I presented a similar talk at the May 4, 2003 meeting of AMSAT-DC.  I added
the fact that a codeless GPS receiver could remove the bi-phase modulation
and yield a CW carrier for frequency standard applications.  Amateur radio
at the forefront, again.  After a recent move, I am in the process of adding
a four-foot diameter fixed position dish antenna to my set up.

John  WA4WDL

From: "Robert Harmon" harmon52@mchsi.com
Sent: Thursday, October 07, 2010 9:02 PM
To: time-nuts@febo.com
Subject: [time-nuts] GPS backup for the stationary time and frequency user

On 10/08/2010 03:02 AM, Robert Harmon wrote:

Notice footnote 12 (on page 9).

Will not a standard GPS antenna have the wrong circular polarisation
when looking into the mirror image of the offset antenna?

I would expect the thing to work due to antenna gain and leakage in the
wrong antenna mode. Ah well...

Cheers,
Magnus

You are correct.  The dish feed should be LHC.  The feed would work for WAAS
or other GPS satellites while in the beam pattern, but reject satellites
seen through spill over, etc.

John  WA4WDL

From: "Magnus Danielson" magnus@rubidium.dyndns.org
Sent: Thursday, October 07, 2010 9:28 PM
To: time-nuts@febo.com
Subject: Re: [time-nuts] GPS backup for the stationary time and frequency
user

When I said the feed would work, I was meaning it would work if LHC.  The
illustrations and text imply you could just place a normal GPS receiver at
the feed location, but the polarization would be wrong.

John  WA4WDL

From: "jmfranke" jmfranke@cox.net
Sent: Thursday, October 07, 2010 9:32 PM
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Subject: Re: [time-nuts] GPS backup for the stationary time and
frequencyuser

In another paper, at the FEI web site, the author emphasizes that he did not
change the polarization, but did see a 12 dB gain with the dish.  There is
even a nice image with a typical timing GPS antenna mounted at the feed
slide 27 in
http://www.frequencyelectronics.com/gps_gnss/waas_for_telecom_2-07.pdf .

John  WA4WDL

From: "Magnus Danielson" magnus@rubidium.dyndns.org
Sent: Thursday, October 07, 2010 9:28 PM
To: time-nuts@febo.com
Subject: Re: [time-nuts] GPS backup for the stationary time and frequency
user

On 10/08/2010 03:35 AM, jmfranke wrote:

Which was what I reacted on...

I am by no means a practical antenna expert, and the EM-theory is a bit
fuzzy on the edges, but I do distinctly recall that signal is RHC and
reflections becomes LHC so an antenna with RHC orientation will provide
some first-degree damping of the LHC reflections. For this antenna setup
the intended RHC signal is reflected and should become LHC... just as
the interference... so it relies on the antenna gain of the dish to
out-perform the other reflections for the half-space receiver that a
normal GPS antenna is. The choke ring for a dish head has a distinct
different pattern (forming an inner cone rather than flat space).

So, a normal antenna would kind of work since the antenna gain would
overcome the poor LHC supression of a simple RHC antenna... yay.

If an LHC antenna was used instead... now we are talking.

Cheers,
Magnus

The other thing is that something like a quad helix or patch doesn't have the same cross-pol over the hemisphere.  It could be real good in one direction and not so good in others.  Just like isotropic antennas, you can't physically realize the same cp in all directions (cf hairy ball theorem)

On Oct 8, 2010, at 1:48 AM, Magnus Danielson magnus@rubidium.dyndns.org wrote:

Magnus Danielson wrote:

So a dish reflector and a sub reflector and the GPS receiver at the dish
would work? What is that
configuration called? I can't remember at this early hour.

Bill K7NOM

Yep, a Cassegrain antenna would work.

John  WA4WDL

From: "Bill Janssen" billj@ieee.org
Sent: Friday, October 08, 2010 11:52 AM
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Subject: Re: [time-nuts] GPS backup for the stationary time and
frequencyuser

Bill Janssen wrote:

Depends on the relative curvatures and focal points:

Cassegrain if the subreflector convex.
Gregorian if the subreflector is concave parabolic.
Dragonian if the subreflector is concave hyperbolic

IEEE Ant and Prop Magazine a few years back had a series of articles on
designing them all.

All of them can be done offset or coaxial

Any would conceivably work..  It's all about what your pattern looks
like, what sort of efficiency you need, any mechanical constraints, etc.

Hi Jim:

I've got a spare Ku band satellite dish and would like to use it for GPS.
In an ideal application the GPS antenna would be mounted in the normal
manner and above it would be a sub-reflector aimed at the Ku dish.
That way the antenna might pickup sats near the horizon directly and
from a narrow part of the sky by means of the dish.
The dish might be aimed at a WAAS GPS sat.
I've heard that you can just use the TV dish with a normal GPS antenna,
and it gas gain even though the polarization is reversed.

Have Fun,

Brooke Clarke
http://www.PRC68.com

jimlux wrote:

--
Have Fun,

Brooke Clarke
http://www.PRC68.com

I've been half following this thread and can't make out the reason for a
less than hemispheric antenna pattern. GPS needs several birds to lock up,
and if you look at a single bird, Dopplar will make the signal useless as
a frequency reference.

Best,

-John

=============

The Doppler is dramatically reduced by looking only at the WAAS bird(s).

John WA4WDL

From: "J. Forster" jfor@quik.com
Sent: Friday, October 08, 2010 1:27 PM
To: brooke@pacific.net; "Discussion of precise time and frequency
measurement" time-nuts@febo.com
Subject: Re: [time-nuts] GPS backup for the stationary time and
frequencyuser

On Oct 8, 2010, at 10:27 AM, J. Forster wrote:

To get a position fix, this is true; 3 birds minimum for a 2D fix, 4 birds minimum for a 3D fix.

Only if you don't already know your position. Once your position has been determined accurately and you have the current ephemeris data for the bird you're listening to, you can factor out the predictable doppler and use a single bird for timing and frequency.

--
Mark J. Blair, NF6X nf6x@nf6x.net
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.

KISS. This seems like a lot of work for not particularily good results.
Among other things, you have no closure so no measure of how good your
measuremwent is.

FWIW,

-John

================

On 10/8/2010 10:44 AM, J. Forster wrote:

I think the point is that the results are amazingly better than the
alternative if the medium-orbit GPS sats are all destroyed and/or
there's jamming coming from all over the sky.

What bothers me is that I haven't figured out yet how the WAAS satellite
configuration itself isn't usable as a wide-area GPS jammer. It is just
a bent-pipe transponder down from 6 GHz or so to L1, so if you can
overpower the uplink with spoofed data that interferes with reception of
the other satellites and jam the C&C channels that would be used to
shut the transponder off, you're able to jam a whole continent at once.

I'm sure this must have been considered before deployment, but I can't
find any references to the countermeasure(s).

Matthew Kaufman

Hi John:

The WAAS birds may be too weak to receive with an omni antenna, hence
the desire to get some gain.
The normal GPS sats will pass through the beam and the GPS antenna will
pick up some sats directly so you do get some TRAIM.
Doppler is not an issue in timing mode (i.e. position is known) and only
one sat is required.

Have Fun,

Brooke Clarke
http://www.PRC68.com

J. Forster wrote:

--
Have Fun,

Brooke Clarke
http://www.PRC68.com

Brooke Clarke wrote:

Give it a shot.  The other thing is that if you have your GPS antenna
facing straight up, at the focus of the dish, you're looking at the side
of the gps antenna, where the polarization might be less circular.

But one thing to think about here... a standard Ku dish isn't very big..

At GPS frequencies, you're looking at 20 cm wavelength.  The dish is
perhaps 2, maybe 3 wavelengths across.  That's not a huge amount of gain.

You might do just as well with a flat cookie sheet.

J. Forster wrote:

If you're looking at boosting the level of the WAAS signal, it's
broadcast from a Clarke Orbit satellite, and has no doppler.

Matthew Kaufman wrote:

Most spacecraft have a "command loss timer" that would shut off the
transmitter in the event that a valid command isn't received every so
often, in case the transmitter is malfunctioning and jamming the uplink.

Maybe because WAAS is basically a civilian use system to do precision
approaches?

On Oct 8, 2010, at 10:44 AM, J. Forster wrote:

Single-satellite timing mode is already commonly implemented in timing receivers; there's no work to be done other than turning it on. Closure is only lacking if all of the USAF ground stations are taken out such that the ephemeris and health information can no longer be updated on any intact birds; otherwise you know where every existing bird is at any instant in time, and each bird indicates whether it should be trusted.

Bottom line: If there's still one intact bird and one intact ground station to control it, you can get accurate time and frequency whenever it's visible (assuming no local jamming/spoofing) if you've already surveyed your position, using off-the-shelf hardware like a TBolt.

--
Mark J. Blair, NF6X nf6x@nf6x.net
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.

On 10/08/2010 07:27 PM, J. Forster wrote:

If you target the WAAS, EGNOS or SBAS birds you have significant less
doppler since they sit on geostationary orbit. However, since these
orbits is not perfect circular but slightly elliptic, so a 24 h pattern
evolves. Just squaring up, as proposed earlier in the thread, will
suffer from these shifts. Also, ionospheric and tropospheric delays also
plays in, but the point of these birds is to provide improved
predictions so a good receiver should be able to handle it. You also
rely on the time of the base-station, but that is obvious I hope.

Cheers,
Magnus

On 10/08/2010 08:07 PM, Matthew Kaufman wrote:

I would not be supprised if they had not considered such a threat.

This is a common threat for all bent-pipe birds. They have been jammed
before and we can expect them to be jammed again. However, I do not
think the WAAS or any similar is highest up on the target list as for
most uses it is a support-function rather than main function system.
Birds which has been jammed is typically TV signal relays.

Cheers,
Magnus

On 10/08/2010 08:22 PM, jimlux wrote:

Well, a 1 m dish gives you 48 dB gain at L1 if I calculate correctly.
The normal antenna is at 6 db of antenna gain?

Even if less than perfect, not too big size is needed to get meaningful
antenna gain.

Cheers,
Magnus

48dBi is way way too big

18 I might go for

Look at aperture.  Say it's six square wavelengths(40x60 cm). A dipole is about 1/8th square wave lengths..so the gain is 48 times that of a dipole.  Say about 17dB +2dB or 19 dBi

On Oct 8, 2010, at 3:05 PM, Magnus Danielson magnus@rubidium.dyndns.org wrote:

Magnus Danielson wrote:

I don't think so..48dBi would be huge...A numerical gain of 48 I can
believe:

let's say we have an antenna that is 2x3 wavelengths (40x60cm, 1.5 GHz
= 20 cm) or 6 square wavelengths. the gain is 4 pi * A, or about 75.
But in reality, the efficiency is typically no better than 50% (because
of sidelobes, illumination, etc.)... Call 66% to be sporty, and you've
got a numerical gain of 50, or 17dBi.

20 log(base10) (diameter in meters) + 20log(frequency in ghz) +17.8 = dbi

On 10/8/2010 8:39 PM, jimlux wrote:

Brian Kirby wrote:

That assumes some nominal efficiency?

I'll have to remember that one.. 17.8

It's like the 32.44 dB for free space loss between isotropes 1 km apart
at 1 MHz
(+20log10(dist in km) + 20log10(freq in MHz))

The received WAAS signal strength is designed to be similar to that from
regular GPS satellites.  And, I find that to be true in practice.

John

From: "Brooke Clarke" brooke95482@att.net
Sent: Friday, October 08, 2010 2:09 PM
To: jfor@quik.com; "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Subject: Re: [time-nuts] GPS backup for the stationary time and
frequencyuser

These were formulas I had written in my notes about 30 years ago, when I
worked in Satcom for the military and NASA.

I think they used an efficiency of 55 percent

another was 20log D(feet) + 20log F(mhz) - 52.4 = dbi

and the main formula of gain in db = k(pi*D/wave)squared ....showing
k=0.55, d is diameter, wave is the operating frequency

On 10/8/2010 9:47 PM, jimlux wrote:

On 10/08/2010 08:09 PM, Brooke Clarke wrote:

The WAAS/EGNOS/SBAS power should be equal to the GPS birds, such that a
normal GPS receiver can receive them on the same antenna, the code is
such that a standard GPS receiver channel can lock to it, the big
difference lies in the rate and encoding of the data-channel. Calls for
some extra software including a standard Viterbi-decoder.

Cheers,
Magnus

On 10/8/2010 1:48 PM, Magnus Danielson wrote:

Threat model is different here. The end-user terminals for TV sats have
narrow antennas looking at just that satellite, so it affects just that
satellite/transponder.

The end-user terminals for GPS are looking at the entire hemisphere of
sky, so if you can send spoofed signals from the WAAS transponder (same
RF frequency that all the rest of the GPS satellites are using to talk
to single-frequency receivers) and get the receivers to lock to these
signals instead of the legitimate ones, you can interfere with all GPS
reception, not just the WAAS signal, for the entire coverage area of WAAS.

Matthew Kaufman

On 10/09/2010 04:47 PM, Matthew Kaufman wrote:

Indeed.

Hmm. Yes. Creative! Once demonstrated essentially all WAAS/EGNOS/SBAS
sats need to develope some protective measure.

To pull it off, a standard GPS simulator and some minor frequency
conversion is needed. Should not stop the handy man.

It would be an interesting legal aspect to attempt to charge the guilty...

Cheers,
Magnus

Hi Magnus:

Stanford Telecom built GPS simulators to test their GPS ICs.  It's made
where each wire wrap PCB is based on a page from ICD-200.
http://www.prc68.com/I/5001A.html

Have Fun,

Brooke Clarke
http://www.PRC68.com

Magnus Danielson wrote:

--
Have Fun,

Brooke Clarke
http://www.PRC68.com

On 10/9/2010 8:00 AM, Magnus Danielson wrote:

Unless, of course, such protective measures already exist. I can think
of several ways right off the top of my head... one would be to have the
transponder be able to shut itself down if it doesn't appear to be
relaying a single valid code stream... another would be to have ensured
that the maximum power density was low enough that anything with enough
real satellites in view couldn't be practically interfered with...
another is to have ground-based monitoring and a transponder enable
system that can't simply be jammed into the "on" mode but rather
requires a key be sent require regularly to keep the transponder powered
up...

I just haven't seen anything in the literature noting that the threat
had been looked at and/or addressed in any way.

Indeed.

If there's >1 space-based receiver on the uplink frequency, you can
fairly easily find any source that has an uplink beam wide enough to
illuminate more than one of them. Failing that there's other ways to
find the source that take longer. Once found, I think there's adequate
law and precedent for going after someone who interferes with
safety-of-life transmissions. But there could be quite a bit of damage
(even as simple as lost productivity from truck drivers who couldn't
make timely deliveries until they found some printed maps) in the meantime.

Matthew Kaufman

Matthew Kaufman wrote:

that would assume that the WAAS transponders are actually purpose
designed, and not just a generic bent pipe that was repurposed from
something else.

If it's purpose designed, it's easy.. the uplink signal doesn't have to
look anything like the downlink signal. You could, for instance, send up
two signals which have to be combined on board to make the downlink signal.

For the initial WAAS, they may have just leased transponders that
already existed..  which would be vulnerable, after a fashion.  There
are a variety of AJ measures taken by the satellite operators (no more
Capt. Midnight stuff)

Unlikely that this would be in the open literature.  It's pretty clearly
one of those things that falls under export control.

On 10/9/2010 8:22 AM, jimlux wrote:

Quite likely. The threat itself hasn't been mentioned as far as I can
tell, either... as someone who relies on GPS timing for several telecom
and network applications, including one where time disruptions can be
security disruptions, it is definitely something to think about and add
to the list of risks.

Matthew Kaufman

Hi

The Russians have been willing to sell GPS jammers for quite a while. They aren't terribly expensive. Their export controls are a bit different than the US's.

Bob

On Oct 9, 2010, at 12:00 PM, Matthew Kaufman wrote:

But unlike the threat I described, they're not pre-mounted in geo orbit over the US... As far as we know.

Matthew Kaufman

(Sent from my iPhone)

On Oct 9, 2010, at 9:16 AM, Bob Camp lists@rtty.us wrote: