Hi

There is a limited tracking range for Doppler. You would need to stay inside that.

Bob

Sent from my iPhone

It doesn't matter, so long as the first LO is in the ballpark so that the
Doppler search is not needlessly large.  I'm not so familiar with the early
receivers, but I imagine a single reference oscillator serves for
everything---there would seem to be no reason to have more than one unless
the antenna/downconverter were physically separate from the rest of the
receiver.  If an older receiver used a physical source at 10.23 MHz, it
would still need to be offset slightly for each satellite because of "code
doppler", but this choice of frequency might slightly simplify the
circuitry.  Current receivers would use any convenient physical rate, then
synthesize the code rates.

Cheers,
Peter

On 3/30/17 10:32 AM, Bob Camp wrote:

Doppler is pretty big when the spacecraft is coming or going at the
horizon, about 5 kHz (out of 1.5 GHz, so 4-5 ppm).
Relatively speaking, GPS satellites are moving slowly (a few km/s)

in LEO you're buzzing along at 7km/s, which is about 20-25 ppm.  That is
the usual limiting case for bandwidth/tracking loops; you might want to
go up to 11-12 km/s so you can get things moving at escape velocity.
(there just aren't many people putting GPS on hypersonic projectiles -
if you've got the bucks to shoot something at Mach 45, you can probably
afford a custom GPS receiver)

This is a bit tricky for older receivers because their tracking loop has
to acquire in the face of the Doppler uncertainty and the range (code
phase) uncertainty - there's a whole lore of optimum search strategies
and how to get the fastest time-to-first-fix.

Does the first LO have to be locked to something?  the signal you're
acquiring is MHz wide, so a 10ppm error in the LO frequency isn't a big
deal. Short term stability does help, while you're acquiring.

But one of the things about GPS that made it attractive is that the
local clock can be pretty crummy.

On 3/30/17 11:06 AM, Peter Monta wrote:

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 from zero, even at max negative Doppler)

Hi

So somewhere in the baseband processor code somebody said “we’ll handle +/- 5 KHz”. If your LO is < (say) 500 Hz it’s still inside the likely doppler handling range.

If you want to do carrier phase then maybe you want to get a bit fancier ….

Bob

Thanks everyone for your comments. It will be a GPSDP TBolt or Z3801
reference.
I just wanted to eliminate some variables at this stage.
Regards
Paul
WB8TSL

On Thu, Mar 30, 2017 at 7:56 PM, Bob kb8tq kb8tq@n1k.org wrote:

True.  I've been wanting to try this with an FPGA transceiver; even the
cheap ones go to 6 Gb/s now, but binary only.  The newest transceivers
support PAM-4, which would be great, but they're not affordable yet.  Also
that's a lot of gain at one frequency.

Cheers,
Peter

On Thu, 30 Mar 2017 19:56:31 -0400
Bob kb8tq kb8tq@n1k.org wrote:

You have a carrier PLL anyways. What we call "carrier phase" is just
using the information from that subsystem to get a better estimate
for the time differences.

		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, Neil Stephenson

On Thu, 30 Mar 2017 20:00:31 -0700
Peter Monta pmonta@gmail.com wrote:

I guess you know of [1] already?

Alternatively, instead of using a MAX2021, you can use a discrete
mixer and use the high analog bandwidth of todays ADCs to use them
as downmixers. [2] and [3] describe how to do this in detail.

The epitome of this is using direct sampling of the signals without
previous downmixing (e.g. [4]). Though I have no idea how easy or hard that
is with todays electronics. It will definitely need good preselection
filters to keep SNR high.

		Attila Kinali

[1] http://www.aholme.co.uk/GPS/Main.htm

[2] "A Prototyping Platform for Multi-Frequency GNSS Receivers",
by Akos, Ene and Thor, 2003
http://waas.stanford.edu/~wwu/papers/gps/PDF/AkosIONGPS033FreqRX.pdf

[3] "Design of a GPS and Galileo Multi-Frequency Front-End",
by Parada, Chastellain, Botteron, Tawk, Farine, 2009
http://202.194.20.8/proc/VTC09Spring/DATA/04-04-01.PDF

[4] "Design and Implementation of a Direct Digitization GPS Receiver Front End",
by Akos, and Tsui, 1996

--
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, Neil Stephenson

On 3/31/17 4:35 AM, Attila Kinali wrote:

Our space GPS receiver flying on SCaN Testbed/ISS is a direct sampling
at 38 MHz. Filters are nothing special as I recall.  A chain of
amplifiers with filters - Lots of gain into the thresholders, so good
layout is important.

https://trs.jpl.nasa.gov/handle/2014/41781

https://trs.jpl.nasa.gov/bitstream/handle/2014/41781/11-0046.pdf?sequence=1&isAllowed=y

Hi,

On 03/30/2017 03:46 PM, paul swed wrote:

If you only do a code receiver, you can do this, but the carrier
tracking will need to compensate the LO. This is naturally possible, but
some of the precision will be lost. You can play some tricks to average
this from the channels and sort things out.

If you want to do a carrier phase receiver, you would suffer too much,
then you would need to lock the LO to get the gain.

For old receivers, I'd just assume the LO is locked, as it saves
compensation tricks later, which would save computation cycles.
However, there where some crude receivers back in the days.

Cheers,
Magnus

Hi Jim,

On 03/31/2017 01:07 AM, jimlux wrote:

Still fills the function of LO, as the sample and hold operates as a
mixer and the fold-down can be seen as an overtone mix followed by a
sampling of the mix product, so well, it's about the same thing.

Cheers,
Magnus

God kväll Magnus,

On Fri, 31 Mar 2017 21:19:00 +0200
Magnus Danielson magnus@rubidium.dyndns.org wrote:

"Harmonic mixer" is the word you are looking for :-)

		Attila Kinali

--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

God natt Attila,

On 03/31/2017 11:29 PM, Attila Kinali wrote:

Not necessarily. It could be a locked oscillator too.
Harmonic mixer is another way to go.

Cheers,
Magnus

Hi

There are a lot of GPS chips that do an I/Q mix down to a low IF. It’s then (re) sampled from there. The “LO” in this case would down convert to the low IF ….

Bob

Thanks everyone but I am working on an austron 2201a so all the discussions
on modern methods won't help. Whats is interesting is indeed the 2201 down
converts to 80KHz and the does sample in an IQ fashion. Its all discreet
chips and such.
Easily traceable and logical.
I think I have what I asked for and am experimenting with active mixers and
IFs made of minicircuit gain stages.
I am using a commercial antenna with 34 db of gain. It says 50db I question
that.
But lots of gain to a HP IAM 81008 active mixer low drive LO. Then a 40db
at least 75 MHz IF. (Pretty sure this is overkill.)
The LO is a HP 8660c for now. Locked to a TBolt.
Thats the reason for the question. I can shift the 8660 to the ausytron 10
MHz.
Regards
Paul.
WB8TSL

On Fri, Mar 31, 2017 at 7:47 PM, Bob kb8tq kb8tq@n1k.org wrote:

Hi

If you have a free running VCO in the head end, then indeed you need to lock it to
something. An unlocked VCO will not be close enough to frequency to do you any good.
The 8660 may or may not be close enough in free run mode. It depends a lot on
what standard you have in yours.

That is a very normal antenna gain spec and it was quite common in the era the
device you have was designed. I run a number of GPS gizmos that need a 50
db antenna on them. They might work with a 40 db setup. They do not work
with something in the 20 to 30 db range. If you watch for a while (as in 6 to 12
months) you can indeed get good old 50 db gain antennas on eBay pretty cheap.

Bob

Dave,

I was able to find [2] here:

http://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Akos_IONGPS_2003_3FreqRX.pdf

[3] was harder, and I don't have a link but a google search for the title
in quotes got me a link on semanticscholar that let me download the PDF.

Interesting stuff!

Hope that helps,
-Logan

On Fri, Mar 31, 2017 at 9:17 AM, David C. Partridge <
david.partridge@perdrix.co.uk> wrote:

On 4/3/17 9:32 PM, Logan Cummings wrote:

So those folks were trying to use 1 ADC for all three bands, so they had
to choose a sampling rate that lets them separate the signals later in
software.

But that ADC is a MAX104 - a 1GSPS, 8 bit converter - that draws 5 Watts!!!

I'm not sure that's a good trade against a 1 or 2 bit converter for each
band, in terms of the downstream data rate and processing.

Perhaps someone could start a new thread with a different title since this
has evolved to something else then my request.
By the way the response to my question was answered by the group and its
true the first LO does not need to be locked for at least code tracking. I
really didn't prove its needed for carrier tracking.
Thanks everyone.
Paul
WB8TSL

On Tue, Apr 4, 2017 at 9:55 AM, jimlux jimlux@earthlink.net wrote:

On Tue, 4 Apr 2017 06:55:24 -0700
jimlux jimlux@earthlink.net wrote:

Honestly, I don't think the direct sampling approach is a good idea.
It folds a lot of noise into the signal band. I'd rather use a single
heterodyne with an LO frequncy of around 1000MHz, or something between
L2 and E5, such that the bands stay still seperated. Here I would add
a filterbank to get rid of as much noise as possible. And after that
use an ADC sampling frequency to fold the signals down again.
(Effectively forming a super-heterodyne receiver)

You don't need a 1Gsps ADC for that, but if you want to keep all
frequency bands completely seperate, even after sampling, a relatively
high sampling rate is necessary. L1C/E1OS needs at least 14MHz,
L2C needs 2MHz, E5 needs 50MHz. Ie to keep them separated, at least
66MHz of (un-aliased) bandwidth or a sample rate of 132Msps is needed
(alternatively, an I/Q ADC with 66Msps). There are plenty of ADCs
that go up to 120Msps with 10-14bits resolution available, and a couple
that go higher (up to 200MHz are easy to find). 8bit ADCs with >100Msps
are available, but not so many with >120Msps. So, it should be "easy"
to build such a system, if one can find a nice pair of LO frequency
and sampling rate. Alternatively, if one can accept a slightly decreased
SNR one can choose a pair of frequencies where all the signals fall ontop
of eachother, making the 50MHz of E5 the only real requriement. My guess
would be that the CDMA character of the codes would make them easily
seperatable, resulting in an (additional) SNR los of maybe 1-3dB, which
can be compensated by using an ADC with 8bit or even 12bit instead of
the 2-4bit that are now common for GPS receivers. The frequency requirement
can further reduced if one drops the E5b signal and just works with the E5a.
Then 21MHz of bandwidth would be enough.

Looking at the frequency band values, an LO frequency between
1367MHz (L1C touches E5) and 1405MHz (L1C touches L2) would be
the most sensible range. The IF would be below 240MHz and it
seems like the maximum needed bandwidth would be around 70MHz
(eyeballing the graph, no real calculation). I'd say the best
compromise would be using an LO of 1398MHz (IF=170-230MHz)
and using a sampling rate between 120Msps and 160Msps.

The advantage of such a system would be that there is only a single
path through the system for all signals, especially through the filters.
Thus the variability of the differential phase shift between the
frequency bands would be significantly reduced, which would result
in better stability. Of course, that's the theory. Whether things work
out this way in reality is a different question. What can be said for
sure is, because of the high IF frequency of >200MHz, the standard tuner
chips cannot be used anymore and the RX chain has to be build from
"discrete" components, which increases the BOM cost quite considerably.

		Attila Kinali

--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

On 4/4/17 4:21 PM, Attila Kinali wrote:

in most of the designs, the noise is from the LNA, and is band limited,
so the additional noise from the amplifier chain is less. COnsider if
the LNA has 40dB gain and a 2 dB NF. Let's say all the other amps in the
chain have 5 dB NF.

The thermal noise into the next amp is -132 dBm/Hz.  In order for the
5dB NF noise (-169 dBm/Hz) to get up high enough to be noticeable, say,
30dB, you'd have to fold 1000 times the sampling bandwidth. if the
sampling bandwidth is 40 MHz, to get the noise up high enough it would
have to extend to 40 GHz... I'll bet it doesn't<grin>

I don't think keeping the bands together buys you much - you don't need
a multibit ADC for a signal that is below the noise floor. (unless
you're trying to reject strong interference signals, but that's a
different kind of receiver).

Oh, I'm not sure about that. It would depend on the filter kind and
topology.

If it's a SAW or BAW filter, it's all one "brick", but I think you'd
still need to calibrate the differential phase shift vs temp.  And it
might be very predictable in a "measure 10 of them, and now you know the
characteristics of the next 1000"

Of course, that's the theory. Whether things work

There's a ton of integrated demodulator/ADC parts out there these days
that go up to 6GHz.
AD9361 for example

it will do 56 MHz BW through the IF, with 12 bit ADC feeding a 128 tap
FIR filter, etc.

which increases the BOM cost quite considerably.

On 04/05/2017 01:21 AM, Attila Kinali wrote:

Regardless you already have SAW filters on the LNA to provide selectivity.

Also, you don't really need to keep the bands fully separate in their
mixed-down form, since they do not correlate except for the P(Y), but
keeping enough frequency difference, such that doppler shift does not
remove correlation margin, they remain uncorrelated. Some of the
literature pay much attention to the band not wrapping around the
band-edge, but I'm not convinced it is such a big issue.

A direct sampler of 100 MHz would work well for GPS for instance, but
not for GLONASS, but 90 MHz would work there. The S/H would need to have
the BW of the top frequency, but then the S/H action will act as the
first mixer.

Since you don't really need to keep signals very separated, you can pack
them relatively tight. It's the E5 of GALILEO which is wide.

Using a 1,4 GHz range LO1 to pick L1 and L2 has been known to be used
before. There is even existing chips which uses 1.4 GHz on LO1, which
with a different set of filters could almost support L2, will have to
check the details. While that front-end would be neat, I would not use
that chip since it is no longer in production.

The fun thing about these types of receivers, is that there is so many
ways to do it, that it allows for many different approaches to be tried
as technology develops. There is no single one "right" way of doing it.

Cheers,
Magnus

Hi

Galileo E5 is a bit of a strange case. It’s really E5a and E5b. You can either grab it all as one
giant signal or as two separate signals. You may (or may not) care about the data on E5a or
b depending on what you are trying to do. Getting the entire very wide signal likely has some
interesting benefits when it comes to working out very small differences in location or … errr…
time.

One way to do the E5 signal would be a dual (duplicate) IF ISB downconverter. How practical that turns out
to be is an open question. The more conventional approach is to take a monstrous chunk of
L band down to a high speed sampler.

Bob

Hi,

There are many things to be done before attempting the full E5 approach
anyway, so I would not have make it a make or break for a first design.

Cheers,
Magnus

On 04/05/2017 02:27 PM, Bob kb8tq wrote:

On Tue, 4 Apr 2017 17:58:11 -0700
jimlux jimlux@earthlink.net wrote:

The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference. And at the same time, because the IF frequency
being low, you don't need any specialized filter components that
might not be available in a couple of months.

Of course, this doesn't really work that way when significantly
wider signals (E5) have to be caught together with "narrow band"
signals (L1 C/A or L2C).

Unfortunately, the AD9361 does not offer the IF bandwith necessary.
Even though it has a high sample rate and can offer high bandwidth
capture of signals, the zero-IF nature of its design doesn't work
for this design approach. The IF of the AD9361 has a low pass filter
of at most 56MHz, ie it offers to capture a bandwith of 56MHz of
frequency space (using both I and Q channels). But the above approach
would need an IF of >200MHz, but it would be enough to only have a
single channel.

I looked up the prices for the components and figured that the prices for
mixer and IF amplifiers are actually quite low (a 2-4 USD per IC) so it
isn't that much more expensive to build such a system than using a 3 tuner
approach (eg using MAX2120 as Peter Monta did with the GNSS Firehose).

		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, Neil Stephenson

On Wed, 5 Apr 2017 10:37:07 +0200
Magnus Danielson magnus@rubidium.dyndns.org wrote:

If part of the signal wraps because you are at the bandedge,
then you lose this part of the signal and the part it wraps over.
This is due to the signal coherently overlapping in frequency space.
As far as I understood the math, there isn't a way to seperate them
again (at least there isn't any I am aware of). Thus this signal energy
is lost for the decoding process.

		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, Neil Stephenson

On Wed, 5 Apr 2017 08:27:58 -0400
Bob kb8tq kb8tq@n1k.org wrote:

I wouldn't call it strange, but rather neat :-)
The E5 signal is created as a single, 8-PSK signal(see [1]), which is
modulated such, that the positive and negative frequency parts get
a specific signal structure. This is done in order to allow an extremely
wide band signal to be demodulated in parts. I guess they feared that a
receiver for a 50MHz wide signal would be too expensive for the
commercial market and made it possible to process the signal as two
20MHz wide pieces. There is a slight loss in correlation energy in this
case, but for most applications it should not matter. The bigger issue
is that the path delays for the two receiver channels would need to be
calibrated and tracked during operation in order to make full use of
the E5 signal.

BTW: I have been told, that using the full E5 signal makes the use
of any other signal kind of unnecessary as its extremely wide bandwidth
allows a very fine tracking of the signal. Thus the use of any other signal
(e.g. E1 OS) would actually degrade the receivers timing performance than
improve it.

As I have written above, to be able to do this is the reason for the E5's
signal structure. And apparently the designers thought that this would be
the way how most users would decode it. I am currently not aware of any
commercial E5 receiver that is already on the market, so it is kind of moot
to ask what the common way to decode E5 is.

BTW: Rodriguez' PhD thesis[2] (which is the basis of navipedia) gives a very
nice overview of the trade-off's that went into the Galileo signals and
gives a few hints where future GNSS signals could further improve things.

		Attila Kinali

[1] Galileo OS SIS ICD Issue 1 Revision 2,
Section 2.3.1.3 "Equivalent Modulation Type"

[2] "On Generalized Signal Waveforms for Satellite Navigation",
by José Ángel Ávila Rodríguez, 2008
https://athene-forschung.unibw.de/node?id=86167

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, Neil Stephenson

HI

Why would you substitute an expensive IF filter for a cheap front end filter?

Bob

Without a “second frequency” you can’t do local ionosphere corrections. That’s
true regardless of the bandwidth of the signals …..

Bob

Hi Attila,

On 04/09/2017 10:29 PM, Attila Kinali wrote:

Your generated signal would have the same wrapping. A single bit sampler
would be feasible to loose in, but for multibit ADCs I'm more skeptic.

Cheers,
Magnus

God Morgon Attila,

On 04/09/2017 10:28 PM, Attila Kinali wrote:

You got it backwards.

You need to protect your LNA and mixer from other signals, not to be
blocked out by out of band signals which is strong. That's why you have
SAW filters to start with. This has become a larger issue these days.

So, considering that you already have them, then what good do they do
for the different scenarios.

Only if you need the Galileo E5.

Cheers,
Magnus

Hi

The other point with E5 is the nature of the data on the various sub signals. Galileo has three
classes of service and only one of them is free (open). As with traditional  L1 / L2 survey receivers, you
don’t have to recover full data from a signal for it to be useful. That said, the free (open) service
is only on one of the two sub signals. If you are building a L1 / L2 / L5 GNSS receiver, you might
well opt to only grab the lower part of the E5 signal.

You might also decide on a setup that only used two of the three bands. That would give you all
the data and ionospheric correction. It is a bit unclear what the third band would add other than a “cool factor”
if traditional criteria are used for the receiver design. There are various arguments for L1 / L5 and L1 / L2. One
could even make a case for L2  /  L5.

Even if ionospheric correction is not a bit issue in your design, jamming probably should be for a design
targeted to run for many years into the future.. A broadband jammer (intentional or accidental) can fairly
easily take out one of the bands. It’s quite a bit harder to take out all of them at once. A lot would depend
on just how nasty an environment you intend to operate in, and how sensitive you are to occasional issues.

Lots of choices ….

Bob

Hi Bob,

Even quicker RTK convergence.
http://www.navipedia.net/index.php/Carrier_phase_ambiguity_fixing_with_three_frequencies

--

Björn

Hi,

On 04/10/2017 03:00 PM, Bob kb8tq wrote:

L2 and L5 is so close, that bringing them down together is relatively
simple.

If you make a receiver today, it should be able to use any set of bands,
including L2 only or L5 only. Even more refined than that, any set of
signals. One should attempt to get any of the transmitted signal, so
there might be L1 from one, L2 and L5 from one, L1 and L5 from one etc.
Each signal contributes. Signal pairs and tripples allow for ionospheric
estimation, with the added benefit.

Indeed. I have been advocating for use of multiple bands, besides L1,
and for multiple GNSS. This helps to build signal redundancy. Such
receivers should not be too expensive eventually.

Cheers,
Magnus

On Sun, 9 Apr 2017 18:13:48 -0400
Bob kb8tq kb8tq@n1k.org wrote:

Availability: Although there are L1/L2 filters available, they are not
easy to get unless you buy them in bulk. The standard L1 filters you
can buy are rather narrow band (just 2-4MHz) and don't allow the
reception of the modern signals. L5 filters are very rare and E5 filters
simply do not exist yet.

And keep in mind that the IF filter does not need to be a special
ultra-steep filter. The high sampling rate of the ADC allows to place
the input signal such, that the stop band can be quite far from the
pass band. Also, the filter is only really necessary to filter out
narrow band interference, which is hopefully far from the signal anyways.
Having a bit of noise fold in is, as Jim noted earlier, not a problem at all.

		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, Neil Stephenson

On Mon, 10 Apr 2017 07:00:27 +0200
Magnus Danielson magnus@rubidium.dyndns.org wrote:

I don't think they are necessary anymore. Todays LNAs have a very high
IP3 (in the order of 10-30dBm) and even IP1dB is usually around 0-10dBm.
Ie unless there is a very strong, narrow band interference, the LNA will
not cause any problems. Same goes for modern mixer.

Or, to make it a bit more practical: if you take an RTL-SDR dongle,
then you have a cheap, zero-IF system that has no frontend filter
and relies solely on selectivity of the antenna (which is often
a cheap puck without any filter) and its IF low-pass filters.
I have used this a few times and have not seen it fail. I took my
bladerf a few times and looked at the spectrum around 1575GHz and
haven't seen any strong interferer yet.

With only L1 and L2 it is still ~180MHz. The two bands are ~350MHz appart.
You cannot get around that without introducing a second down-mix step.

			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, Neil Stephenson

On Mon, 10 Apr 2017 08:08:10 +0100
"David J Taylor" david-taylor@blueyonder.co.uk wrote:

As far as I am aware of, that is the current state.
The GPS/Galileo was signed slightly before that thesis, IIRC.
The L2C and L5C signals are "old-school" LFSR generated PSRN BPSK
signals. The only "special" signals are L1C, E1 OS and E5.

QZSS uses GPS L1 C/A signals, IIRC
Beidu and IRNS I don't know.

If I am not mistaken, navipedia is up to date with everything,
but I have not read everything, nor checked against the standards
what I have read there.

		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, Neil Stephenson

On Mon, 10 Apr 2017 09:00:17 -0400
Bob kb8tq kb8tq@n1k.org wrote:

Yes. Thats why we do not talk about E6, or E1 PRS.
On E5 there are OS signals and CS signals. More accurately, the F/NAV data
which is part of the OS signal, is on E5a, and the I/NAV data, which is
both part of OS and CS is on E5b. Additionally, there is a dataless pilot
on both E5a and E5b.

As far as I am aware of, neither the CS nor the PRS specifications are
public yet. If someone has any information on those, I would be interested.

Yes. But semi-codeless tracking only works because:

1. A lot of the signal structure is known and it is actually such
   that you can correlate quite a bit of it without knowing the P(Y) code.
2. The P(Y) code is send out on both L1 and L2, which allows correlating
   both signals possible.

Without these two points, the use of L2 would not have been possible.
And yes, the US military learned from this and made the M code without
the strucutre that helped correlating it in 1). And they also learned
that not documenting it is the best protection against people using it.
Though I wonder how long it will take until someone figures out what
the signal structure is.

No. See above.

E5a overlaps with L5, The center frequncy of L2 is a mere 20MHz
from the E5b center frequency. So, if you are building an L1/L2/L5
receiver, there is very little point in not investing a little bit
in getting E5b as well.

E5 allowes, due its large bandwidth, a supperior multi-path supression.
But Galileo is not yet fully functional, so using L1 C/A & L2C for now is
the best option. Supporting L5 is a good idea, to make the receiver
future proof (again supporting a large bandwith for multi-path supresion)
but it is not yet known, when L5 will reach full constellation (there
only 12 satellites transmitting, yet). Also L2 was only recently marked
as GNSS band and thus there are still radar systems working in this band,
causing interferences.

		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, Neil Stephenson

This approach is known as “security through obscurity”, and is deprecated in the professional of information security. What one invents, another can discover.

The most secure systems use well-documented algorithms with open-source software — widely scrutinized for bugs or implants, and therefore with well-understood performance limitations. The secrecy comes from good crypto key management.

— Eric

depend how much in-band loss could you afford it is relative easy to
make cavity filters if you have a network analyzer available

73

Alex

On 4/10/2017 9:11 AM, Attila Kinali wrote:

On 4/10/17 2:08 PM, Eric Scace wrote:

The M-code is described in a fair amount of detail here:
www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA456656

On Mon, 10 Apr 2017 16:49:24 -0700
jimlux jimlux@earthlink.net wrote:

Not really. All it says that it's a BOC(10,5) signal using some
code that allows direct aquisition. It doesn't even mention what
the rate of the data on the new signal is. From [1] one can see
that the modulation is a BOC_sin (and not a BOC_cos). And that
is all I could find out about the M code so far.

Unfortunately, all the other papers concerning the M code are
behind the ION paywal, to which I do not have access.
But guessing what else I have seen on the M code, I do not think
they contain much technical detail on the spreading code itself
or the data transmitted or the encryption.

			Attila Kinali 

[1] "Design and Performance of Code Tracking for the GPS M Code Signal"
by John Betz, 2000
http://www.dtic.mil/get-tr-doc/pdf?AD=ADA460257

--
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, Neil Stephenson