> ... One value is "code phase" (along with PRN, sample length, sig level,
> dopple, and time-of-measurement). This is a single precision floating
> point number in units of 1/16 of a chip. Does anybody know how to massage
> this value into either a carrier phase (in cycles) or a pseudorange
>
>
> Results from the Z12 in L1/L2 mode were... iffy... then the receiver died.
>
The teqc tool from UNAVCO can help with diagnosing iffy data. Can you put
your RINEX somewhere? With the antenna in an unobstructed area my Z12
routinely gives 1 cm-ish horizonal, few-cm 3D, using OPUS-S or
> Well, my Z12 stopped tracking sats yesterday. It passes all self-tests.
> I then replaced the memory backup batteries... it fixed the bootup error
> problem, but still won't track sats.
>
Rats. Is the antenna known to be good? Is the Z12 providing bias on the
antenna cable? Did the Z12
>
> Does the Z12 have an internal memory backup battery?
Yes, there are two lithium thionyl chloride cells, Tadiran TL-5104. They
are 3.6 V primary cells, cylinders roughly AA in size, with spot-welded
axial leads. That part is obsolete, but Digikey has an equivalent,
TL-5903/P, which worked
Hi Tom,
> It that's still not close enough to 0.3 m, is one then forced to use more
> expensive multi-frequency (L1/L2) or multi-band (GPS, GLONASS, Galileo) to
> achieve this level of precision? If so, how cheaply can one do this? Or is
> the learning curve more expensive than just hiring an
Jim Lux writes:
> But another poster did comment on "why not use the telescope" you could
> precision point to a series of stars and calculate using celestial nav
> where you are. Although, that might be painful to the 1 meter sort of
> accuracy - the "tables" probably don't really account for
>
>
> Also the new iphones now support Galileo in addition to GPS and Glonass.
>
Any word on whether iOS will support raw observables, as Android has for a
little while now? I gather the APIs need to support continuous tracking
better so that phase observables are meaningful across epochs.
Will
>
> BTW a lot of GPS receivers don't have a "first LO".. they are more like a
>> Tuned RF receiver - an input BPF for L1, L2, or L5, then direct sampling at
>> around 30-40 MHz - something that makes the GPS signals alias down
>> somewhere convenient (and always have positive frequency offset
> I am curious if the first local oscillator on a GPS receiver must actually
> be locked or coherent to the reference oscillator in the GPS receiver
> typically running at some 10 MHz approximately. Or as long as the first LO
> is quite stable it doesn't matter because the receiver can track the
Hi Tom,
> [ USB time transfer ]
>
> It seems to me that if the read path and the write path are different it
breaks down.
>
> ... But turning that into precise time requires some kind of calibration
of the actual code path delays. In other words, it sounds to me like your
method is valid for
Time transfer over USB can be improved by timestamping on both ends, then
using a robust estimator for the clock offset. For example, imagine the
USB is a small microprocessor peripheral. It has a local timer, freely
incrementing, based on its local clock. When it gets a USB interrupt from
the
Hi Dave,
The Harrisons are indeed at the observatory; also look for a regulator
pendulum clock in the octagon room. I'm not quite sure whether it was
running when I was there some years back.
Could it hurt to petition the observatory's powers-that-be for a little hut
or something at the ITRF
Hi Attila,
>
> Galileo E1 and GPS L1C are not BOC(1,1). They both use a variant
> of the MBOC(6,1), CBOC(6,1,3/11) for E1 and TMBOC(6,1,4/33) for L1C.
> You can view those as a weighted combination of a BOC(1,1) signal with
> a BOC(6,1). This gives an additional peak at 7MHz offset, compared to
>
> What should the IF pass band bandwidth be?
>
For GPS C/A with wide correlator, about 2 MHz; if you want Galileo BOC and
(eventually) GPS L1C, or legacy C/A with narrow correlator, about 8 MHz;
for GPS P code about 20 MHz. Books on GNSS software receivers will detail
the many tradeoffs
>
> Further, would you be able to see the phase of the moon and the tides?
> This
> is using the pendulum as a gravimeter. Would it be sensitive enough for
> that?
>
Yes. There's a book called "My Own Right Time", by a skilled amateur
clockmaker (Philip Woodward), that shows some periodograms
And for the receiver:
https://github.com/pmonta/GNSS-DSP-tools
Cheers,
Peter
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Hi Tom,
Based on mass and radius, a clock here on Earth ticks about 6.969e-10
slower than it would at infinity. The correction drops roughly as 1/R below
sea level and 1/R² above sea level. For practical and historical reasons we
define the SI second at sea level.
Yes, the change in clock
Chris Albertson writes:
But you are right, no two clocks will ever agree at that level because they
will experience different gravitational fields. At this level the reason
to have a clock is no longer to tell time. It is to measure the
gravitational field.
I have a question about
How hard is the beam steering relative to everything else?
It's a weighted sum of the antenna signals (as with any phased array), so
the cost is the extra arithmetic to do this (on a per-satellite basis).
The weights can be computed open-loop from the positions of the satellites,
provided
Hi Jayson,
You may already be aware of it, but there's a set of historical recordings
of WWV and WWVH, covering 1955 to 2005:
http://www.myke.me/atthetone/
As for the simulation, I'm sure it would be easy to do the tones and
clicks, but the voice announcements would need a considerable amount
I've got a raw wideband GPS capture, dual frequency L1/L2, on this
page about my GPS/GNSS front-end board project:
http://pmonta.com/blog/2012/06/04/gnss-firehose/
I plan to do a new spin of this board with some minor improvements as
soon as I have the time. (For those who want to build the
IMHO the transfer mode of choice for this purpose should
be the Isochronous Transfer (in USB 2.0 and 3.0) because
it happens periodically and thus can achieve a guaranteed
maximum latency (for high speed this means 125us).
If 1 ms or 125 us is good enough, then this would be fine; but for
Hi Hal,
Why are X-Ray pulsars better than radio pulsars for navigation?
My impression is that it's easier to manage all-sky coverage at x-ray with
a small spacecraft package (I think millisecond pulsars generally emit at
both microwave and x-ray). Also there's some interstellar
Hi Magnus,
Would not an antenna with a deep zero focus on the earth center help to
reduce earth-noise (ground temperature noise as well as man-made noise)?
It might, although you'd need a large antenna to generate the angular
resolution needed to reject Earth noise while listening to a GPS bird
Hi Attila,
The goal is to make two lasers locked with about 7GHz of offset to
eachother.
So far, i figured out that PIN photodiodes can go up to several 100MHz
transition frequency and avalanche photodiodes are available up to 2GHz.
If you need photodiode response only near 7 GHz, as
Let me put in a recommendation for python / scipy / numpy / matplotlib as a
replacement for Matlab or octave. I used Matlab as a student and octave
since, and they do their job well, but the python tools have the advantage
of a better, more modular, less idiosyncratic language.
The scipy
Have you actually tried it and gotten it working, except possibly in a
very strong signal area?
This is precisely the issue. Squaring the WWVB signal results in a
significant SNR penalty. At high SNR it doesn't matter that much; at
low SNR you are in a world of hurt.
I had suggested to John
I'm not sure about residual carrier aiding the tracking process. A Costas
loop recovers the carrier pretty well, and a symbol aided loop (where the I
channel has a hard limiter, for instance) does even better.
Yes, these work (and a soft tanh() limiter improves on the hard
limiter a little
Hi John,
Thank you for clarifying the openness of the transmission format.
Could I ask whether there is any scenario under which aspects of the
signal transmission design might be patented? If companies or
individuals wish to patent aspects of receiver design, that's fine,
but I'd be
Jim Lux writes:
It won't be state of the art (I think tvb's cesium wrist watch does that..
but it doesn't have the non-digital display you want)
One would think wristwatches based on the Symmetricom CSAC would be on
the market by now. Surely the prices some are willing to pay for
high-end
Hi Michael,
What is the function of the clock/PPS inputs?
They could be used to compare a master station clock with the GPS
signals. It seems like a good idea to put the clock and PPS through
the same path as the satellite signals. The goal was to permit a good
comparison between GPS and any
Hi Attila,
* Connect all free pins of the FPGA to a 2.54mm header pin connector
This would make extensions to the system a lot simpler.
Up to the point of using a simple uC board for a full fledged
GPSDO system. Additonally put onto this connecter an unused output
of the LMK03806 and
On Wed, Jun 6, 2012 at 9:33 AM, Eric Garner garn...@gmail.com wrote:
On Tue, Jun 5, 2012 at 12:53 PM, Attila Kinali att...@kinali.ch wrote:
On Tue, 05 Jun 2012 15:40:48 -0400
Michael Tharp g...@partiallystapled.com wrote:
On 06/05/2012 03:31 PM, Attila Kinali wrote:
* The XC6SLX9 is10USD
Oops, sorry, hit the send button too soon.
Another nice book on GPS signal processing is the one by James Tsui,
_Fundamentals of Global Positioning Receivers: A Software Approach_.
I learned a lot from it.
Cheers,
Peter
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Hi Attila,
Could you also add just the option of a power splitter at the input?
I'm not really sure what the most appropriate RF interface is. I was
planning to make another much simpler board with just a bias tee, a
3:1 splitter, and some hairpin microstrip filters. Then it would be
three
[ Borre GPS book ]
However, it does not appear that they let me download the
matlab code that comes on the CD that comes with the book. do you know
if it's available somewhere?
I think it's here:
http://kom.aau.dk/~borre/easy2/
The last entry, the zip file, looks to be the most recent.
http://kom.aau.dk/~borre/easy2/
Oops, sorry, that was for the previous book. For the
software-receiver book, you want this page:
http://kom.aau.dk/project/softgps/
I'm not sure the source code is in the demo CD image, though (the GPS
SDR demo link).
Cheers,
Peter
is
limited and it seems geared toward high-spectral-efficiency signals
with many (=8) bits per sample.
Cheers,
Peter Monta
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Everyone should take ten seconds and look at this animated GIF:
http://leapsecond.com/pages/ut/ut-ani-v2.gif
Very nice. I guess a comparable plot with ephemeris time would be a
lot noisier and sparser, something like the graphs in the Markowitz
1988 paper.
I wonder if there's some sort of
So, I've looked at several dozen helibowls and talked to makers of said
items..
Thanks for posting this; maybe a homemade helibowl is a good way to
get a low-cost GPS antenna with full frequency coverage down to L5.
In my search for survey-grade antennas at hobbyist prices, I ran
across this,
Are there better estimates of the ET second nowadays (relative to the
SI second)? It would be interesting to know what the cesium frequency
should have been if much better estimates of the ephemeris-time
second were available at the time. One would think that with all the
solar-system data JPL
Okay. A little 3586B hacking was required, but here are some wide-band
results: http://www.jks.com/wwvb/wwvb.html#wideband
Thanks very much. This data shows the full-bandwidth WWVB signal very
well. Attached are some plots and an octave script.
The first plot shows the demodulated WWVB
The transitions (where the phase change!) are what you correlate,
the more, the better S/N you get.
Yes---it's too bad that the proposed WWVB changes don't increase the
number of transitions at all. Could they not do the
low-modulation-index DCF77-like signal on top of the BPSK? That is,
put
Attached are some more renderings of John Seamons' WWVB data. This is
what one might expect from a receiver that knows when the phase
reversals happen and takes them out noiselessly---re-reversing the
out-of-phase bursts to recover an approximation of the usual WWVB
signal.
The first plot shows
So I gather from this they are working on something, but no details were
given.
One possibility for a workaround to keep the classical receivers happy
is to leave some residual carrier. Instead of a 180-degree phase
shift, make it 90 or 120, so that the signal can be regarded as the
sum of a
Many A/D converter systems use a sample and hold before the A/D converter.
If you do the same before your sound card (your A/D converter) and drive the
SH with an audio output from your sound
card, say at 6.1 kHz you would get a 1 kHz signal into your sound card to
process. You can call it
I'm not clear how accurately one can resolve the phase transition
in the new scheme, but I suspect probably unambiguously to 1 cycle of
the 60 KHz... and from there is merely a function of how accurately one
can resolve the phase of the 60 KHz. This potentially can supply a
much
In thinking about it a bit further, one might be able to take the 60 kHz
received sine at some point in the receiver, full wave rectify and HP
filter it (which doubles the frequency) then divide by two in a Flip-Flop
and heavily filter the resultant. This is a hybrid solution... analog and
Part of the processing gain comes directly from the BPSK modulation and that
amounts to a little over 10 dB improvement, but there's a further 18 dB gain
to be had by accumulating an hours worth of data and processing that.
That part of the paper bothered me. There's nothing preventing a
I believe what they were doing is applying a simple 180 deg phase shift to
the carrier (BPSK) during certain one second timecode intervals.
I suppose this will help coherent receivers somewhat, but I was
expecting something DCF77-like. A pity.
Thanks for posting your data. You certainly
... or I suppose we could ask.
I did ask, using the contact email on the NIST page (John Lowe). I'll
forward any replies to the list.
Perhaps we could encourage a more ambitious DCF77-like scheme, if
that's really not in the current plans for WWVB. Especially given the
recent demise of
Interesting that WWVB is running a phase-modulation test---thanks for
the links. Is the signal format known? A quick search shows nothing
specific, just we're testing.
Could someone record a few minutes of the broadband signal so that
those of us without ready-to-hand LF receivers can have a
... but then they get taken indoors and plugged into the network, and
probably never get a clear view of a GPS or GLONASS satellite again.
A high-sensitivity GPS receiver might still give useful results here,
especially if it has a high-quality reference oscillator like an OCXO.
Even 20 or 30
Time is money...
I wonder if long-distance neutrino links might be attractive to the
financial community. The SNRs are currently way too low, but with
aggressive engineering, a link through the Earth would shave off many,
many milliseconds (even at not greater than the speed of light :-) ).
The
I think, a specialized GPS SDR can be build for less than 500 USD
in low (a dozen at max) volumes.
The USRP works for GPS L1 (though P/Y is a little undersampled at 8
Ms/s complex), but I didn't find a way to acquire both L1 and L2
simultaneously at useful sample rates (maybe current USRP
What if the crystal driving the DSP changes frequency with temperature?
I believe it would have no effect. The local clock should drop out of
any navigation or timing solution, so long as the changes are slow
enough to avoid loss of lock in carrier tracking.
Let's take a GPS module with 1 PPS
Hi Attila,
So, that'd mean there would be an automatic calibration system inside
the device, because i dont have any equipment with which i could
calibrate delays over a temperature range.
I suppose they could do that---provide a weak broadband source (say a
comb) combined with the antenna
Hi Attila,
[ L1 / L2 timing differences ]
How do dual frequency receivers deal with that?
I've also been toying with the idea of an inexpensive dual- or
tri-band GPS SDR, especially since there are now quite a few
satellites emitting L2C, the civil L2 signal. (Though I'd still like
to try my
Speaking of GPS's L2C signal, it turns out there's a timing difference
between L2C and L2, namely 90 degrees of phase (nominally).
Apparently there was some controversy over whether to include this
correction in the RINEX files. The RINEX 3.0 document has details.
Even with all this complexity,
Paul Swed writes:
I agree this does not make sense. There is a divider in the way so its not a
preamp.
I'll bite: what does a small-signal model of a prescaler look like?
As a guess, it might be a (flat? highpass?) attenuator (with a great
deal of loss) up to some threshold amplitude, then
in other respects. The mountaintop could also be
passive if that helps: pairs of antennas connected by a transmission
line and appropriately pointed.
Cheers,
Peter Monta
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Cheers,
Peter Monta
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Satellite laser ranging using LAGEOS and friends?
On second thought, this wouldn't work anyway (besides being too
expensive)---stations would have to be very close together to have
common view.
Cheers,
Peter Monta
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wrong
time, then put it back to the automatic setting, causing it to
reacquire the time. Now my phone is within 3 seconds of NTP.
Could there be some huge hysteresis/dead zone within which
the iPhone doesn't bother to trim its clock to the cell tower? Sigh.
Cheers,
Peter Monta
sources or against GPS if the phone has it (or both).
Then any photos can be batch-corrected later if desired. Apple, give
me control over the time on my own phone, and please don't force me to
resort to these schemes :-).
Cheers,
Peter Monta
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