[time-nuts] GPS Modulation and 10 MHz Delay Lock (Jim Lux)

Wed Oct 3 03:40:47 UTC 2012

Hi Jim -

Thanks for the update on the modern GPS receivers. I was aware that the 
modern ones
do not have a classical analog tracking loop, much less a bunch of them.
However it is a useful concept for purposes of explanation that you do 
not need the 1 pps to
lock up the 10 MHz VCXO - which was my main point.

The Tbolt block diagram in the manual Figure 5-10 shows the 10 MHz VCXO 
output going to
the receiver and also to the output. The 1 pps comes from the "cpu and 
support" circuit.

While the diagram is clearly simplistic - it implies that the Receiver 
circuitry first locks up the
10 MHz oscillator and the the 1 pps is derived from that.  My only 
point was that it is possible
and perhaps even better to discipline an oscillator using a code 
correlator (however implemented)
rather than steering using the 1 pps. I believe this is why the T bolts 
work so well. For one thing the
loop should work better and faster if the input is at the chipping rate 
rather than at 1 pps as the
information rate is higher.

On doppler - I believe that since the spread spectrum sidebands are 
coherent with the suppressed carrier, their
relationship to it is unchanged by doppler and thus it should be 
possible to achieve a code correlation on
a  doppler shifted signal. The recovered carrier would be shifted in 
frequency by the doppler but it would still be
recovered - at least by a classical IF correlator.

- On jamming - maybe so, but the effect of the receiving correlator is 
to spread the energy of a CW interferer
and concentrate the energy of the signal with the matching PN 
modulation is it not????

good fun anyway!

-73 john k6iql

-----Original Message-----
From: time-nuts-request <time-nuts-request at febo.com>
To: time-nuts <time-nuts at febo.com>
Sent: Tue, Oct 2, 2012 9:53 pm
Subject: time-nuts Digest, Vol 99, Issue 19

Send time-nuts mailing list submissions to
	time-nuts at febo.com

To subscribe or unsubscribe via the World Wide Web, visit
	https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
or, via email, send a message with subject or body 'help' to
	time-nuts-request at febo.com

You can reach the person managing the list at
	time-nuts-owner at febo.com

When replying, please edit your Subject line so it is more specific
than "Re: Contents of time-nuts digest..."

Today's Topics:

   1. Re: GPS Modulation and 10 MHz Delay Lock (Jim Lux)
   2. Re: GPS Jammer (Jim Lux)
   3. Re: GPS Jammer (johncroos at aol.com)
   4. Re: Best counter setting for ADEV? (Jim Lux)
   5. Re: RFX GPSDO - Anybody played with one of these? (Jim Lux)
   6. Centering ocxo (Bill Dailey)
   7. Re: GPS Jammer (Jim Lux)
   8. Re: Google's Spanner uses GPS and atomic time (paul swed)

----------------------------------------------------------------------

Message: 1
Date: Tue, 02 Oct 2012 19:30:20 -0700
From: Jim Lux <jimlux at earthlink.net>
To: time-nuts at febo.com
Subject: Re: [time-nuts] GPS Modulation and 10 MHz Delay Lock
Message-ID: <506BA33C.1010606 at earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 10/2/12 3:39 PM, johncroos at aol.com wrote:
> Hello All -
>
> Here is a link that describes the GPS modulation. You do not need the
> 1 pps to lock the 10 MHz oscillator to the atomic clock in the 
satellites.
>
> http://www.kowoma.de/en/gps/signals.htm
>
> If you look at the block diagram you see PN code modulates the 
carrier at
> the 1.023 MHz chip rate. This is done by BPSK modulation of the 
carrier
> with the PN code. It can be done simply with a double balanced mixer.
>
> This spreads the signal with PSK at the chip( i.e. code clock) rate.
>
> Note also the modulo - 2 addition of the data to the code sequence. 
This
> called code inversion
> modulation. After de-spread of the code in the receiver - the signal 
is
> then simple BPSK and
> may be demodulated by a Costas or Squaring Loop to get at the data 
message.
>
> The obtain precision frequency needed I believe the T bolt simply 
locks
> to the chipping rate
> using some form of Delay Lock Loop. It is NOT at PLL. There is no need
> what ever to
> deal with the 1 pps using this method. The internal 10 MHz oscillator 
is
> controlled by this locking circuit and
> is part of the code correlation loop.

That's not quite how it works.. It would work for terrestrial links
where there is no Doppler, but in the GPS case, there is significant
Doppler shift on all the signals.  Since the carrier and the chips are
generated from a common source on the spacecraft (the carrier frequency
is a multiple of the chip rate, in fact), you can recover carrier and
chips at the same time.

But.. most receivers these days don't actually have an analog tracking
loop at all.  They digitize the input signal (1 bit quantizer) at a 
rate
that makes the carrier alias down to something convenient (a few 
hundred
kHz is typical.. you want it far enough away from zero that Doppler
never makes it go negative).  In the experimental receiver in SCaN
Testbed flying on ISS it's about 39 MHz sample rate.

Once you've got your one bit samples, you do some sort of combined
Doppler/Code phase acquisition (these days, often using an FFT), then
track both together digitally using some form of NCO.  The tracking
loops for all the satellite signals aren't necessarily independent and
might be part of a Kalman filter that estimates all the observables
together.

Finally, from all that, you have an estimate of your local clock offset
and timing offset, and from that you can generate your 1pps, typically
with another NCO (with granularity of your clock rate).  Since it's
unlikely that your clock is EXACTLY an even number of cycles per 
second,
at each second, a bit of error accumulates, until you have an whole
cycle's worth leading to the familiar sawtooth error.

That sawtooth error is predictable, of course, so you can generate a
"time error" estimate for each 1pps pulse (or, even, control a variable
delay to line it up).

The important thing is that in modern receivers, nowhere is there a
signal at the GPS carrier frequency, nor is there a signal at the chip
rate.  There *is* probably a signal (with low precision) at the code
epoch (every millisecond), but it's different for each satellite 
signal,
of course.

------------------------------

Message: 2
Date: Tue, 02 Oct 2012 19:32:48 -0700
From: Jim Lux <jimlux at earthlink.net>
To: time-nuts at febo.com
Subject: Re: [time-nuts] GPS Jammer
Message-ID: <506BA3D0.4090609 at earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 10/2/12 4:48 PM, Charles P. Steinmetz wrote:
>
>> The Power Output is 0.5 Watts and it claims a jamming range of 1-10
>> Meters.
>> Anybody think there is something wrong?
>
> I'd expect a much greater range with a 0.5 W jammer.  But note that 
0.5
> W is the "total output power" -- the "transmit power" is only 10 dBm
> (0.01 W).  Whatever those terms mean.  (Does "total output power"
> include far IR and heat?)
>

maybe it has some real bright LEDs to indicate it's on?

------------------------------

Message: 3
Date: Tue, 2 Oct 2012 22:33:28 -0400 (EDT)
From: johncroos at aol.com
To: time-nuts at febo.com
Subject: Re: [time-nuts] GPS Jammer
Message-ID: <8CF6F269AD88F67-1D20-7F60D at webmail-d139.sysops.aol.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

In considering the effect of a simple jammer on a GPS receiver, a
simple link analysis
is insufficient.

What must also be considered is the anti-jam capability of the receiver
which due to spread spectrum processing gain will reject any simple
jamming signal even though is it 10's of dB stronger than the desired
signal.

73 -john k6iql

------------------------------

Message: 4
Date: Tue, 02 Oct 2012 19:34:47 -0700
From: Jim Lux <jimlux at earthlink.net>
To: time-nuts at febo.com
Subject: Re: [time-nuts] Best counter setting for ADEV?
Message-ID: <506BA447.5090704 at earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 10/2/12 7:00 PM, shalimr9 at gmail.com wrote:
> The Thunderbolt is a special case that does not provide sawtooth 
correction
because it does not need it.
>
> It uses the OCXO as the clock for the processor while disciplining it 
to GPS
so there is no nominal timing error between where the 1PPS is versus 
where it
should be.
>
> The processor is able to bring the PPS edge exactly where it wants 
it, instead
of the typical 25 to 40 ns granularity of most other GPS receivers that 
operate
on a separate clock.
>
> Pretty simple and elegant solution.
>

But it does depend on having a good oscillator that can be shoved
around.  That costs money and power.

------------------------------

Message: 5
Date: Tue, 02 Oct 2012 19:37:32 -0700
From: Jim Lux <jimlux at earthlink.net>
To: time-nuts at febo.com
Subject: Re: [time-nuts] RFX GPSDO - Anybody played with one of these?
Message-ID: <506BA4EC.2070403 at earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 10/2/12 2:36 PM, SAIDJACK at aol.com wrote:
> Hello Paul,
>
> thanks much for the feedback!
>
> Yes, we think we have identified a nice combination of oscillators, 
GPS,
> and firmware that seems to work pretty well. The GPSTCXO units cannot 
be
> compared to a lower cost $150 Thunderbolt in terms of phase noise or
stability
> of course, and they have CMOS 10MHz outputs, but then the  TB's cost 
around
> $1500 new I guess.
>
> We think the GPSTCXO's and LC_XO type units will work quite well  
wherever
> standard OCXO's are used today, and power/size/weight are an  issue.
>

Intriguing.. Can it handle the Doppler, etc., for a cubesat in LEO?
(7km/s)  The total Doppler isn't usually the issue (the GPS satellites
are moving faster, after all), but the receiver may not work for high
velocities, high altitudes?

------------------------------

Message: 6
Date: Tue, 2 Oct 2012 21:37:53 -0500
From: Bill Dailey <docdailey at gmail.com>
To: Time Nuts <time-nuts at febo.com>
Subject: [time-nuts] Centering ocxo
Message-ID: <21F2B49D-72FA-46A5-956E-B45ACBD6EEEB at gmail.com>
Content-Type: text/plain;	charset=us-ascii

I am ok for awhile but how do you center the efc of an ocxo?  I 
understand there
is something (screw) to adjust the ocxo so it is approximately on freq 
with 2.5v
efc.

Specific oscillator datum-1111c. I have he datasheet but doesn't say 
"coarse
frequency adjust this screw" or some such.

Doc
KX0O

Sent from my iPad

------------------------------

Message: 7
Date: Tue, 02 Oct 2012 19:45:32 -0700
From: Jim Lux <jimlux at earthlink.net>
To: time-nuts at febo.com
Subject: Re: [time-nuts] GPS Jammer
Message-ID: <506BA6CC.2040702 at earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 10/2/12 7:33 PM, johncroos at aol.com wrote:
> In considering the effect of a simple jammer on a GPS receiver, a 
simple
> link analysis
> is insufficient.
>
> What must also be considered is the anti-jam capability of the 
receiver
> which due to spread spectrum processing gain will reject any simple
> jamming signal even though is it 10's of dB stronger than the desired
> signal.

not most simple GPS receivers which have very little AJ capability. 
They
have a single bit quantizer (or maybe a 1.5 or 2 bit) after the LNA.  
If
the LNA doesn't saturate, then the quantizer is captured by the strong
CW carrier.

This is a classic problem with DSSS receivers and led to a lot of
research in the 80s on things like "adaptive excisers" to remove CW
carriers.

If you built a linear receiver with a lot of dynamic range, then, yes,
the process gain will suppress the CW tone, but you still have to
acquire the code, and as Dixon says (paraphrasing) "acquisition is the
secret sauce in spread spectrum systems".  Back when I was doing this
kind of thing seriously (mid to late 80s), acquisition, particularly
robust techniques, were literally SECRET (in the DoD sense).

There have been a nice series of articles in GPS World over the past 
few
months about the variety of inexpensive GPS jammers out there. (and the
problems they cause).

------------------------------

Message: 8
Date: Tue, 2 Oct 2012 22:53:18 -0400
From: paul swed <paulswedb at gmail.com>
To: jfor at quikus.com, Discussion of precise time and frequency
	measurement	<time-nuts at febo.com>
Subject: Re: [time-nuts] Google's Spanner uses GPS and atomic time
Message-ID:
	<CAD2JfAi3Q_BgbBhSGK3dfz_kdG2-_3vmhAkNv0pQJbaaS19KNA at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

And yes thank god we are cutting funding to those pesky satellites. 
Seems
we can't afford them anymore. But fortunately other countries are 
filling
our gap slow but surely.
There was an article this month in GNSS about the funding cuts.
Regards
Paul
WB8TSL

On Tue, Oct 2, 2012 at 9:11 PM, J. Forster <jfor at quikus.com> wrote:

> The US, foolishly IMO, has killed LORAN-C and is killing WWVB. Hence 
my
> comment. Essentially, GPS is soon going to be the sole source of a
> standard of time interval.
>
> It is going to take a disaster of some kind to return sanity.
>
> -John
>
> ===========
>
>
>
> > John writes:
> >
> >> Seems like a good reason to have LORAN-C or some other 
backup/sanity
> >> check.
> >
> > What LORAN? I thought the U.S. had shut down all LORAN transmissions
> > in order to enhance the vulnerability of navigation systems in the
> > U.S. (?).
> >
> > --
> > Anthony
> >
> >
> > _______________________________________________
> > time-nuts mailing list -- time-nuts at febo.com
> > To unsubscribe, go to
> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > and follow the instructions there.
> >
> >
>
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>

------------------------------

_______________________________________________
time-nuts mailing list
time-nuts at febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

End of time-nuts Digest, Vol 99, Issue 19
*****************************************