GPS failure
is not that big a deal. How long it’ll take before that sort of design is common in the US…. years and years …

If you are going to come up with a time source at the ~ 10 ns level, that’s not going to happen from WWVB
or WWV. They never were good enough to get to that level and it’s not on the transmit end. You would need
a very different system. It’s been a long time since any of these services (internet, finance, cell )  were in the
millisecond or even the microsecond range. The modern stuff in all theses areas  is  < 100 ns.

How long would it take to change all this? Well first some random Senior Member of the IEEE would
have to start writing papers about the various issues. Various organizations in various countries would
need to hold meeting after meeting after meeting talking things over. Somebody eventually would have
to come up with funds to actually try a few things. Maybe they work in the real world / maybe they don’t
work.

Once you prove you have a system that can do “good enough", you would need laws / regulations passed to
make the “new thing” part of the required designs. You also need funding bills to deploy the “source” end
of things and time to get that up and running. Once it’s running, you then give manufacturers some amount of time
to get it in the field ….. and extensions when that doesn’t happen. Twenty years? Thirty years? Maybe longer?
This stuff does not go very fast.

Best bet on what the “new thing” would be? Something like IEEE-1588 over fiber. It cuts out a bunch of this and
that in terms of experiments and testing the basic system. We know most of how to do it already. It’s just a matter
of a  billions of dollars in tax money to get the gaps filled in and then a few tens of billions in tax money to get
the backbone gear in place. Once that’s done you ramp up to the really expensive part of the deal ….Is it paid
for by your tax return in April or by a higher price on every cell call / transaction you make? … who knows … it’s
a tax that you are paying either way.

Bob

On Thu, 30 Aug 2018 18:54:17 -0700
Brooke Clarke brooke@pacific.net wrote:

I have somewhere a paper (which i cannot find currently, sorry) that
used a dish trained at one of the EGNOS satellites and used it as the
only source for timing. IIRC the results were promising, but not
spectacular. The problem being that all the ionospheric and tropospheric
effects limited the performance, which also could not be averaged
over several satellites. Hence most people today focus on whole
constelation systems and try to get the best out of that, even under
multipath and jamming.

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson

On Thu, 30 Aug 2018 23:05:48 +0000
Gregory Maxwell gmaxwell@gmail.com wrote:

"Just DSP work" is a tad bit more than you think. You are dealing
with sevaral 1Msps of data, even for a simple L1 C/A receiver.
If you are going multi-band-multi-GNSS you are usually in the 50MHz BW
at L1 and 80MHz BW at L2/L5 range, which means you are dealing with
something in the order of 100Msps of data per channel (either as
a single stream of sample or two streams of samples with half rate).
Then you add to it that you will need at least 4bit ADCs to get
somewhat jaming proof, probably even 10bit or more and suddenly
you are dealing with 200-400Mbyte/s data per antenna. Constantly.
To be able to do reasonable beam forming, you probably need a 4 by 4
grid at least, that makes 16 antennas which brings us into the
3Gbyte/s to 6Gbyte/s region. And that's just the raw input datarate
you have to handle....  A modern GNSS receiver has something in the
order of 50-100 correlators per band, each of which needs to receive
the full data rate mentioned above. So inside the chip, the data rate
gets multiplied as well.

Now take into considerations that beside running the correlators,
after you phase shifted and weighted the inputs correctly, you
have to run some fancy algorithms (on the raw data) to figure
out what these phase shifts and weights are. For each satellite
you are tracking individually. All this toghether means you have
run some pretty heavy computation, that is very likely not going
to fit into an FPGA, so you need to build a custom ASIC.

You still need the lower satellites to survey your position accurately.
Besides that you lose a lot of in terms of timing accuracy, if you
have only a limited number of satellites. So you cannot decrease the
lower elevation angles too much. Going below -10dB is probably not
agood idea. Also, going from ~10 birds in view down to ~5 means that
your PPS jitter just increased by a factor of 5 to 10.
(source: experiment i've done here some time ago)

Besides, a narrow band jammer trips up most of the commercial receivers
badly (adds a correlation peak where it does not belong).
And for that to be effective you only need to be 5-10dB above the noise
level. Which is pretty easy to achieve, even if you have very directive
antenna (the sidelobes are usually only 10-30dB down from the main lobe)

Fortunately, narrow band jammers are also pretty easy to mask, given
you have enough bits in your ADC.

There is a reason why Microsemi is building more 5071 these days than
ever before (rumors have it that they are are 3-4 devices per week).

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson

I/We track down things that jam weather radars.  Mostly WiFi access points misconfigured.

Which share many of the characteristics of GPS jammers

1 - small low powered
2 - one can ruin a pilots entire day
3 - distributed
4 - can literally be anywhere

Stuff like this is why FCC blocked anyone but chip manufacturers from updating WiFi radio firmware.

The overwhelming majority of the mods were attempts to improve performance.  But the firmware hackers were unaware of the band sharing and also did not understand that signal characteristics need to be closely controlled and the limits the FCC applies are not there to ‘ruin my performance’ but to allow others to play nicely in that sandbox.

So many things in life should have been learned in kindergarten.  Sadly many have forgotten those lessons.

On Aug 31, 2018, at 10:55 AM, Attila Kinali attila@kinali.ch wrote:

On Thu, 30 Aug 2018 23:05:48 +0000
Gregory Maxwell gmaxwell@gmail.com wrote:

"Just DSP work" is a tad bit more than you think. You are dealing
with sevaral 1Msps of data, even for a simple L1 C/A receiver.
If you are going multi-band-multi-GNSS you are usually in the 50MHz BW
at L1 and 80MHz BW at L2/L5 range, which means you are dealing with
something in the order of 100Msps of data per channel (either as
a single stream of sample or two streams of samples with half rate).
Then you add to it that you will need at least 4bit ADCs to get
somewhat jaming proof, probably even 10bit or more and suddenly
you are dealing with 200-400Mbyte/s data per antenna. Constantly.
To be able to do reasonable beam forming, you probably need a 4 by 4
grid at least, that makes 16 antennas which brings us into the
3Gbyte/s to 6Gbyte/s region. And that's just the raw input datarate
you have to handle....  A modern GNSS receiver has something in the
order of 50-100 correlators per band, each of which needs to receive
the full data rate mentioned above. So inside the chip, the data rate
gets multiplied as well.

Now take into considerations that beside running the correlators,
after you phase shifted and weighted the inputs correctly, you
have to run some fancy algorithms (on the raw data) to figure
out what these phase shifts and weights are. For each satellite
you are tracking individually. All this toghether means you have
run some pretty heavy computation, that is very likely not going
to fit into an FPGA, so you need to build a custom ASIC.

You still need the lower satellites to survey your position accurately.
Besides that you lose a lot of in terms of timing accuracy, if you
have only a limited number of satellites. So you cannot decrease the
lower elevation angles too much. Going below -10dB is probably not
agood idea. Also, going from ~10 birds in view down to ~5 means that
your PPS jitter just increased by a factor of 5 to 10.
(source: experiment i've done here some time ago)

Besides, a narrow band jammer trips up most of the commercial receivers
badly (adds a correlation peak where it does not belong).
And for that to be effective you only need to be 5-10dB above the noise
level. Which is pretty easy to achieve, even if you have very directive
antenna (the sidelobes are usually only 10-30dB down from the main lobe)

Fortunately, narrow band jammers are also pretty easy to mask, given
you have enough bits in your ADC.

There is a reason why Microsemi is building more 5071 these days than
ever before (rumors have it that they are are 3-4 devices per week).

       Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson

time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.

Maybe this one is equivalent?
http://freqelec.com/gps_gnss/waas_for_telecom_2-07.pdf
On Fri, Aug 31, 2018 at 4:55 PM Attila Kinali attila@kinali.ch wrote:

On Fri, Aug 31, 2018 at 2:56 PM Attila Kinali attila@kinali.ch wrote:

I have done beamforming in software for wifi signals, which is quite
similar a task.

You are probably underestimating somewhat how fast modern systems have
become.  There are now people using GPUs for SDR now as well, which is
perhaps more fitting to a GNSS timestamper than many other SDR
applications, since that application doesn't need to be low latency.
There are now consumer GPUs that do 14 trillion single precision
multiply-adds per second.

There I agree.  It seems to me that a interesting architecture would
be to run a sinusoid killer on a FPGA immediately behind the DACs,
then spit out a 2 or 4 bit stream.

The best processing tool I have for estimating sinusoid parameters is
https://arxiv.org/abs/1602.05900  the computationally hungry parts
(measuring correlations with candidate sinusoids and the signal) are
easily implemented on an FPGA but still would be pretty impressive to
see running at 50msps. For GNSS applications something far stupider
probably is adequate: FFTs on windowed overlapping blocks, null out
outlying peaks and ifft.  It would not take a very large FFT to get
enough frequency selectivity to kill only 1% of signal power per
eliminated sinusoid-- maybe 256 samples.

There are many papers showing interesting GNSS beamforming results
with far fewer antennas:

4 antennas, 14-bit, 40msps, and a GPU based decoder:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3231503/pdf/sensors-11-08966.pdf

This one has simulated polar patterns for 7 antennas that look pretty
impressive https://web.stanford.edu/group/scpnt/gpslab/pubs/papers/DeLorenzo_IONGNSS_2004.pdf

This one has measurements with 7 antennas on actual hardware:
https://pdfs.semanticscholar.org/5531/aa89f81e4cdfa15e74e98c264dcb697395ca.pdf
(and also talks a bit about their SSE correlators needed to get
realtime performance in a software implementation).

Yes, but that is potentially a one time operation.

But, if your SNR on those 5 remaining birds increases substantially
from increased antenna gain might it not offset some of that loss? I
would worry that getting a consistent phase center on some weird high
gain antenna design would be hard, however.

If you consider the cost of a new 5071 ... that would pay for a heck
of a beam-forming anti-jamming receiver-- at least hardware wise. :)

Hi Bob:

Do you have and info on that article that would allow me to read it?

--
Have Fun,

Brooke Clarke
https://www.PRC68.com
https://www.end2partygovernment.com/2012Issues.html
axioms:

1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------

On 8/31/18 9:38 AM, Gregory Maxwell wrote:

AJ is a slightly different problem than straight up beamforming

You need N+1 receivers to suppress N point source jammers - it's more of
an adaptive canceller than a beamformer.

The literature has been around since the 60s on how to do this in a
variety of cases and signals, with plenty of hardware demonstrations
over the decades ranging from analog combiners to very sophisticated
digital approaches using algorithms like ESPRIT or MUSIC.

There's also extensive work on AJ techniques for PN signals, although I
don't know how much is in the open literature.

One wants to think about the jamming/interference signal

In general, you're probably not going to design to "notch" a sinusoid -
sure that might be a common jammer, but the sinewave generated by the
jammer might be very noisy and unstable in frequency - sort of the
opposite of the sinusoids desired by list members :) - the spurious
oscillation of the TV antenna amplifier was in this bucket.

The jamming signal is likely to be somewhat broadband = pulsed, or
harmonics of some lower frequency signal, which is none too clean - most
power supply designers do not lock their PWM rate to a maser, after all.

If you're talking about a sophisticated jammer - typically they'll
attack some design feature - For instance, way back (more than 50
years), a strategy for radars was to transmit two tones separated by
60MHz (Radar IF chains were at 60 MHz) and pulse it, in hopes that you'd
hose up the AGC, or create intermods. Similarly, transmitting on some
ham bands in the  50s would get into the TV receiver IF.

Then you get into sophisticated approaches which are more like spoofing

• transmit a PN modulated signal that replicates the desired signal,
  then pull it away and turn off, forcing the receiver to be in
  "acquisition" mode all the time.

Can you build a receiver which is immune to all of these and sell it for
$10?  Probably not.  Can you do it for $100-1000, almost certainly.

So now it comes down to the classic tradeoff - what is the value of the
thing you are protecting.

On time-nuts, we're used to "better performance for its own sake" - it's
cool, it's nifty, it's educational, for most of us, we don't necessarily
have to justify our desire to get that next digit. We're also often very
thrifty - as part of the challenge - I know I can get accuracy X by
spending $100k, but can I get there spending only $200.

The commercial world, which has to consider the slings and arrows of
these threats does put a value on it, and that drives the budgets for
the fixes.  I would venture that in most cases, the cost of a fancy
holdover GPSDO is a tiny, tiny part of the cost of a cell site -
Installation of the concrete pad for the tower probably costs more.

Sure, if you've got 1000 sites, and you need to upgrade them all at
$10k/each, that's a $10M hit you need to explain to the shareholders,
but, it is a cost of doing business.

And, realistically, it's much more likely your network failure will be
because some idiot dug in the wrong place, or someone rolled out a
software update with a bug, rather than a freak ionization event
blocking GPS.

Check out the telephone switch bug that shutdown phone service for a day
in 1990 -
https://users.csc.calpoly.edu/~jdalbey/SWE/Papers/att_collapse.html